Floral induction in Eucalyptus nitens (Deane and Maiden) Maiden in South Africa.
Gardner, Robin A. W.
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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.
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