Leaf Water Potentials Measured with a Pressure Chamber

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Abstract
Leaf water potentials were estimated from the sum of the balancing pressure measured with a pressure chamber and the osmotic potential of the xylem sap in leafy shoots and leaves. When leaf water potentials were compared with those measured with a thermocouple psychrometer known to indicate accurate values of leaf water potential, determinations were within [image] 2 bars of the psychrometer measurements with yew and sunflower. In rhododendron, determinations were 2.5 bars less negative to 4 oars more negative than psychrometer measurements. The discrepancies in the rhododendron measurements could be attributed, at least in part, to the filling to tissues other than xylem with xylem sap during measurements with the pressure chamber. Although stem characteristics may affect the measurements, pressure chamber determinations were sufficiently clost to psychrometer measurements that the pressure chamber may be used for relative measurements of leaf water potentials, especially insunf lower and yew. For accurate determinations of leaf waterpotential , however, pressure chanber measurements must be calibrated with a thermocouple psychrometer.