13.3.3  Temperature

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Temperature effects on plant growth and development raise two sets of issues within a context of interactions between CO2 enrichment and environmental factors. First, will the rate of net photosynthesis at any given temperature be altered by CO2 enhancement and, second, will a growth response to CO2 enrichment be temperature dependent?

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Figure 13.7 Predicted light-saturated rates of assimilation with leaf temperature for three different CO2 concentrations. (Based on Long 1991)

The dual carboxylase/oxygenase function of Rubisco results in a reduction in net photosynthesis as O2 competes with CO2 for access to the key CO2-acceptor molecule of C3 photosynthesis, ribulose-1,5-bisphosphate (RuBP). As temperature rises, both CO2 solubility and specificity of Rubisco for CO2 decreases. Moreover, CO2 solubility decreases to a greater extent than does O2 solubility. Taken together, these changes favour oxygenation of RuBP, so for any given CO2 concentration increased temperature will increase RuBP oxygenation relative to carboxylation. Consequently the proportional enhancement of assimilation in response to CO2 enrichment increases because of a progressively larger benefit of inhibiting photorespiration as temperature increases (Figure 13.7). (See Long (1991) and Eamus (1996) for further discussion.)

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