Relationship between photosystem II activity and CO2 fixation in leaves

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO 2 fixed. A measure of the quantum yield of photosystem II, Φ II (electron/photon absorbed by PSII), can be obtained in leaves under steady‐state conditions in the light using a modulated fluorescence system. The rate of electron transport from PSII equals Φ II times incident light intensity times the fraction of incident light absorbed by PSII. In C 4 plants, there is a linear relationship between PSII activity and CO 2 fixation, since there are no other major sinks for electrons; thus measurements of quantum yield of PSII may be used to estimate rates of photosynthesis in C 4 species. In C 3 plants, both CO 2 fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO 2 , or per O 2 reacting with RuBP). The rates of PSII activity associated with photosynthesis in C 3 plants, based on estimates of the rates of carboxylation (v o ) and oxygenation (v o ) at various levels of CO 2 and O 2 , largely account for the PSII activity determined from fluorescence measurements. Thus, in C 3 plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO 2 fixation.