Electron flux from water via photosystem II (PSII) and PSI to oxygen (water–water cycle) may provide a mechanism for dissipation of excess excitation energy in leaves when CO2 assimilation is restricted. Mass spectrometry was used to measure O2 uptake and evolution together with CO2 uptake in leaves of French bean and maize at CO2 concentrations saturating for photosynthesis and the CO2 compensation point. In French bean at high CO2 and low O2 concentrations no significant water–water cycle activity was observed. At the CO2 compensation point and 3% O2 a low rate of water–water cycle activity was observed, which accounted for 30% of the linear electron flux from water. In maize leaves negligible water–water cycle activity was detected at the compensation point. During induction of photosynthesis in maize linear electron flux was considerably greater than CO2 assimilation, but no significant water–water cycle activity was detected. Miscanthus × giganteus grown at chilling temperature also exhibited rates of linear electron transport considerably in excess of CO2 assimilation; however, no significant water–water cycle activity was detected. Clearly the water–water cycle can operate in leaves under some conditions, but it does not act as a major sink for excess excitation energy when CO2 assimilation is restricted.
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