Potassium Starvation Affects Growth More Than Photosynthesis in Soybean across Ambient and Elevated CO2.

To evaluate combined effects of potassium (K) deficiency and elevated carbon dioxide (eCO₂) on soybean photosynthesis, growth, and yield, plants were grown under controlled environment with an adequate (control, 5.0 mM) and two deficient (0.50 and 0.02 mM) levels of K under ambient CO₂ (aCO₂; 400 µmol mol^-1) and eCO₂ (800 µmol mol^-1). Results showed that K deficiency limited soybean growth traits more than photosynthetic characteristics. Over 50% reduction in leaf K concentration under 0.5 mM K versus the control caused about 45% less leaf area, biomass, and yield without decreasing photosynthetic rate (P_net). In fact, the steady photochemical quenching, efficiency, and quantum yield of photosystem II, chlorophyll concentration (TChl), and stomatal conductance under 0.5 mM K supported the stable P_net. The biomass decline was primarily attributed to the smaller plant size, lower leaf area, decreased pod numbers and seed yield in K-deficient plants. Under severe K deficiency (0.02 mM K), photosynthetic processes declined roughly parallel with growth and productivity. Increased specific leaf weight, greater biomass partitioning to the leaves, decreased photochemical quenching and TChl, and smaller plant size to reduce the nutrient demands appeared to be the means by which plants adjusted to the severe K starvation. The enhancement of soybean growth by eCO₂ was dependent on the level of K supply. A lack of eCO₂-mediated growth and photosynthesis stimulation under severe K deficiency underscored the importance of optimum K fertilization for maximum crop productivity under eCO₂. The eCO₂ compensated, at least partially, for the reduced soybean growth and seed yield under 0.5 mM K supply, but severe K deficiency completely suppressed the eCO₂-enhanced seed yield.