Temperature

Temperature directly affects plant water use by its effect on VPD and it indirectly affects transpiration by altering the size of the transpiring leaf area. VPD generally increases with diurnal and seasonal cycles of temperature. Thus, transpiration and water use closely follow these temperature cycles. If the water demand of the plants cannot be met, stomata close to reduce water loss. At elevated [CO2], leaf transpiration decreases, thus alleviating temporal water-deficit stress. A simultaneous increase in [CO2] and temperature, which some climate models predict will occur in the future, might partly offset the beneficial effect of elevated [CO2] on plant growth under drought stress. A temperature increase by 2-3°C will have beneficial effects on plant growth in cooler parts of the world. However, if increasing temperatures increase potential water use to an extent that cannot be met by the supply, then an increasing water deficit would certainly reduce plant growth.

When temperature was increased by 2.5°C, gs decreased at ambient (-15%) and elevated (-9%) [CO2], but only at the top of the canopy (Nijs et al., 1997). The increased VPD at the elevated temperature, however, more than compensated for the reduced gs, and leaf transpiration increased by 28% and 48% at ambient and elevated [CO2], respectively. Leaf WUE decreased as temperature increased, but the temperatures of the air and the canopy were supra-optimal during these measurements.

A temperature increase of 3°C at elevated [CO2] increased canopy transpiration by 8% (Casella et al., 1996). The soil water content decreased considerably at the elevated air temperature. This was especially pronounced in summer, when plant growth was reduced at supra-optimal temperature. However, in spring and autumn (i.e. suboptimal temperature), yield and water use increased, additionally reducing soil moisture and leading to the earlier onset of water deficit in summer. These effects were stronger in high-N than in low-N plots, due to the larger canopy's greater demand for water. The beneficial effect of a 3°C temperature increase on photosynthesis was more than offset in summer by the increased drought stress. Increased temperature did not significantly affect the annual mean of canopy WUE. However, WUE showed strong seasonal variation, with a maximum in spring and autumn and a minimum in summer. When both [CO2] and temperature increased simultaneously, the annual mean WUE increased compared with both factors at ambient [CO2]. In summer, the simultaneous increase in [CO2] and temperature did not change WUE compared with both factors at ambient levels.

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