Combined Effect of Potassium Nutrition and Carbon Dioxide on Soybean Growth and Development.

Potassium (K) is one of the major plant nutrients. Potassium deficiency limits crop yield worldwide due to its direct effect on the plant water relation, leaf area expansion, photosynthesis, and plant growth. The crop demand for nutrients such a K is expected to be higher due to the use of high yielding cultivars and increased plant growth under rising atmospheric carbon dioxide (CO₂) concentration. To evaluate the combined effects of K and CO₂ on soybean growth,  development, and biomass partitioning, 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. Plant height and mainstem nodes at maturity were not significantly affected by either CO₂ or K supply. Resutls showed that, K deficiency tended to decrease whereas eCO₂ tended to increase soybean growth. As compared to optimum K supply, the averaged 65% lower biomass under K-deficiency was associated with decreased leaf area (73%) and both above (65%) and below (36% roots) ground biomass production. The contribution of seeds to the total biomass was also highly reduced especially in the lowest K supply (6%) as compared to the optimum K supply (48%). However, biomass partitioning to the leaves stems and roots were higher under K deficiency. Elevated CO₂ significantly increased leaf area (8-70%) and plant biomass (11-58%) across K supply. Thus, the K deficiency altered the biomass partitioning among the plant parts. In addition, eCO₂ increased the seed production mainly at the two higher K supply (17-37%); however, showed about 6% decrease at the lowest K supply. Leaf K concentration decreased with the K supply and was lower at eCO₂. However, results showed that total amount of K extracted was greater in eCO₂ versus aCO₂. Plant growth and seed parameters exhibited a significant curvilinear (polynomial second order) relationship with the tissue K concentration and increased at eCO₂ roughly above K concentration of 8 mg g^-1. Thus, the degree of beneficial effect of eCO₂ on soybean growth and seed yield was dependent upon the K nutrition. A severe K deficiency highly reduced the positive effect of eCO₂ on biomass production and completely offset the seed yield.