Physiological Basis for C3C4 Plant Distributions

Photorespiration impacts both maximum photosynthetic rate and photosynthetic light-use efficiency (Bjorkman, 1966; Ehleringer and Bjorkman, 1977; Ehleringer and Pearcy, 1983; Sage and Monson, 1999). One consequence is that light-use efficiency or quantum yield for C02 uptake differ between C3 and C4 taxa (Ehleringer and Bjorkman, 1977). The quantum yield is defined as the slope of the photosynthetic light-response curve at low light levels. As the total leaf area within a canopy increases, an increasing proportion of the canopy-level carbon gain is influenced by light-use efficiency because the light level that the average leaf within the canopy is exposed to reduces with increasing total leaf area. The reduced quantum yield values in C4 taxa are temper-tature independent and reflect the additional ATP costs associated with operation of the C4 cycle (Hatch, 1987; Kanai and Edwards, 1999). In contrast, the quantum yield values of C3 taxa are reduced as temperatures increase, because Rubisco oxygenase activity increases with temperature. As a consequence, for any given set of atmospheric C02 and 02 conditions, the light-use efficiency of C3 plants will exceed that of C4 plants at lower air temperatures and will fall below that of C4 plants at higher temperatures.

Cerling et al. (1997) and Ehleringer et al. (1997) modeled the effects of variations in C3 /C4 quantum yields on predicted photosynthetic carbon gain under different environmental combinations of atmospheric C02 and temperature. They predicted that as atmospheric C02 levels decreased, C4 photosynthesis should become increasingly more common because of its higher light-use efficiency (Fig. l).This model predicts that C3 plants predominated during periods of the earth's history when atmospheric C02 levels were above ~ 500 ppmV. Plants with the C4 pathway are predicted to have a selective advantage only in the warmest ecosystems at atmospheric C02 levels close to 500 ppmV. However, as atmospheric C02 levels decrease further, the advantage of C4 photosynthesis and C4 dominance are predicted to drift toward cooler habitats.

Ci favored

predicted fcr

6-8 million years ago_

present day pre1850 Last Glacial Maximum -

C4 favored

Daytime growing-season temperature ['C]

FIGURE 1 Modeled crossover temperatures of the quantum yield for C02 uptake for C, and C4 plants as a function of atmospheric C02 concentrations. The boundary conditions shown are for NADP-me C4 plants (upper boundary) and NAD-me C4 plants (lower boundary). The crossover temperature is defined as the temperature (for a particular atmospheric C02 concentration) at which the quantum yields for C02 uptake are equivalent for both the C, and the C4 plant. Figure is modified from Ehleringer et al. (1997).

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