ANALYSIS OF THE DIURNAL CHANGE IN OSMOTIC POTENTIAL IN LEAVES OF FRAXINUS-EXCELSIOR L

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 Iser e river plain, On sunny days, the stomatal conductance tends to remain close to its maximum value allowing a high transpiration rate and diu rnal variations in leaf water potential, psi(w), which may fall as low as -2 MPa at solar noon, These variations of psi(w) are closely corre lated with changes in malate, mannitol and the concentration of the we ll-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 psi(s), evalua ted by the Boyle-Van't Hoff relation, to the osmotic potential psi(pi) measured by thermocouple psychrometry. These experiments have led us to suspect some errors in the measurement of psi(pi), presumably due t o experimental artefacts and the ability of Ca2+, present in high leve ls in leaves, to form chelates with malate once the cells have been de compartmented by freezing and thawing, Since significant changes of Ca 2+ also occurred during the diurnal variations of psi(w), the possible mechanisms by which Ca2+ may be implicated in controlling the water s tatus of the tree are discussed.