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-rateof subterranean clover plants (cv. Mt. Barker), even those dueto moderate nitrogen deficiencies, were reflected in reductionsof the leaf-area ratio and particularly of the net assimilationrate. A decline in nitrogen supply in the culture solutions was foundto depress net rates of carbon dioxide uptake per unit leafarea and leaf expansion per plant to about the same extent,even at moderate levels of nitrogen stress. Four days aftertransfer of plants grown with adequate nitrogen to solutionswithout nitrogen, leaf area and net carbon dioxide uptake haddeclined to 84 per cent and 89 per cent of the values for thecontrol plants. After a further 4 days these values had decreasedto 71 per cent and 52 per cent respectively. When net carbon dioxide uptake was expressed per unit weightof chlorophyll, the effect of changes in nitrogen supply onnet photosynthesis largely disappeared, indicating a close relationshipwith the chlorophyll content of the leaves. However, anotherand perhaps more direct effect of nitrogen on photosynthesiswas suggested by the fact that, during the early stages of recoveryfrom a severe nitrogen stress, photosynthesis began to increasebefore the chlorophyll content of the leaves.