Species Characteristics in Whole Plant Resistances to Water Vapour and CO 2 Diffusion

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Abstract
Fluxes of carbon dioxide and water vapor were measured for whole intact plants of Helianthus annuus and 7 desert species, under a range of environ-mental conditions in the laboratory and in a natural desert habitat in the Negev desert in Israel. Epidermal resistance to diffusion of H2O (re) (comprising stomatal and cuticular resistances, rs and rc respectively) were estimated for a number of species, under low (10 ppm) and ambient (320 ppm) CO2 concentrations in the light, and high (800-1000 ppm) CO2 concentration in the dark. The smallest values were recorded for H. annuus and Kochia indica (2.0 sec cm-1), a summer-growing deserTannual, while the desert shrub species, Atriplex hallmus, and A. vesicaria, Haloxylon articulatum, Zygophyllum dumosum, Raeumuria hirtella, Artemisia herba-alba and Salsola inermis had generally high re values (3-6 sec cm-1). When the stomata of K. indica were induced to open (so that re fell to minimum values) by low CO2 concentrations, the leaves became desiccated and were damaged. In the field, the rates of assimilation fell to some extent throughout the day in all the species. The falls were not simply related to increasing re. This may be an effect of increasing leaf temperatures. Mesophyll resistance to diffusion of CO2 (rm) was particularly small in Haloxylon (0.75 sec cm-1), and small in Raeumuria (1.5 sec cm-1) and Zygo-phvllum (2.5 sec cm-1) in comparison with other published values. A plant which makes most effective use of water to grow should combine a small rm with a large rc, and its stomata should respond sensitively to changes in water stress. H. articulatum approaches this ideal, but K. indica was equally efficient, even though it has small rc and re so that its transpiration rate is large, because it has a characteristically large rate of assimilation.