Stomatal closure is induced rather by prevailing xylem abscisic acid than by accumulated amount of xylem-derived abscisic acid

We have studied the stomatal response in relation to the xylem-derived absc isic acid (ABA) accumulation in sunflower leaves. When ABA was introduced i nto detached leaves of the sunflower through xylem flux, stomatal conductan ce was regulated, water flux was changed as a result and at the same time t he xylem-derived ABA was metabolised in the leaves. We computed the xylem-d erived ABA accumulation in the leaves as a function of time by taking into account the variation of ABA flux into the leaves (the product of water flu x and ABA concentration) and a continuing ABA metabolism. We found that ABA accumulation was rapid during an initial lag phase, much slowed down durin g the decreasing phase of stomatal conductance, but still substantial when stomatal conductance reached a new stable state. The results show a poor li nk between the kinetics of ABA-induced stomatal closure and the xylem-deriv ed ABA accumulation. Xylem-derived ABA was metabolised rapidly in the leave s. Tetcyclacis, as an inhibitor, substantially inhibited this process. Two hours after ABA was fed into a leaf, about 70% of the fed ABA was metabolis ed, but when tetcyclacis was added into the feeding solution, less than 30% of ABA. was metabolised, even after 24 h of incubation. The inhibition of ARA metabolism by tetcyclacis did not lead to more stomatal closure, which was still concentration-dependent. Since the accumulation of xylem-derived ABA was enhanced substantially by the presence of tetcyclacis, these result s strongly indicate that stomata mainly respond to the prevailing ABA conce ntration in the xylem stream, rather than to the accumulated amount of xyle m-derived ABA in the leaves.