Effect of winter chilling and paclobutrazol on floral bud production in Eucalyptus nitens

Abstract

Flower bud production of Eucalyptus nitens is comparatively abundant at specific high altitude sites in the South African summer rainfall region. This suggests that cumulative cold may be implicated in the floral induction process. Therefore, three chill models were used to investigate whether winter temperature data can be related to E. nitens flower bud production at sites differing in chill accumulation. 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. Although the potential chilling requirement of E. nitens can be calculated with similar accuracy by each of the chill models tested, the Dynamic Model performed best under the particular range of experimental conditions. At low to moderate levels of winter chill (43 to 81 Chilling Portions (CPs) of the Dynamic Model), paclobutrazol application hastened first flowering and increased the percentage reproductive trees by between 5–27% in the case of seedlings, and between 2–62% in the case of grafts. At high levels of winter chill (> 87 CPs), paclobutrazol had a negligible effect on the percentage reproductive trees in the case of either seedlings or grafts. At four and five years after planting, very high levels of accumulated winter chill (96 CPs) stimulated a high percentage of seedlings (25–50%) and grafts (55–64%) to produce flower buds. Considerable variation in precocity and chilling requirement (for floral induction) is evident within the South African E. nitens breeding population. A better accuracy in relating E. nitens floral bud production to amount of prior winter chilling could be achieved by further experimentation involving fewer genotypes and a wider range of chilling conditions.