13.2.1  C3 plants versus C4 plants

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When water supply does not restrict growth, C3 plants can accumulate dry matter more rapidly in elevated CO2 concentrations than ambient CO2 concentrations. This is primarily because the rate of net photosynthetic carbon reduction by Rubisco is increased by increased substrate availability and partly because the rate of oxygenation by Rubisco (photorespiration) is reduced. The different mechanisms of C3, C4 and CAM photosynthetic pathways (Section 2.1) confer contrasting characteristics to the CO2 response of photosynthesis and growth of these plant groups. The CO2-concentrating mechanism in C4 plants suppresses photorespiration. This causes a steeper initial slope to the CO2 response curve of net photosynthesis than in C3 plants and a sharper transition to saturation (Figure 2.4). At an atmospheric CO2 concentration of around 350 µmol CO2 mol–1 the CO2 partial pressure inside a leaf (pi) is, for C4 species, close to the zone on the curve where there is a transition to the saturating value. In C3 species the normal operating pi supports net carbon fixation at about half the saturating rate. Accordingly, growth experiments in well-watered conditions show little or no response of C4, but substantial responses of C3, species to CO2 enrichment. However, in experiments where CO2 concentration is reduced below about 350 µmol CO2 mol–1, growth of both C3 and C4 species respond strongly. CAM plants (Figure 2.7) also show little or no photosynthetic or growth response to CO2 enrichment. Their photosynthetic mechanism involves either daytime CO2 fixation by Rubisco at high intercellular CO2 concentrations, or night-time assimilation via phosphoenolpyruvate (PEP) carboxylase with a high affinity for CO2.

There is a wide range in the growth responses of plants grown in twice ambient CO2 concentration (Figure 13.2). The overall mean response of C3 species (+41%) exceeds those of C4 (+22%) and CAM (+15%) plants. Wide variation in growth enhancement by C3 plants, and responses of C4 and CAM plants that were somewhat larger than predicted from leaf photosynthetic considerations alone, imply plant responses to CO2 beyond those of photosynthesis and photorespiration. Such features include interactions between CO2 and other growth-limiting environmental variables, duration of intervals between harvests, photosynthetic acclimation to high CO2 and changes in carbon partitioning. These topics are dealt with below.

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