Nitrogen Use Efficiency in Growth of Polygonum cuspidatum Sieb. et Zucc

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Abstract
The growth of Polygonum cuspidatum in sand culture was analysed under varying nutrient conditions. Nitrogen availability influenced nitrogen uptake of plants through the uptake rate per unit root weight rather than the amount of root. In turn, the different amounts of nitrogen taken up affected plant growth through their effects on the rate of leaf expansion. Net assimilation rate (NAR) increased with nitrogen content per unit leaf area (C), but further increase in leaf nitrogen caused diminishing returns of NAR Optimal nitrogen content per unit leaf area (Copt) to maximize dry-matter production of a leaf could be determined by drawing a tangent from the ongin to a curvilinear relation between NAR and C. This optimal content divides a nitrogen-limiting range (C < Copt) from a carbon-limiting one (C> Copt) along the axis of nitrogen content. Under nitrogen limitation, efficiency of nitrogen use in dry-matter production could increase if the plant had a larger carbon sink. This gives a qualitative explanation to reduced shoot-to-root ratio under limited availability of nitrogen.

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