Does electron transport to oxygen protect plants against excess light?

Abstract

In addition to assimilatory linear electron flow, a number of auxiliary pathways of electron transport occur in the chloroplast. It has been suggested that these may play an important role in the protection of the photosynthetic apparatus under conditions where the light energy absorbed by plants exceeds their metabolic capacity. In particular electron flow to O2 in photorespiration or the Mehler reaction may help to minimise the damaging production of singlet O2 by acting as a sink for excess reductant, thus preventing the build-up of reduced intermediates in the electron transport chain. In addition, the Mehler reaction may enhance DpH-dependent quenching processes in PSII by increasing DpH. We have examined the extent to which electron transport to O2 occurs in vivo in barley under a range of physiological conditions and its capacity to protect the photosynthetic apparatus through the enhancement of high-energy state non-photochemical quenching. Electron transport to O2 was absent at low temperatures, but was increased at high temperatures. However, over the same temperature range NPQ remained constant and was insensitive to O2 concentration, suggesting that any additional electron transport in this way is not necessary for the generation of DpH. Measurements of PSI electron transport rate indicate that cyclic electron transport occurs and we suggest that this may be more important than O2 as a sink for excess reductant under conditions where CO2 fixation is limited.