Abscisic acid – an intraleaf water‐stress signal

Physiological aspects of abscisic acid (ABA) as a drought signal directed specifically at guard cells are topical research foci. Most investigations concentrate on the importance of remote sources of ABA against the background knowledge that leaves are also a source of ABA. Foliar compartmentation of ABA and water‐stress‐induced release of cellular ABA into the apoplast imply additional levels of complexity. In the present study, we established that the pre‐existing foliar ABA pool can be moved to guard cells. We detached leaflets of Vicia faba L. to eliminate an external source of ABA. The leaflets were then pretreated with cycloheximide (CHI), an inhibitor of protein biosynthesis that abolished ABA accumulation in the leaflets. Next, the leaflets were water stressed. After incubation, leaflets were snap‐frozen and freeze‐dried. Guard cells were individually dissected from the leaflets; such guard‐cell samples contain both apoplastic and symplastic ABA. The ABA content of guard cells of CHI‐treated, water‐stressed leaflets was less than that of control water‐stressed leaflets, but higher than that of control water‐sufficient leaflets, indicating that guard cells are targets of intraleaf ABA redistribution under stress.