Carbon dioxide exchange rates in soybeans during podfilling as a function of potassium supply

Abstract

Photosynthetic and dark respiration rates of single leaflets and whole plant canopies were measured during podfilling of soybean plants that were grown under low and high K regimes. Dark respiration rates of detached seed from these plants were also determined during the latter part of seed development. The study was carried out to test the hypothesis that low oil/protein ratios of seed from K-deficient plants resulted from the reduction in carbon availability within the plant, as a result of lower carbon assimilation rates and higher rates of respiratory carbon loss. Photosynthetic rates of upper canopy leaflets during early podfilling were depressed under K deficiency but this effect did not occur with whole plant canopies. In fact, towards the latter part of the podfilling period canopy photosynthetic rates were higher in K-deficient plants as nitrogen was exported earlier from the leaves in high-K plants, resulting in earlier leaf senescence in these plants. The level of K supply had no consistent effect on dark respiration rates of single leaflets or plant canopies, and had no effect on CO₂ evolution rates from detached, developing seed. The findings do not substantiate the hypothesis that reduced photosynthetic efficiency or enhanced respiratory carbon loss are responsible for lower oil/protein ratios in seed from K-deficient soybean plants.