Effect of high-irradiance stress on primary photochemistry and light regulated enzymes of photosynthetic carbon metabolism

Abstract

The photosynthetic electron transport, chlorophyll fluorescence and light regulated CO2 reduction cycle enzymes of chloroplasts isolated from photoinhibited Sorghum bicolor L. (R 109A) were determined. The leaves were exposed to a range of irradiance (850-2500 mu tmol m-2 s-1) at 5 degrees C and 20 degrees C. The photosynthetic electron transport and chlorophyll fluorescence (Fv/Fm ratio) decreased with increasing irradiance. This decrease was more at 5 degrees C than at 20 degrees C. Exposure of seedlings to higher irradiance caused damage to the oxidizing side of PS II while PS I remained comparatively unaffected. The activities of light-regulated CO2 reduction enzymes, NADP-malate dehydrogenase, PEP-carboxylase and phosphoribulokinase in leaves increased upon exposure to higher irradiance. Increase in the activities of enzymes was temperature dependent. Extending the photoinhibitory treatment for longer than 2 hours showed little further increase in the enzyme activities. The results show that photoinhibition of leaves affects the photosynthetic electron transport of their chloroplasts with primary damage to the oxidizing side of PS II, and also effects the CO2 fixation properties of the chloroplasts.