Since the Industrial Revolution, atmospheric carbon dioxide (CO2) concentration has been increasing. According to the IPCC, the CO2 concentration is predicted to double the current level by yhe end of this century. The effect of the increase of CO2 concentration would be very important for the future crop production. The objective of this study was to determine the effects of raised CO2 concentration on photosynthesis in soybean cultivars. Following results were obtained from this study. 1.When CO2 concentration was raised to 700 ppm, the rate of apparent photosynthesis (AP) increased by 39% aat vegatative growth stage whereas by 59% at reproductive growth stage compared with that under 350 ppm CO2 concentration. Asignificant positive relationship, which the earlier maturity in the cultivar was accompanied with the greater increase in the rate of AP, was observed only at vegetative growth stage, except cultivar Tanbakuro. 2.Since the increase in AP by raising CO2 concentration could be divided into rhe effect of the inhibition of photo-respiration (Ⅰ.Pr) and the direct effect of CO2 concentration (DE. CO2), the increase of AP by raised CO2 consisted of 63% by Ⅰ.Pr and 37% by DE. CO2 at vegetative growth stage. However, at reproductive growth stage, the increase of AP by raised CO2 was contributed mainly by DE. CO2 (66%). 3.There were almost no changes in Ⅰ.Pr between vegetative and reproductive growth stages when CO2 concentration was raised, thus the increase of AP byaraised CO2 at reproductive growth stage was almost due to the increase of DE. CO2
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