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 excels ior 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 irri gated and non-irrigated ash seedlings. As the leaf water potential decrease d from -0.3 to -3.0 MPa, there was a significant increase in malate and man nitol xylem concentrations, and a concomitant decrease in maximal stomatal conductance. The functional significance of the increased malate and mannit ol concentrations was investigated by using a transpiratory bioassay with m ature detached leaves which exhibited, for stomatal conductance, the typica l pattern showed by expanded leaves during dark/light transitions. Supplyin g detached leaves with mannitol in a range of concentrations found in the x ylem sap had no effect on stomatal movements, but malate, for concentration s between 0.5 and 3 mM, was effective in preventing stomatal opening. The a bility of malate to inhibit stomatal opening appeared to be rather nonspeci fic. Two structural malate analogues, citrate and aspartate or an unrelated anion, shikimate, also inhibited this process. Given the drought-induced i ncrease in xylem malate concentrations, and the fact that the range of mala te levels required to close stomata was very similar to that of the concent rations found in the xylem sap, it has been hypothesized that malate is inv olved in the stomatal closure of ash leaves under drying conditions.