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 C₃, C₃-C₄ intermediate, and C₄ species grown in the low CO₄ and hot conditions proposed to favo r the evolution of C₄ photosynthesis. Closely related C₃ , C₃ -C₄ , and C₄ species in the genera Flaveria, Heliotropium, and Alternanthera were grown near 380 and 180 µmol CO₂ mol⁻¹ air and day/night temperatures of 37/29°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 C₃ and C₃–C₄ species, indicating that, during past episodes of low CO₂ , individual C₃ 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 C₃–C₄ intermediacy. The C₃–C₄ species had approximately 50% greater rates of net CO₂ assimilation than the C₃ species when measured at the growth conditions of 180 µmol mol⁻¹ and 37°C, demonstrating the superiority of the C₃–C₄ pathway in low atmospheric CO₂ and hot climates of recent geological time.