Relationship between photosystem II activity and CO₂ fixation in leaves

Abstract

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.