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

Abstract

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.

Keywords: CO2 enhancement; Down regulation; N availability; Non-structural carbohydrates; Soybean crops; Stomatal traits.

Fig. 1

Effects of elevated CO₂ concentrations on (a) leaf net photosynthesis rate (A_n), (b) stomatal conductance (G_s), (c) intercellular CO₂ concentration (C_i), and (d) dark respiration rate (R_d). Parameters are measured at ambient or elevated CO₂ of their growing condition for each treatment and values given are mean ± standard deviation for n = 5 leaves. The arrow indicates the optimal CO₂ concentration for leaf R_d of soybean plants

Fig. 2

Effects of elevated CO₂ concentrations on the V_cmax (a), J_max (b) and V_cmax/J_max ratio of soybean plants

Fig. 3

Changes in the morphological traits of stomata on the adaxial leaf surface (a-g) and abaxial leaf surface (A-G) of soybean leaves grown at CO₂ concentrations of 400, 600, 800, 1000, 1200, 1400 and 1600 ppm observed with scanning electron microscopy. Bars = 5 μm

Fig. 4

The spatial distribution pattern of stomata on the adaxial surface (a) and abaxial surface (b) of soybean leaves under elevated CO₂ concentrations. The upper and lower 95% boundaries were obtained by Monte Carlo simulation of 100 replicates

Fig. 5

Light micrographs of cross-section through leaves of soybean. Note that cross-section micrographs show leaf thickness (LT), palisade mesophyll (PM), and spongy mesophyll (SM) of soybean leaves grown at ambient (a-b) and elevated CO₂ concentrations (c-h). Bar = 50 μm

Fig. 6

Effects of elevated CO₂ concentrations on the relationships between net photosynthetic rate and stomatal conductance (a) as well as stomatal area (b and c) of soybean plants

Fig. 7

Effects of elevated CO₂ concentrations on the relationships between net photosynthetic rate and spongy cell area (a) or palisade cell area (b) of soybean plants

Fig. 8

Effects of elevated CO₂ concentrations on the relationships between net photosynthetic rate and non-structural carbohydrate concentration of soybean plants