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 c onditions for a long period of time (more than 2 d). The maintenance r espiration is also affected by the nitrogen status in plant, because p rotein turnover is one of the major energy consumption sources under m aintenance process. Therefore, to determine whether respiratory substa nces 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 und er 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-1 4 respiratory rate was monitored during 24 h, then the C-14 distributi on to organic acids, free amino acids, proteins, sugars, and polysacch arides 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 l eaf of rice and soybean plants, the C-14 release rate by respiration w as 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 tr eatment. 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 a ffected by the nitrogen treatment. When C-14-sucrose was introduced to rice leaf, the C-14-distribution ratio to sugars and proteins was hig her and that to polysaccharides was lower in the natural light treatme nt than in the dark treatment. The C-14-distribution ratio was less af fected by the light or nitrogen treatment in case of soybean leaf. 3) Although it was assumed that maintenance metabolism was dominant in th e lower leaf (counted from the bottom), the C-14-distribution ratio wa s similar to that of upper leaf. 4) Nitrogen content of leaf was not d ifferent between rice and soybean in the high N treatment, unlike the C-14-distribution ratio. In rice, the nitrogen content of leaf was abo ut twice as high in the high N treatment compared with the low N treat ment, while the C-14-distribution ratio in leaf was stable regardless of nitrogen treatment. Based on the above results, it is suggested tha t since the C-14-distribution ratio into each chemical component did n ot change regardless of light treatment, nitrogen treatment, or leaf a ge, it was impossible to separate respiration into two components, suc h as growth and maintenance respiration. The results also indicated th at current photosynthates and storage substances were not used only fo r growth and maintenance, respectively.