Abstract
BackgroundUnderstanding 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.ResultsThis 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 (An) with elevated CO2 concentrations. This down-regulation of leaf photosynthesis was evident in biochemical and photochemical processes since the maximum carboxylation rate (Vcmax) and the maximum electron transport rate (Jmax) 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 (Gs) 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.ConclusionsDown-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.
Highlights
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
CO2 effects on leaf photosynthesis, stomatal conductance, water use efficiency, and dark respiration We found a negative quadratic relationship between leaf photosynthesis and CO2 concentration (R2 = 0.83) with the minimum leaf photosynthesis occurred at the CO2 concentration of 1200 ppm (Fig. 1a)
Stomatal diffusion processes explain the down-regulation of leaf photosynthesis In addition to biochemical and photochemical processes, our results showed that enhancing CO2 concentrations generally decreased stomatal density on both leaf surface, especially the stomatal density on the adaxial leaf surface was substantially decreased by about 50% with increasing CO2 concentration from 400 ppm to 1600 ppm (Table 1)
Summary
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. Kanemoto [29] found that leaf photosynthesis of soybean plants was substantially decreased with elevating CO2 concentration from about 400 ppm to 1000 ppm for 27 days of treatment. This down-regulation of An may be attributed to the lower Rubisco concentration and activity [30,31,32,33,34] or/and the source-sink imbalance due to leaf carbohydrates accumulation under elevated CO2 concentration [29, 35,36,37,38]. Xu [47] found that the decline in biomass of winter wheat at high CO2 concentration might be attributed to the decrease of Gs mainly due to the reduction in stomatal length and stomatal density
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