Soybean Photosynthesis and Growth Responses to Temperature during Reproductive Stage Under Elevated CO2.

The high or low temperatures stresses are highly detrimental to soybean during reproductive stage of growth. However, an increased atmospheric CO₂ concentration may compensate the losses caused by temperature stresses. To evaluate the interactive effect of temperature and CO₂ on growth and photosynthetic processes of soybean during reproductive stage, soybean (cv. NC-Roy) was grown in six environmental control growth chambers at 28/24°C temperature. Half of the chambers assigned to either at ambient (400 µmol mol^-1; aCO₂) or elevated (800 µmol mol^-1; eCO₂) CO₂ concentrations under optimum water and nutrient conditions until flowering. Thereafter, day/night temperature treatments of 28/24°C (Optimum temperature; OT) and two temperature stresses (TStress) as 36/32°C (High; HT), and 22/18°C (Low; LT) were initiated till maturity. The soybean growth and photosynthesis were either decreased due to Tstress or increased at eCO₂. Compared to the OT, Tstress showed 3-9% decreased plant height at aCO₂ whereas 8-28% increases at eCO₂ at maturity. Leaf area was not significantly affected by temperature. However, eCO₂ caused over 45% increased leaf area at a given temperature. Compared to the OT, the averaged photosynthesis (P_net) and stomatal conductance decreased more at the LT (26-34%) than at HT (6-18%). However, eCO₂ increased P_net at all temperature treatments with the highest increase (79%) observed at the HT. The light energy harvesting efficiency of photosystem II (Fv'/Fm') was not significantly affected by either temperature or CO₂ treatments. Regardless of the growth CO₂, the maximum carboxylation efficiency (V_Cmax) of photosynthesis was either higher by 73-88% at HT or lower by approximately 46% at LT. Moreover, photosynthetic acclimation of soybean to CO₂ enriched environment was evident from the reduced V_Cmax and maximum electron transport. The total biomass and seed production were negatively affected by TStress whereas stimulated by eCO₂. The maximum biomass and seed production were observed at the combination of OT and eCO₂. However, highest stimulation by eCO₂ was observed at HT when compared to the plant grown at the combination of HT and aCO₂. Thus, a temperature dependent response of soybean growth and photosynthesis to eCO₂ was evident where maximum productivity was observed at the ambient temperature. It is worth mentioning that eCO₂ partially compensated the negative effect of temperature stress on growth and photosynthesis in soybean.