Increased cyclic electron transport under drought ¿ a possible mechanism for avoiding oxidative stress?

Abstract

Plants subjected to drought stress have a decreased rate of photosynthesis and, therefore, are unable to use absorbed light energy. As a consequence, the plant is at risk of oxidative damage through the formation of reactive oxygen species (Asada, 1999). Regulation of electron transport to minimise the production of these radicals is a mechanism whereby protection may be achieved (Genty and Harbinson, 1996; Ott et al., 1999). Non-photochemical quenching (NPQ) dissipates excitation energy from the reaction centres of PSII and so protects against photoinhibition under conditions of excess energy (Heber and Walker, 1992). Cyclic electron transport around PSI has also been suggested as a protective mechanism (Clarke and Johnson, 2001) and is capable of generating the DpH required to support NPQ. The response of the electron transport chain to moderate and severe drought has been studied in Barley (Hordeum Vulgare c.v Chariot). Measurements of gas exchange, chlorophyll fluorescence and absorbance changes at 830 nm have been made in plants subjected to varying sorbitol concentrations for 24 hours. It was found that PSII electron transport capacity is unaffected by moderate drought but is inhibited by severe drought. By contrast, PSI electron transport rate is increased under both conditions. This increase in PSI electron transport rate indicates increased cyclic electron transport, which may be important in generating the DpH required to support NPQ. NPQ is also increased under drought.