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Lipase activity was studied during endodormancy in low-chilling-requiring `Anna' and high-chilling-requiring `Northern Spy' apples (Malus domestica Borkh.). Lipase activity greatly increased in bud axes when the chilling requirement of buds was almost satisfied regardless of the absolute chilling needed. Lipase activity greatly increased in `Anna' after 400 chill units (CU) and in `Northern Spy' after 2600 CU. This corresponded with an increase in budbreak at 22 to 24C. The increase in lipase activity also coincided with the release of water in buds from the bound to the free form. We propose that lipase(s) activity is an integral part of breaking dormancy and that lipase participates in causing changes in membrane lipid composition that coincides with releasing water into the free form.
Membrane lipids and cellular water states were studied in endodormant and paradormant apple buds. Paradormancy was overcome by thidiazuron while endodormant buds were forced to break after a certain period of chilling. Nuclear magnetic resonance imaging was used to determine water states in buds of different stages of dormancy. In endodormant buds, the changes in water states from a more tightly-bound to a more free form were correlated with changes in membrane fatty acid composition. The ratio of saturated/unsaturated fatty acids decreased with chilling, especially in C18:l/C18:3 molecular species of phosphatidylcholine and phosphatidylethanolamine. Bud lipase activity, which was assayed by in vitro hydrolysis of triglycerides, showed an abrupt increase after chilling treatments.