Analysis of the diurnal change in osmotic potential in leaves of Fraxinus excelsior L.

Abstract

The daily cycle of the transpiration rate, stomatal conductance, water potential, and the concentration of the main osmoticum identified in ash leaves, malate and mannitol, were monitored in a field on the Isere river plain. On sunny days, the stomatal conductance tends to remain close to its maximun value allowing a high transpiration rate and diurnal variations in leaf water potential, ψw, which may fall as low as - 2 MPa at solar noon. These variations of ψw are closely correlated with changes in malate, mannitol and the concentration of the well-known osmoticum K+, which agree with the involvement of an osmotic adjustment to counteract the evaporative demand during daylight hours. How malate, mannitol and K+ contribute to the osmotic adjustment was analysed subsequently by comparing the solute potential ψs, evaluated by the Boyle-Van't Hoff relation, to the osmotic potential ψπ measured by thermocouple psychrometry. These experiments have led us to suspect some errors in the measurement of ψπ, presumably due to experimental artefacts and the ability of Ca2+, present in high levels in leaves, to form chelates with malate once the cells have been decompartmented by freezing and thawing. Since significant changes of Ca3+ also occurred during the diurnal variations of ψw, the possible mechanisms by which Ca2+ may be implicated in controlling the water status of the tree are discussed.