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% O-2 and in 2% O-2, to limit electron transport to O-2 in the Mehler reaction. Measurements were made in the presence of saturating CO2 concentrations to suppress photorespiration. It was observed that the O-2 dependency of PSII electron transport is highly temperature dependent. At 10 degreesC, the quantum yield of PSII (Phi PSII) was insensitive to O-2 concentration, indicating that there was no Mehler reaction operating. At high temperatures (> 25 degreesC) a substantial reduction in Phi PSII was observed when the O-2 concentration was reduced. However. under the same conditions, there was no effect of O-2 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 O-2 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.