2.2.3 - Energetics of C₄ photosynthesis

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Figure 2.7. Generalised light response curves for leaf photosynthesis show that C₄ plants assimilate comparatively faster at high temperature (35°C), but that C₃ plants are advantaged at low temperature (10°C). Photorespiration increases with temperature, and is largely responsible for this contrast. C₄ plants are equipped with a CO₂-concentrating device in their bundle sheath tissue which both enhances Rubisco’s performance at that location, and forestalls photorespiratory loss. (Original drawings coustesy M.D. Hatch).

One disadvantage of the C₄ pathway is that an energy cost is incurred by C₄ plants to run the CO₂ ‘pump’. This is due to the ATP required for recycling PEP from pyruvate by the chloroplastic enzyme pyruvate, Pi dikinase in the mesophyll cells (Figure 2.4 and Hatch 1987). Under ideal conditions, five ATP and two NADPH are required for every CO₂ fixed in C₄ photosynthesis (two ATP are required to run the CO₂ pump, i.e., regenerate PEP). In addition, a proportion (20-30%) of CO₂ fixed by PEP carboxylase in the mesophyll is not fixed by Rubisco in the bundle sheath, and subsequently leaks back to the mesophyll. This leaked (or overcycled) CO₂ represents an additional, inherent energetic cost of the C₄ pathway.

From the previous section, the C₄ pathway is obviously energetically more expensive than the C₃ pathway in the absence of photorespiration. However, at higher temperatures the ratio of RuBP oxygenation to carboxylation is increased and the energy requirements of C₃ photosynthesis can rise to more than five ATP and three NADPH per CO₂ fixed in air (for these calculations see Hatch 1987).

Representative light response curves for photosynthesis in C₃ cf. C₄ plants (Figure 2.7) can be used to demonstrate some of these inherent differences in photosynthetic attributes. At low temperature (10°C in Figure 2.7) a C₃ leaf shows a steeper initial slope as well as a higher value for light-saturated photosynthesis. By implication, quantum yield is higher and photosynthetic capacity is greater under cool conditions. In terms of carbon gain and hence competitive ability, C₃ plants will thus have an advantage over C₄ plants at low temperature and especially under low light.

By contrast, under warm conditions (35°C, upper curves in Figure 2.7) C₄ photosynthesis in full sun greatly exceeds that of C₃, while quantum yield (inferred from initial slopes) remains unaffected by temperature. Significantly, C₃ plants show a reduction in quantum yield under warm conditions (compare 10°C and 35°C curves; right side of Figure 2.7). At 35°C C₃ plants also show lower rates of light-saturated assimilation compared with C₄ plants. Increased photorespiratory losses from C₃ leaves at high temperature are responsible (Section 2.3). C₄ plants will thus have a competitive advantage over C₃ plants under warm conditions at both high and low irradiance.