Elevated Carbon Dioxide Effects on Reproductive Phenology and Seed Yield among Soybean Cultivars

Seed yield increases in soybean [Glycine max (L.) Merr.] caused by increased growth at elevated carbon dioxide concentrations primarily result from increased numbers of pods. Reasons for differences among cultivars in the increases in pod number are not clear. In experiments in controlled environment chambers, cultivars varied in how much elevated carbon dioxide delayed the transition of the apical node of the main stem from vegetative to reproductive growth. The longer the delay in the transition, the more main stem and total nodes and pods were produced, increasing the relative stimulation of seed yield. In free-air carbon dioxide enrichment (FACE) systems in the field, the timing of the first open flower, and the rate of progression through reproductive stages was faster, slower, or unchanged by elevated carbon dioxide in three indeterminate cultivars. However, independently from these effects on the rate of progression through the flowering stages, elevated carbon dioxide concentration also differentially affected the overall duration of vegetative growth, and the number of main stem and total nodes and pods produced. The extension of the duration of vegetative growth caused by elevated carbon dioxide was highly correlated with the relative seed yield increase in the field, as well as in the controlled environment chamber experiments.