Dynamic response of O2-dependent alternative electron flow to the growth irradiance in pea leaves

Abstract

O2-dependent alternative electron flow (AEF)(Mehler reaction) is proposed to have a physiological function of dissipating the excess photon energy by producing electron flux at the limited photosynthesis conditions. We studied its function by comparing the activities of O2-dependent AEF in the leaves of pea grown at different irradiance. If O2-dependent AEF has the ability of dissipating the excess photon energy, the pea grown at high irradiance should have higher O2-dependent AEF activity, compared to the pea grown at lower irradiance. Pea was grown at different irradiance [50(LL), 400(ML) and 1,000(HL) mmol photon m-2 s-1], 16h light/8h dark and 30°C, until their 4th leaves fully expanded. The activity of O2-dependent AEF in their 4th leaves was evaluated as the O2-dependent alternative electron flux [Ja(O2-depend)] determined by the simultaneous measurements of both gas-exchange and Chl fluorescence [Plant Cell Physiol. (2000) 41: 335-343]. Amount of ferredoxin (Fd), which catalyzes the photoreduction of O2, the limiting step of O2-dependent AEF and the activity of chloroplastic ascorbate peroxidase (APX) in each plants were assayed. We found that Ja(O2-depend) in HL-plant was higher than those in both LL- and ML-plants. Ja(O2-depend) had a positive linear correlation with the amount of Fd and the activity of chloroplastic APX. We concluded that the plant grown at higher irradiance required the higher activity of O2-dependent AEF, and both the high amount of Fd and the high activity of chloroplastic APX support the functioning of O2-dependent AEF in dissipating the excess photon energy.