TEMPERATURE-DEPENDENCE OF THE LINKAGE OF QUANTUM YIELD OF PHOTOSYSTEM-II TO CO2 FIXATION IN C-4 AND C-3 PLANTS

The temperature dependence of quantum yields of electron transport from photosystem 11 (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, NADmalic enzyme, and phosphoenolpyruvate carboxykinase subgroups, the ratio of PHI(II)/PHI(CO2) was constant over the temperature range from 15 to 40-degrees-C at high light intensity (1100 mumol quanta M-2 s-1). A similar response was obtained at low light intensity (300 mumol quanta m-2 s-1), except the ratio of PHI(II)/PHI(CO2) increased at high temperature. When the true quantum yield for CO, 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(CO2)* 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 biochemical mechanism of concentrating CO, in the bundle sheath cells. The results suggest that PSII activity is closely linked to the true rate of CO, 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 C, 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.