Abstract
In C3 photosynthesis, electrons from photosystem II are consumed by processes such as CO2 assimilation, photorespiration and oxygen reduction in the Mehler reaction. By measuring modulated chlorophyll fluorescence and CO2 uptake in leaves of Nicotiana tabacum in air with 350 ppm CO2, about 65 % of electrons transported was estimated to be utilized for CO2 assimilation under high light. Electron consumption by photorespiration was suppressed significantly under low light, but became larger under intermediate light where CO2 assimilation approached saturation. The increase of photorespiratory electron flow was accompanied by low wave, a transient drop of fluorescence yield after application of saturating pulse. The low-wave phenomenon was shown to be an indicator of cyclic electron transport around photosystem I (PSI). PSI-mediated cyclic electron flow was suggested to be necessary for photorespiration.
