Comparison of root growth and nitrogen absorbing ability between Gramineae and Leguminosae during the vegetative stage

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Abstract
Eight Gramineae crops (rice, spring wheat, barley, oat, maize, sorghum, redtop, and orchardgrass), and seven Leguminosae crops (soybean, field bean, adzuki bean, lupin, pea, alfalfa, and red clover) were grown in a field. The characteristics of nitrogen absorption were parametrized as root size (root length or dry weight), root activity (specific absorption rate of nitrogen per unit root dry weight; SARN), and photosynthetic factors (crop growth rate (CGR), leaf area index and net assimilation rate). The results were as follows. 1) The rate of nitrogen accumulation by plant (ΔN) was highly correlated with the crop growth rate (CGR), and dry matter accumulation per unit amount of nitrogen accumulated in plant was lower in Leguminosae than in Gramineae. 2) In Leguminosae, the length or weight of root (root size) was generally shorter or smaller than in Gramineae. However, SARN of Leguminosae was higher than that of Gramineae. Therefore, the increase in the root size was considered to contribute more significantly to the increase in the shoot weight in Leguminosae compared to Gramineae 3) Throughout the growth, in Gramineae, SARN and CGR were correlated with ΔN, whereas in Leguminosae, LAI and CGR were correlated with ΔN. Thus, nitrogen absorption was assumed to be basically regulated by the requirement of nitrogen in shoot. 4) Root function was found to be different among crops; in Gramineae at both early vegetative and late vegetative stages, SARN was correlated with ΔN, and in Leguminosae at the early vegetative stage, the root size was correlated with ΔN, then at the late vegetative stage, the root activity (SARN) became an important factor for the control of ΔN.