13.3.1  Light

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Figure 13.5 Relative growth rate (RGR) for Acmena smithii and Doryophora sassfras. A. smithii achieves higher RGR than D. sassfras, especially at higher light levels. Data points are the average of two CO2 treatments, 350 and 700 umol CO2 mol -1. (2 × standard errors of means are shown) (D.J. Wiggins, M.C. Ball, R.M. Gifford and G.D. Farquhar, pers. comm. 1996)

The nature of CO2 enrichment differs from increasing the availability of light or any other environmental variable in one important respect. Besides increasing the availability of a limiting resource, CO2 enrichment reduces inefficiency through a reduction in the photorespiratory loss of carbon. So even under extreme light-limiting conditions, CO2 enrichment can be expected to enhance net photoassimilation. This is most frequently expressed as an increased quantum efficiency. In addition, a frequently observed consequence of increasing the supply of photoassimilate in response to CO2 enrichment is increased leaf area. This has the feed-forward effect of causing increased radiation interception, which further amplifies the initial response to CO2 enrichment. Increased light and elevated CO2 can therefore interact positively to affect relative growth rate via an initial effect on rate of leaf expansion (Figure 13.5).