Turgor and the transport of CO2 and water across the cuticle (epidermis) of leaves

Leaf photosynthesis relies on CO2 diffusing in while water vapour diffuses out. When stomata close, cuticle waxes on the epidermal tissues increasingly affect this diffusion. Also, changes in turgor can shrink or swell a leaf, varying the cuticle size. In this study, the properties of the cuticle were investigated while turgor varied in intact leaves of hypo stomatous grape (Vitis vinifera L.) or amphistomatous sunflower (Helianthus annuus L.). For grape, stomata on the abaxial surface were sealed and high CO2 concentrations outside the leaf were used to maximize diffusion through the adaxial, stoma-free cuticle. For sunflower, stomata were closed in the dark or with abscisic acid to maximize the cuticle contribution to the path. In both species, the internal CO2 concentration was measured directly and continuously while other variables were determined to establish the cuticle properties. The results indicated that stomatal closure diminished the diffusion of both gases in both species, but for CO2 more than for water vapour. Decreasing the turgor diminished the movement of both gases through the cuticle of both species. Because this turgor effect was observed in the adaxial surface of grape, which had no stomata, it could only be attributed to cuticle tightening. Comparing calculated and measured concentrations of CO2 in leaves revealed differences that became large as stomata began to close. These differences in transport, together with turgor effects, suggest calculations of the CO2 concentration inside leaves need to be viewed with caution when stomata begin to close.