The objective of this study is to elucidate the effects of CO 2 enrichment during different growth periods on flowering, pod set and seed yield in soybean (Glycine max (L.) Merr.). Soybean cultivar 'Fukuyutaka' was grown in a growth chamber of the Institute of Bioresources Research Center of Kyushu Electric Power Co., Inc. at Saga, Japan (33°17′-N, 130°18′-E) under natural light. The CO 2 concentrations were maintained at 350 μmol mol -1 for ambient CO 2 and at 700 μmol mol -1 for CO 2 enrichment. CO 2 concentration was elevated during the whole growth period (WP), vegetative growth period (VP) or reproductive period (RP). Seed yield was increased by CO 2 enrichment during RP or WP due to the increase of pod number, but not by CO 2 enrichment during VP. Although CO 2 enrichment had no effect on the number of flowers, CO 2 enrichment during RP increased the pod number on all raceme orders and that during WP increased the pod number in the secondary and tertiary racemes. It is suggested that an increase of seed yield by CO 2 enrichment is mostly brought by the improvement of pod set, mainly on the high-order racemes that opened later during flowering period, and that the response of seed yield to CO 2 enrichment is mainly attributed to the response during RP.
!!!All Science Journal Classification (ASJC) codes