Abstract
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, 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, probably 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.