Analysis of the diurnal change in osmotic potential in leaves of Fraxinus excelsior L. |
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Authors: | Marigo, G. Peltier, J.P. |
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Abstract: | The daily cycle of the transpiration rate, stomatal conductance,water potential, and the concentration of the main osmoticumidentified in ash leaves, malate and mannitol, were monitoredin a field on the Isere river plain. On sunny days, the stomatalconductance tends to remain close to its maximun value allowinga high transpiration rate and diurnal variations in leaf waterpotential, w, which may fall as low as 2 MPa at solarnoon. These variations of w are closely correlated with changesin malate, mannitol and the concentration of the well-knownosmoticum K+, which agree with the involvement of an osmoticadjustment to counteract the evaporative demand during daylighthours. How malate, mannitol and K+ contribute to the osmoticadjustment was analysed subsequently by comparing the solutepotential s, evaluated by the Boyle-Van't Hoff relation, tothe osmotic potential measured by thermocouple psychrometry.These experiments have led us to suspect some errors in themeasurement of , presumably due to experimental artefacts andthe ability of Ca 2+ , present in high levels in leaves, toform chelates with malate once the cells have been decompartmentedby freezing and thawing. Since significant changes of Ca2+ alsooccurred during the diurnal variations of w, the possible mechanismsby which Ca2+ may be implicated in controlling the water statusof the tree are discussed. Key words: Fraxinus excelsior L, osmotic adjustment, thermocouple psychrometry, malate, calcium |
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