CO2 and Water Vapor Exchange across Leaf Cuticle (Epidermis) at Various Water Potentials

Abstract

~ ~Cuticular properties affect the gas exchange of leaves, but little is known about how much CO, and water vapor cross the cuticular barrier or whether low water potentials affect the process. Therefore, we measured the cuticular conductances for CO, and water vapor in grape (Vitis vinifera 1.) leaves having various water potentials. l h e lower leaf surface was sealed to force all gas exchange through the upper surface, which was stoma-free. In this condition both gases passed through the cuticle, and the CO, conductance could be directly determined from the interna1 mole fraction of CO, near the compensation point, the externa1 mole fraction of CO,, and the CO, flux. The cuticle allowed small amounts of CO, and water vapor to pass through, indicating that gas exchange occurs in grape leaves no matter how tightly the stomata are closed. However, the CO, conductance was only 5.7% of that for water vapor. This discrimination against CO, markedly affected calculations of the mole fraction of CO, in leaves as stomatal apertures decreased. When the leaf dehydrated, the cuticular conductance to water vapor decreased, and transpiration and assimilation diminished. This dehydration effect was largest when turgor decreased, which suggests that cuticular gas exchange may have been influenced by epidermal stretching.The waxy cuticle of leaves serves to inhibit water loss and thus to decrease the dehydration of the underlying cells (Scott, 1964(Scott, ,1966Norris and Bukovac, 1968; Leon and Bukovac, 1978). The waxes vary in thickness and composition, and the inhibition of water loss varies accordingly (Bengtson et al., 1978; OToole et al., 1979; OToole, 1982; Jordan et al., 1983 Jordan et al., , 1984 von Wettstein-Knowles, 1989; Jenks et al., 1994). Holmgren et al. (1965) found that water loss through the cuticle was 1.7 to 28.6% of that through open stomata, depending on the species. This indicates that gas exchange through the cuticle can be a small or substantia1 fraction of the exchange through open stomata. As stomata close, the fraction becomes larger and the control of gas exchange shifts increasingly to the cuticle. Leaves that are darkened or that dehydrate and close their stomata experience this shift from stomatal control to cuticular control.As water loss becomes increasingly dependent on the cuticle, the exchange of CO, becomes similarly dependent on cuticular properties. Dugger (1952) found that 14C02 passes through the cuticle. Woolley (1967) observed that artificial films discriminate against CO, and predicted that the cuticle would show the same behavior. However, the C0,-exchange properties have not been reported for cuticles. This issue is important because CO, in the intercellular spaces is the substrate for photosynthesis and the concentration is generally calculated from the diffusion of water vapor through the leaf (Moss and Rawlins, 1963; von Caemmerer and Farquhar, 1981). Similarly, the diffusive conductance for CO, is calculated from water vapor diffusion (Gaastra, 1959; von Caemmerer and...

Reduced nighttime transpiration is a relevant breeding target for high water-use efficiency in grapevine

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