Oxygen-sensitive Differences in the Relationship between Photosynthetic Electron Transport and CO₂ Assimilation in C₃ and C₄ Plants during State Transitions

Abstract

Wheat (C₃) and maize (C₄) leaves were exposed to light treatments that were limiting for CO₂ assimilation and which excite preferentially photosystem I (PSI) or photosystem II (PSII) and induce State 1 or State 2, respectively. In order to examine the relationships between linear electron transport and CO₂ in leaves during State transitions, simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PSII and PSI photochemistry. In wheat leaves with photorespiratory activity, no significant change in quantum efficiency of CO₂assimilation was observed during State transitions. This was not the case when photorespiration was inhibited with either 2% O₂ or 1000 ppm CO₂ and transition from State 2 to State 1 was accompanied by a large decrease (c. 20%) in the quantum efficiency of CO₂ assimilation which was not associated with a decrease in the quantum efficiency of electron transport through PSII. Photorespiration appears to buffer the quantum efficiency of CO₂ assimilation from changes associated with decreases in the rate of CO₂ fixation resulting from imbalances in PPFD absorption by PSI and PSII.

When maize leaves were subjected to similar State transitions, no significant change in the quantum efficiency of CO₂ assimilation was observed on transition from State 2 to State 1, but on switching back to State 2 a very large decrease (c. 40%) was observed. This decrease could be prevented if leaves were maintained in either 2% O₂ or 593 ppm CO₂. The possible occurrence of photorespiration in maize leaves on transition from State 1 to State 2, which could result from an inhibition of the CO₂ concentrating mechanism, cannot account for the decrease in the quantum efficiency of CO₂ assimilation since the relationship between PSII electron transport and CO₂ assimilation remained similar throughout the State transitions. Also changes in the phosphorylation status of the light-harvesting chlorophyll _a/b protein associated with PSII cannot be implicated in this phenomenon.