Problems in the methods of estimation of growth and maintenance respiration

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, ¹⁴C- U] -sucrose or a mixture of ¹⁴C- 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 ¹⁴C-compounds, the ¹⁴CO₂ respiratory rate was monitored during 24 h, then the ¹⁴C distribution to organic acids, free amino acids, proteins, sugars, and polysaccharides was analyzed. Following results were obtained.

1. When ¹⁴C-U]-sucrose or a mixture of ¹⁴C-U]-amino acids was introduced to the leaf of rice and soybean plants, the ¹⁴C release rate by respiration was not affected by the nitrogen and light treatments except when ¹⁴C-sucrose was introduced to soybean in the low N plot. The ¹⁴C release rate from the ¹⁴C-compounds introduced into leaf in the low N plot of soybean was higher in the dark treatment than in the natural light treatment.

2. ¹⁴C-distribution ratio after introduction of ¹⁴C-sucrose and a mixture of ¹⁴C-amino acids to the leaf was not significantly affected by the nitrogen treatment. When ¹⁴C-sucrose was introduced to rice leaf, the ¹⁴C-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 ¹⁴C-distribution ratio was less aifected 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 ¹⁴C-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 ¹⁴C-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 ¹⁴C-distribution ratio in leaf was stable regardless of nitrogen treatment.

Based on the above results, it is suggested that since the ¹⁴C-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.