Water Content-Potential Relationship in Soya Bean: Changes in Component Potentials for Mature and Immature Leaves under Field Conditions*

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Abstract
Changes in turgor and osmotic potentials of soya bean leaves (Glycine max.) with changes in water content were measured throughout a season using the pressure-volume technique. Two distinct reponses to water loss were found. When water was expressed from leaves in the pressure chamber their osmotic behavior was described by a concentration effect based on the osmotic volume. The osmotic fraction of the total water content averaged 0·72 and 0·84 for mature and immature leaves, respectively. The changes in turgor pressure in the chamber were described by a volumetric modulus of elasticity which increased linearly with turgor pressure. The changes in total potential at high turgor pressures were almost exclusively due to changes in turgor due to the high modulus (high tissue rigidity) in that range. Responses were different, however, for leaves drying in the field. For these, the osmotic changes were always large and dominated by solute adjustment. Diurnal changes in osmotic potential were as much as 5 bars (500 kPa), or around 50 per cent, and were about the same magnitude as the changes in turgor pressure for both mature and immature leaves. The elastic modulus at the time of sampling showed the normal turgor dependence for immature leaves but for mature leaves the initial modulus was apparently constant at about 180 bars. The different behaviour in the pressure bomb and the field is interpreted in terms of a rate dependence for turgor and osmotic response to water loss.