Leaf-level transpiration rate (E, mmol m-2 s-1) can be expressed as:
where K is a slightly temperature-dependent, physical constant for the conversion of vapour pressure (kPa) to gas concentration (mmol mol-1); e*(7L) is the saturation vapour pressure at leaf temperature, TL; ea is the vapour pressure of air; and ra and rs are the boundary layer and stomatal diffusive resistances of water vapour (mol m-2 s-1), respectively. A similar relationship to that shown in equation 5.2 also holds for canopy transpiration, with the substitution of bulk air (rb) and canopy (rc) resistances instead of ra and rs, respectively.
It is now well established that for many plants, including rice, an increase in [CO2] increases the stomatal resistance (rs) or, inversely, decreases the conductance (gs = 1/rs) through a reduction in stomatal aperture (Akita, 1980; Morrison and Gifford 1983, 1984a; Baker et al., 1990a; Nakagawa et al., 1997; Homma et al., 1999). A 56% increase in rs (Morrison and Gifford, 1984a) and 40-49% increase in rc (Homma et al., 1999) were reported for rice subjected to long-term doubled [CO2] treatments. However, doubling [CO2] does not reduce Eto a similar extent as the rs increase. This is because an increase in rs causes a rise in TL which leads to an increase in E caused by the increase in the vapour pressure gradient between leaf and air (e*(TL) - ea). Thus, the effect of elevated [CO2] on E depends not only on rs but also on evaporative demand of the environment, which in turn is determined by factors such as temperature, solar radiation, humidity and wind speed.
Morrison and Gifford (1984a) found that daily E for rice grown in pots was similar for both ambient and doubled [CO2]. However, Wada et al. (1993) measured season-long E for rice grown at different temperatures under field-like conditions in TGCs (Fig. 5.3). They found that CO2 enrichment reduced seasonal total E by 15% at 26°C but increased E by 20% at 29.5°C. These results suggest that rs in the ambient [CO2] was not affected by air temperature, but rs decreased with increasing temperature under CO2 enrichment. Indeed, it has been shown from the energy budget analysis of remotely sensed canopy temperatures and microclimates for rice grown in TGCs that rc declined at doubled [CO2] with the rise in growing temperature, whereas rc was unaffected by temperature at ambient [CO2] (Homma et al., 1999). Further, they also found a much stronger relationship between rcand air temperature under CO2 enrichment for the rice cultivar IR36, an indica type rice, compared with Akihikari, a japonica type rice. A reduction in rs with increasing air temperature at elevated [CO2] has also been reported by Imai and Okamoto-Sato (1991). These results indicate that although [CO2] effects on E are much smaller than [CO2] effects on rs, at elevated [CO2], rs depends strongly on air temperature.
Rice, as with many other C3 species, displays higher crop water-use efficiencies (WUE) under elevated [CO2], due primarily to increased biomass production and partly to reduced transpiration. Morrison and Gifford (1984b) found that CO2 enrichment for rice grown in pots increased WUE by 53-63%. For rice grown in a TGC, with [CO2] enrichment at air temperatures between 24 and 26°C, WUE increased by 40-50% (Fig. 5.5). However, with further increases in air temperature, WUE decreased sharply to about a 20% enhancement at 30°C (Fig. 5.5). This declining CO2 enrichment effect on WUE with increasing air temperature is similar to whole-canopy photosynthetic WUE reported by Baker and Allen (1993). Similar declines in the CO2 enrichment effects on photosynthetic WUE at higher growth temperatures have also been reported for individual rice leaves: Nakagawa et al. (1997) found that leaf WUE at ambient [CO2] was not affected by growing temperature; but at elevated [CO2], WUE declined with increasing temperature.
20 22 24 26 28 30 32
Acclimated air temperature (°C)
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Acclimated air temperature (°C)
Fig. 5.5. Effect of daily mean temperature on relative enhancement of crop water use efficiency (WUE) for rice grown at ambient and enriched [CO2] (350 and 700 mmol mol-1, respectively) in TGCs. The WUEs at 700 mmol mol-1 are normalized to the values at ambient [CO2]. (Adapted from Nakagawa et al., 1997.)
We suggest that a simple modelling approach can help to explain these differential temperature responses in WUE for the elevated and ambient [CO2]. Since net photosynthetic rate (Pn) per unit leaf area can also be expressed by a gas diffusion equation similar to equation 5.2, the transpirational WUE (Pn/E) as given by Sinclair et al. (1984) is:
where Ca and Ci are [CO2] surrounding the leaf and in the leaf intercellular space, respectively. A constancy in the Ci/Ca ratio over a wide range of [CO2] has been reported for many plant species (Goudriaan and van Laar, 1978; Wong et al., 1979), including rice (Morrison and Gifford, 1983; Imai and Okamoto-Sato, 1991). This implies that stomatal aperture and hence rs respond to [CO2] in parallel with the response of leaf photosynthesis to [CO2] (Wong et al., 1979). At ambient [CO2], the observed conservative response of Pn/E ratio to temperature indicates that the C/Ca ratio remains constant with temperature and, further, that both rs and photosynthesis vary in tandem with temperature. However, at elevated [CO2], the decline in both rs and Pn/E ratio with increasing temperature indicates that the Ci/Ca ratio increased with temperature, and/or that e*(7L) - ea increased with temperature. Since the difference in TL between elevated and ambient [CO2] was very small at higher air temperatures (Homma et al., 1999), it seems that the Ci/Ca ratio increased at higher temperatures despite the reduction in rs. This suggests that at higher [CO2] and temperatures, rice stomata open independently of photosynthetic activity. This effect could be a mechanism employed by rice to increase leaf transpirational cooling at high temperatures. In any event, it appears that doubling [CO2] increases crop WUE in rice by about 50% over that at the optimal temperature range for growth. However, this increase in WUE declines sharply as temperature increases beyond the optimum.
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