Soybean Gas Exchange and Chlorophyll Fluorescence As Affected By K Supply Under Ambient and Elevated CO2 Concentration.

Potassium (K) deficiency adversely affects many of the vital processes inside the plant such as water relation, photosynthesis, cell expansion, assimilates transport, and enzyme activation. The K availability in soil nutrients may exert a major control over the plant response to rising atmospheric carbon dioxide (CO₂) . To evaluate the combined effects of K and CO₂ on soybean gas exchange and chlorophyll fluorescence, plants were grown in controlled environment growth chambers with three levels of K supply (optimum, 5.0 mM; K-stress, 0.50 and 0.02 mM) under ambient (aCO₂, 400 ppm) and elevated (eCO₂ 800 ppm) CO₂. Photosynthesis (P_net) and chlorophyll a fluorescence (Fvʹ/Fmʹ) were measured at ambient oxygen (O₂ ≈21%) and low O₂ (2%) exhibiting photo-respiratory (PR) and non photo-respiratory (NPR) conditions, respectively. At the lowest K supply, P_net decrease by 68% but did not differ between the two higher K supply. Elevated CO₂ increased P_net by 11-25% across the K nutrition. Stomatal conductance consistently decreased due to K deficiency  (45%) and eCO₂ (37%).  However, Fvʹ/Fmʹ was not affected by ether of the treatments. Both the photo-respiration (differences in P_net at NPR vs. PR condition) and the day-respiration increased 50-150% under K deficiency across CO₂ level. However, eCO₂, decreased the photo-respiration by 54% but increased the day-respiration by 36%, when averaged across K supply. Compared to the optimum K supply, the maximum carboxylation efficiency (V_Cmax) and the maximum rate of photosynthetic electron transport (J_max) either increased by 10-34% at 0.50 mM K or decreased by 30% at 0.02 mM K across CO₂ levels. However, mesophyll conductance decreased markedly under K deficiency by 41-81% and eCO₂ by 30%. Irrespective of the K supply, photosynthetic acclimations of soybean plants to CO₂ enriched environments were evident from the reduced quantum yields and carboxylation efficiency of the photosynthetic processes. . The results from this study clearly indicated that the amount of available soil K will affect soybean photosynthetic processes independent of CO₂ concentration. Moreover, the beneficial effect of eCO₂ on photosynthesis will be offset by severe K deficiency.