14C-allocation of14C-compounds introduced to a leaf to carbon and nitrogen components in rice and soybean during ripening

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Abstract
14CO2, 14C-[U]-sucrose, and 14C-[U]-asparagine were introduced to the flag leaf of rice or fully-expanded leaf of soybean. The 14C-distribution to respired CO2 and each organ as well as to crude chemical components (sugars, amino acids, organic acids, protein, and the “others”) was determined in the flag leaf of rice and fully-expailded leaf of soybean and the sink organ. The results obtained were as follows. 1. A large amount of 14CO2 was released rapidly from the leaf which assimilated 14CO2 in soybean both under dark and light conditions compared to rice. 2. A larger amount of 14C in the flag leaf which assimilated 14CO2 was distributed to sugars after 24 h in rice than in soybean, regardless of light conditions. However, a larger amount of 14C was distributed to protein and organic acids in soybean. 3. The 14CO2 release rate from the leaf to which 14C-sucrose and 14C-asparagine had been introduced was similar in rice and soybean, regardless of nitrogen supply. The 14CO2 release rate from the sink organ was higher in soybean than in rice when sucrose was introduced, but similar in both crops when asparagine was introduced. The above results indicate that the current photosynthates during CO2 assimilation were actively catabolized and respired in the source leaf of soybean compared to rice. As the utilization of 14C-sucrose or 14C-asparagine in the leaves did not differ significantly between rice and soybean, it is assumed that the use of these translocating compounds for respiration in the leaves was similar in rice and soybean. The low growth efficiency of soybean was partly due to the high respiratory loss of current photosynthates in leaves compared to rice, regardless of light or nitrogen conditions.