According to Thornley, J.H.M. (Nature, 227, 304-305, 1970) and McCree, K.J. (Crop Sci., 14, 509-514, 1974), respiratory substances are used only for maintenance respiration when plants are exposed to the dark conditions for a long period of time (more than 2 d). The maintenance respiration is also affected by the nitrogen status in plant, because protein turnover is one of the major energy consumption sources under maintenance process. Therefore, to determine whether respiratory substances are used only for maintenance, C-14-[U]-sucrose or a mixture of C-14-[U]-amino acids was introduced to rice and soybean plants from the tip of leaf. Plants were grown under natural light conditions and under dark conditions for 4 d with 2 nitrogen levels (0.2 and 0.02 g N L(-1) soil). After the introduction of the C-14-compounds, the (CO2)-C-14 respiratory rate was monitored during 24 h, then the C-14 distribution to organic acids, free amino acids, proteins, sugars, and polysaccharides was analyzed. Following results were obtained. 1) When C-14-[U]-sucrose Or it mixture Of C-14-[U]-amino acids was introduced to the leaf of rice and soybean plants, the C-14 release rate by respiration was not affected by the nitrogen and light treatments except when C-14-sucrose was introduced to soybean in the low N plot. The C-14 release rate from the C-14-compounds introduced into leaf in the low N plot of soybean was higher in the dark treatment than in the natural light treatment. 2) C-14-distribution ratio after introduction of C-14-sucrose and a mixture of C-14-amino acids to the leaf was not significantly affected by the nitrogen treatment. When C-14-sucrose was introduced to rice leaf, the C-14-distribution ratio to sugars and proteins was higher and that to polysaccharides was lower in the natural light treatment than in the dark treatment. The C-14-distribution ratio was less affected by the light or nitrogen treatment in case of soybean leaf. 3) Although it was assumed that maintenance metabolism was dominant in the lower leaf (counted from the bottom), the C-14-distribution ratio was similar to that of upper leaf. 4) Nitrogen content of leaf was not different between rice and soybean in the high N treatment, unlike the C-14-distribution ratio. In rice, the nitrogen content of leaf was about twice as high in the high N treatment compared with the low N treatment, while the C-14-distribution ratio in leaf was stable regardless of nitrogen treatment. Based on the above results, it is suggested that since the C-14-distribution ratio into each chemical component did not change regardless of light treatment, nitrogen treatment, or leaf age, it was impossible to separate respiration into two components, such as growth and maintenance respiration. The results also indicated that current photosynthates and storage substances were not used only for growth and maintenance, respectively.
