Difference in system of current photosynthesized carbon distribution to carbon and nitrogen compounds between rice and soybean

14CO2 was assimilated during 10 min in leaf of rice and soybean under 21 kPa O2 (21% O2 treatment) and 2 kPa O2 (2% O2 treatment) at the vegetative growth stage and flowering stage. The 14C distribution ratio to respired CO2 and crude chemical components (sugars, polysaccharides, amino acids, organic acids, and proteins) was determined. In this paper, since emphasis was placed on the 14C distribution mechanism to carbon compounds and nitrogen compounds, the terms carbon metabolism pool (C-pool) composed of sugars and polysaccharides, and nitrogen metabolism pool (N-pool) composed of organic acids, amino acids and proteins were used. The results obtained were as follows. 14C distribution ratio to N-pool at 0 min after 14C assimilation was higher in soybean than in rice regardless of the treatments and stages, and that at 30 min after 14C assimilation under light condition markedly decreased both in rice and soybean. Therefore, especially in soybean, a large amount of photosynthesized 14C was once distributed to the N-pool, then 14C compounds in the N-pool were reconstructed into the C-pool. During this reconstruction process, 14C compounds in the N-pool were actively respired. 14C distribution to N-pool at 0 min after 14C assimilation changed slightly or did not change by the N treatment. 14C distribution to N-pool in the - N treatment of soybean (13–29 mg N g-1 content in leaves) was higher than that in the + N treatment of rice (31–48 mg N g-1 content in leaves). Photosynthesized carbon distribution to N-pool in rice decreased with growth, while it remained constant in soybean. Accordingly, in soybean, photosynthesized carbon was predominantly distributed to the N-pool through photorespiration and/or Calvin cycle (supplying triose-P), which was less affected by nitrogen nutrient and aging. Thus, the mechanism of photosynthesized carbon distribution to carbon and nitrogen compounds was basically regulated by inherited characters of each plant more than by the nitrogen status of leaves. By the 2% O2 treatment, 14C distribution to N-pool decreased in both crops regardless of N treatment, indicating that photorespiration plays an important role in the supply of the preliminarily photosynthesized carbon compounds to N-pool. In the 2% O2 treatment, 14C distribution to N-pool was higher in soybean than in rice, indicating that triose-P transported from chloroplast was preferentially distributed to N-pool in the case of soybean.