CO_2 assimilation-governing mechanisms based on the balance in chemical energy produced and consumed in the photosynthetic process.

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This study was conducted to understand the CO_2 assimilation-governing mechanism by relating the leaf CER (CO_2 exchange rate) with the chemical energy production-consumption balance between photosystem and CO_2 assimilation. (1) CER of mungbean leaves were determined as a time course under the intermittent illumination with 20 second-intervals of light on/off in the different combinations of CO_2 and O_2 concentrations and light intensities. Based on the calculation of the energy produced at the light-on (20s) moments and the weight of CO_2 assimilated at the light-off (20s) moments, the effect of the intermittent illumination on the Rubisco (RuBP Carboxylase/Oxygenase) activity was estimated. CER or light utilization efficiency was increased at the intermittent illumination in high light intensities, because CO_2 was possible to be assimilated at the light-off moments by using the accumulated energy at the light-on moments. Light activation level of Rubisco varied with CO_2 and O_2 concentrations. The relative activity between carboxylase and oxygenase differed depending on the environmental conditions, affecting the CER and light utilization efficiency at the intermittent illumination. (2) Then, to get more clear explain for the action balance between photosystem and CO_2 assimilation system, the relationships between CER and chlorophyll fluorescence quenching were studied. The improved measurement tools were skillfully assembled to make a simultaneous determination of both factors. It was evident in mungbean leaves that the energy consumption by photorespiration significantly affected, in relation to environmental factors, both CER and the potimal energy utilization efficiency in photosynthesis.