Effects of low atmospheric CO 2 and elevated temperature during growth on the gas exchange responses of C 3 , C 3 –C 4 intermediate, and C 4 species from three evolutionary lineages of C 4 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 CO 2 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 Flaveria , Heliotropium , and Alternanthera were grown near 380 and 180 μmol CO 2 mol −1 air and day/night temperatures of 37/29°C. Growth CO 2 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 CO 2 on photosynthetic and stomatal responses to intercellular CO 2 concentration. These results demonstrate little ability to acclimate to low CO 2 growth conditions in closely related C 3 and C 3 –C 4 species, indicating that, during past episodes of low CO 2 , 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 CO 2 assimilation than the C 3 species when measured at the growth conditions of 180 μmol mol −1 and 37°C, demonstrating the superiority of the C 3 –C 4 pathway in low atmospheric CO 2 and hot climates of recent geological time.