Effects of Nitrogen Nutrition on Leaf Expansion and Photosynthesis of Trifolium subterraneum L. 1. Comparison between Different Levels of Nitrogen Supply

Growth analysis showed that reductions in the relative growth-rate of subterranean clover plants (cv. Mt. Barker), even those due to moderate nitrogen deficiencies, were reflected in reductions of the leaf-area ratio and particularly of the net assimilation rate. A decline in nitrogen supply in the culture solutions was found to depress net rates of carbon dioxide uptake per unit leaf area and leaf expansion per plant to about the same extent, even at moderate levels of nitrogen stress. Four days after transfer of plants grown with adequate nitrogen to solutions without nitrogen, leaf area and net carbon dioxide uptake had declined to 84 per cent and 89 per cent of the values for the control plants. After a further 4 days these values had decreased to 71 per cent and 52 per cent respectively. When net carbon dioxide uptake was expressed per unit weight of chlorophyll, the effect of changes in nitrogen supply on net photosynthesis largely disappeared, indicating a close relationship with the chlorophyll content of the leaves. However, another and perhaps more direct effect of nitrogen on photosynthesis was suggested by the fact that, during the early stages of recovery from a severe nitrogen stress, photosynthesis began to increase before the chlorophyll content of the leaves.