Effects of low atmospheric C[O.sub.2] and elevated temperature during growth on the gas exchange responses of [C.sub.3], [C.sub.3]-[C.sub.4] intermediate, and [C.sub.4] species from three evolutionary lineages of [C.sub.4] photosynthesis

This study evaluates acclimation of photosynthesis and stomatal conductance in three evolutionary lineages of [C.sub.3], [C.sub.3]-[C.sub.4] intermediate, and [C.sub.4] species grown in the low C[O.sub.2] and hot conditions proposed to favo r the evolution of [C.sub.4] photosynthesis. Closely related [C.sub.3], [C.sub.3]-[C.sub.4], and [C.sub.4] species in the genera Flaveria, Heliotropium, and Alternanthera were grown near 380 and 180 µmol C[O.sub.2] [mol.sup.-1] air and day/night temperatures of 37/29°C. Growth C[O.sub.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 C[O.sub.2] on photosynthetic and stomatal responses to intercellular C[O.sub.2] concentration. These results demonstrate little ability to acclimate to low C[O.sub.2] growth conditions in closely related [C.sub.3] and [C.sub.3]-[C.sub.4] species, indicating that, during past episodes of low C[O.sub.2], individual [C.sub.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.sub.3]-[C.sub.4] intermediacy. The [C.sub.3]-[C.sub.4] species had approximately 50% greater rates of net C[O.sub.2] assimilation than the [C.sub.3] species when measured at the growth conditions of 180 µmol [mol.sup.-1] and 37°C, demonstrating the superiority of the [C.sub.3]-[C.sub.4] pathway in low atmospheric C[O.sub.2] and hot climates of recent geological time.