Chilling accumulation, dormancy release temperature, and the role of leaves in olive reproductive budburst: Evaluation using shoot explants

Winter rest and release of axillary buds from dormancy have been frequently noted but scarcely studied systematically in the olive tree (Olea europaea L). A series of experiments was carried out over three consecutive fall-winter periods, with one-node and three-node leafy and defoliated explants from shoots sampled from nonbearing (OFF) and bearing (ON) trees and forced in a growth chamber at different constant temperatures (12.5, 15, 20 and 30 °C). Buds acquired reproductive budburst capacity after a period of chilling accumulation on the tree for both OFF and ON explants, with percentage of reproductive budburst substantially higher in the OFF explants. Reproductive budburst occurred on explants sampled from early January to the second half of February, with slight variation in dates among the different experiments and seasons. During this period, forcing temperatures and defoliation influenced bud growth and development. Reproductive budburst activity indicated that 12.5 °C forcing temperature added effective chilling and promoted earlier budburst, while 20 °C forcing was only effective in promoting reproductive budburst after buds had accumulated enough natural chilling and 30 °C forcing produced a delay in reproductive budburst. From early sampling dates defoliation promoted vegetative budburst, which was then substituted by reproductive budburst as natural chilling accumulation increased. In histological comparisons of the buds a “shell-zone” of active morphogenetic activity was observed in the third node axils of OFF buds at the start of reproductive budburst capacity, later forming an inflorescence branch by the end of the sampling period. Starch presence shown by staining increased during the natural chilling accumulation period and then progressively disappeared with reproductive budburst. These results indicate dormancy release in the potentially reproductive buds following chilling and prior to inflorescence initiation and development. In addition to the developmental information they provide, the results permit standardizing the protocol to determine the period of dormancy release in olive tree reproductive buds.