COMPARISON OF GROWTH EFFICIENCY BETWEEN RICE AND SOYBEAN AT THE VEGETATIVE GROWTH STAGE

To investigate the cause of the low productivity per unit amount of nitrogen absorbed in Leguminosae compared to Gramineae, the respiratory rates of shoot, root, and a single leaf of rice and soybean were monitored during the vegetative growth stage at 3 levels of nitrogen application using hydroponic culture. The results obtained were as follows. 1) The respiratory rate of shoot and a single leaf in soybean was higher than that in rice. The growth efficiency [accumulated dry matter/(accumulated dry matter+respiration)] of whole plant of soybean was lower than that of rice, regardless of nitrogen treatment. The low productivity based on nitrogen accumulation in soybean during the vegetative growth stage was ascribed mainly to the higher respiratory activity of shoot and single leaf. 2) As the effect of nitrogen application on the growth efficiency in both rice and soybean was negligible, and the nitrogen (protein) content of each organ and whole plant was not always high in soybean, it is suggested that the high respiratory rate in soybean is not due to energy supply for the construction of protein, which is normally considered to account for a major part of growth respiration. 3) Nitrogen application rate and temperature did not exert an appreciable effect on growth efficiency. Since, maintenance respiration is generally considered to be affected by the temperature and protein turnover, the contribution of maintenance respiration to total respiration may be negligible.