Soybean grown under elevated CO benefits more under low temperature than high temperature stress: Varying response of photosynthetic limitations, leaf metabolites, growth, and seed yield.

To evaluate the combined effect of temperature and CO on photosynthetic processes, leaf metabolites and growth, soybean was grown under a controlled environment at low (22/18°C, LT), optimum (28/24°C, OT) and high (36/32°C HT) temperatures under ambient (400μmolmol; aCO) or elevated (800μmolmol; eCO) CO concentrations during the reproductive stage. In general, the rate of photosynthesis (A), stomatal (g) and mesophyll (g) conductance, quantum yield of photosystem II, rates of maximum carboxylation (V), and electron transport (J) increased with temperature across CO levels. However, compared with OT, the percentage increases in these parameters at HT were lower than the observed decline at LT. The photosynthetic limitation at LT and OT was primarily caused by photo-biochemical processes (49-58%, L) followed by stomatal (27-32%, L) and mesophyll (15-19%, L) limitations. However, at HT, it was primarily caused by L (41%) followed by L (33%) and L (26%). The dominance of L at LT and OT was associated with the accumulation of non-structural carbohydrates (e.g., starch) and several organic acids, whereas this accumulation did not occur at HT, indicating increased metabolic activities. Compared with OT, biomass and seed yield declined more at HT than at LT. The eCO treatment compensated for the temperature-stress effects on biomass but only partially compensated for the effects on seed yield, especially at HT. Photosynthetic downregulation at eCO was possibly due to the accumulation of non-structural carbohydrates and the decrease in g and A (standard A measured at 400μmolmol sub-stomatal CO concentration), as well as the lack of CO effect on g, V, and J, and photosynthetic limitation. Thus, the photosynthetic limitation was temperature-dependent and was primarily influenced by the alteration in photo-biochemical processes and metabolic activities. Despite the inconsistent response of photosynthesis (or biomass accumulation) and seed yield, eCO tended to fully or partially compensate for the adverse effect of the respective LT and HT stresses under well-watered and sufficient nutrient conditions.