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 inresponse to water deficit. Xylem sap was collected by the pressure methodfrom the petiole of leaves sampled on irrigated and non-irrigated ashseedlings. As the leaf water potential decreased from -0.3 to -3.0 MPa,there was a significant increase in malate and mannitol xylemconcentrations, and a concomitant decrease in maximal stomatal conductance.The functional significance of the increased malate and mannitolconcentrations was investigated by using a transpiratory bioassay withmature detached leaves which exhibited, for stomatal conductance, thetypical pattern showed by expanded leaves during dark/light transitions.Supplying detached leaves with mannitol in a range of concentrations foundin the xylem sap had no effect on stomatal movements, but malate, forconcentrations between 0.5 and 3 mM, was effective in preventing stomatalopening. The ability of malate to inhibit stomatal opening appeared to berather non-specific. Two structural malate analogues, citrate and aspartateor an unrelated anion, shikimate, also inhibited this process. Given thedrought-induced increase in xylem malate concentrations, and the fact thatthe range of malate levels required to close stomata was very similar tothat of the concentrations found in the xylem sap, it has been hypothesizedthat malate is involved in the stomatal closure of ash leaves under dryingconditions.Key words: Fraxinus excelsior: L., malate,mannitol, xylem sap, stomata, water deficit.