The impact of elevated carbon dioxide on the growth and gas exchange of three C-4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C-4 speci es can be stimulated by increasing CO2 concentration, [CO2], even under opt imal water and nutrients.To determine the basis for the observed photosynth etic stimulation, we tested the hypothesis that the CO2 leak rate from the bundle sheath would be directly related to any observed stimulation in sing le leaf photosynthesis at double the current [CO2]. Three C-4 species that differed in the reported degree of bundle sheath leakiness to CO2, Flaveria trinervia. Panicum miliaceum, and Panicum maximum, were grown for 31-38 da ys after sowing at a [CO2] of 350 mu l l(-1) (ambient) or 700 mu l l(-1) (e levated). Assimilation as a function of increasing [CO2] at high photosynth etic photon flux density (PPFD, 1 600 mu mol m(-2) s(-1)) indicated that le af photosynthesis was not saturated under current ambient [CO2] for any of the three C-4 species. Assimilation as a function of increasing PPFD also i ndicated that the response of leaf photosynthesis to elevated [CO2] was lig ht dependent for all three C-4 species, The stimulation of leaf photosynthe sis at elevated [CO2] was not associated with previously published values o f CO2 leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in day time leaf water potential for the species tested in this experiment. In sp ite of the simulation of leaf photosynthesis, a significant increase in gro wth at elevated [CO2] was only observed for one species, F. trinervia. Resu lts from this study indicate that leaf photosynthetic rates of certain C-4 species can respond directly to increased [CO2] under optimal growth condit ions, but that the stimulation of whole plant growth at elevated carbon dio xide cannot be predicted solely on the response of individual leaves.