Temperature Dependence of the Linkage of Quantum Yield of Photosystem II to CO2 Fixation in C4 and C3 Plants

The temperature dependence of quantum yields of electron transport from photosystem II (PSII) (phi]II, determined from chlorophyll a fluorescence) and CO2 assimilation (phi]CO2, apparent quantum yield for CO2 assimilation) were determined simultaneously in vivo. With C4 species representing NADP-malic enzyme, NAD-malic enzyme, and phosphoenolpyruvate carboxykinase subgroups, the ratio of phi]II/phi]CO2 was constant over the temperature range from 15 to 40deg]C at high light intensity (1100 mu]mol quanta m-2 s-1). A similar response was obtained at low light intensity (300 mu]mol quanta m-2 s-1), except the ratio of phi]II/phi]CO2 increased at high temperature. When the true quantum yield for CO2 fixation (phi]CO2*) was calculated by correcting for respiration in the light (estimated from temperature dependence of dark respiration), the ratio of phi]II/phi]C02* remained constant with varying temperature and under both light intensities in all C4 species examined. Because the phi]II/phi]CO2* ratio was the same in C4 monocots representing the three subgroups, the ratio was not affected by differences in the bio-chemical mechanism of concentrating CO2 in the bundle sheath cells. The results suggest that PSII activity is closely linked to the true rate of CO2 fixation in C4 plants. The close relationship between phi]II and phi]CO2* in C4 species under varying temperature and light intensity conditions is apparently due to a common low level of photorespiration and a primary requirement for reductive power in the C3 pathway. In contrast, in a C3 plant the phi] II/phi]CO2* ratio is higher under normal atmospheric conditions than under nonphotorespiratory conditions and it increases with rising temperature. This decrease in efficiency in utilizing energy derived from PSII for CO2 fixation is due to an increase in photorespiration. In both the C3 and C4 species, photochemistry is limited under low temperature, and thus excess energy must be dissipated by nonphotochemical means.