Horticultural Science

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Aspects of improving cold hardiness of tomato (Lycopersicon esculentum) var. Rossol.
(2004) Ghebretinsae, Amanuel Ghebrehiwet.; Bertling, Isa.; Bower, John Patrick.
Tomatoes, particularly those of the determinate type, are one of the most popular vegetables in the East African country Eritrea. The crop is a source of income to small farm operators as well as commercial growers, and plays an important role in the nutritional supply of the population. Nonetheless, tomato production is limited during the cool season resulting - on the one hand - in a reduced nutritional supply during this period and - on the other hand - in an increased economic potential of the crop. Although performance of tomato plants under low temperature conditions is genetically influenced, prevailing temperatures as well as management practices also affect growth and development. In order to understand the long-term effects of a cold spell on tomato plants, experiments were carried out to determine the effect of a cold spell on plant vegetative and reproductive characteristics of determinate type ''Rossol'' tomatoes. Plants were moved for two to five subsequent nights from a tunnel to a cold room (4°C). The fruit set stage of "Rossol" tomatoes was found to be most sensitive to cold temperature, followed by the flowering stage. The juvenile stage was, of all the developmental stages examined, the least sensitive to cold. Furthermore, subjecting tomato plants for five subsequent nights (cumulative 60hrs) to 4°C resulted in a significant retardation of growth and development and in yield reduction. However, tomatoes exposed to a two-night cold spell during either the vegetative or the flowering stage recovered quickly and, ultimately, performed well. Furthermore, trials were carried out under tunnel and field conditions to investigate the impact of potassium application as well as mulching on cold tolerance of "Rossol" tomatoes. Under low temperature conditions, increasing the level of potassium to up to 150% of the recommended level (157.5kg*ha-1 ) resulted in quicker ontogenetic development and increased yields significantly. On the other hand, supplying tomato plants with 50% and 200% of the recommended potassium level reduced growth, delayed development and decreased yield and yield attributes. Using black plastic mulch also increased plant growth and speeded up plant development. Maize stover mulch, however, retarded plant growth and development so that certain stages ofthe phenological cycle were reached later than by non-mulched plants. However, yield and yield attributes increased significantly using stover mulch. Therefore, the management practises potassium fertilization and mulching provide excellent tools to increase the tolerance of tomato plants to low temperature conditions. Furthermore, organic mulches can be used to delay crop development and time maturity to achieve high retail process of the commodity in the cool winter months.
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.
Development of a sulphur free litchi storage protocol using sealed polypropylene bags.
(2006) Archibald, Alison Joy.; Bower, John Patrick.; Bertling, Isa.
The use of sulphur as a method of postharvest disease control and colour retention in litchis is soon to be restricted by the European Union. It is therefore essential that new postharvest treatments and packaging techniques be developed in order to retain internal and external fruit qualities and thus allow for export. Good litchi quality is not only important for the export market but also for use on the local market. In this study, alternative methods for postharvest quality control were investigated with the aim of extending the litchi storage life to 40 days under modified storage. Packaging the fruit in polypropylene bags significantly decreased fruit water loss and resulted in an increase in shelf life, as determined by red colour and overall rind appearance. There was no distinct advantage of amodified atmosphere. The use of a punnet, lined with absorbent sheeting and placed within the sealed polypropylene bag, further improved the shelf life. The absorbent sheeting reduced the amount of free water and resulted in little pathogen infection, while the punnet was effective in protecting the fruit from damage. It was notable that most water loss occurred within the first 10 days of storage and that the majority would actually take place during the cooling phase. A hydrocooling technique was therefore investigated and was found to not significantly decrease water loss, possibly due to not hydrocooling the fruit for a long enough period of time. Temperature management was extremely important for both colour retention and pathogen control. It was found that treatments stored at 5.5QC showed better colour retention after the 40 days storage than the 1QC storage treatment. The higher storage temperature, however, enhances the potential for postharvest diseases. Three compounds, namely ISR 2000, 'Biosave' and F10, were tested for pathogen control. 'Biosave' showed the best results with the most effective concentration being 100 mill water and good pathogen control occurred when storage was at 10 C. Polyphenol oxidase (PPO) activity in the litchi rind was evaluated as it is thought to be closely related to browning of litchi fruit, probably due to the degradation of phenolics by PPO. Brown fruit had a high PPO activity whilst red fruit had much lower activity. It was also shown that PPO activity decrease over storage time, possibly due to product inhibition of the enzyme. The internal quality of the fruit was determined using the T88: acid ratio of the pulp, as it is well correlated to mean eating quality. For fruit to have excellent taste, it must have a T88: acid ratio of between 31:1 and 60:1. All the fruit had a ratio that met this criterion and would therefore ensure good eating quality.
Effect of coloured shade nets and plastics on Eucalyptus hybrid mini-hedge stock plant morphology and subsequent cuttings rooting potential.
(2014) Gilbert, Donna Louise.; Bertling, Isa.
Eucalyptus grandis (W. Hill) × E. nitens (Maiden) (G × N) clonal hybrids are bred to produce trees more cold-tolerant than E. grandis alone due to the cold-resistant E. nitens parent. Some of these selected hybrid clones have superior wood and pulp properties but are considered “difficult-to-root”; thus, any technique that improves their rooting percentage is valuable to the industry. As the radiation spectrum can affect growth and development of plants, manipulation of the spectrum received by mini-hedge stock plants through cultivating them under certain shade nets and plastics could alter the rooting potential of the subsequent mini-cuttings. Therefore, the effects of the covering factor at eight levels (shade nets: black 30 %, green 40 %, Apple blue 20 %, Photo red 30 % and silver Aluminet® 40 % and plastics: Clarix E Blue® as well as Patilite®, plus the control (no covering)) on morphology and subsequent rooting potential on the G x N clone factor at two levels (GN018B: difficult-to-root or PP2107: easy-to-root) were evaluated. A further factor, fertilizer, at two levels (inorganic or organic) was also evaluated. Statistical analysis was carried out using GenStat®. Data were analysed using analysis of variance (ANOVA) where data were orthogonally distributed. Where data were not orthogonally distributed the algorithm restricted maximum likelihood (REML) was used to estimate variance parameters in the multivariate linear mixed model. To pinpoint the effect of the covering employed, firstly, the alteration of environmental parameters (temperature, relative humidity (RH) and radiation spectrum) were analysed as well as the seasonal effect on stock plants and rooting of mini-cuttings and root system quality of mini-cuttings were analysed. One greenhouse was used for stock plants and one for mini-cuttings. In the stock plant tunnel the shade nets and plastic were draped over wire trellising over bricked beds where the top and two sides were covered but the ends left open to allow sufficient ventilation. Thermometers and HOBOs® were hung above the plants and a spectroradiometer placed centrally under a specific covering to measure environmental parameters. It was found that Aluminet® and black nets and Patilite® plastic act as neutral covers with regard to radiation transmission, while the blue, red and green shade nets as well as Clarix E Blue® plastic covers altered the transmission spectrum; thus, they can be considered photoselective. Similarly, the red to near infrared ratios (R:NIR) were altered significantly by the coverings, however, this did not significantly affect the shoot internode length or leaf area (LA) of stock plants. Leaf area was determined with a leaf area meter using ten of the first fully expanded leaves collected per treatment and averaging them to run analyses based on LA per leaf. The irradiance levels in the stock plant tunnel were lowest under Aluminet® 40 % (PPFD of 204.6 μmol s-1 m2) during winter, resulting in low rooting percentages, while higher irradiance under blue 20 % shade net and control (no cover) (PPFD 604.4 and 931.5 μmol s-1 m-2, respectively) during spring 2011, achieved average rooting percentages. There was no clear trend of an optimal radiation intensity to expose G × N stock plants to, in order to achieve high rooting percentages; however, a tendency was documented, whereby black 30 % and green 40 % at 200 to 450 μmol s-1 m-2 PPFD gave good rooting percentages.
Effect of elevated CO2 concentration on growth, development and postharvest characteristics of sweetcorn (Zea mays L. var. saccharata)
(2022) Dlulisa, Balungile Precious.; Bertling, Isa.; Clulow, Alistair David.
Abstract available in PDF.
Effect of methyl jasmonate and salicyclic acid on chilling injury of 'eureka' lemons.
(2010) Siboza, Xolani Irvin.; Bower, John Patrick.; Bertling, Isa.
South Africa is the second largest exporter of citrus fruit in the world. There has recently emerged a strong demand for lemons in the world market due to their nutritional value, culinary and non-culinary uses. During exportation, fruit are subjected to low temperature (-0.5°C) for varying periods of time as an obligatory quarantine treatment. However, lemons are sensitive to low temperatures and easily develop chilling injury during this obligatory quarantine treatment. This has become a major limitation to the expansion of South Africa’s lemon industry. Postharvest treatments with methyl jasmonate (MJ) and / or salicylic acid (SA) have been successfully used in horticultural crops to reduce chilling injury. A similar treatment was applied to ‘Eureka’ lemons. During the 2008 harvest season, postharvest fruit were either dipped in 10 or 50 μM MJ or 2 or 2.5 mM SA solutions. A control or no dip treatment was also applied. Three replicates of 15 fruits per treatment were used. During the 2009 harvest season the following postharvest treatments were applied as dips: 10 μM MJ, 2 mM SA, 10 μM MJ & 2 mM SA, 1 μM MJ & 0.2 mM SA, or 0.1 μM MJ & 0.02 mM SA solutions. A control or no dip treatment was also applied. Three replicates of 15 fruits per treatment were used. Subsequently fruit were stored at -0.5ºC for 0, 7, 14, 21, 28, 35, and 42 days, before being transferred to room temperature (25°C) for 7 days where after chilling injury was rated. Treatments with 10 μM MJ and / or 2 mM SA reduced chilling injury symptoms in lemons harvested during the 2009 season. Although no visual symptoms of chilling injury were observed during the 2008 harvest season, treatments with 10 μM MJ and / or 2 mM SA reduced fruit mass loss, delayed the occurrence of stress symptoms such as lipid peroxidation and suppressed accumulation of ROS in the rind. Treatments with 10 μM MJ and / or 2 mM SA were more effective in inducing antioxidant capacity and other defence compounds such as phenolics, ascorbic acid, carbohydrates and chilling injury responses such as accumulation of proline in the rind. This may have increased the chilling tolerance of fruit during the cold storage. Therefore, this study revealed that MJ and SA have the potential to reduce and delay symptoms of chilling injury in lemons. This lead to the suggestion that both, MJ and SA dips should be further tested as treatments to mitigate chilling injury in lemons. Future studies should focus more on preventing the injury itself or preventing the primary event of chilling injury. This could probably reduce the chances of secondary events to take place.
The effect of moringa leaf extract (MLE) on growth and development, mineral composition and antioxidant properties of radish (raphanus sativus) and green beans (phaseolus vulgaris)
(2019) Mabaso, Makungu Charmaine.; Bertling, Isa.
Besides enhancing food production, one of the major challenges of the agricultural sector is to provide essential minerals and nutrients to humans for the maintenance of a healthy body, not only from a caloric perspective, but also through the provision of antioxidant compounds. It is believed that two-thirds of the world’s plants have medicinal properties and many of these plants have high antioxidant potential. Natural antioxidants, such as flavonoids, vitamin C, tocopherols and other phenolic compounds are known to be present in many plants. Moringa oleifera is one of such plants that has been identified to contain natural antioxidants; particularly the leaves of moringa are a good source of natural antioxidants due to the presence of phenolics, carotenoids, ascorbic acid and flavonoids. While the effect of such plant material on human health has become common subject of investigation, little is known on the effect of moringa leaf extracts applied to plants to enhance their resistance and antioxidant potential. The aim of the experiment was to evaluate the effect of moringa leaf extract (MLE) on the growth and development, mineral composition and antioxidant properties of radish (Raphanus sativus) and green beans (Phaseolus vulgaris). The experiment was laid out in a completely randomized design with five replications and comprised of three treatments, viz. (Control, only inorganic fertilizer Calmag+B (5 g/plant) (T1), common fertilizer plus MLE 100% (T2) (20 g/L dried moringa powder (obtained from Run KZN, Pietermaritzburg, South Africa)) and MLE 50% (T3) (T2 diluted to 50% with 100% methanol). Applications of MLE was carried out during the flowering stage, pod formation stage and prior to harvest. The obtained results demonstrate that MLE applications increased growth and development of both crops (leaf size, pod size, number of flowers, number of matured leaves and, at final harvest, above and below ground fresh and dry mass). Among the various MLE treatments, MLE 50% resulted in higher growth development and yield parameters on both radish and green bean plants compared with the MLE 100% and control plants. The mineral composition of radish leaves, storage roots and green bean pods was carried out by an independent laboratory. Applications of the treatments had significant influence (p < 0.05) on plants, with MLE-treated plants obtaining higher mineral concentrations compared with the control plants. Treatment with MLE also significantly (p < 0.05) increased antioxidant properties, particularly total antioxidants, anthocyanin, ascorbic acid and total chlorophyll concentrations, with MLE 50% producing plants of the highest overall antioxidant properties. This treatment could, therefore, be possibly employed as a method to obtain healthier, organic vegetables.
Effect of postharvest silicon application on 'hass' avocado (Persea americana Mill.) fruit quality.
(2010) Kaluwa, Kamukota.; Bertling, Isa.
The South African avocado industry is export-orientated with forty percent of total production sold overseas. The avocado fruit is a highly perishable product with a relatively high rate of respiration which results in the quick deterioration of fruit quality. Good phytosanitary procedures are a necessity in ensuring good product quality. Due to the threat of pests and diseases becoming resistant to the conventional chemicals currently used to control them, there has been a great need to diversify from their usage. Silicon (Si), being the second most abundant element (28%) in the earth’s crust after oxygen, is a major constituent of many soils and has been associated with disease resistance in plants for a long time. It has been used in a number of crop species to provide resistance against pathogenic agents. In some horticultural crops Si has been found to offer protection against fungal infections by strengthening cell walls, thus making it more difficult for the fungi to penetrate and colonize the plant. The aim of this research was to investigate the effects of postharvest silicon application on the quality of ‘Hass’ avocado fruit. The specific objectives included investigating the effect of silicon on the ripening pattern as well as the metabolic physiology of the avocado fruit. Avocado fruit were obtained from two locations in the KZN Midlands (Everdon Estate in Howick and Cooling Estate in Wartburg). Fruit were treated with different forms of Si (potassium silicate (KSil), calcium silicate (CaSil), sodium silicate (NaSil) and Nontox-silica® (NTS)) at concentrations ranging from 160 ppm to 2940 ppm. After dipping for 30 minutes in the silicon treatments, the fruit were stored at -0.5°C, 1°C, 5°C or at room temperature (25°C). Energy dispersive x-ray (EDAX) analysis was then conducted on the exocarp and mesocarp tissues to determine the extent of silicon infiltration within each treatment. Firmness measurements, ethylene evolution and CO2 production were recorded as fruit approached ripening. The CO2 production of fruit that were stored at room temperature was analysed daily until they had fully ripened, while fruit from cold storage were removed weekly to measure respiration. Mesocarp tissue from each fruit was extracted using a cork borer and subsequently freeze-dried and stored for physiological analysis. The freeze-dried mesocarp tissue was then finely ground and later analysed for sugar content, total anti-oxidant capacity (TAOC), total phenolic (TP) content and phenylalanine ammonia lyase (PAL) activity using their respective assays. Statistical analyses were carried out using GenStat® version 11 ANOVA. Treatment and storage temperature means were separated using least significant differences (LSD) at 5% (P = 0.05). The experimental design in this study was a split-plot design with the main effect being storage temperature and the sub-effect being treatments. Each replication was represented by a single fruit. EDAX analysis revealed that Si passed through the exocarp into the mesocarp tissue in fruit treated with high concentrations of silicon, i.e., KSil 2940 ppm. Significant differences (P < 0.001) were observed in temperature means with regards to firmness. Fruit treated with KSil and NTS only and stored at 5°C were firmer than fruit stored at other temperatures. Fruits treated with Si in the form of KSil 2940 produced the least amount of CO2, while non-treated fruits (Air) had the highest respiration rate. Fruit stored at room temperature (25°C) produced significantly higher amounts of CO2 and peaked much earlier than fruit stored at other temperatures. Ethylene results showed that there were differences (P < 0.05) between temperature means with the highest net ethylene being produced by fruit stored at 25°C. There were also significant differences amongst treatment means (P < 0.001), with fruits treated with KSil 2940 ppm producing the least ethylene. There were significant differences (P < 0.001) in temperature means with regards to the total phenolic concentration with fruits stored at 1°C having the highest TP concentration (26.4 mg L-1 gallic acid). Fruit treated with KSil 2940 ppm had the highest total phenolic concentration whilst the control fruit (Air and Water) had the lowest. There were also differences (P < 0.05) in storage temperature means with respect to the total antioxidant capacity. Fruit stored at -0.5°C had the highest TAOC (52.53 μmol FeSO4.7H2O g-1 DW). There were no significant differences in TAOC (P > 0.05) with regards to treatment means although fruit treated with KSil 2940 ppm and stored at -0.5°C showed the highest TAOC of 57.58 μmol FeSO4.7H2O g-1 DW. With regards to the concentration of major sugars in avocado, mannoheptulose and perseitol (mg g-1), no significant differences (P > 0.05) were observed in temperature means. However, fruit stored at -0.5°C had the highest concentration of these C7 sugars compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) showing that fruit treated with KSil 2940 ppm had the highest concentration of both mannoheptulose (18.92 mg g-1) and perseitol (15.93 mg g-1) in the mesocarp tissue. Biochemical analyses showed differences (P < 0.05) in storage temperature means with respect to PAL enzymatic activity. Fruit stored at 5°C had the highest PAL activity (18.61 mmol cinnamic acid g-1 DW h-1) in the mesocarp tissue compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) with regard to PAL activity. Fruit treated with KSil 2940 ppm had the highest PAL activity (23.34 mmol cinnamic acid g-1 DW h-1). This research has demonstrated the beneficial effects, particularly applications of 2940 ppm Si in the form of KSil. This treatment successfully suppressed the respiration rate of avocado fruit. Biochemical analyses of total antioxidants, total phenolics and PAL activity in the mesocarp tissue have shown the usefulness of Si in improving the fruit’s metabolic processes. The C7 sugars (D-mannoheptulose and perseitol) also seem to be more prevalent in avocado fruit treated with Si (particularly KSil 2940 ppm) than in non-treated fruit. This suggests that an application of Si to avocado fruit can aid in the retention of vital antioxidants (C7 sugars).
Effect of systemic resistance inducers applied pre- and postharvest for the development of a potential control of colletotrichum Gloeosporioides on Persea Americana (Mill.) CV 'Fuerte'.
(2012) Bosse, Ronelle Joy.; Bertling, Isa.
Avocados are one of the major food sources in tropical and subtropical regions and are an important horticultural crop in South Africa. Avocados are exported over long distances and may have storage times of up to 30 or more days at temperatures of about 5.5oC. This procedure increases the risk of poor fruit quality, including physiological disorders, early softening and postharvest disease incidence. A major component of the postharvest diseases is Anthracnose caused by Colletotrichum gloeosporioides. Anthracnose infects unripe fruit and once infected, the fungus remains dormant in the fruit until ripening begins. This leads to a problem for producers and packers, as the presence of the disease cannot be detected on the pack line, and fruit is not removed. Anthracnose control is normally done through pre-harvest treatment with copper-based fungicides. While effective such treatment needs to be repeated frequently, resulting in copper residues on the avocados. The study was conducted to investigate the effects of phosphoric acid and potassium silicate on known antifungal compounds and critical enzymes of the pathways elemental for systemic resistance inducers, so as to evaluate the potential for using them as alternatives to or in conjunction with, copper fungicides in the control of Anthracnose in avocado fruit. The study included storage temperature and time variations, to take account of the logistics in shipping avocado fruit to distant markets. Pre- and postharvest applications of phosphoric acid and potassium silicate were used, and after harvest, fruit were either ripened at room temperature (22oC) without storage or stored for 28 days at temperatures of 5.5oC or 2oC before analysis. Concentrations of phenolics, activity of the enzyme phenylalanine ammonia lyase (PAL) and a known antifungal diene were determined in the fruit exocarp. Pre-harvest treatments of phosphoric acid showed that the highest phenolic concentration was found in fruit harvested 14 days after application for fruit stored at room temperature. For fruit stored at 5.5°C it was seen that as fruit softened, phenolic concentrations increased compared with hard fruit immediately after storage, with the highest increase noted for fruit harvested 7 days after application. When comparing the three storage temperatures, phenolic concentrations were enhanced most when fruit was stored at 2°C. Postharvest treatments showed a significant increase in phenolic concentrations for potassium silicate treated fruit stored at room temperature and 2°C when determined immediately after storage. Fruit stored at 5.5°C showed an increase in phenolic concentrations as it became softer. When considering PAL enzyme activity, it was found that postharvest treatments of both potassium silicate and phosphoric acid influenced enzyme activity, with potassium silicate having greater effects. Similarly, an increase in PAL activity was noted in the pre-harvest phosphoric acid treatment harvested 14 days after application for fruit ripened immediately as well as fruit stored at 5.5°C. Fruit stored at 2°C showed the highest PAL activity for fruit harvested 7 days after application. No results were obtained in the analysis of antifungal compounds for both pre- and postharvest treatments. However, it is suggested that the antifungal diene could follow similar trends to those found for phenolics. It is concluded that applications of both phosphoric acid and potassium silicate do create changes in phenolic concentrations and the activity of the enzyme PAL which is involved in the synthesis of phenolic compounds known to possess antifungal properties. It is therefore possible that phosphoric acid and potassium silicate may be used as part of an integrated programme for Anthracnose control, and should be tested as potential alternatives for high volume copper-based fungicides.
Effectiveness of pre- and postharvest silicon and phosphorous acid applications in inhibiting Penicillium digitatum on citrus fruit.
(2014) Mkhize, Nopayi.; Bertling, Isa.
Citrus is, by tonnage, globally the most-produced fruit. Although technological advances have greatly improved storage life and quality of citrus, postharvest decay remains a major problem. Penicillium digitatum (green mold) and P. italicum (blue mold) are the most economically important postharvest pathogens. Over the years, fungicides belonging to the benzimidazole, thiabendazole (TBZ), benomyl, and imidazol (IMZ) groups have been used extensively to control these diseases; however, the development of fungicide-resistant strains of the pathogens together with the withdrawal of effective chemicals from the market has led to the search for more integrated methods of disease control. Silicon and phosphorus are able to trigger some ‘systemic responses’ that enhances fruit resistance to pathogen attack. The aim of this study was to ascertain the changes in biochemical composition of fruit after application of these two chemicals with the intention to improve the current understanding of their mechanisms of action. A proper understanding of these mechanisms could allow for the manipulation of fruit metabolism to improve the level of disease control. Two orange cultivars (‘Delta’ Valencia and ‘Washington’ navel) as well as one lemon cultivar (‘Eureka’) from Ukulinga Research Farm, Pietermaritzburg, were used. Fruit were treated both, pre- and postharvest with three different concentrations of potassium silicate (Si, 1250mg ℓ-1, 2675mg ℓ-1 and 5350mg ℓ-1) and one concentration of phosphorous acid (P, 500mg ℓ-1) as well as a combination of each of the Si concentrations together with P (1250 + 500, 2675 + 500 and 5350+ 500 mg ℓ-1). For the pre-harvest experiment, trees bearing fruit were treated by a soil drench around the base of the trunk with 5 ℓ treatment solution. This treatment was carried out for four consecutive weeks leading up to harvest. As a postharvest treatment, fruit were immersed in treatment solutions for a period of 90 s. Control fruit as well as control trees were treated with water. Following these applications, fruit were inoculated with a 1 x 104 mℓ spore suspension of P. digitatum, stored at 5.5oC, and sampled for 4 biochemical analysis ten days later. Petroleum jelly was applied over the area where the peel tissue had been sampled in order to prevent fruit desiccation and allow for disease monitoring. Disease lesion size (mm), as well as total rind phenolic and flavonoid concentrations were determined. Results were compared by analysis of variance (ANOVA) followed by Fisher’s Protected Least Significant Difference test (P< 0.05) using GenStat® Version 14. When applied at the lowest concentration (1250 mg ℓ-1), the Si treatment provided the most effective disease inhibition, as these fruit developed the smallest average lesion size. The two higher Si concentrations (2675mg ℓ-1 and 5350mg ℓ-1) were not significantly different (P>0.05) from each other and the control. Phosphorous acid provided less disease control than all other treatments and the control. Although the treatment combinations did not have a synergistic effect on disease suppression, they delayed disease onset and sporulation compared with the treatments alone. There were significant differences in the level of disease inhibition achieved by the treatments, but differences in phenolic and flavonoid concentration between treatments were not consistently significant; it can, however, be concluded that there was a correlation between disease control and increased rind phenolics. Increasing the concentration of Si did neither result in a significant increase in the level of disease control, nor in an increased production of rind phenolics or flavonoids. Separate Si and P treatments proved to be more effective in hindering disease spread than the combination of these treatments. Further research is required to fully understand the biochemical changes that these chemicals induce and to determine which mode of action they follow.
Effects of Litsea glutinosa (Lour.) C.B. Rob. plant biowaste-derived media on plant growth and development of thyme and rocket.
(2020) Anumanthoo, Thagen.; Bertling, Isa.
Enhancing and sustaining agricultural productivity is critical, as soil quality in many parts of the world deteriorates becoming unsuitable for agriculture. Plant bio-waste derived from composted alien invasives could be recycled and reused to enrich media used for plant production. This bio-waste could improve soil fertility and thereby enhance agricultural productivity. KwaZulu-Natal (KZN) is threatened by numerous alien invasive plants which negatively impact on the natural environment, human welfare and quality of life. Biological plant invasion is a natural process; however, human intervention has accelerated the rate of spread and naturalisation of many species across a multitude of landscapes. Composting some species of such alien invasives into bio-waste has been reported as a viable source of nutrients and organic matter. Farmers can, therefore, use these outputs as livestock-feed products and/or fertilizer for crops. The purpose of this study was to assess the effects of compost, derived from the IAPs (invasive alien plant species) - Litsea glutinosa – (Lour.) C.B. Rob., as enrichment for plant growth and development of two herb species, Thymus vulgaris (thyme) and Eruca sativa (rocket). This experimental study was conducted using three media into which rocket and thyme were planted: control medium (Gromor® Potting Soil, PS); experimental medium (composted Litsea, EM) and a combination (1:1) of control and enriched medium (PSEM). This study was carried out over three growing periods: eight-week experiments between April and May (autumn to winter), between September and October (winter into spring) and between February and March (summer into autumn). Composting of Litsea glutinosa plants was started at a vacant site in Verulam (KZN) before being moved to the experimental study site, at the Durban University of Technology Horticultural Practical Centre. Five replicates per treatment of the rocket and thyme plants were planted in the three media (PS, EM and PSEM). The following measurements were taken to assess plant growth and development: leaf diameter and plant height (rocket) and length of side shoot and plant height (thyme). Fresh and dry mass (g) were determined and the concentrations of total chlorophylls and carotenoids were measured spectrophotometrically. The growth of the thyme plants was positively influenced by cultivating the plants in EM and PSEM media resulting in increased plant height and length of side shoots, growth parameters significant for the culinary and cosmetic thyme industry. The leaf diameter of rocket was positively influenced when grown in the winter to spring period, particularly when cultivated in the PSEM medium compared with PS. Rocket displayed the most vigorous growth (fresh and dry mass of rocket leaves) during the winter to spring period when grown in PSEM. Results showed that herbs grew similarly in PS and PSEM media. It is, therefore, feasible to use PSEM as a medium for thyme production. Thyme grew best in EM in the autumn season (April-May), while PSEM performed best when used in summer/autumn (February-March). Thyme, therefore, grows well in this composted IAP in the summer and autumn months, rather than in winter or spring. The chlorophyll concentration of rocket plants was also affected by the season (highest concentration in plants grown during summer months) and medium (highest concentration in plants grown in PS) compared with PSEM and EM, as plants grew slowly and showed low values of pigment concentrations. Growing rocket and thyme in the composted Litsea glutinosa did not affect the taste and texture of the leaves determined by the consumer evaluation panel. Litsea glutinosa compost used to enrich potting soil (PSEM) was beneficial to the growth and development of rocket as well as thyme. Therefore, this study recommends the use of composted IAPs mixed in a 1:1 ratio with a general potting soil which would benefit the environment, the ornamental industry, as well as nurseries/wholesalers. A higher dosage of the composted Litsea glutinosa in a PSEM medium should be experimented with to grow thyme plants, while the potting soil is better suited to grow rocket plants. This study, therefore, highlights the usefulness of composted plant bio-waste derived from alien invasive plants as enrichment of media for growing herbs for human consumption.
Effects of organic and inorganic fertilisers on the growth of pseuderanthemum atropurpureum, soil fertility and leachate composition.
(2018) Constance, Duane Wayne.; Bertling, Isa.; Odindo, Alfred Oduor.
The use of fertilisers in agricultural production systems, particularly nitrogen and phosphorous, has been shown to be one of the causes of eutrophication as a result of the excessive enrichment of freshwater systems through surface runoff and soil infiltration. The contamination of freshwater bodies from horticultural production systems in South Africa has, however, been rarely studied, although influx from such systems are considered highly polluting elsewhere. Eutrophication is particularly considered a major problem in areas with limited water resources. Phosphate is especially limiting in contributing to eutrophication in South African rivers and dams. The development of harmful algal blooms, particularly from cyanobacteria, has been a concern for a long time due to toxins introduced into freshwater systems from these algae. This study investigated whether the use of organic fertilisers compared with inorganic fertilisers was potentially less detrimental to freshwater systems as a result of leachate nutrient and algal microorganism composition; further it was examined, if organic fertiliser was more beneficial to plant growth of Pseuderanthemum atropurpureum. Liquid and soluble granular organic and inorganic N equilibrated fertiliser treatments were applied at low, medium and high concentrations based on recommended label rates. Plant growth parameters were determined from mean height, number of leaves, size of leaves, number of nodes, internode length and number of branches. The species was grown over a period of three months and the experiment was repeated three times. Leaf tissue was analysed for mineral nutrient content and chlorophyll a, b and total chlorophyll. Leachate was analysed for mineral nutrient content including total phosphate, orthophosphate and chlorophyll a. Growth media was analysed for total nitrogen, ammonium and nitrate. A phase contrast light microscope was used to identify larger algal microorganisms and a scanning electron microscope (SEM) to identify smaller algal microorganisms from growth media extracted leachate. One specimen of green algae and some diatoms were identified, including two which may be found in eutrophic waters, but would not pose a threat similar to some species of cyanobacteria, if leached into freshwater systems over a period of time. Further, results showed that total phosphate and orthophosphate concentrations were significantly higher in leachate extracts from bark-based growth media across all fertiliser treatments and at all rates of treatment compared with soil-based growth media. This may have been due to a lack of binding sites in soilless media such as bark. Nitrate concentrations from organic soluble granular treatments were higher in both growth media types, whilst other treatments were similar. Ammonium and leachate nitrogen concentrations were found to be also similar. This may explain why plant growth traits assessed together were similar across all parameters tested. No single fertiliser compared with any other, produced plants that were superior in all growth characteristics measured. It is, therefore, suggested that the fertiliser treatments used in this study be applied at the half rate and plants be rather grown in randles growth medium than gromor for the production of Pseuderanthemum atropurpureum.
Enhancement of 'Hass' avocado shelf life using ultra-low temperature shipping or 1-MCP treatment and cold chain management.
(2011) Kok, Richard Dean.; Bower, John Patrick.; Bertling, Isa.
Avocados are becoming an increasingly important crop in South Africa, where the main producing areas include Limpopo, Mpumalanga and KwaZulu-Natal provinces. The South African avocado industry faces considerable challenges including increasing competition exporting avocados, particularly to the European market. The processes involved to export avocados has markedly improved over the past two decades, however there is always room for improvement and it is necessary to remain competitive on a global scale. Issues such as fruit being partially soft on arrival, quality defects and cold chain management breakdown are still present. It is necessary to investigate new aspects of cold storage such as extending the storage period and understanding the physiological aspects involved. To improvement such issues, an investigation was conducted on ultra-low temperature shipping (1°C) as well as the use of 1-MCP; the implementation of deliberate cold chain breaks to achieve a better understanding as to the quality influences involved; an extended storage period of 56 days to assess the quality issues and benefits involved; as well as investigating the physiological aspects involved with all above treatments on 'Hass' avocados. An initial study saw early-, mid- and late-season 'Hass' avocados stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. Storage at 1°C was comparable with 1-MCP treatment for both fruit softening in storage and extending the ripening period. Storage at 5.5°C resulted in partial in-transit ripening, if 1-MCP was not used. Early-season fruit incurred the most external chilling injury but overall levels were minimal and not concerning. Mid-season fruit were the most sound in terms of quality. It is suggested that 1°C can be used as a viable economic alternative to 1-MCP for long distance shipping of 'Hass' up to 28 days. The cold chain break trial included a 24 hour delay before cold storage, a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage and a control of 28 days cold storage where no break was involved. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. It was found that cold chain breaks do influence the amount of water loss, fruit softening and days taken to ripen. Storage at 1°C did not entirely negate the effects of cold chain breaks compared with 5.5°C, but did result in fruit which were harder at the end of storage and took longer to ripen. The use of 1-MCP also had advantageous effects with respect to significantly lengthening the ripening period, even when a cold chain break occurred, compared with fruit not treated with 1-MCP. As results of the study differed in some respects to those of previous studies, it is recommended that further work be conducted to determine what fruit or pre-harvest factors affect the fruit physiological changes which take place when cold chain breaks occur. Having the option to make use of an extended storage period would be of benefit to the industry if delays occur and fruit have to be maintained under cold storage. Extended storage of South African avocados, especially at the end of the season would also allow for the option of strategically holding back fruit from the export market in order to extend the supply period. It would not only benefit export options, but would also be highly beneficial to local pre-packers, as it would reduce the need to import fruit from the Northern hemisphere production areas during the South African off-season. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 56 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The combination of 1°C with the use of 1-MCP resulted in a good shelf life as well as maintenance of internal quality and integrity. External chilling injury is of concern for early-season fruit, however, mid- and late-season fruit did not incur extensive damage. It is, therefore, advised that fruit placed in extended storage are marketed through the 'Ready ripe' program to mask any chilling injury on the 'Hass' fruit. Avocados are renown as a "healthy food" due to their nutritional value as well as containing relatively high concentrations of antioxidants. The fruit also contain high amounts of C7 sugars which can act as antioxidants. Additionally, C7 sugars and other antioxidants play important roles in fruit quality. Therefore, it is important to understand how varying storage conditions and treatments affect the levels of these physiological parameters. Treatments of cold chain break/delay included a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage, a 24 hour delay before cold storage and a control of 28 days where no break was involved. A 56 day extended storage period was also used. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The use of 1-MCP maintained higher levels of antioxidants, ascorbic acid and C7 sugars for both the 28 day and the 56 day storage periods. The 24 hour delay had a tendency to increase consumption of anti-oxidant and sugar reserves. The use of 1°C resulted in antioxidant and ascorbic acid levels decreasing while maintaining higher sugar levels. Overall, high stress imposed on fruit decreased reserves resulting in poor quality fruit. The use of 1°C and 1-MCP treatments maintained fruit quality.
Evaluating the effect of phenolic compounds on the growth of phyllosticta citricarpa, the casual organism of citrus black spot.
(2016) Hlatshwayo, Zinhle Valitha.; Bertling, Isa.; Yobo, Kwasi Sackey.
Abstract available in PDF file.
Evaluation of best practices for local chicory production.
(2017) Manyoni, Nonduduzo Nelly.; Bertling, Isa.; Odindo, Alfred Oduor.
Chicory roots obtained from Cichorium intybus are commonly used to produce a caffeine-free coffee substitute. Although the crop has been produced in South Africa for many decades, the country still relies on imported chicory roots to meet its chicory needs, as satisfactory yields are often not achieved. The low yields are associated with the use of poor quality seed, which often results in poor crop establishment. In addition, there are limited options for weed control in chicory since only one herbicide is currently registered for use with chicory in South Africa. Chicory seeds vary in seed coat colour and research has indicated that seed coat colour maybe associated with seed quality of chicory. Results by various authors showed dark chicory seeds to have better performance than light coloured seeds however, contrary findings showing poor performance of dark coloured seeds compared to light coloured seeds have also been reported. There is a need to gain a deeper understanding of the possible association between seed coat colour variation and seed performance in chicory so as to come up with best management practises in order to obtain maximum crop establishment and optimum yields. The aim of the study was to evaluate the use of the image analysis in determining seed coat colour differences in chicory and to gain a deeper understanding of possible associations between seed coat colour variation and seed quality with respect to germination and vigour. In addition, the study assessed the effect of seed coat colour on germination, seedling growth and development of chicory in response to different priming solutions and durations. Lastly, a field experiment was conducted to identify the optimal planting density of chicory with respect to seed coat colour and weed management strategies. Seeds (cv. Orchies) were obtained from Nestle®, KwaZulu-Natal. In the first experiment (chapter three) seeds were separated visually into eight seed colours and then separated and assigned to a certain group using an image analysis system. This analysis system indicated that two colour categories could be separated with respect to hue. These groups were categorized as light and dark coloured seeds. Results also showed significant interactions (P < 0.05) between seed colour and seed quality test with respect to germination percentage and mean germination time. There were highly significant interactions (P < 0.001) between seed coat colour and seed quality test as detected by the germination velocity index (GVI) and imbibition time. Electrolyte leakage from the seeds was not significantly different (P > 0.05) between the seed colour groups. Results from chapter four showed osmo- and hydro-priming to improve seed quality of chicory through improvements in germination velocity index (GVI) and mean germination time (MGT). Osmo-priming resulted in relatively high improvements in seed quality compared with hydro-priming. Priming improved seedling establishment (mean emergence time (MET), seedling length, shoot length, root length, fresh mass and, root/ shoot ratio). Results from the field trial showed the interaction of planting density, seed coat colour, and weeding method to be significant for total plot yield. This suggested that, no optimal crop stand exists with regards to weeding methods and seed coat colour. On the other hand, if the agronomic parameter of interest is biomass plot yield, the optimal plant density would be 200 000 plants ha-1. Herbicide application tended to reduce agronomic performance of dark coloured seeds.
Evaluation of fruit growth and development over a very extended harvesting period of 'Hass', ‘Fuerte’, ‘Gem’ and ‘Ryan’ avocado fruit.
(2017) Mbele, Nosipho Precious.; Bertling, Isa.; Tesfay, Samson Zeray.
Assessing avocado fruit growth and development by measuring fruit diameter during ontogeny may, therefore, offer clues to better understand whole plant behaviour. Plant sampling was carried out over different developmental stages from early to an extended growing season on four cultivars (‘Hass’, ‘Fuerte’, ‘Gem’ and ‘Ryan’). Mesocarp, exocarp and seed fruit tissues were used to determine internal parameters such as sugars, antioxidant, oil content, dry matter, and calcium). The sugars were extracted and analysed by isocratic HPLC. D-Mannoheptulose in mesocarp+exocarp tissues was found in significant amounts (‘Hass’ = 16.47±1.140 mg/g DM, ‘Fuerte’ = 11.92±1.780 mg/g DM, ‘Gem’ = 9.35±1.410 mg/g DM, ‘Ryan’ = 7.52±1.271 mg/g DM), with perseitol also being significant for all cultivar (‘Hass’ = 4.87±0.662 mg/g DM, ‘Fuerte’ = 5.77±0.650 mg/g DM, ‘Gem’ = 5.09±0.577 mg/g DM, ‘Ryan’ = 3.86±0.227 mg/g DM). D-Mannoheptulose was found in high levels in the mesocarp and exocarp compared to the seed. Perseitol was predominantly found in the seed for all cultivars (‘Hass’ = 7.31±0.486 mg/g DM, ‘Fuerte’ = 6.71±0.842 mg/g DM, ‘Gem’ = 6.76±0.224 mg/g DM, ‘Ryan’ = 8.62±0.473 mg/g DM). The C6 common sugars sucrose and glucose were detected in low concentrations in the mesocarp+exocarp fruit tissue, with sucrose being dominantly present in the seed. Calcium was determined by fruit ashing using HCl/HNO3 for digestion and strontium buffer solution for calcium extraction. Calcium concentration was significantly different during the ontogeny of each cultivar (‘Hass’ p = 0.007, ‘Fuerte’ p < .001, ‘Gem’ p < .001, and ‘Ryan’ p < .001). The calcium uptake peak is mostly reached during early fruit set stages of avocado fruit, followed by a decline and constant continuous low concentrations as approaching maturity. When fertilizer is applied during maturity calcium uptake in the avocado fruit tends to increase. Maturity indicators such as oil content, dry matter and fruit are significantly different across all fruit developmental stages. Oil content percentage (p < .001 all cultivars), dry matter (p < .001 all cultivars) and fruit size for both low and high tree fruit load (p < .001 all cultivars, except ‘Hass’ with p = 0.812 for high tree load fruits). During the extended hanging period maturity indices accumulation had a continually increased per cultivar, Oil% (‘Hass’ = 18.1%, ‘Fuerte’ = 12.74%, ‘Gem’ = 13.41%, and ‘Ryan’ = 17.41%), dry matter (‘Hass’ = 40.37 mg/g DM, ‘Fuerte’ = 24.01 mg/g DM, ‘Gem’ = 44.29 mg/g DM, and ‘Ryan’ = 35.39 mg/g DM), and size (‘Hass’ = 69.73mm, ‘Fuerte’ = 68.46mm, ‘Gem’ = 75.34mm, and ‘Ryan’ = 76.75mm), all significantly increased. Overall this study revealed that avocado fruit development does not necessarily end at the commercial harvesting period, but continues on fruits still attached to the tree after the single sigmoidal growth curve. When fruit harvesting is prolonged, the internal parameter for fruit growth, and C7 sugars, content contributes significantly throughout fruit ontogeny but varies in levels between cultivars. Calcium concentration uptake is in higher demands at early fruit set, where peak accumulation is reached almost at similar period with C7 sugars per cultivar. Therefore, C7 sugars and calcium in avocado are correlated during fruit growth and development. By extending fruit harvesting it allows the avocado fruit to mature by accumulating higher concentrations of sugars and, calcium immature harvest which result in negative market outcomes. This is especially true for late maturing cultivars which are less susceptible to poor postharvest quality. Therefore, avocado fruit development does not only follow a single sigmoidal growth curve but a double sigmoidal one.
Evaluation of maturity parameters of 'Fuerte' and 'Hass' avocado fruit.
(2014) Olarewaju, Olaoluwa Omoniyi.; Bertling, Isa.; Magwaza, Lembe Samukelo.
Avocado fruit is one of the most important horticultural crops produced in South Africa. The fruit does not give obvious indication of maturity as it does not ripen as long as it remains attached to a tree. Harvesting avocado fruit at full physiological maturity, a stage at which it will continue normal development, plays a vital role in the postharvest physiological processes and the successful postharvest management of the fruit. Common maturity parameters used in various avocado fruit industries include mesocarp oil content, moisture content (MC) and dry matter (DM). However, the difficulty of measurement (oil content) and unreliability (MC and DM), can result in immature fruit reaching the consumer. To ensure that avocado fruit of good quality are delivered to the market and for growers to maximise profits, possible factors indicating optimal harvest maturity were investigated during the South African 2013 and 2014 avocado growing season. Additionally, the growth pattern of fruit, beyond what is currently regarded as physiological maturity, was examined for the possibility of the fruit exhibiting a double sigmoidal growth pattern, typical of nut crops. Fruit were harvested from two commercial orchards in the cool subtropical area of KwaZulu-Natal, South Africa. Fruit were harvested bi-weekly from February to March and then monthly from April to October, 2014. The MC decreased over the harvesting period, (p < 0.001), while oil content increased (p < 0.001). The study of the pattern of avocado fruit growth and development over the eight months observation period revealed that the fruit exhibits a single sigmoidal growth pattern. It could also be deduced from the experimental results that MC is a better indicator of maturity compared with oil content. In a quest to find an alternative maturity indicator that could provide a more reliable measure of avocado harvest maturity, total soluble solids (TSS) was evaluated for the possibility of providing an objective maturity index. Seven carbon (C7) sugars, D-mannoheptulose and perseitol, are dominant sugars in avocado fruit and have been suggested as likely indicators of avocado fruit maturity. D-mannoheptulose, a major component of mesocarp TSS, has been suspected to be responsible for the continued growth of the fruit. 'Fuerte‟ and „Hass‟ avocado fruit were harvested during the early, mid and late harvesting period in 2013 from Bounty Farm and during the 2014 season (February to August) from Bounty Farm and Everdon Estate. Sample ws taken along the equatorial region of each fruit and analysed for TSS, measured by squeezing juice out of the mesocarp using a garlic press and determining its °Brix using a digital refractometer. A high level of significant difference was observed between TSS and harvesting period for „Fuerte‟ during both seasons (p < 0.001) and a significant difference was found between the two production locations during the 2014 growing period (p < 0.001). There was no significant difference (p = 0.344) between production sites for „Hass‟ fruit harvested during the 2014 season. The results of the study reveal that TSS cannot be used as an indicator of avocado fruit maturity. In an attempt to non-destructively predict maturity parameters of avocado fruit, a total of 150 intact avocado fruit were scanned in reflectance mode of near-infrared spectroscopy (NIRS) during the 2013 and 2014 growing seasons. Reference maturity parameters, including MC, DM and oil content were measured using conventional destructive methods. Calibration models developed during 2013 season were used to predict the dataset acquired during 2014. NIRS prediction results showed that MC and DM were predicted with significant accuracy compared with oil content, prediction of which was not accurate. The prediction statistics for NIRS predicted MC and DM content demonstrated the potential of this system for non-destructive evaluation of avocado fruit maturity parameters (MC and DM). The high prediction accuracy recorded when models developed in 2013 were used to predict maturity of fruit harvested during the 2014 season demonstrated robustness of partial least square (PLS) models. Where speed and accuracy are required for assessing the maturity status of individual, intact avocado fruit, the method developed in this study is recommended.
Floral induction in Eucalyptus nitens (Deane and Maiden) Maiden in South Africa.
(2003) Gardner, Robin Arthur William.; Bertling, Isa.
Eucalyptus nitens (Deane & Maiden) Maiden is an important, commercial eucalypt planted predominantly for pulpwood in several southern hemisphere countries. In South Africa, the erratic and sparse flowering habit of E. nitens severely impedes genetic improvement and commercial seed production in the species. The comparatively abundant flower bud production at specific high altitude sites in the summer rainfall region suggested that cumulative cold may be implicated in the floral induction process. Series of field trials and semi-controlled environment trials were undertaken between 1996 and 2001 to investigate this. Three chill models were used to investigate whether winter temperature data can be related to E. nitens flower bud production. In the field trials, not only was the relationship between winter chilling and subsequent flower bud crop investigated, but also the relationship between cumulative winter drought conditions and floral bud production. In the trials under semi-controlled environmental conditions, the effect of applied winter chilling on floral bud production and photosynthetic efficiency was investigated. In the field trials, amount of accumulated winter chill, in conjunction with paclobutrazol treatment, was able to explain between 66 and 72 % of the variation in E. nitens flower bud production at four and five years after planting. Very high levels of accumulated winter chill (≥ 88 Chilling Portions (CPs) of the Dynamic Model) stimulated a high percentage of seedlings (25 - 50 %) and grafts (55 -77 %) to produce flower buds. At low to moderate levels of winter chill (41 to 72 CPs), paclobutrazol application increased flower bud production significantly, but at high levels of winter chill (> 76 CPs) paclobutrazol had a negligible effect. Cumulative winter drought did not promote floral bud production. In the semi-controlled environment trials, cold suppressed vegetative growth and induced flowering in pac!obutrazol-treated 18-month old grafted trees. Cold without paclobutrazol did not promote floral bud production. The results suggest that accumulated winter chill units (according to the Dynamic Model) are more effective than accumulated cold hours (hours below 5 degrees C). A high number of cold hours (1366 hours) reduced photosynthetic efficiency, but did not induce flowering. Furthermore, photosynthetic efficiency remained high for the moderate cold treatments which did induce flowering, suggesting that stress is not correlated to flowering in E. nitens. The results of the field and semi-controlled environment trial series suggest that precocity and floral productivity in E. nitens are under strong genetic control. Better accuracy in predicting flower bud crops in E. nitens could probably be achieved by excluding genetic variability and increasing the range of chilling conditions in such trials in future. The results indicate that future research should focus on the identification of optimum chilling (temperature) criteria for floral induction in E. nitens, the use evaporative cooling in seed orchards to reduce warm winter daytime conditions, the possible use of low-chill rootstocks, and the location of orchards as far south as possible in the winter rainfall region to achieve maximal exposure to temperatures which fulfil the chilling requirement of the species.
Horticultural manipulation techniques to improve yield, fruit size and quality in 'Wai Chee' litchi (Litchi chinensis Sonn.)
(2010) Froneman, Izak J.; Bower, John Patrick.; Bertling, Isa.; Bijzet, Zelda.
In South Africa, a shortage of litchi cultivars to expand the very short harvesting period has seriously threatened the viability of the local export orientated Litchi Industry. Only two cultivars have dominated commercial plantings for more than a century, namely ‘HLH Mauritius’ and ‘McLean’s Red’. The marketing period of litchis from South Africa coincides with large export volumes from competitor countries such as Madagascar, resulting in lower returns for local growers. To address this situation, the late season cultivar ‘Wai Chee’ was imported amongst others from Australia. ‘Wai Chee’ is harvested at a time in South Africa when there is a gap in worldwide litchi production, making the cultivar potentially very profitable. However, its potential and subsequent use in the industry is affected by small fruit size and questionable internal quality. As the South African Litchi Industry is export orientated and the qualities of ‘HLH Mauritius’ fruit currently dictate export requirements, solutions need to be found to improve fruit size and fruit quality in ‘Wai Chee’ litchi. In this study, a number of horticultural manipulation techniques were investigated with the aim of enhancing yield, fruit size and quality in ‘Wai Chee’ litchi. The use of chemical applications of foliar nutrients and plant growth regulators were found to improve certain fruit characteristics in ‘Wai Chee’. Foliar nutrient applications of nitrogen, potassium and calcium during the early stages of fruit set and -development improved fruit set and subsequently yield, and also increased fruit mass, fruit size and flesh mass. Treatments with potassium nitrate (KNOз), calcium nitrate (CaNOз) and calcium metalosate proved to be the most enhancing nutrient applications. Applications of synthetic auxins and auxin-like substances during the 2-3g stage of fruit development improved fruit size, fruit mass and flesh mass in ‘Wai Chee’. The combination treatment of Tipimon® (2,4,5-TP), applied at the 2-3g stage, followed by Maxim® (3,5,6-TPA) a week later, yielded the best results in this regard. With biological practices, pollination was found to have an influence on litchi tree- and fruit characteristics. Pollen source proved to have an influence on fruit set and fruit retention at harvest in ‘Wai Chee’. Initial fruit set was lower when using cross-pollination compared to the use of self-pollination in female flowers of ‘Wai Chee’, whereas final fruit retention was higher with the use of cross-pollinators when compared to retention of fruit with self-pollination. Although some beneficial effects with different pollen donors on fruit characteristics were observed, these effects were not significant, and would therefore necessitate further investigation. Pollen donor effect on quality parameters such as Titratable acid (TA)- and Total Soluble Solid (TSS) content of fruit was not significant. Cultivar differences regarding fruit characteristics and maturation rate were detected with the use of cultural practices such as bunch covering materials in ‘HLH Mauritius’ and ‘Wai Chee’. Beneficial effects on fruit size were obtained with thicker covering materials with nominal mass of 70 and 80 g/m² respectively on ‘HLH Mauritius’, while with ‘Wai Chee’, thinner covering materials (60 and 65 g/m²) showed enhancing effects. Maturation rate was significantly delayed only on ‘Wai Chee’ with the use of thicker covering materials (70 and 80 g/m²). Differences in colour were detected amongst different covering materials, but these should be verified with chromameter technology. Covering of fruit bunches for better fruit size and a later harvest date would, especially for ‘Wai Chee’ as a late season cultivar, be beneficial, since better prices are realised towards the end of the season.

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