Multiple signals and mechanisms that regulate leaf growth and stomatal behaviour during water deficit

We highlight the novel observation that the responses of maize leaf growth to abscisic acid (ABA) signals can be amplified both by mild water deficits and by nutrient stress. Under our experimental conditions these stresses alone had no effect on leaf growth rate. In most cases leaf growth responses were not attributable to changes in the turgors of growing cells, focusing attention on a regulatory role for cell wall biochemistry. Roles for xyloglucan endotransglycosylase (XET), expansins and peroxidases are discussed. An effect of drought on the activity of expansins seems particularly attractive if xylem sap pH is considered as a chemical signal. We show how changes in xylem sap pH can also modify local accumulation of ABA and thereby modify the apparent sensitivity of guard cells to ABA signals.