Drought-induced increase in xylem malate and mannitol concentrations and closure of Fraxinus excelsior L. stomata

Changes in the malate and mannitol composition of ash leaf (Fraxinus excelsior L.) xylem sap were studied in response to water deficit. Xylem sap was collected by the pressure method from the petiole of leaves sampled on irrigated and non-irrigated ash seedlings. As the leaf water potential decreased from -0.3 to -3.0 MPa, there was a significant increase in malate and mannitol xylem concentrations, and a concomitant decrease in maximal stomatal conductance. The functional significance of the increased malate and mannitol concentrations was investigated by using a transpiratory bioassay with mature detached leaves which exhibited, for stomatal conductance, the typical pattern showed by expanded leaves during dark/light transitions. Supplying detached leaves with mannitol in a range of concentrations found in the xylem sap had no effect on stomatal movements, but malate, for concentrations between 0.5 and 3 mM, was effective in preventing stomatal opening. The ability of malate to inhibit stomatal opening appeared to be rather non-specific. Two structural malate analogues, citrate and aspartate or an unrelated anion, shikimate, also inhibited this process. Given the drought-induced increase in xylem malate concentrations, and the fact that the range of malate levels required to close stomata was very similar to that of the concentrations found in the xylem sap, it has been hypothesized that malate is involved in the stomatal closure of ash leaves under drying conditions.