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

Abstract.

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 °C, the quantum yield of PSII (ΦPSII) was insensitive to O₂ concentration, indicating that there was no Mehler reaction operating. At high temperatures (>25 °C) a substantial reduction in ΦPSII was observed when the O₂ concentration was reduced. However, under the same conditions, there was no effect of O₂ concentration on the ΔpH-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.