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 ni trogen absorbed in Leguminosae compared to Gramineae, the respiratory rates of shoot, root, and a single leaf of rice and soybean were monit ored during the vegetative growth stage at 3 levels of nitrogen applic ation using hydroponic culture. The results obtained were as follows. 1) The respiratory rate of shoot and a single leaf in soybean was high er than that in rice. The growth efficiency [accumulated dry matter/(a ccumulated dry matter+respiration)] of whole plant of soybean was lowe r than that of rice, regardless of nitrogen treatment. The low product ivity 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 th e growth efficiency in both rice and soybean was negligible, and the n itrogen (protein) content of each organ and whole plant was not always high in soybean, it is suggested that the high respiratory rate in so ybean is not due to energy supply for the construction of protein, whi ch is normally considered to account for a major part of growth respir ation. 3) Nitrogen application rate and temperature did not exert an a ppreciable 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 respir ation may be negligible.