Abscisic acid in leaves and roots of willow: significance for stomatal conductance.

Excised leaves and roots of willow (Salix dasyclados Wimm.) accumulated abscisic acid (ABA) in response to desiccation. The accumulation of ABA was greater in young leaves and roots than in old leaves and roots. In mature leaves, ABA accumulation was related to the severity and duration of the desiccation treatment. Water loss equal to 12% of initial fresh weight caused the ABA content of mature leaves to increase measurably within 30 min and to double in 2.5 h. The drying treatment caused significant (P = 0.05) reductions in leaf water potential and stomatal conductance. Recovery of leaf water potential to the control value occurred within 10 min of rewatering the dehydrated leaves, but recovery of stomatal conductance took an hour or longer, depending on the interval between dehydration and rewatering. The addition of ABA to the transpiration stream of well-watered excised leaves was sufficient to cause partial stomatal closure within 1 h and, depending on ABA concentration, more or less complete stomatal closure within 3 h. When the ABA solution was replaced with water, stomatal conductance increased at a rate inversely related to the concentration of the ABA solution with which the leaves had been supplied.