ABSTRACT
This study was carried out for two successive seasons on "Flame, Superior and Thompson" Seedless grapevine cultivars grown in sandy soil under drip irrigation system. Dormancy types and periods, changes in endogenous hormones and thickness of scale layer in relation to de-scaling, exposure to various chilling and ODD regimes were investigated, in addition to the thermal requirements for some phenological stages. Results indicated that grape buds exhibit a short shallow dormancy period, Part of it is attributed to Endo-dormancy and other to Para-dormancy which are basically due to the scales. Scale layers diameter increased markedly during the dormancy period. Such increment might restrict bud growth and expansion, it may be a source of ABA. On the other hand, dormancy period of the three considered grape cultivars may depend on the interaction between both chilling and ODD with various degrees. It was clear that scaling increased bud burst percentage in most sampling dates. Results concerning endogenous hormones emphasized general increase in the promoting hormones [IAA and GA3] and general decrease in ABA at post dormancy, whereas the reverse was detected in the deep dormancy. GA3 was not detected at both Pre- and deep dormancy phases