Elevated CO₂ concentration induces photosynthetic down-regulation with changes in leaf structure, non-structural carbohydrates and nitrogen content of soybean

Background: Understanding the mechanisms of crops in response to elevated CO₂ concentrations is pivotal to estimating the impacts of climate change on the global agricultural production. Based on earlier results of the "doubling-CO₂ concentration" experiments, many current climate models may overestimate the CO₂ fertilization effect on crops, and meanwhile, underestimate the potential impacts of future climate change on global agriculture ecosystem when the atmospheric CO₂ concentration goes beyond the optimal levels for crop growth. Results: This study examined the photosynthetic response of soybean (Glycine max (L.) Merr.) to elevated CO₂ concentration associated with changes in leaf structure, non-structural carbohydrates and nitrogen content with environmental growth chambers where the CO₂ concentration was controlled at 400, 600, 800, 1000, 1200, 1400, 1600 ppm. We found CO₂-induced down-regulation of leaf photosynthesis as evidenced by the consistently declined leaf net photosynthetic rate (A _n) with elevated CO₂ concentrations. This down-regulation of leaf photosynthesis was evident in biochemical and photochemical processes since the maximum carboxylation rate (V _cmax) and the maximum electron transport rate (J _max) were dramatically decreased at higher CO₂ concentrations exceeding their optimal values of about 600 ppm and 400 ppm, respectively. Moreover, the down-regulation of leaf photosynthesis at high CO₂ concentration was partially attributed to the reduced stomatal conductance (G _s) as demonstrated by the declines in stomatal density and stomatal area as well as the changes in the spatial distribution pattern of stomata. In addition, the smaller total mesophyll size (palisade and spongy tissues) and the lower nitrogen availability may also contribute to the down-regulation of leaf photosynthesis when soybean subjected to high CO₂ concentration environment. Conclusions: Down-regulation of leaf photosynthesis associated with the changes in stomatal traits, mesophyll tissue size, non-structural carbohydrates, and nitrogen availability of soybean in response to future high atmospheric CO₂ concentration and climate change.