• C3 plants;
  • C4 plants;
  • chlorophyll fluorescence;
  • CO2 fixation;
  • photosynthesis;
  • photosystem II activity;
  • quantum yield

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO2 fixed. A measure of the quantum yield of photosystem II, ΦII (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 ΦII times incident light intensity times the fraction of incident light absorbed by PSII. In C4 plants, there is a linear relationship between PSII activity and CO2 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 C4 species. In C3 plants, both CO2 fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO2, or per O2 reacting with RuBP). The rates of PSII activity associated with photosynthesis in C3 plants, based on estimates of the rates of carboxylation (vo) and oxygenation (vo) at various levels of CO2 and O2, largely account for the PSII activity determined from fluorescence measurements. Thus, in C3 plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO2 fixation.