Masters Thesis

Efficacy trials of new dormancy-breaking treatments in pistachio

This thesis focused on the problem of adapting pistachio cultivation to warm winters such as the Central Valley of California is likely to face in the future. Warm winters negatively affect pistachio yields through two known mechanisms: bloom asynchrony and the accelerated depletion of energy reserves in winter. The California pistachio industry currently has no dormancy-breaking agents (DBAs) purposefully developed to counter the physiological effects of low chill. This thesis work concerns efficacy trials of new candidate DBAs (GA3, ethephon) in pistachio, a comparison of the new DBAs with oil sprays, an investigation of DBAs’ effects on the mobilization and utilization of non-structural carbohydrates by pistachio trees, and the prototyping of new plant tissue analysis methods to predict effective DBA application times. A suite of carbohydrate movements that signify growth initiation in spring has been identified. A new diagnostic procedure has been prototyped to assess the physiological transition from endodormancy to ecodormancy in pistachio shoots. Single large doses of GA3 can break endodormancy in pistachio shoots, and the minimum effective dose of GA3 that breaks endodormancy is a proxy of endodormancy depth. Even though no tested treatment protected yield against the threat of low chill, more is now known about proper concentrations and application times for the new DBAs to avoid adverse side effects. The monitoring techniques developed in this thesis contribute to filling in the 5-month tissue-testing gap between leaf fall and bloom with direct indicators of a tree's endodormant, ecodormant, or active status.

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