In addition to physiological traits, leaf structural and biochemical characteristics may also play a pivotal role in plant response to high CO 2 concentration. Xu found that the decline in biomass of winter wheat at high CO 2 concentration might be attributed to the decrease of G s mainly due to the reduction in stomatal length and stomatal density. In addition, the down-regulation of A n at high CO 2 concentration may also be caused by the decline of stomatal conductance.
This down-regulation of A n may be attributed to the lower Rubisco concentration and activity or/and the source-sink imbalance due to leaf carbohydrates accumulation under elevated CO 2 concentration. For example, Kanemoto found that leaf photosynthesis of soybean plants was substantially decreased with elevating CO 2 concentration from about 400 ppm to 1000 ppm for 27 days of treatment. However, other studies reported that the A n was not marginally enhanced and even declined when plants exposed to long-term elevated CO 2 concentrations. Meanwhile, the enhanced A n may also be resulted from the reduced photorespiration and dark respiration and enhanced carboxylation efficiency under high CO 2 concentrations. Many previous studies have shown that elevated CO 2 generally stimulated the net photosynthetic rate ( A n) of plants, namely “CO 2 fertilization effect”, especially for the C 3 species, because the ribulose-1, 5-bisphophate carboxylase/oxygenase (Rubisco) of C 3 plants is not CO 2-saturated at the current atmospheric CO 2 concentration. Plant responses to elevated CO 2 concentration are fundamentally mediated by leaf photosynthesis, which is closely associated with the changes in leaf structure, chemical composition and carbon balance depending on plant species and/or functional types. The elevated atmospheric CO 2 concentration may lead to drastic impacts on the structure and function of natural and managed ecosystems. The most recently released report by the Inter-governmental Panel on Climate Change (IPCC) showed that global atmospheric CO 2 concentration has been dramatically increased from 280 ppm (the pre-industrial level) to over 400 ppm (the present level) with the growth rate of CO 2 concentration by ∼1.0 ppm per year, and may even be over 1000 ppm at the end of this century. It is well known that human activities have dramatically increased atmospheric concentrations of greenhouse gases, particularly the elevated atmospheric carbon dioxide concentration due to fossil fuel combustion and land use change following the nineteenth century industrial revolution. Conclusionsĭown-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 CO 2 concentration and climate change.
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 CO 2 concentration environment. Moreover, the down-regulation of leaf photosynthesis at high CO 2 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. 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 CO 2 concentrations exceeding their optimal values of about 600 ppm and 400 ppm, respectively. We found CO 2-induced down-regulation of leaf photosynthesis as evidenced by the consistently declined leaf net photosynthetic rate ( A n) with elevated CO 2 concentrations. This study examined the photosynthetic response of soybean ( Glycine max (L.) Merr.) to elevated CO 2 concentration associated with changes in leaf structure, non-structural carbohydrates and nitrogen content with environmental growth chambers where the CO 2 concentration was controlled at 400, 600, 800, 1000, 1200, 1400, 1600 ppm.
Based on earlier results of the “doubling-CO 2 concentration” experiments, many current climate models may overestimate the CO 2 fertilization effect on crops, and meanwhile, underestimate the potential impacts of future climate change on global agriculture ecosystem when the atmospheric CO 2 concentration goes beyond the optimal levels for crop growth. Understanding the mechanisms of crops in response to elevated CO 2 concentrations is pivotal to estimating the impacts of climate change on the global agricultural production.
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