Doctoral Degrees (Horticultural Science)

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Aspects of avocado fruit growth and development : towards understanding the 'Hass' small fruit syndrome.
(1997) Moore-Gordon, Clive Scott.; Cowan, Ashton Keith.; Wolstenholme, B. Nigel.
Persea americana Mill. cv. Hass is predisposed towards producing a high proportion of undersized fruit. Reasons for phenotypically small 'Hass' fruit are obscure, but it does appear to be aggravated by adverse growing conditions. A detailed study of the metabolic control of avocado fruit growth was carried out to determine the underlying physiological reasons for the appearance of the 'Hass' small fruit phenotype. Furthermore, the application of a mulch was evaluated as a possible management strategy to increase 'Hass' fruit size. Anatomical and morphological comparisons were made between normal and small 'Hass' fruit in an attempt to characterise the 'Hass' small fruit phenotype. Small fruit always contained a degenerate seed coat and fruit size was closely correlated with seed size. Kinetic analysis of changes in cell number and size during fruit development revealed that growth was limited by cell number in phenotypically small fruit. Analysis of endogenous isopentenyladenine (iP) and abscisic acid (ABA) revealed that ABA concentration was negatively correlated with size of similarly aged fruit. Calculation of the iP:ABA ratio showed a linear relationship with increasing fruit size. Qualitative and quantitative differences in mesocarp sterol composition were observed between normal and phenotypically small fruit. Both the normal and small-fruit phenotypes were used to probe the interaction between end-products of isoprenoid biosynthesis and activity of mesocarp 3-hydroxy-3- methylglutaryl coenzyme A reductase (HMGR) in the metabolic control of avocado fruit growth. In phenotypically small fruit, a 70% reduction in microsomal HMGR activity was associated with a substantial rise in mesocarp ABA concentration at all stages of development. Application of mevastatin, a competitive inhibitor of HMGR, via the pedicel reduced growth of phenotypically normal fruit and increased mesocarp ABA concentration. These effects were reversed by co-treatment of fruit with either mevalonate, iP or the synthetic cytokinin (CK) analogue, N-(2-chloro-4-pyridyl)-N-phenylurea, but were unaffected by gibberellic acid. Likewise, in vivo application of ABA reduced fruit growth and HMGR activity, and accelerated abscission at all stages of development, effects that were reversed by co-treatment with iP. In contrast, the effect of sterols on mevastatin-induced inhibition of fruit growth was temporally different. Application of either stigmasterol or cholesterol during phase I caused a decline in growth, accelerated fruit abscission and exacerbated the effects of mevastatin whereas during phase II and III, stigmasterol reversed inhibition of fruit growth. Stigmasterol did not however, reverse the inhibitory effect of mevastatin on HMGR activity - presumably as a result of mevastatin-induced increased endogenous ABA. It was therefore concluded that ABA accumulation downregulates mesocarp HMGR activity and that in situ CK biosynthesis modulates the effect of ABA during phase I of fruit growth whereas, both CK and sterols perform this function during the later stages to sustain the developmental programme. The effect of an altered CK:ABA ratio on solute allocation, cell-to-cell communication and plasmodesmatal structure was investigated in 'Hass' avocado fruits to determine the relationship between a change in hormone balance and expression of phenotypically small fruit. Exogenous application of ABA induced early seed coat senescence and retarded fruit growth, and these effects were negated in fruit co-injected with ABA and iP. The underlying physiological mechanisms associated with ABA-induced retardation of 'Hass' avocado fruit growth included: diminution of mesocarp and seed coat plasmodesmatal branching; gating of mesocarp and seed coat plasmodesmata by deposition of apparently proteinaceous material in the neck region; abolishment of the electrochemical gradient between mesocarp and seed coat parenchyma; and arrest of cell-to-cell chemical communication. In addition, solute allocation in ABA-treated fruit resembled closely that of phenotypically small fruit confirming that elevated ABA concentration had contributed to the decline in postphloem symplastic continuity. In a field trial in the KwaZulu-Natal midlands, root growth was substantially increased throughout three seasons by the application of a coarse composted pinebark mulch. Mulching resulted in a significant 6.6% increase in mean fruit mass, in spite of 14.7% more fruits per tree. The combined effect was a 22.6% increase in overall yield. Differences in productivity between treatments closely correlated to levels of bark carbohydrate reserves. Data collated during this study to suggest that mulching at least partly ameliorated tree stress included: a reduction in the incidence of premature seed coat senescence and pedicel ring-neck, both of which are considered to be advanced symptoms of the stress syndrome; a lowering of mean foliage temperatures; and a reduction in the degree of photoinhibition during the heat of the day.
Assessing gaseous ozone and edible coatings as postharvest treatments for mango (mangifera indica L.) fruit.
(2021) Bambalele, Nonjabulo Lynne.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.
This research examines the potential of gaseous ozone and edible coatings in preserving postharvest quality and extending the shelf-life of ‘Keitt’ mango fruit. A critical review of the literature focused on the recent postharvest technologies used to preserve the quality of mango fruit. The prospects of using non-chemical postharvest treatments such as gaseous ozone and edible coatings were also reviewed. A screening study was conducted to determine the optimum ozone (O3) application time for effectively maintaining fruit quality and extending shelf-life. Mango fruit were intermittently exposed to gaseous ozone for twelve, twenty-four, thirty-six, or forty-eight hours, and the control fruit were untreated. Fruit were stored at 10℃ for twenty-one days and seven days shelf-life at ambient temperature. The findings showed that the O3 treatment should be applied at the pre-climacteric stage to achieve optimum results. Ozone treatment for 24 or 36 hours effectively maintained firmness and carotenoids content, delayed color changes, decay incidence, and mass loss. Therefore, an ozone exposure time of 24 and 36 hours was adopted for the study. These exposure times were incorporated into edible coatings (moringa leaf extract and carboxymethyl cellulose) for further investigation. The study on the effect of gaseous O3 incorporated with edible coatings on sensory attributes and physicochemical parameters showed that EC and EC + O3 (36 h) were more effective in delaying the ripening process and maintaining the postharvest quality. Overall, consumers preferred the fruit coated with EC due to its attractive color, smell, and sweetness. The study on the postharvest effect of gaseous O3 and EC on antioxidants and the biochemical properties of mango fruit is discussed in Chapter Five. The findings of this study demonstrated that the treatment combination of EC and gaseous O3 (36 h) effectively maintained antioxidants, membrane integrity and enhanced the quality of mango fruit during storage. The effect of gaseous O3 and EC on postharvest diseases of mango fruit, specifically Colletotrichum gloeosporioides (anthracnose) and Lasiodiplodia theobromae (stem-end rot), was also investigated. This study revealed that EC + O3 (24 h) effectively controlled stem-end rot and anthracnose in mango fruit. The treatment combination of EC and O3 (36 h) reduced the mycelial growth and disease incidence of Lasiodiplodia theobromae and Colletotrichum gloeosporioides. The study of EC and O3 in the volatile compounds of mango fruit is discussed in Chapter Seven. The fruit treated with EC had a high content of volatile compounds compared to other treatments. The treatment combination of EC + O3 (24 h) was not effective in maintaining the volatile compounds of mango fruit during storage. The findings of the current study suggest that EC + O3 (36 h) can be used as postharvest treatment of mango fruit. Additional research is required to gain more insights in understanding the EC+ O3 mode of action in maintaining volatile compounds and controlling mango postharvest diseases.
Avocado seed physiology aspects.
(2021) Abdalla, Mamoun Ahmed Arabi.; Bertling, Isa.
The avocado seeds/seedling is needed as rootstock for other economic trees and loss of tress stand in orchids after establishment is of great commercial loss in avocado orchids around the worldwide and South Africa. The aim of this study was to evaluate and compare avocado seeds development of various seeds ages by investigating seeds germination percentage over three generations, as there is little information on avocado seeds growth and development, despite the importance of the seeds in avocado propagation. Seed harvesting was carried out over various developmental stages, from early fruit development to two-year-old seeds (Generation 1, 24 to 29 months after full bloom MAFB). Seed from current season (Generation 2, 12 to 17 MAFB) and newest seeds (Generation 3, 0 to 5 MAFB) of two cultivars (‘Hass’ and ‘Fuerte’) was analysed. Seed of three generations were analysed: ‘Hass’ Generation 1seed (seed from the oldest, commercially over-mature, fruit full bloom in July/ August 2017); Generation 2 (full bloom in July/ August 2018) and Generation 3 (full bloom in July/August 2019). Similarly, ‘Fuerte’ fruit of three generations were compared: from the avocado fruit, (Generation 1, full bloom in June/July 2017), to Generation 2 (full bloom in June/July 2018) to Generation 3 (full bloom in June/July 2019). Seed were extracted from fruit to determine seed parameters, such as germination percentage, seed viability, seed moisture content and seed respiration rate. Further, seed physiological parameters, such as cotyledonal sugars and starch concentrations, seed coat phenolic compound concentrations and polyphenol oxidase (PPO) concentrations were determined. Anatomical features of the seed coat, such as seed coat thickness and seed coat ultrastructure were also observed. In both cultivars, the germination percentage was higher in Generation 2, 12 to 18 MAFB), than in Generation 1, 24 to 29 MAFB) seed from June to September. From October to November Generation 3 (0 to 5 MAFB) had a higher germination percentage than Generation 2. Seed viability was higher in Generation 2 of both cultivars and lower for the Generation 1; similar results were found for the germination percentage, with seed from Generation 2 having a higher germination rate than seed from the Generation 1. Seed viability differed significantly between seed age, and the interaction between generations and months was statistically significant (P ˂ 0.001). The seed collected from fruit of the Generation 2 of both cultivars had a slightly higher moisture content and a higher germination percentage than the Generation 1. Seed moisture content ranged between 54.5 and 62.1 % in ‘Hass’ (Generation 2 seed age 12 to 15 MAFB), harvested in June to September, while the Generation 1 seed age 23 MAFB) seed had a lower moisture percentage (39.2%) in June. ‘Hass’ seed of (Generation 3 seed age 4 MAFB) harvested from October to November had a higher seed moisture than seed from (Generation 2,15 MAFB). ‘Fuerte’ seed showed a similar pattern with the highest moisture percentage (60.5%) in July and the lowest in June (33.2%). (Generation 2’ seed age 13MAFB seed had higher moisture percentages than Generation 1 from June to September, and (Generation 3, 3 MAFB) had higher moisture percentage than Generation 2. Seeds respiration rate, determined following fruit harvest, decreased over the time. Generally, Generation 2 respired more than the Generation 1, from June to September. From October to November 2019 the Generation 3 respired more than Generation 2 seed. The respiration rate of seed extracted from June to September 2018 Generation 2 declined rapidly. The Generation 3 (collected October to November 2019, seed age 4 to 5 MAFB) were characterized by a higher respiration rate than seed of Generation 2, seed age 16 to 17 MAFB; therefore, younger seeds generations respired more than older ones. It is concluded that the contribution of seeds respiration rate to avocado whole fruit respiration decreases with development over the time. The ability of the avocado seed to germinate quickly and produce seedlings is dependent on the carbohydrate reserves in the cotyledons, which make up the bulk of the avocado seed. In seed coats of both cultivars, phenolic concentrations inhibited seed germination of Generation 1, probably due to the higher level of phenolic concentrations in older seed coats. Seed coats generally contained high amounts of phenolics (2.3 mg GAE*g-1 DM for ‘Hass’ and 2.02 mg GAE* g-1 DM for ‘Fuerte’). Seed extracted from Generation 1 fruit in June to September 2018, had a higher amount of seed coat phenolics than those from Generation 2 fruit. In fruit from October to November 2019 the Generation 3 seed coat had lower phenolic concentrations than Generation 2 seed coats, confirming that older seed coats contain more phenolics than younger seed coats. Germination percentages of Generation 3 seed were higher than those of Generation 2 seed. The high phenolic concentration in the seed coats seems to be aligned with the seed turning dark brown upon maturation, probably due to sufficient oxygen present in the fruit to allow phenolic oxidation of the seed coat; the seed coat becoming entirely brown and very thin, could, therefore, be used as an indication that the fruit has reached physiological maturity. Seed at this stage of maturation are, however, characterized by a low germination percentage, possibly due to the seed coat phenolic compounds interfering with germination. This is supported by the positive correlation between lower seed coat phenolic compound concentration and higher seed germination rate for both cultivars (r = 0.11, P ˂ 0.61). Seed coat thickness of Generation 1 (24 to 28 MAFB) and Generation 2 (12 to 16 MAFB) ‘Hass’ seed coats differed, with the younger seed generation displaying thicker seed coats than the older ones (0.51 versus 0.11 mm, respectively). In ‘Fuerte’, in June and July older seed coats Generation 1, 24 to 25 MAFB, respectively) were thicker than Generation 2 (12 to 13 MAFB) (0.46 and 0.15 mm, respectively. There was, however, negative relationship between seed coat thickness and germination percentage (r = -0.11). Polyphenol oxidase (PPO) and phenolic concentrations of avocado seed coats were also investigated in the seed coat of Generation 1and Generation 2 ‘Hass’ and ‘Fuerte’ seed. Polyphenol oxidase (PPO) and phenolic concentrations of the avocado seed coats of the two avocado cultivars of Generation 1, 27 to 26 MAFB) and Generation 2, 15 to 14 MAFB) respectively, seed coats were investigated. During the colder (winter) season (June-August), Generation 1, fully mature ‘Hass’ seed coats showed higher polyphenol oxidase (PPO) concentrations than seed coats from the Generation 2. From October to November the Generation 3 seed coat also had a lower PPO concentration than those of Generation 2. Generation 2 ‘Hass’ seed coats had relatively low PPO concentrations in June /July, when fruit were 12 to 14 MAFB, but PPO concentrations increased thereafter and remained at a higher-level until October/ November. Generation 1 ‘Fuerte’ seed coat had a similar PPO concentration during all investigated months. Phenolic compounds were present in seed coats of both avocado cultivars, with seed coats of older seeds containing a much higher phenolic concentrations than the seed coats of the newer generation. The seed (cotyledons plus embryo) sugar profile was dominated by the C7 sugar perseitol, followed by the C6 sugar, sucrose, while mannoheptulose and glucose were present in very small amounts. Perseitol was present in in both cultivars with 14 months-old ‘Hass’ (September) cotyledons containing 9.8 mg*g-1 DM and 15-months-old ‘Fuerte (September) containing 10.3 mg*g-1 DM. Avocado cotyledons were found to also be a large starch source, probably providing carbohydrates for seed development and germination. The Generation 2, 14 to 15 MAFB) of ‘Fuerte’ and ‘Hass’ had a higher starch concentration than the Generation 1, 26 to 27 MAFB) and similarly, Generation 3, 4 to 5 MAFB, respectively, had higher starch concentration than Generation 2 for both cultivars, indicating the use of this carbohydrate reserve to sustain embryo development. The highest concentration of starch in ‘Hass’ seeds was detected in August as 88.8% of seed DM (Generation 2, seed age 13 MAFB), while for ‘Fuerte’ seed the highest starch concentration was in August at 90.5% of seed DM (Generation 2, 14 MAFB). Starch seems, therefore, more related to avocado seed development than to avocado fruit growth and development. Delaying fruit harvest to October (seed age 16 to 18 MAFB) allows seed to fully mature and to continue accumulating sugars and starch. To improve percentage and velocity of germination, seeds were soaked in various concentrations of aqueous moringa leaf extract (MLE, 0, 2.5, 5.0 and 7.5 % w/v) over different periods (0, 10, 30 or 120 minutes). Younger seed were stronger affected by the increasing MLE concentration. Soaking in 2.5% MLE tended to enhance the germination percentage more so than the other MLE concentrations. The lowest germination percentage was determined for seeds soaked in 7.5% MLE for 120 minutes, indicating that younger seed (from10 to 12 months after fruit set ‘Fuerte’ fruit harvested April to June) should be used as ‘nurse seed’. Overall, this study revealed that avocado seed germination and development do not coincide with the commercial fruit harvesting period, the avocado fruit needs 15 to 18 months to change from its flowering blooming period to a full harvest, and seed age12 MAFB can germinate for both cultivars. The study further confirmed perseitol as the dominant free storage sugar that assists in seed development, while starch is also an important energy provider for the developing embryo.
The cascade of physiological events leading to chilling injury : the effect of post-harvest hot water and molybdenum applications to lemon (citrus limon) fruit.
(2012) Mathaba, Nhlanhla.; Bertling, Isa.; Bower, John Patrick.
New emerging markets such as Japan and the United States require cold sterilisation of South African citrus fruit as a phytosanitary standard against fruit fly. However, citrus fruit are chilling susceptible, with lemons being the second-most chilling susceptible after grapefruit. Chilling injury is a physiological rind disorder; the occurrence of which is despite its prevalence in horticultural commodities, not well understood. Therefore, the aim of this study was to investigate physiological compounds regulating chilling susceptibility or resistance in citrus fruit, with special emphasis on lemons. Furthermore, the potential of hot water dips or “molybdenum soaks” to maintain a certain level of physiological compounds which determine manifestation of chilling injury symptoms in citrus fruit was investigated. Moreover, it was attempted to create an understanding of the order in which physiological compounds mitigate chilling injury. Lemon fruit from different farms known to be chilling susceptible or resistant were obtained during the 2007 and 2008 harvest season. Thereafter, fruit were treated by soaking for 30 min in 1μM NaMo04.2H20 solution followed by a 2 min HWD 47 or 53°C. Treated fruit were waxed, weighed and stored at -0.5°C for up to 28 days and sampled for chilling injury evaluation 7, 14, 21, or 28 days into cold storage. A second evaluation was carried out five days after withdrawal from cold storage to allow development of chilling injury symptoms as a shelf-life simulation. After the second evaluation fruit were peeled, peel freeze-dried, milled using mortar and pestle and stored at -21°C for further physiological analysis. Freeze-dried peel was analysed for soluble sugars (glucose, fructose, sucrose), vitamin C (ascorbic acid), vitamin E (α-tocopherol), β-carotene, polyamines (putrescine, spermine, spermidine), specific flavanones (naringin and hesperidin) using HPLC-UV-Vis detector and proline, total antioxidant assays (FRAP, ABTS, DPPH), total phenolics, total flavonoids, lipid peroxidation using spectrophotometry, as well as for the heat shock protein (HSP70) using electrophoresis and silver-staining. Chilling susceptibility of lemon fruit varied with fruit source; those sourced from Ukulinga and Eston Estates were chilling resistant, while fruit from Sun Valley Estates showed chilling injury symptoms after 28 days of cold storage plus five days shelf-life. Furthermore, hot water dips (HW) 53°C, 1 μM Molybdenum (Mo) and 10 μM Mo plus HW 53°C significantly reduced chilling injury symptoms compared with the control and HW 47°C. In addition, Sun Valley Estates fruit also showed higher fruit weight loss compared with non-chilling resistant lemons. The alignment of higher fruit weight loss during storage with chilling susceptibility ascertains the use of weight loss as a non-destructive parameter for chilling susceptibility. With respect to flavedo sugars, glucose was found to be the dominant soluble sugar with multi-functional roles during cold storage. This plays a significant role in mitigating cellular stress. Chilling susceptible lemons from Sun Valley Estates had low flavedo glucose concentrations and, therefore, little conversion of glucose to ascorbic acid was possible resulting in a low antioxidant capacity. However, treatments with HW 53°C and Mo soaks seemed to enhance the enzymatic conversion of glucose to ascorbic acid leading to a higher antioxidant capacity in the flavedo of such treated fruit. Furthermore, glucose also feeds into the pentose phosphate pathway which is coupled with the shikimate pathway synthesizing secondary metabolites, especially of the phenolics group. The decrease in glucose was aligned to the levels of total phenolics, but not to that of β-carotene, naringin and hesperidin through 28 days into cold storage period. Moreover, as glucose also feeds into shikimate pathway, simultaneously an increase in proline flavedo concentration was observed. Proline is an antioxidant synthesized from glutamate; as cellular glucose decreases so does the total antioxidant capacity during cold storage. Ascorbic acid is a dominant and potent antioxidant in lemon flavedo as proven with the FRAP, ABTS and DPPH assays. Chilling resistant fruit have significantly higher ascorbic acid conversion. Furthermore, ascorbic acid also acts to generate the α-tocopheroxy radical to further important membrane-bound antioxidant, vitamin E (α-tocopherol equivalent). Furthermore, the DPPH assay was found to be effective in quantifying total antioxidants in lemon flavedo since it detects both lipophilic and hydrophilic antioxidants compared with the ABTS and FRAP assays which are bias to the estimation of liphophilic or hydrophilic antioxidants, respectively. The hot water and molybdenum treatments increased total antioxidants (DPPH assay) with reduced lipid peroxidation 7 days into cold storage and therefore, reduced chilling symptoms in fruit from Sun Valley Estates. The capacity of antioxidant to scavenge reactive oxygen species (ROS) was increased during cold storage and membrane stability significantly improved. Furthermore, putrescine as low valency polyamine was reduced as such compound acted as precursor to the synthesis of the high valency polyamines, spermine and spermidine. Chilling susceptible lemons from Sun Valley Estates showed increased soluble-conjugated polyamines as a response to stress. Furthermore, HW 53°C, 1 μM Mo and 10 μM Mo plus HW 53°C significantly increased the protein concentration and, therefore, likely also the occurrence of proteins with 70kDa (as estimator of HSP70). Additionally, the concentration of conjugated high valency polyamines was also increased, resulting in reduced chilling injury symptoms. The effect of ROS has only been viewed as damaging, while recently their role has also been viewed as stress acclamatory signalling compounds when produced concentrations below critical damaging threshold. Therefore, hot water dips seems to signals synthesis of total protein which include HSPs which then act throughout cold stress to protect other protein and channel other damaged proteins towards proteolysis. While molybdenum increased ROS production below damaging critical threshold, with ROS signalling stress acclimation by further signalling production of bioactive compound with antioxidant properties.
Characterisation of ganoderma species using morphological, molecular and biochemical markers and evaluation of substrate enhancement influence on their development and biochemical profile.
(2024) Sihlangu, Sydwell Mcebo.; Magwaza, Lembe Samukelo.; Mditshwa, Asanda.; Tesfay, Samson Zeray.; Ramachela, Khosi.; Mbili, Nokwazi Carol
Ganoderma, also known as Reishi mushroom, is used for its potential health benefits in several countries. The current study characterised Ganoderma species using molecular and biochemical markers and evaluated the substrate enhancement influence on its development and biochemical profile. The overall research study consisted of four objectives. The first objective focused on the isolation and characterisation of fifteen fungal specimens collected from the three provinces of South Africa, namely, Mpumalanga, KwaZulu-Natal, and North- West. Five fungal specimens were collected in each province and growth media potato dextrose agar (PDA), malt extract agar (MEA), and sabouraud dextrose agar (SDA) were used to grow sample isolates. After 8 days of incubation, MEA recorded the highest mycelial diameter followed by PDA and SDA. Samples were identified using comparative morphology traits and supported by internal transcribed spacer region (ITS) and phylogenetic analyses. Based on the ITS of ribosomal DNA, fungal samples KG3SY219 and MG1SY119 were found to be closely related to Ganoderma resinaceum and Ganoderma austroafricanum, respectively. The species were further characterised by biochemical compounds, including antioxidants, proteins, essential elements, and heavy metals. The antioxidant capacity exhibited a higher radical scavenging activity in G. austroafricanum compared to G. resinaceum. The concentrations of total phenolics, flavonoids, proteins, essential elements and heavy metals were more abundant in G. austroafricanum compared to G. resinaceum. This study also evaluated the effect of different growth conditions on mycelial growth and development of Ganoderma austroafricanum and Ganoderma resinaceum. The experimental treatments included three levels of pH (4, 6 & 8), temperature (20, 25 & 30 °C), and different types of plant residues namely; beech sawdust (BS), sugarcane bagasse (SB), and buffalo grass (BG). Three independent in vitro experiments were conducted, PDA and MEA were used as standard growth media to grow each fungal species. Mycelial growth and development were measured over 9 days where they reached maximum growth. The culture media pH results demonstrated that the maximum growth for mycelia was reached on day 9 for both species. Typically, G. resinaceum showed the highest mycelial growth for both cultures except for days 6 and 9 where the mycelial growth of the species was decreased by low levels (pH 4) and high levels (pH 8), respectively. The addition of BG to the growth media delayed the mycelial growth of G. resinaceum for both growth media (PD+MEA). Experiment three investigated the effect of different substrates on the development, total biomass, and biochemical profile of Ganoderma species. The experiment involved growing G. austroafricanum (GA) and G. resinaceum (GR) on different substrates; beech sawdust (BS), sugarcane bagasse (SB), and buffalo grass (BG), and suspension of all substrates (BSSBBG). The growth parameters such as pileus size, weight, total biomass, and biological efficiency were measured. Additionally, each substrate was analysed for biochemical composition. The biochemical composition of the harvested samples was also analysed to determine the levels of biochemical compounds such as minerals, antioxidants, and protein. The substrate pH levels demonstrated that all substrates were within the optimal growth pH range (5-6). SB exhibited greater levels in the majority of essential elements such as Zn and K, also, heavy metals Pb and Hg. The results on the development and total biomass production of Ganoderma species revealed significant variations across different substrates. In terms of development, GRBS was faster to reach the 100% rate of all production parameters in 40-52 days after inoculation. However, GASB exhibited higher quantities in total yield and biological efficiency. In addition, pileus from GASB demonstrated higher concentrations of all evaluated biochemical compounds. GASB also yielded higher levels of DPPH, phenolic compounds, flavonoids, and protein. Experiment four examined the impact of substrate fortified with essential elements on the development, total biomass, and biochemical compounds of Ganoderma species. The experiment involved growing G. austroafricanum (GA) and G. resinaceum (GR) on beech enhanced with elements; no element (Control), Zn(NO3)2.6H2O (Zn), Fe2SO4 7H2O (Fe), Na2SeO3 (Se), and suspension of all essential elements (ZnFeSe). The growth parameters such as the pileus size, weight, total biomass, and biological efficiency were measured. In addition, the biochemical profile of Ganoderma spp. was analysed to evaluate the concentration of compounds. The development and total biomass production findings for the substrate fortified with essential elements exhibited significant differences. GRZn developed expeditious, reaching 100% of all production parameters in 52 days after inoculation. In comparison to all treatments, GAFe showed larger quantities in total yield and biological efficiency. The substrate enhancement with Zn had a significant increase in the majority of minerals. GAZn exhibited higher concentrations of essential elements such as Zn, K, and Mg. Higher levels of heavy metals such as Cd, Pb, and As were recorded from GAControl. GASe produced higher levels of DPPH, phenolic compounds, flavonoids, and protein reading. These findings demonstrate the variability of morphological characteristics, biochemical compounds, and growth conditions requirements between Ganoderma species. These findings provide valuable insights into the diversity, taxonomy, and potential therapeutic applications of Ganoderma species in South Africa. Further investigation is required to identify Ganoderma species and its pharmaceutical properties.
Ecophysiological studies and tree manipulation for maximisation of yield potential in avocado (Persea americana Mill.)
(1994) Whiley, Anthony W.; Wolstenholme, B. Nigel.; Schaffer, B.
Tree fruit crops generally consist of scion and rootstock components, which through interactive synergism affect tree performance. Coupled with tree architecture, sink/source relationships (both spatial and temporal), genotypic responses to environments, and carry-over seasonal effects present a high level of complexity which often confounds research results. The development, description and use of pheno/physiological models as research and crop management tools is a new holistic approach to reduce complexity and improve understanding of the critical factors which influence crop productivity. A pheno/physiological model is described for cv. Hass avocado growing in a cool, mesic subtropical environment in S.E. Queensland, Australia. Seasonal shoot and root growth had bimodal periodicity with root growth offset and delayed with respect to shoot growth. The priority sink strength of developing shoots compared with roots was confirmed with 14(C) studies. Root growth in summer extended through until late winter when there was a substantial decline following anthesis - a critical time in fruit development with competition between reproductive and vegetative sinks for limited resources. Delayed harvesting of fruit over several seasons resulted in alternate bearing patterns, while removal of fruit at the minimum legal maturity of 21 to 24% dry matter sustained successive high yields. With cv. Hass, production was directly related to starch concentrations in trunks or shoots in July (midwinter) immediately prior to anthesis. However, seasonal starch concentration fluxes in trunks were much lower in coastal subtropical Australia compared with those previously reported from interior areas in more southerly latitudes (7.5% vs. 18% maximum). Current assimilate from over-wintered leaves was necessary to bridge the gap in early spring between the depletion of starch reserves by new reproductive and vegetative shoot growth, and the sink/source transition of the spring shoot growth. Net CO2 assimilation of summer grown leaves reached ca. 17 µmol CO2 m(-2) s(-1), approximately twice as high as previously reported rates on container-grown plants or trees in minimum temperatures were < 10⁰C for 50 days, this being the first report of this phenomenon in field-grown avocado trees. Partial recovery occurred prior to senescence of previous season's leaves in spring after minimum temperatures increased above 10⁰C. The plasticity of the light response was high with the compensation point for net CO2 assimilation at 30 µmol quanta m(-2) s(-1) and the light saturation point at 1270 µmol quanta m(-2) s(-1). Net CO2 fixation from fruit photosynthesis was always less than losses through respiration but was highest during the first few weeks of ontogeny, perhaps contributing to the fruit's own carbon economy at a time when competition for assimilates was greatest. In general, CO2 assimilation studies with current technology applied to orchard trees in non-restrictive soils have elucidated efficiencies more akin to deciduous than evergreen trees - thereby compensating for short-lived leaves and energy expensive fruits. Pheno/physiology models were used to substantiate the most effective timing for trunk injection of ambimobile phosphonate fungicides for the control of Phytophthora root rot, a serious disease of avocados, viz. at the completion of the leaf expansion phases when leaves were strong net exporters. Preliminary studies demonstrated potential yield increases when the assimilation efficiency of photoinhibited over-wintered leaves was improved through increased nitrogen concentration, and spring shoot growth was partially suppressed with foliar sprays of the growth retardant paclobutrazol.
Effect of canopy position and non-detructive determination of rind biochemical properties of citrus fruit during postharvest non-chilling cold storage.
(2017) Olarewaju, Olaoluwa Omoniyi.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.; Fawole, Olaniyi Amos.; Opara, Umezuruike L.
No abstract provided.
Efficacy of enhanced freshness formulation as a novel postharvest treatment for gold kiwifruit (actinidia chinensis)
(2023) Mthembu, Sisanda Sibusiso Luyanda.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray .
When exporting kiwifruit, the main limiting factors are excessive fruit softening and fungal decay. Furthermore, exposing kiwifruits to low temperatures induces the mechanisms involved in the softening process. In addition, kiwifruit become much more susceptible to fungal decay as they soften, which exacerbates deterioration of kiwifruit during transit. Fruit firmness is an important criterion for the market value of kiwifruit and storage life for the wholesale and retail trade. Therefore, loss in firmness is a serious problem resulting in postharvest and economic losses. Given that cold storage alone is not enough to optimise firmness retention of kiwifruit, the current study aimed to investigate the capacity of enhanced freshness formulation (EFF) to reduce kiwifruit softening. Kiwifruits were subjected to eight weeks of storage at 0 °C and 90 % relative humidity, followed by one week at ambient storage. The efficacy of EFF to regulate the mechanisms behind the softening process such as cell wall degradation, membrane deterioration and fungal infection was evaluated. Furthermore, the efficacy of EFF to preserve bioactive compounds was also evaluated to assess the effect of this treatment on these highly appreciated quality attributes. The first study evaluated the efficacy of EFF to delay softening of kiwifruit harvested at both the mid and late maturity stage. The influence of EFF on cell wall polysaccharides (CWPs) and cell wall degrading enzymes (CWDEs) was assessed. The findings showed that EFF optimised firmness retention of kiwifruit by preserving CWPs through the suppression of CWDEs which are responsible for initiating fruit softening. Furthermore, EFF maintained fruit quality and marketability, characterised by significantly lower mass loss, total soluble solids and higher titratable acidity. The second study examined the capacity of EFF to inhibit the excessive production of reactive oxygen species (ROS) which induce membrane damage, resulting in softening and senescence. EFF treatment effectively delayed the progression of kiwifruit senescence by reducing membrane deterioration and the accumulation of ROS. The mechanism by which EFF reduced membrane damage caused by ROS was attributed to its capacity to enhance the activities of antioxidant enzymes involved in neutralising ROS and suppressing Phospholipase D which destroys membrane integrity, resulting in improved storability of treated fruit. The third study assessed the efficacy of EFF to inhibit the fungal growth of Botrytis cinerea, which one of the major postharvest pathogens of kiwifruit that contribute to excessive softening. The findings show that EFF effectively suppressed the fungal growth of Botrytis cinerea in a dose -dependent manner. The results demonstrate that EFF can serve as a potential disease control strategy for kiwifruit at the postharvest stage. The mechanism by which EFF suppressed fungal decay can be attributed to the treatment’s capacity to induce disease resistance against Botrytis cinerea, by enhancing the synthesis of secondary metabolites and the action of defence-related enzymes, thus conferring greater protection against pathogen attack. The fourth study investigated the ability of EFF to retain the antioxidant quality of kiwifruit during prolonged storage. The bioactive compounds of EFF-treated kiwifruits were effectively maintained throughout the storage period. The findings of the present study further revealed that the mechanism by which EFF optimises antioxidant retention is due to its ability to regulate enzymes involved in the synthesis and oxidation of bioactive compounds. Despite the positive results, further research on transcriptome analysis is needed to elucidate how EFF regulates softening and antioxidant enzymes.
Ethephon-induced changes in macadamia nuts: implications for dietary indicators, nutritional quality, and postharvest shelf life.
(2024) Aruwajoye, Noluthando Noxolo.; Tesfay, Samson Zeray.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.
The thriving macadamia nut industry, prized for its nutritional value, grapples with the challenge of inconsistent abscission seasons, leading to variable harvest times and increased operational costs. To address this, the traditional application of ethephon induces uniform nut abscission and streamlines harvesting. However, it is crucial to investigate how ethephon impacts the nutritional quality and postharvest shelf life of macadamia nuts. This study focuses on the effect of ethephon on the postharvest quality of the ‘788’ and ‘Beaumont’ cultivars, comparing various categories: nuts that successfully abscised due to ethephon (ED), those manually picked from ethephon-treated trees (ET), nuts naturally dropped from control trees (CD), and those manually picked from control trees (CT). Guided by industryrecommended doses, Ethephon 480 SL® was administered using a Cima mist blower to facilitate nut abscission, specifically targeting physiologically mature nuts. The research explores the influence on fatty acids, dietary indicators, biochemical components, and polyphenol oxidase (PPO) activity. Various parameters, including total phenolics, flavonoids, antioxidant activities, sucrose, and total protein, were assessed across treatments using standardized procedures. samples were collected over a 72-day accelerated storage period. The study further assessed the effect of Ethephon on the physical attributes of macadamia during a 56-day accelerated storage, analyzing mass, diameter, colour (L*, a*, b*, and hue angle), and texture. Kernels were roasted at 125 °C for 15 minutes using a hot air oven dryer post drying and dehusking. Models were then developed to predict the firmness and colour parameters of macadamia nuts harvested with ethephon from the ‘788’ and ‘Beaumont’ cultivars based on drying days and storage conditions. The aim was to determine the minimal processing requirements for achieving satisfactory postharvest nut appearance. Input parameters considered included storage temperature, drying temperature, and storage days, vi while the resulting outputs were firmness and colour parameters (L*, a*, b*, and hue angle). The range of the input parameters spanned from -22℃ to 25℃ for storage temperature, 35 to 57℃ for drying temperature, and 0 to 70 days for storage duration. Significant changes were observed in the fatty acid profiles of macadamia nuts exposed to ethephon treatment. Notably, ethephon led to a substantial increase in stearic acid content, reaching 24,622 µg/g in (ED) after 72 days, compared to 16,764 µg/g in (CD), thereby impacting their nutritional quality. Conversely, ED nuts exhibited a reduction in unsaturated fatty acids (USFAs). Hierarchical clustering analysis identified strong correlations between ED and saturated fatty acids (SFAs) in both ‘Beaumont’ and ‘788’ cultivars, with coefficients of 0.78 and 0.80, respectively. This was accompanied by an increase in atherogenic indices, thrombogenic index, and saturation index, along with a decrease in the hypocholesterolemic/hypercholesterolemic ratio. Concurrently, a robust correlation (0.97) between sucrose and Polyphenol Oxidase (PPO) was observed in ‘Beaumont’, underscoring the influence of ethylene treatment. Notably, ED ‘Beaumont’ nuts exhibited the highest sucrose content at 18.63 mg/g, coupled with elevated PPO activity at 1.06 U g-L. For the physical attributes, ED exhibited a notable influence on the ‘Beaumont’ cultivar, particularly affecting parameters such as a*, b*, mass, and diameter. Conversely, in the ‘788’ cultivar, Principal Component Analysis (PCA) revealed that CD treatment held prominence, showing elevated values for firmness, a*, and b*. Furthermore, model development demonstrated significance, with high coefficients of determination ranging between 0.83 and 0.97. Notably, under reduced drying days and optimal storage temperature, the models predicted maximum L* values of 76.253 and 79.748 for the ‘788’ cultivar. This study provides a theoretical framework for identifying optimum set points crucial for the effective preservation of both the ‘788’ and ‘Beaumont’ varieties of macadamia nuts, particularly in preventing external discolouration. The proposed set points aim to enhance the quality and overall market value of these macadamia nut varieties. The overall findings offer a comprehensive perspective on the effects of ethephon application on macadamia nuts. Stakeholders, producers, and processors in the macadamia nut industry can leverage this research to make informed decisions that enhance overall efficiency and quality while ensuring optimal nutritional value and extended shelf life for macadamia nuts.
Factors affecting mesocarp discolouration severity in 'Pinkerton' avocados (Persea americana MILL.)
(2005) Van Rooyen, Zelda.; Bower, John Patrick.
The susceptibility of the 'Pinkerton' avocado cultivar to mesocarp discolouration, after storage, has seriously threatened its export from South Africa. This disorder has proven to be complex, requiring a better understanding of the fruit's physiology. The purposes of this study were to identify the role of pre- and postharvest factors, or their interactions, in the development of the problem. This was done by obtaining fruit from several production areas of varying mesocarp discolouration histories (referred to as "high", "medium" or "Iow risk" areas) during the 2000 and 2001 seasons. Fruit were stored at 8, 5.5 and 2°C for 30 days, as well as ambient (20°C). Evaluations of fruit quality were made before and after storage, as well as after softening. Once removed from storage the weight loss (during storage) was determined, and fruit firmness and carbon dioxide (C02) production rates monitored daily. It was found that temperatures below the recommended shipping temperature of 5.5°C, i.e. 2°C, produced the best internal fruit quality. This was supported by the membrane integrity studies that showed less membrane stability at the warmer storage temperature of 8°C. Furthermore, remained hard during storage and subsequently had an extended shelf life. Fruit origin was also found to play a major role in browning potential, with discolouration being consistently more severe in fruit from "high risk" areas and increasing in severity as the season progressed. The rate of CO2 production was found to follow a similar trend, with rates increasing as the season progressed, and also being slightly higher in fruit from "high risk" areas. The higher CO2 production rates were thought to be related to a decrease in membrane integrity as the season progressed. While storage temperature was not found to have a significant effect on the rate of CO2 production after storage, it did affect the time taken to reach the maximum rate, with fruit stored at 2°C taking longer. Biochemical analyses to determine the concentration of total phenolics and the activity of the enzyme polyphenol oxidase (PPO) also showed that the potential for browning was initiated by preharvest conditions. While no significant differences were found between growers with regards to total phenol concentrations, the PPO activity was found to be higher in fruit from poor quality areas, and subsequently browning potential was expected to be higher in these fruit. It was, however, found that the potential for browning could be reduced by storing fruit at 2°C, as this decreased the total phenolics concentration. This evidence further emphasized the idea that storage at 2°C could be highly advantageous. Fruit mineral analysis showed that certain key elements played a significant role in the severity of mesocarp discolouration, with excessive fruit nitrogen and decreasing copper and manganese concentrations appearing to play major roles. The high fruit nitrogen concentrations were suspected to reflect fruit grown on very vigorous trees, resulting in shoots competing with fruit for available reserves. It is suggested that 'Pinkerton' of a quality acceptable to the market, can be produced by manipulating source:sink relationships, particularly through decreasing the availability of nitrogen, followed by low temperature (24° C) shipping. Future work should concentrate on manipulation of source:sink relationships, to take account of both climatic conditions and leaf to fruit ratios. The evaluation of chlorophyll fluorescence as a tool for predicting mesocarp discolouration potential in 'Pinkerton' proved to be unsuccessful in this study and future studies may require modifications to the current technique. It is suspected that differences in chlorophyll content, for example, between fruit from different origins, will have to be taken into account when interpreting results. The success of using 2°C storage to improve the internal quality on 'Pinkerton' fruit prompted further studies, during 2004, to ensure that the development of external chilling injury would not decrease the marketability of the cultivar. Low temperature conditioning treatments, prior to storage, proved to be highly successful in reducing the development of external chilling injury, thus further improving fruit quality as a whole. Preconditioning treatments consisted of fruit that were kept at either 10°C, 15°C or 20°C for 1 or 2 days before being placed into storage for 30 days at 2°C or 5.5°C. All preconditioning treatments were compared to fruit that were placed directly into storage. The effect of fruit packaging on moisture loss (as determined by weight loss) and chilling injury was also investigated using unwaxed fruit, commercially waxed and unwaxed fruit individually sealed in micro-perforated polypropylene bags with an anti-mist coating on the inside (polybags). Holding 'Pinkerton' fruit, regardless of packaging treatment, at 10°C for 2 days prior to storage at 2°C or 5.5°C significantly decreased the severity of external chilling injury. The use of polybags during preconditioning and storage showed potential in further reducing the development of external chilling injury, although the higher incidence of fungal infections in these fruit needs to be addressed. The determination of proline concentrations in fruit exocarp tissue after storage was helpful in determining the level of stress experienced by fruit that were subjected to different packaging and preconditioning treatments. In this study waxed fruit subjected to 1 d preconditioning at 10°C, 15°C or 20°C or placed directly into storage at 2°C showed very high proline concentrations and also displayed more severe external chilling injury, despite unwaxed fruit losing more weight during these treatments. The role of moisture loss thus needs further investigation. The thickness and method of wax application was thought to play an important role in the higher external chilling injury ratings in this study as waxed fruit often developed chilling injury symptoms around the lenticels and it was suspected that either the lenticels were damaged by the brushes used to apply the wax or that the lenticels became clogged thus resulting in reduced gaseous exchange. Nevertheless, the success of low temperature conditioning in reducing external chilling injury, while maintaining sound internal quality, may enable storage temperatures to be dropped even further, thus enabling South Africa to export avocados to countries that require a cold disinfestation period prior to entry to eliminate quarantine pests (e.g. fruit fly).
Gladiolus scabridus - the road to conservation and commercialisation.
(2005) Campbell, Tracey Barbara.; Bower, John Patrick.; Van Niekerk, Renate.
There is at present a growing concern and awareness of the endangered status of many indigenous South African plants in the wild, a number of which have potential for commercial production. One such example is Gladiolus scabridus, a vulnerable species endemic to the mountains of northern KwaZulu-Natal and southern Swaziland. It has considerable potential due to its floral characteristics. However, little is known about its horticultural requirements. Thus propagation and cultural practices were investigated with the aim of both conservation and commercialisation of the species. The ideal conditions for G. scabridus seed germination were determined. The presence or absence of light had no significant effect. Optimum germination was achieved at 20 QC of both winged and dewinged seeds. Higher temperatures appear to have a negative effect on germination and none of the winged seeds germinated at 30 QC. Under ideal conditions, fresh seed showed significantly higher vigour and viability than stored seed although the germination of stored seed was hampered by a higher internal fungal content. Successful tissue culture protocol was established for G. scabridus axillary bud and cormel halve explants. During the initial shoot initiation and proliferation stages (stage I and 11), the significantly higher shoot numbers occurred in the absence of growth regulators in both explant types. However, shoots with 1.0 mg 1 -1 6-benzyl-amino-purine (BAP) and 0.5 mg 1 -1 1-naphthalene-acetic-acid (NAA) were healthier in appearance. Higher levels of 5.0 mg 1 -1 NAA inhibited shoot production and encouraged root development in cormel halve explants. During stage 11, axillary bud explants showed root and cormlet development. More roots were initiated without growth regulators, whilst 5.0 mg 1 -1 NAA resulted in significantly better cormlet development. Shoot and cormlet growth of cormel halve explants during stage II was not significantly affected by the presence or absence of NAA and BAP. Significantly more roots were produced with 5.0 mg 1 -1 NAA. During stage III (rooting), the presence of activated charcoal (AC) was essential for the initiation and development of roots in vitro. Root and cormlet development in all explant types was significantly affected by the interaction between the previous treatments from stages I and 11 and the new treatments. During stage IV (hardening off) , most explants died down in the hardening off media leaving resting cormlets. There was a significant interaction between 1.0 mg 1 -1 NAA and 0.3 % AC from stage III, resulting in significantly more cormlets in both axillary bud and cormel halve explants. There was successful cormlet growth after cold storage which is advantageous in reducing the need for acclimatization. G. scabridus corms were successfully forced out of their normal flowering period. Temperatures of 10,5.5 and 2 QC showed successful corm initiation although the corms need to be stored for longer at warmer temperatures. It is suggested that a treatment of 2 QC for 6 weeks is the optimal condition for forcing G. scabridus corms. Although originating from a stressful environment, G. scabridus appears to show capacities for improved growth under controlled conditions. Fertilizer applications enhanced growth and reduced the time to flowering. Nitrogen (N) was found to be important for vegetative growth, flowering and daughter corm development, whereas potassium (K) influenced cormel production. Fertilizer with higher N and lower K is appropriate for the beginning and middle of the growing season and then adjusted to a lower N and higher K fertilizer to promote cormel formation. G. scabridus was found to produce prolific numbers of cormels which is an important source of plant material. Mineral leaf analysis showed that optimum levels for wild species are lower than those for hybrid gladioli with the optimum levels affected by physiological corm maturity and subsequent plant growth. Norms for postharvest handling of cut G. scabridus spikes have been developed. Spikes held in 2 % sucrose had a longer vase life and better floret opening and quality than those kept in distilled water, Prolong, Chrysal, 2 % sucrose and 2 % ethanol, 1 % sucrose, 4 % sucrose, 1 % sucrose and 0.5 % JIK, 2 % sucrose and 0.5 % JIK and 4 % sucrose and 0.5 % JIK, 2 % fructose and 2 % glucose. The use of commercially available solutions should be used with caution. Florets produced a climacteric-like CO2 peak, but levels of ethylene were unmeasurable. Cold storage and the use of polypropylene sleeves delays senescence. G. scabridus spikes secrete droplets of a sticky substance which was confirmed to be extra floral nectar through HPLC analysis. Market research revealed a positive response to the species from consumers and retailers alike with potential for cultivation as a cut flower and bedding plant. However, the cost will determine supply and demand. A field study conducted at Bivane Dam, northern KwaZulu-Natal, confirmed that G. scabridus colonies prevail in rocky, quartzite outcrops where they become wedged between the rocks. Plants were found at different stages of development with populations of not more than 108 plants per colony. Soil data of G. scabridus sites was compared to that of two sites nearby. It was found that G. scabridus soils are higher in phosphorus (P), zinc (Zn) and organic carbon. Leaf analysis confirmed that they have adapted their growth to low nutrient levels. The G. scabridus studies have clearly shown that the species can be successfully moved from a wild plant to a commercially viable one and in so doing its conservation status can also be improved.
Induced polyploidy as a tool for the development of novel South African indigenous crops.
(2015) Hannweg, Karin Fiona.; Bertling, Isa.
Polyploidy is a naturally-occurring phenomenon in plants and has been reported as an important pathway for evolution and speciation; it is estimated that a large percentage of flowering plants are polyploid in origin. Although the first plant polyploid was discovered over a century ago, the genetic and evolutionary implications of polyploidy have not been fully elucidated. On a more practical level, there are many opportunities for utilizing induced polyploidy as a valuable tool in traditional plant breeding programmes. South Africa has the highest recorded plant species density in the world, however, many of these species have only marginal potential due to size and other constraints. Induced polyploids may be expected to exhibit one, or more, of the following characteristics resulting in the improvement or the development of new economically important plants: larger tuber, rhizome or root size; increased flower or fruit size; enhanced flower colour intensity, improved drought tolerance, increased bio-mass; improved photosynthetic capacity; larger and/or thicker leaves; dwarfism; increased secondary metabolite production, e.g. medicinal compounds. Several plant species (Crocosmia aurea, Tetradenia riparia, Siphonochilus aethiopicus and Plectranthus esculentus) were selected for the induction of polyploidy and various horticultural characteristics evaluated. Methods for the successful induction of polyploidy were developed for all selected species. By evaluating various horticultural characteristics of the induced polyploids it was determined that flower size, plant vigour and nematode resistance, as well as essential oil content and bioactivity could be significantly improved in all tested species. Induced polyploidy could, therefore, have a significant impact on the development of economically-viable novel crops indigenous to southern Africa.
Investigating the effect of thermal processing on biochemical composition and kernel shelf-life of macadamia (macadamia integrifolia).
(2018) Buthelezi, Nana Millicent Duduzile.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.
Rancidity is a major limiting factor affecting the postharvest quality and consequently, the storability and market value of macadamia nuts. Initial high moisture content accelerates the primary stages of rancidity where hydroperoxides accumulate as main oxidation products, eventually breaking down to form low molecular weight oxygenated constituents such as alcohols, aldehydes, ketones and free fatty acids, eventually resulting in the development of off-odours and off-flavours. Hydroperoxides can also react with amino acid residues in the Maillard reaction, thereby initiating excessive browning. Kernel browning may be evident as surface discolouration or internal as ‘concealed damage’ of nuts. Internal browning may be accompanied by off-odours and off-flavours and is impossible to detect during processing, often with no visible signs. Such kernels are unacceptable to both the export and local market. The aim of this chapter was to review the potential of thermal processing on delaying the onset of rancidity and therefore, improving kernel shelf life and nutritional quality of macadamia nuts; and to review the reliability of visible to near infrared spectroscopy (Vis/NIRS) to non-invasively predict kernel rancidity. Keywords: Rancidity, browning, free fatty acids, near infrared spectroscopy
Management of avocado postharvest physiology.
(2011) Blakey, Robert John.; Bower, John Patrick.; Bertling, Isa.
Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction.
Methyl jasmonate and salicylic acid enhance chilling tolerance in lemon (citrus limon) fruit.
(2013) Siboza, Xolani Irvin.; Bertling, Isa.; Odindo, Alfred Oduor.
South African ‘Eureka’ lemon fruit must be exposed to chilling temperatures (± 0.6°C) as a mandatory quarantine treatment against insect pests for all its overseas markets. Chilling lemon fruit at such temperatures may develop chilling injury (CI) symptoms on the flavedo. This negative effect on fruit quality reduces fruit marketability. This study evaluated postharvest factors influencing physiological, biochemical and ultra-structural mechanisms involved in alleviating CI in lemon fruit. It was hypothesised that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by maintaining cellular integrity and inducing synthesis of enzymatic and non-enzymatic antioxidants. Furthermore, fruit susceptibility to CI was associated with the source of fruit. Lemon fruit were harvested from three locations representative of moderate subtropical, warm temperate and cool subtropical environments. Harvested fruit were treated either with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, stored either at -0.5, 2 or 4.5°C for 0, 7, 14, 21, or 28 days and afterwards transferred to 23°C for a week as shelf-life simulation. Thereafter, fruit were evaluated for alterations in physiological, biochemical and ultra-structural features involved in the manifestation of CI symptoms. Chilling damage was more severe in untreated lemon fruit than in treated lemon fruit. Storing lemon fruit at 4.5°C accelerated the manifestation of CI symptoms more so than at 2°C while storage at -0.5°C delayed the manifestation of CI symptoms. Lemon fruit of moderate subtropical origin were more chilling-tolerant than lemon fruit of warm temperate and cool subtropical origin. Treatment with 10 μM MJ plus 2 mM SA significantly (P < 0.05) improved chilling tolerance in lemon fruit. This treatment effectively maintained membrane integrity, thereby retarding electrolyte leakage and membrane lipid peroxidation as well as mass loss and respiration rate. Treatment with 10 μM MJ plus 2 mM SA was also effective in enhancing the antioxidant concentrations of vitamin E and carotenoids. The production of these antioxidants could have been part of a defence system against chilling damage, reducing CI and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA enhanced the concentration of compounds involved in chilling resistance, such as proline, soluble sugars, ascorbic acid and total phenolics as well as the enzyme phenylalanine ammonia-lyase (PAL). The enhancement of the defence mechanisms may have played a role in enhancing chilling tolerance in lemon fruit. The treatment also inhibited certain enzymes involved in tissue browning, such as peroxidase (POD) which might have contributed to delaying manifestation of symptoms. Polyphenol oxidase (PPO) was found to not be a good biochemical marker of the occurrence of CI. Treatment with 10 μM MJ plus 2 mM SA appeared to be able to enhance chilling tolerance in lemon fruit by maintaining the ultra-structure of the cuticle, cell wall integrity, cell membrane of parenchyma cells of the flavedo. This treatment also preserved the mineral nutrients of the flavedo (carbon, oxygen, phosphorus, potassium, calcium, magnesium, sulphur, sodium, silicon and aluminium) during cold storage. This could have played a role in protecting the fruit against chilling stress and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA reduced ROS production, while the activity of enzymatic antioxidants such as catalyse (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and accumulation of essential proteins was enhanced. This increase in activity of enzymatic antioxidants and the presence of stress-responsive proteins in the lemon flavedo could have been directly involved in enhancing chilling tolerance. The CI symptoms were accompanied by an increase in membrane permeability, membrane lipid peroxidation as well as phospholipase D (PLD) and lipoxygenase (LOX) activity; however, treatment with 10 μM MJ plus 2 mM SA effectively reduced the membrane permeability, membrane lipid peroxidation, and PLD and LOX activity induced by the cold treatment. This could have contributed to the efficacy of 10 μM MJ plus 2 mM SA in inhibiting the manifestation of CI symptoms. Treatment with 10 μM MJ plus 2 mM SA enhanced flavedo total antioxidant capacity measured by ferric reducing ability of plasma; 2,2-diphenyl-1-picrylhydrazyl; 2,2-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) and the oxygen radical absorption capacity assays. The enhancement of antioxidant capacity in lemon flavedo could have contributed to the fruit’s chilling tolerance. Therefore, the effect of 10 μM MJ plus 2 mM SA treatment, enhancing chilling tolerance, may be attributed to its ability to enhance enzymatic and non-enzymatic antioxidants; activate essential proteins and mitigate the effect of ROS accumulation. With the use of 10 μM MJ plus 2 mM SA treatments, the South African citrus industry will be able to meet the quarantine temperature requirements for exportation of lemon fruit whilst reducing economic losses, depending on the preharvest conditions experienced by the fruit in each shipment.
A multi-factor study of cabbage production in the Umlaas River valley.
(1995) Askew, Derek James.; Smith, Irwin Elcoate.; Le Roux, Fanie.; Wolstenholme, B. Nigel.
A research and extension programme monitored 59 cabbage crops and many factors associated with cabbage production in the Umlaas River Valley of KwaZulu-Natal, from July 1991 - June 1993. A large data bank was developed, accumulating all information regarding soils, weather patterns, farming practices, management, crop water stress, yields, plant populations, weeds, pests, diseases, soil fertility, leaf nutrient concentrations and the presence of hormone herbicides in the rain. Analysis of this large data bank of information enabled the identification of limiting factors and optimized production practices. Excessively high or low levels of soil nutrients and soil acidity problems, reduced cabbage yields in the Umlaas River Valley. Identification of critical values and an ideal cation ratio in soils, and recommendations to rectify limiting factors, could drastically improve yields. Wide variation and excesses of soil nutrients were seen from soil analysis of 59 cabbage crops in the Umlaas River Valley. Soil P > 155, K > 486 and Mg > 568 mg 1(-1), exchangeable acidity> 0.11 cmol(c) 1(-1), acid saturation> 2 % and pH < 4.51 resulted in lower yields. Top yields were found between a certain range of soil nutrient levels and critical values lay within this range. Optimal yields resulted with an ideal ratio of Ca, Mg and K (65 :25: 10) in plots monitored. A soil acidity complex affected cabbage yields in the Umlaas River Valley. High exchangeable acidity, acid saturation and low pH increased Mn and Zn availability in the soil. Both Mn and Zn were absorbed in large quantities by the plant and maximum levels of 406 mg kg(-1) and 114 mg kg(-1) respectively were recorded in most recently mature (MRM) cabbage leaves at headform. Mn concentrations > 100 mg kg(-1) at headform and 50 mg kg(-1) at harvest reduced yields. Nutrient analysis results of a MRM leaf at headform, and a 1/8 head slice of a mature head at harvest, generated leaf nutrient norms and crop removal figures for cabbage. Adequate nutrient levels of MRM leaf at head form were: N = 3.3 - 4.8 %, P = 0.32 - 0.55 %, K = 2.1 - 4.2 %, Ca = 1.3 - 2.5 %, Mg = 0.25 - 0.65 %, Mn = 15 - 100 mg kg(-1), Zn = 18 - 60 mg kg(-1) and Cu = 4 - 100 mg kg(-1). Nutrient concentrations above or below adequate ranges resulted in lower yields. Nutrient removal figures were: 1.9 kg N, 0.3 kg P, 2.03 kg K, 0.43 kg Ca, 0.19 kg Mg, 2.8 g Mn, 2.6 g Zn and 0.4 g Cu t(-1) fresh material. These leaf nutrient norms were developed for reliable identification of soil fertility problems and the crop removals for improved fertilizer recommendations. A study of fertilization practices showed that most farmers supplied enough (+ 200kg) N to cabbage crops but used incorrect pre- and postplant proportions (1/4 and 3/4) for top yields. For maximum yields, 2/3 of the total N should be applied preplant and 1/3 at 4-6 weeks. Topdressings of N should be applied once only and not split as is common practise in the area. Application of P on cabbage crops was proportional to soil requirements but was generally excessive. Sufficient quantities of K were applied but scant attention was given to soil analysis results for K, Ca and Mg requirements. Input costs could be decreased and high yields maintained if soils were fertilized up to critical values and no further nutrients added when soil test values exceeded critical values. Fertilization was not generally based on results of soil analysis and few farmers limed their soils in spite of soil acidity problems. The more acidifying NH4 containing fertilisers were mostly used in preplant applications. The study recommended using more N0(3) based fertilisers to slow down acidification or, at least, corrective applications of lime. Good pest and disease control is essential for maximum yields in the Umlaas River Valley. This study identified optimum infection periods and maximum and minimum temperatures for infection by blackrot, club root , ringspot, damping-off, sclerotinia, downy mildew and alternaria leaf spot; and evaluated disease effects on yields. Clubroot, blackrot and damping-off significantly reduced cabbage yields. Clubroot incidence was generally associated with soil acidity problems, waterlogged lands or sandy soils subjected to slight over-irrigation. Blackrot was reduced by adequate N and K fertilization, increased with higher concentrations of Mn in most recently mature leaves (MRM) at head form (also associated with soil acidity) and always occurred with the warm, wet conditions of summer. Aphids and thrips occurred throughout the year; cutworm, bollworm, greater cabbage moth, leafminer and diamond-back moth, occurred mostly during the warmer season; and webworm over the cooler period in autumn. Aphids, thrips and webworm were the most important cabbage pests, causing serious losses especially during the first six weeks after transplanting. Factors such as amounts of fertilizer applied, soil acidity problems, slope of the land, infiltration rate, soil nutrient status, water stress, leaf concentrations of each element, weeds and disease were all associated with the incidence of various pests. This study highlighted the importance of good management practices, which affect all these aspects of cabbage production, and therefore pest incidence. An awareness of these factors and recommendations made in this study, could improve cabbage management practices in the Umlaas River Valley and therefore, reduce pest infestation. The study also evaluated relationships between weather patterns, hormone herbicide-like symptoms which occurred simultaneously on various garden trees and shrubs and vegetable seedlings, and pest and disease outbreaks or hormone herbicide symptoms on field crops of cabbage. Hot days with temperatures> 30°C, high VPD and high solar radiation followed by a rapid drop in temperature, high relative humidity and low solar radiation (stress weather cycles) were characteristic of weather conditions with an approaching frontal low in the Umlaas River Valley. All the dates of hormone herbicide-like symptoms on some garden shrubs and trees and vegetable seedlings in the nursery (indicator plants) were correlated with stress weather cycles, however not all stress weather cycles were correlated to these symptoms on indicator plants. It appeared that an unknown factor (Factor X) occurred with these weather cycles, and together with the stress induced by the harsh weather conditions, caused these symptoms on the indicator plants. A Canary creeper (Senecio tamoides DC.), fig tree (Ficus natalensis Hochst.) and an indigenous Cape chestnut (Calodendrum capense (L.F.) Thunb.) showed symptoms of leaf bubbling, twisting, burning, deformities and leaf drop. Lettuce and cabbage seedlings were twisted, etiolated and suffered from downy mildew outbreaks. All trees, shrubs and nursery crops experienced these symptoms 1 - 4 days after the stress weather cycles and Factor X. It is possible that low levels of hormone herbicide < 25 ng 1(-1)) deposited during dynamic fumigations (associated with approaching frontal lows), were Factor X. However, when examining the relationship between field cabbages and all related factors, no clear correlation could be established between weather patterns, Factor X, cabbage plant health, and pest and disease incidence. An overall study examined the effects of a range of factors on cabbage yield including: soil analysis data; management and crop water stress ratings; total nutrients supplied; weed, pest and disease indices; tillage operations; soil characteristics; plant populations; percentage marketable yield and headmass. Poor water management, high incidence of weeds, pest and disease, excessive nitrogen and phosphorus applied as fertilizer, and too many tillage operations, were associated with low yields. Conversely, good water management, good liming and soil fertility practices, higher levels of soil potassium and better weed, pest and disease control, resulted in higher yields. A greater percentage of marketable cabbage heads was more important than head mass for higher cabbage yields. The methodology used in this study provided an efficient, reliable method of identifying factors which limited and maximized vegetable crop production and provided data for the compilation of production guidelines for cabbage. The research and extension aspects of this study also provided an opportunity for the rapid transfer of technology to the farmer and excellent farmer participation.
Optimization of flower and seed crop production in temperate eucalyptus orchards in South Africa through site selection and environmental manipulation.
(2014) Gardner, Robin Arthur William.; Bertling, Isa.; Savage, Michael John.
Temperate eucalypts are an important part of the commercial forestry landscape in South Africa, comprising approximately 50% of the total Eucalyptus planted area. The majority of the commercial temperate eucalypts grown in South Africa are reticent, shy flowerers, and subsequently erratic seed producers. Disadvantages associated with sub-optimum (inconsistent and sparse) flowering in Eucalyptus orchards include decreased levels of out-crossing and compromised quantity and (genetic) quality of the seed produced. Genotype, physiological age and a range of environmental factors are known to influence flower bud production in temperate eucalypts. To date, winter cold and paclobutrazol (PBZ), a plant growth regulator, remain the most effective treatments for encouraging early and prolific flowering in temperate eucalypts. Disadvantages associated with the use of PBZ in the outdoor environment include the toxicity and recalcitrant nature and persistence of the chemical in soils, the high cost of PBZ and its orchard application, and the need to re-apply the chemical approximately every five years. The main aim of this study was to provide a practical solution to the problem of shy flowering and seed crop production in important temperate Eucalyptus species in South Africa. The study focused on investigating key environmental factors associated with optimum flower bud production in temperate eucalypts. The resultant data were used to achieve optimization of flower bud production in temperate eucalypts in the summer rainfall forestry areas of South Africa, via informed site selection and/or manipulation of the environmental conditions. A subsidiary aim of the project was to lessen the dependency on PBZ for achieving satisfactory flowering levels in temperate eucalypts via improved site-orchard matching and environmental manipulation. The key objectives in the study included definition of summer rainfall area site conditions for maximal floral bud production in two important species, viz. E. nitens and E. smithii, investigation of the effects of optimum flowering environmental conditions on post-initiation floral development and seed maturation in temperate eucalypts, and development of a method for supplementing winter cold and increasing flower bud production in orchards located at marginal winter chilling sites.
Optimization, validation and application of radioimmunoassays for plant growth substances in avocado (Persea americana Mill.) fruits.
(1984) Cutting, Jonathan Garth Melville.; Wolstenholme, B. Nigel.; Lishman, Arthur William.
The objective was to develop, optimize and validate radioimmunoassays (RIA) for several plant growth substances (PGS) and then apply the RIA's to determine PGS trends in 'Fuerte' avocado fruits from fruit set to fruit ripening. Antibodies to the cytokinin isopentenyl adenosine (IPA) were obtained from rabbits inoculated with a periodate-derived IPA-BSA conjugate. The antiserum cross-reacted (25%) with only 2iP (isopentenyl adenine). The RIA measuring range was from 0,1 to 100 ng. Anti-IPA serum was used to develop a RIA for 2iP, with a measuring range from 0,5 to 100 ng. Using Dowex 50W-X8 and cellulose TLC purified avocado fruit extract, 20 samples per day could be processed. The RIA for abscisic acid (ABA) was developed from rabbit antibodies from an inoculated carbodiimide-derived, (+ -) ABA-BSA conjugate. The free active ABA component was isolated prior to quantitation by RIA by solvent partitioning and silica gel TLC. The indole-acetic-acid (IAA) RIA was established from sheep-produced antibodies to a formaldehye-derived IAA-BSA conjugate, after repeated inoculations. For both the ABA and IAA RIA's, contaminants in the avocado tissue were removed by solvent partitioning. Developing avocado fruits, and especially young fruits were rich sources of IAA and 2iP in particular with seed concentrations exceeding those of the fruit flesh. The concentration of ABA rose throughout fruit development, reaching 100 ng g(-1) in the flesh at fruit maturity. Just prior to seed and fruit maturity, relatively high levels of IAA, 2iP and IPA were associated with the thick, fleshy testa, rapidly declining to zero as the testa dried. The avocado fruit is physiologically dependent on the seed right up to this stage, and testa maturity correlates well with minimum "legal" maturity of 80% moisture content of flesh. In ripening avocado fruit, the concentration of free ABA rose as softening progressed. Total ABA concentrations fell initially, and then rose after a firmometer reading between 50 and 60 (100 coincides with eating ripeness). Later harvested fruit had double the ABA concentration of early harvested fruit. The ripening stimulus appeared to be related to moisture stress in the fruit. This study has confirmed the prime advantages of sensitivity, specificity and rapidity of RIA, as well as its usefulness in multi-PGS studies and batch-type analysis.
Ovule and seed studies in relation to fruit growth in Carica papaya L. cl. honey gold.
(1979) Pereira, Abel Luiz.; Alan, Peter.
No abstract available.
Plant hormone homeostasis and the control of 'hass' avocado fruit size.
(2002) Taylor, Nicolette Jane.; Cairns, Andrew Lawrence Patrick.; Cowan, Ashton Keith.; Van Staden, Johannes.
The 'Hass' avocado produces two distinct phenotypically different populations of fruit, i.e. normal and small fruit. The small fruit variant is characterized by early seed coat senescence that results in arrested growth, due to dramatically reduced cell cycle activity. This system has been used to study the metabolic control of fruit growth for two reasons. Firstly, the 'Hass' avocado is a major export crop in South Africa and unmarketable small fruit cost the industry millions of rands per season. Secondly, in the absence of evergreen tree-crop mutants with which to dissect controlling mechanisms contributing to the control of final fruit size, the 'Hass' avocado and its small fruit variant provides an ideal system to investigate the physiology, biochemistry and molecular biology of fruit growth in subtropical species. A detailed study was conducted to probe the contribution of hormones in the control of final fruit size by comparing and contrasting tissue distribution and content of hormones in developing 'Hass' avocado and its small fruit variant. In addition the proposal that changes in hormone homeostasis occur as a result of differences in the allocation of the molybdenum cofactor (MoCo) and changes in the activity of xanthine dehydrogenase (XDH) and the aldehyde oxidases (AO) involved in abscisic acid (ABA) and indole-3-acetic acid (IAA) metabolism was evaluated. Activity of XDH, xanthoxal (XAN) oxidase, indole acetaldehyde (IA-aid) oxidase and cytokinin oxidase (CKOX) was related to tissue content and composition of IAA and ABA. Comparisons between normal and small fruit revealed that under conditions where CKOX is elevated, the increased adenine produced inhibits XDH activity, which leads to elevated activity of the AOs involved in ABA and possibly IAA biosynthesis as a result of increased MoCo allocation to these enzymes. Further analyses revealed that both cytokinin (CK) and auxin elevates CKOX activity and that adenine and CK do indeed inhibit XDH activity, which leads to increased AO activity. In addition, application of CK to normal fruit increased IAA in mesocarp tissue but reduced IAA content of seed tissue and reduced ABA in mesocarp tissue but had no effect on ABA in seed tissue. Cytokinin oxidase therefore contributes to the regulation of ABA and IAA metabolism during plant organ growth by modulating the activity of XDH. Low XDH and lA-aid oxidase activity together with high XAN oxidase and CKOX activity early in fruit development combine to reduce both elongation and radial growth, which results in the appearance of the 'Hass' small fruit phenotype. This event was associated with high ABA and low IAA in seed tissue of small fruit, but high ABA and IAA in seed coat and mesocarp tissue of these fruit. Thus, whilst low IAA in seed tissue is associated with reduced growth the . reverse is true in seed coat and mesocarp tissue where high IAA retards tissue growth. Calculation of CK/ABA and CK/IAA ratios revealed that a decrease in these ratios was found in mesocarp tissue of small fruit. However, in seed tissue of small fruit both IAA and ABA were decreased relative to CK. The maintenance of the correct hormonal balance in avocado fruit thus ensures the continuation of cell division cycle activity, with any changes responsible for the high incidence of a small fruit variant in the 'Hass' avocado.

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