Effects of Long term Exposure to Atmospheric Carbon Dioxide Enrichment on Flowering and Podding in Soybean

It is known that increasing atmospheric carbon dioxide （CO_2） concentration resulted in the enhancement of vegetative growth and an increase in seed yield in soybean （Glycine max （L.） Merrill）. However, the way this increase in seed yield comes about is not well known. The objective of this study was to make clear the effects of CO_2 enrichment on flowering and podding, since these are the factors which directly limit the seed yield in soybean. Cultivar \u27Fukuyutaka\u27 were grown in the ambient CO_2 concentration （AC: 350 μmol CO_2 mol^<-1> Air） and an enriched CO_2 concentration （EC: 700 μmol CO_2 mol^<-1> Air） throughout the growth period. The effects of EC on the flowering were significantly different among the individual racemes. On the terminal racemes and the primary racemes, there was no difference in the total number of flowers between AC and EC, although the number of flowers at the peak of flowering differed slightly with the CO_2 regime used. On the secondary racemes with compound leaves, EC condition increased the number of flowers compared with AC. The opposite effect was seen on the secondary racemes without compound leaves. Furthermore, large number of flowers on the tertiary racemes appeared in EC throughout the flowering period. The number of pods and the pod-setting ratio in EC condition on the secondary racemes, the secondary racemes with compound leaves and the tertiary racemes were larger than those in AC condition. Therefore, enrichment of CO_2 increased seed yield because of the increase in the number of flowers and the high pod-setting ratio on the high order racemes （the secondary racemes, the secondary racemes with compound leaves and the tertiary racemes）, which appeared at later time during the flowering period