## Abstract

An apparatus is described for the rapid measurement in the field of the quantum yield of <latex>$CO_2$</latex> assimilation, <latex>$\phi$</latex>, and chlorophyll fluorescence kinetics from attached leaves exposed to ambient <latex>$CO_2$</latex> concentrations. This apparatus was used to measure 0 and the ratio of variable to maximal fluorescence, <latex>$F_v$</latex>/<latex>$F_M$</latex>, of fully expanded leaves of a maize crop in northeast Essex at monthly intervals throughout the growing season. The quantum yield of <latex>$CO_2$</latex> assimilation and <latex>$F_v$</latex>/<latex>$F_M$</latex> increased from May to August and then decreased in September. A linear correlation between <latex>$\phi$</latex> and <latex>$F_v$</latex>/<latex>$F_M$</latex> was observed for the leaves. The relations between light, temperature, the quantum yield of <latex>$CO_2$</latex> assimilation and fluorescence emission kinetics of leaves of a maize crop during the early growing season were also examined. Decreases in <latex>$\phi$</latex> associated with chilling temperatures and high light were observed and identified from analyses of fluorescence kinetics as being attributable to photoinhibitory damage of the photosynthetic apparatus. The possibility of using measurements of <latex>$\phi$</latex> and fluorescence kinetics for screening the photosynthetic performance of crops is considered. Studies with winter rape demonstrated that changes in <latex>$\phi$</latex> during the growing season were correlated with changes in the efficiency of light-energy conversion to dry matter by the crop.