Effects Of Elevated Atmpospheric CO₂ Concentrations On Leaf Structure,Photosynthesis And Water Use Efficiency Of Soybean Plants

It is widely evident that global atmospheric carbon dioxide?CO₂?concentration has dramatically increased since the 19th century industrial revolution,elevating by about 1.6 ppm/yr,during the past five decades.Global atmospheric CO₂ levels have increased from 1960 level of 310 ppm to the present level of nearly 410 ppm.Moreover,it is predicted that the global atmospheric CO₂ concentration may even reach about1000 ppm by the end of this century,and nearly 2000 ppm by the end of the next century if no effective measures controlling global CO₂ with further increase in the cumulative emissions of CO₂ to the atmosphere.This projected global elevated CO₂concentration is expected to have effects on terristral ecosystem structure and function.It is noted that agricultual ecosystem is one of the most important components of terristral ecosystem.Elevated CO₂ concentration may not only alter the carbon balance of terristral ecosystem,but also influence the safety of future food supply all over the world.Numerous studies have investigated CO₂ fertilization effect primarily focusing on the impact of twofold current CO₂ concentration on plants.However,the underlying mechanisms of climate change on the crop production are far from understood,especially futher increase of CO₂ concentration change impacts on stomatal traits and gas exchange of crop such as soybean plants.Therefore,we conducted this experiment with environmental growth chambers controlling multiple high CO₂ levels from 400ppm to 1600 ppm to examine the changes in leaf structure,physiology and water use efficiency of soybean plants.We found that:?1?The stomatal density on the adaxial side was decreased with enhancing CO₂concentration.Our results also showed that elevated CO₂ concentration significantly decreased stomatal area index on both the adaxial and abaxial leaf surfaces except for increasing CO₂ concentration from 400 to 600 ppm,where the stomatal area index on the adaxial side was marginally increased by 15%and reached its maximum value at600 ppm.Nevertherless,high CO₂ concentration had little effect on the stomatal length both the adaxial and abaxial sides.Moreover,increasing CO₂ concentration from 400 to600 ppm caused the stomatal distribution to become more regular at small scales on the adaxial surface.However,the stomata on the abaxial surfaces tend to be less regular than those on the adaxial surface.?2?Elevated CO₂ concentration significantly increased leaf thickness?LT?,mainly due to the larger cell area of palisade and spongy.?3?Enhancing CO₂ concentration from 400 ppm to 800 ppm slightly increased the leaf,stem,and total TNC,whereas the TNC in leaves,stems and total TNC were dramatically reduced with further increasing CO₂ concentration from 800 ppm to 1600ppm.Moreover,elevated CO₂ concentration enhanced the N concentration of stem and root,while the leaf N was significantly decreased from 32.0 mg g^-1 to 30.8 mg g^-1 with increasing CO₂ concentration from 400 ppm to 1200 ppm.?4?We also found a negative quadratic relationship between leaf photosynthesis and CO₂ concentration,indicating down-regulation of leaf photosynthesis did occur when soybean plants subjected to enhanced CO₂ concentrations.The dark respiratory rate?R_d?,maximum carboxylation rate(V_cmax),maximum electron transport rate(J_max)and the V_cmax/J_max ratio all demonstrated a bell-shaped curve in relation to CO₂concentration,peaking at 900 ppm,592.5 ppm?390 ppm and 666.7 ppm,respectively.Moreover,water use efficiency?WUE?and stomatal conductance?G_s?were dramatically decreased with the increase of CO₂ concentrations.?5?Increased CO₂ concentration decreased the total biomass of soybean,whereas increased the relative growth rate of soybean plants.