Carbon Dynamics Under Drought and Recovery in Grapevine's Leaves.

Drought stress reduces leaf net assimilation (A_N) and phloem export, but the equilibrium between the two is unknown. Consequently, the leaf carbon balance and the primary use of the leaf nonstructural carbohydrates (NSC) under water deficit are unclear. Also, we do not know how quickly leaves can replenish their NSC storage and resume export after rehydration. Hence, we dried grapevines to either zero A_N, leaf turgor loss, or complete wilting while following the leaf carbon dynamics. The vines ceased growth and minimized carbon export under drought, conserving the leaves NSC until A_N zeroed. Subsequently, the leaves slowly depleted their NSC storage. However, the NSC depletion rate in the leaves was too slow to support the leaf's energetic requirements, potentially transforming the leaves into carbon sinks. Even under extreme drought (-2 MPa), the leaves had substantial NSC reserves (38% of the controls). After rehydration, all surviving leaves recovered their NSC storage within a week, and even leaves that were later shed had functional phloem export in the week after rehydration. The study reveals the leaf carbon relations under drought, highlighting the preference of the leaf to conserve its NSC storage rather than utilize it.