In vivo temperature dependence of cyclic and pseudocyclic electron transport in barley

The effect of temperature on the rate of electron transfer through photosystems I and II (PSI and PSII) was investigated in leaves of barley (Hordeum vulgare L.). Measurements of PSI and PSII photochemistry were made in 21% O2 and in 2% O2, to limit electron transport to O2 in the Mehler reaction. Measurements were made in the presence of saturating CO2 concentrations to suppress photorespiration. It was observed that the O2 dependency of PSII electron transport is highly temperature dependent. At 10 degrees C, the quantum yield of PSII (phi PSII) was insensitive to O2 concentration, indicating that there was no Mehler reaction operating. At high temperatures (> 25 degrees C) a substantial reduction in phi PSII was observed when the O2 concentration was reduced. However, under the same conditions, there was no effect of O2 concentration on the delta pH-dependent process of non-photochemical quenching. The rate of electron transport through PSI was also found to be independent of O2 concentration across the temperature range. We conclude that the Mehler reaction is not important in maintaining a thylakoid proton gradient that is capable of controlling PSII activity, and present evidence that cyclic electron transport around PSI acts to maintain membrane energisation at low temperature.