Browsing by Author "Gardner, Robin Arthur William."

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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.
Investigation of rootstocks for seed production in Eucalyptus nitens.
(2013) Adejumo, Adewale.; Laing, Mark Delmege.; Gardner, Robin Arthur William.
Rootstock age and size, combined with nutrients, temperature, and humidity are major factors thought to play an important role in grafting success in eucalypts. Research was conducted on the grafting success of interspecies and sub-species rootstocks, and three scions were chosen for their differential flowering ability, combined with six different rootstocks, chosen for their precocity of flowering, or as representatives of Eucalyptus nitens by itself or in a hybrid. The agro-meteorological conditions found inside the greenhouse tunnel were monitored and grafted plants were placed in three positions at different distances from the wet wall. The grafting environment was optimized by controlling the temperature and humidity of the greenhouse tunnel to achieve optimum grafting conditions. Overall, 44% of the grafted plants survived. The best rootstock host for grafting E. nitens scion was the hybrid with a strong E. nitens appearance (R5) at 67% survival rate. The two grafting periods had no significant effect on the grafting success, nor did the position of the plants in the greenhouse tunnel. For flower initiation, environmental factors such as day length (light and dark periods), temperature, and topography may affect the flowering characteristics of E. nitens, factors which are often geographically specific. Research was also conducted on the use of interspecies and sub-species rootstocks for early flowering in E. nitens and to monitor the impact of light, cold temperatures (via chilling units) and site location on floral induction. Six rootstocks were selected from Eucalyptus taxa and provenances, including three species and two hybrids. Three scions were selected from three Eucalyptus provenances: Tallaganda, Barren Mountain, and Barrington Tops. Exposed sites with good air drainage and low winter day/night temperature amplitudes were considered ‘good flowering sites’. Of the six rootstocks selected, amongst the ungrafted trees, the hybrid with a strong E. nitens appearance (R5) had the highest budding percentage while amongst the grafted trees, the hybrid rootstock with a strong E. grandis appearance (R2) induced the highest budding percentage on the E. nitens scions. This study provides further insight into the selection of rootstocks and site conditions that yielded good flowering results within the subtropical climatic conditions found in South Africa. Overall, the study showed that grafting on to dwarfing and precocious rootstocks, as well as suitable planting sites are effective in inducing early flowering in E. nitens. The optimized ii methods developed in this study will be important for E. nitens breeding and future flowering research.
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.