ABSTRACT
We have seen that elevated CO2 reduces stomatal conductance (Chapter 8). We now investigate the effects that this reduction has on transpiration and evapotranspiration.
An increase in stomatal conductance causes an increase in transpiration. This is illustrated in Figure 10.1, where there is a positive sign between leaf conductance and transpiration. However, conductance and transpiration are decoupled to some extent, particularly in greenhouses, where insuf-£cient cooling of the leaves through transpiration causes heat damage that occurs at high radiation (Nederhoff et al. 1992). This is explained in Figure 10.1, which depicts a course of events following an increase in CO2 in a greenhouse (Nederhoff 1994, p. 61). High CO2 causes partial stomatal closure (reduced leaf conductance-note the negative sign between CO2 and leaf conductance in Figure 10.1). This initially reduces transpiration of leaves, which slightly increases leaf temperature. There is a negative sign between transpiration and leaf temperature in Figure 10.1 because the greater the transpiration rate, the cooler is the leaf temperature. Reduction of transpiration also decreases absolute humidity. [Absolute humidity is the mass of water vapor present in unit volume of the atmosphere, usually measured as grams per cubic meter. It may also be expressed in terms of the actual pressure of the water vapor present (Weast 1964, p. F-45).] If the absolute humidity is decreased, the vapor pressure de£cit (VPD) increases, £rst in the boundary layer and later in the surrounding air. Increased leaf temperature and reduced absolute humidity imply an increase of the VPD between the stomatal cavities and the air (VPD in leaf-VPD in air; Figure 10.1, bottom). An increase in VPD reduces the stomatal conductance, thus enforcing the effect of the CO2 concentration on stomatal conductance. [See also Figure 8.1. This £gure shows that when atmospheric VPD is increased, guard-cell turgor is decreased and stomata close.] However, increased VPD increases transpiration through a larger driving force, thus counteracting the effect of stomatal closure through the CO2 concentration on transpiration (Allen et al. 1985, p. 13; Nederhoff 1994, pp. 60-61).
