The relationship between CO₂ assimilation and electron transport in leaves

The inter-relationships between the quantum efficiencies of photosystems I (φ_I) and II (φ_II) and the quantum yield of CO₂ fixation {Mathematical expression} were investigated in pea (Pisum sativum (L)) leaves with differing rates of photosynthesis using both photorespiratory and non-photorespiratory conditions, and in a leaf of Hedera helix (L) under photorespiratory conditions. The results indicate that under photorespiratory conditions the relationship between {Mathematical expression} and both φ_I and φ_II is non-linear and variable. The relationship between φ_I and φ_II under these circumstances remains predominantly linear. Under non-photorespiratory conditions, leaves with a low rate of photosynthesis due to sink limitation exhibit a non-linear relationship between φ_I and φ_II, though the relationship between φ_I and φ_II remains linear suggesting a close relationship between linear electron flow and CO₂ fixation. Leaves irradiated at the CO₂ compensation point also exhibit a non-linear relationship between φ_I and φ_II. These results suggest that for leaves in air linear electron flow is the predominant source of energy for metabolism. The role of cyclic electron transport is considered when the requirement for the products of linear electron transport is depressed.