Interactive Effects of Phosphorus Nutrition and Carbon Dioxide Concentration On Photosynthesis and Fluorescence Characteristics of Soybean.

Phosphate (Pi) is a major plant nutrient that limits photosynthetic processes, thus crop yield. Elevated CO₂ may overcome the limitations to photosynthesis (P_net) and chlorophyll fluorescence (Fvʹ/Fmʹ) resulting from phosphorus deficiency. To evaluate the interactive effects of elevated CO₂ and Pi nutrition, soybean plants were grown in controlled environment growth chambers with three levels of phosphate (Pi) (0.50, 0.10 and 0.01 mM) each at two levels of CO₂ (ambient 400 and elevated 800 µmol mol^-1). The P_net and Fvʹ/Fmʹ tended to decrease over the growth period between 25 and 65 days after planting; however, total chlorophyll concentration (Chl) and C_i/C_a ratio was not affected. Phosphorus stress severely reduced P_net (30-40%) and stomatal conductance (g_s, 25-54%), whereas elevated CO₂ significantly increased P_net especially at 0.50 mM Pi while decreasing g_s up to 56%. The Fvʹ/Fmʹ also reduced by Pi-stress but remained unaffected by growth CO₂. The limitation to P_net under Pi-stress at both CO₂ levels was mainly attributed to factors other than g_s as deduced from unaffected C_i/C_a ratio by either treatment. Photosynthetic acclimation/down regulation was evident under elevated CO₂ across Pi treatments as inferred from inhibition of biochemical process including the carboxylation efficiency. The diffusional limitation to P_net was greater due to mesophyll versus stomatal conductance under Pi-stress. Despite a decrease in phosphorus and nitrogen concentration in leaf tissue and reduced stomatal conductance at elevated CO₂, the P_net when measured at the growth CO₂ was either higher (0.05 mM Pi) or unaffected (0.10 and 0.01 mM Pi). The results indicated a negligible response of soybean photosynthesis to CO₂ enrichment under Pi-stress due to limitations caused in the path of CO₂ diffusion (stomatal and mesophyll), and photo-biochemical processes.