Effects of low atmospheric CO2 and elevated temperature during growth on the gas exchange responses of C3, C3-C4 intermediate, and C4 species from three evolutionary lineages of C4 photosynthesis.

This study evaluates acclimation of photosynthesis and stomatal conductance in three evolutionary lineages of C-3, C-3-C-4 intermediate, and C-4 species grown in the low CO2 and hot conditions proposed to favo r the evolution of C-4 photosynthesis. Closely related C-3, C-3-C-4, and C-4 species in the genera , , and were grown near 380 and 180 mu mol CO2 mol(-1) air and day/night temperatures of 37/29A degrees C. Growth CO2 had no effect on photosynthetic capacity or nitrogen allocation to Rubisco and electron transport in any of the species. There was also no effect of growth CO2 on photosynthetic and stomatal responses to intercellular CO2 concentration. These results demonstrate little ability to acclimate to low CO2 growth conditions in closely related C-3 and C-3-C-4 species, indicating that, during past episodes of low CO2, individual C-3 plants had little ability to adjust their photosynthetic physiology to compensate for carbon starvation. This deficiency could have favored selection for more efficient modes of carbon assimilation, such as C-3-C-4 intermediacy. The C-3-C-4 species had approximately 50% greater rates of net CO2 assimilation than the C-3 species when measured at the growth conditions of 180 mu mol mol(-1) and 37A degrees C, demonstrating the superiority of the C-3-C-4 pathway in low atmospheric CO2 and hot climates of recent geological time.