Cuticular conductance of adaxial (astomatous) and abaxial (stomatous) surfaces ofFagus sylvatica L. leaves was measured under varying vapour pressure deficits (D). Conductance was determined from gravimetric measurements of water flux made using a leaf disc envelope specially designed to maintain leaf relative water content and minimize reduction in cuticular hydration. The adaxial surface provided a determination of ‘true’ cuticular conductance (gc) and transpiration (Ec). The abaxial surface was used to estimate minimum leaf surface conductance (gMINsur) and transpiration (EMINsur). In experiment I, leaf discs were placed under one of a range of water vapour pressure deficits (0.4-2.0 kPa). Both gc and gMINsur decreased approximately 2-fold with an increase in D between 0.4-2.0 kPa. The decrease in gc was linear, but gMINsur declined more steeply at D between 0.4-0.95 kPa than at D between 0.95-2.0 kPa. In experiment II, leaf discs were exposed to a stepwise change in D. After a period of acclimation to D of 0.95 kPa, responses of gc and gMINsur to an increase or decrease in D were recorded. The response time of gc to increasing or decreasing D were similar (<60 min). By contrast, gMINsur responded more slowly to increasing than to decreasing D. These significant responses of gc and gMINsur to increasing and decreasing D are discussed in relation to hydration state of the cuticle and current knowledge of cuticle structure.
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Hoad et al. (1997) studied this question.