Physio-Morphological Analysis of Soybean Lines Showing the Extremely High Photosynthetic Capacity.

The enhancement of the leaf photosynthetic capacity has been expected to contribute to the greater seed yield in crop plats. Targets for enhancing the photosynthetic capacity of soybean [Glycine max (L.) Merr.] remain to be elucidated. In the preliminary experiment in the growth chamber, two soybean cultivars, PI 594409 A (116) and PI 603911 C (125), were shown to have the high photosynthetic capacity. The objective of this study is to identify the factor responsible for the high photosynthetic capacity by analyzing photosynthetic parameters along with physiological and biochemical properties in 116 and 125. 116 and 125 were cultivated at the experimental field in Kyoto University, Japan, with two commercial varieties in US (UA-4805) and Japan (Tachinagaha). Leaf gas exchange was measured with a portable gas-exchange system (LI-6400, LI-COR, Lincoln, NE, USA). The upper-most fully expanded leaf was measured during the reproductive stage. The CO₂ assimilation rate was also measured at seveal values of interceller concentration of CO₂ (C_i) to draw the Pn-Ci curve. The content of soluble protein, chlorophyll, Rubisco and Nitrogen was measured at the same leaf position with the measurement of leaf gas exchange. The CO₂ assimilation rate at a CO₂ concentration of 400 µmol mol^-1 in 125 was greater than other cultivars at all measurements. The Vcmax, the indicator of CO₂ fixation activity, in 125 was also higher than other cultivars. It is concluded that 125 had the greater CO₂ fixation activity, which enabled to achieve the high photosynthetic capacity through the reproductive stage. The content of Rubisco and Nitrogen in 125 was higher than other cultivars. This result suggests that the greater content of Rubisco and Nitrogen in 125 contributed to the high CO₂ fixation activity. The content of Rubisco can be a candidate of the target for enhancing the photosynthetic capacity in soybean.