Quantum Yields of Photosystem II Electron Transport and Carbon Dioxide Fixation in C₄ Plants

Abstract

The quantum yields of non-cyclic electron transport from photosystem II (determined from chlorophyll a fluorescence) and carbon dioxide assimilation were measured in vivo in representative species of the three subgroups of C₄ plants (NADP-malic enzyme, NAD-malic enzyme and PEP-carboxykinase) over a series of intercellular CO₂ concentrations (C_I) at both 21% and 2% O₂. The CO₂ assimilation rate was independent of O₂ concentration over the entire range of C_i (up to 500 μbar) in all three C₄ subgroups. The quantum yield of PS II electron transport was similar, or only slightly greater, in 21% v. 2% O₂ at all Ci values. In contrast, in the C₃ species wheat there was a large O₂ dependent increase in PS II quantum yield at low CO₂, which reflects a high level of photorespiration. In the C₄ plants, the relationship of the quantum yield of PS II electron transport to the quantum yield of CO₂ fixation is linear suggesting that photochemical use of energy absorbed by PS II is tightly linked to CO₂ fixation in C₄ plants. This relationship is nearly identical in all three subgroups and may allow estimates of photosynthetic rates of C₄ plants based on measurements of PS II photochemical efficiency. The results suggest that in C₄ plants both the photoreduction of O₂ and photorespiration are low, even at very limiting CO₂ concentrations.