Growth, Osmotic Adjustment, and Cell‐Wall Mechanics of Expanding Grape Leaves during Water Deficits

The responses of growth, components of leaf water potential, and mechanical properties of leaves to ontogeny and soil water deficits were investigated in wine grape Vitis vinifera L. Vines were cultured in soil in controlled environments, and water deficits were imposed by withholding water. Osmotic adjustment of several hundred kilo‐pascals maintained turgor at or above that of controls, but growth was completely inhibited. The bulk modulus of elasticity increased almost twofold, from 8.1 MPa for a rapidly expanding leaf to 14.7 MPa for a similar leaf in which growth had ceased after withholding water for several days, and continued to increase to 20.3 MPa 2 d after growth had ceased. Stress‐strain relationships of isolated leaf strips were determined with the Instron analyzer. Total extension (elastic and plastic components) under 20‐ or 30‐g loads was linearly related to the rate of leaf expansion determined prior to sampling. Total extension of well‐watered leaves exhibited a diurnal pattern, with a minimum extensibility near midnight and high extensibility in the light. Plastic extensibility was estimated as the difference in extension in successive load cycles. Plastic extension was linearly related to the rate of leaf expansion. The results indicate that the extensibility (as determined by Instron analysis) of grape leaf tissue decreases concomitant with the rate of expansion as leaves age. Similarly, plastic and elastic extensibility decrease when growing leaves are exposed to water deficits. The results are consistent with the hypothesis that decreased cell‐wall extensibility contributes to the inhibition of leaf expansion caused by water deficits.