OSMOTIC ADJUSTMENT IN WATER-STRESSED GRAPEVINE LEAVES IN RELATION TO CARBON ASSIMILATION

The response of grapevine plants to severe water deficit (predawn leaf water potential of - 1.13 MPa), imposed at a rate of about 0.16 MPa d ay-1 was studied in terms of leaf water relations characteristics, sto matal behaviour and gas exchange. Carbohydrate status of leaves was al so analysed in order to assess the contribution of soluble sugars as o smotic solutes during drought. Pressure/volume analysis showed an acti ve osmotic adjustment in water-stressed leaves, which decreased osmoti c potential at full turgor by 0.45 MPa and the apoplastic water fracti on showed a reduction of 19% as compared to the well-watered plants. C ell wall elasticity was not significantly affected by water stress, an d turgor loss point in stressed leaves was reached at lower water pote ntial and relative water content values than in the well-watered contr ols. Photosynthesis was markedly reduced in water-stressed plants. How ever, well-watered and water-stressed leaves had similar concentration s of glucose and fructose. The concentrations of sucrose and starch de creased in water-stressed leaves. This accounted for a marked decrease in the ratio of leaf dry weight to area in droughted plants. The chan ges in concentrations of soluble carbohydrates could not account for t he difference in osmotic potential between water-stressed and well-wat ered leaves.