Petioles of water-sufficient intactUnlike when leaves are stressed, the guard-cell symplast ABA content did not increase because of ABA infusion ( P = 0·48; 3·0 ± ± ± ± 0·5 versus 4·0 ± ± ± ± 1·2 fg guard-cell-pair ----1 ).However, the guard-cell apoplast ABA content increased linearly ( R 2 = 0·98) from ----0·2 ± ± ± ± 0·5 to 3·1 ± ± ± ± 1·3 fg guard-cellpair ----1 ( ª ª ª ª 3·1 m m m m M ) and was inversely related to leaf conductance ( R 2 = 0·82). Apparently, xylem ABA accumulates in the guard-cell wall as a result of evaporation of the apoplast solution. This mechanism provides for integrating transpiration rate and ABA concentration in the xylem solution.

Abscisic acid (ABA) integrates the water status of a plant and causes stomatal closure. Physiological mechanisms remain poorly understood, however, because guard cells flanking stomata are small and contain only attomol quantities of ABA. Here, pooled extracts of dissected guard cells of Vicia faba L. were immunoassayed for ABA at sub‐fmol sensitivity. A pulse of water stress was imposed by submerging the roots in a solution of PEG. The water potentials of root and leaf declined during 20 min of water stress but recovered after stress relief. During stress, the ABA concentration in the root apoplast increased, but that in the leaf apoplast remained low. The ABA concentration in the guard‐cell apoplast increased during stress, providing evidence for intra‐leaf ABA redistribution and leaf apoplastic heterogeneity. Subsequently, the ABA concentration of the leaf apoplast increased, consistent with ABA import via the xylem. Throughout, the ABA contents of the guard‐cell apoplast, but not the guard‐cell symplast, were convincingly correlated with stomatal aperture size, identifying an external locus for ABA perception under these conditions. Apparently, ABA accumulates in the guard‐cell apoplast by evaporation from the guard‐cell wall, so the ABA signal in the xylem is amplified maximally at high transpiration rates. Thus, stomata will display apparently higher sensitivity to leaf apoplastic ABA if stomata are widely open in a relatively dry atmosphere.