A RAPID FIELD METHOD FOR MEASURING NET ASSIMILATION RATE STOMATAL CONDUCTANCE RELATIONSHIP - A FEASIBILITY TEST USING GRAPEVINE LEAVES

Based on the assumption that the response time of the photosynthetic apparatus to changes in light intensity is faster than the response time of stomata, a rapid field method for measuring the assimilation rate (A) stomatal conductance (g(s)) relationship in wine-grapes is proposed. Leaves from the outer canopy of Vitis vinifera L. cultivar 'Cabernet sauvignon' were placed between two pieces of foam for 90 min to reduce light intensity and induce stomatal closure, while allowing water vapor diffusion to occur. Heat load was reduced by shading the foam with a piece of light colored, heavy paper attached to the foam with a 10 mm air space between them. When the cover was removed, A and g(s) were measured several times until no significant changes in g(s) were observed. The A/g(s) relationships of the leaves were similar in spite of differences in the response time between leaves. Very few exceptions to the A/g(s) curves existed, and these leaves were easily identified. This procedure permits an analysis of the A/g(s) relationship without the complications of different plant water potentials that are normally used to achieve different levels of g(s).