Changes in the Concentrations and Contents of Minerals Elements, Oil, and Protein in Soybean Seeds Under P-Stress Across Growth CO2.

Effects of phosphorus (P) stress or elevated CO₂ (eCO₂) on crop growth and productivity have been extensively studied. However, their combined impacts on the seed mineral elements and metabolic profile have received little attention. Since P deficiency and eCO₂ often show an inverse effects on plant growth, their interaction might alter the seed compositions. To investigate the response of seed mineral elements, protein and oil concentrations (g^-1 seed weight) and contents (plant^-1), soybean plants were grown in controlled environments at the sufficient (0.50 mM P) and two deficient (0.10 and 0.01 mM) levels of phosphorus under ambient (aCO₂) and eCO₂ (400 and 800 ppm, respectively). Results showed that seed carbon (C) and nitrogen (N) concentrations, and N/P ratio increased, but both the concentrations and the contents of majority of the minerals elements declined under P deficiency. Averaged across growth CO₂ and deficient P levels, the maximum decline was observed for the concentration of P (57.7%) followed by copper (22%) then potassium (17.5%). However, concentrations of C, iron (Fe), and manganese (Mn), and the contents of almost all mineral elements increased at eCO₂, but this enhancements were limited to the sufficient and 0.10 mM P treatments. Seed protein concentration and Protein/Oil ratio increased, but the oil concentration and the contents of protein and oil declined under P deficiency. On average, eCO₂ increased oil concentration (2%) and the contents of protein (16.8%) and oil (12.5%) in soybean seeds. There were inverse linear relationships of protein concentration with oil concentration (r² = 0.96) and seed yield (r² = 0.97) across P nutrition. However, a positive linear relationship (r² = 0.99) was found between the seed contents of protein and oil. Thus, P deficiency is likely to reduce the concentration and the contents of seed mineral elements and oil regardless of ambient or elevated CO₂ conditions. The eCO₂ failed to compensate for the losses in the majority of seed mineral elements concentration under P deficiency. However, eCO₂ might compensate, at least partially, for the overall contents of the mineral elements due to increased seed yield, especially near sufficient P nutrition.