The percentage of clover in a mixture of Huia white clover and Mellc perennial ryegrass decreased during each of three winters. In the most severe winter, both grass and clover suffered a net loss of weight, clover losing twothirds of its maximum weight while grass lost about one-third. In the mildest winter, grass continued to gain weight throughout, while clover lost weight. Such losses, when combined with conditions such as the use of nitrogen fertilizer, which prevent a recovery of clover percentage during summer, will lead to a longterm decrease in the clover content of a sward.The losses from the clover were contributed to by all plant parts especially leaf, which lost between 60% and 95'^i) of lamina weight. The change from autumn to winter caused a greater decrease in the number of live leaves per shoot, and In the area and weight of individual leaves, in clover than in grass. Other work has shown that there is a greater decrease in photosynthesis in clover than in grass, probably owing to changes in canopy structure; this is one cause of clover's poor performance, though differences between the two species in the effect of winter on the distribution of the carbon Tixed in photosynthesis and in losses of weight in respiration and tissue death may also be involved.

The impact of sustained low external concentrations of NO; (0,10,100 and 1000 mmol m -3) on plant growth and the relative acquisition of N through N2 fixation and NO 3 uptake by established, nodulated white clover (Trifolium repens L. cv. Blanca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and lSN dilution methods. Plants supplied with NO~-achieved higher relative growth rates (~ = 0.091 d -1) compared with 'control' plants dependent on N2 fixation (0.073 d-l). Nitrate plants showed progressive increases in shoot : root d.w. ratios from 4 to 6.5-7.6 between days 0--28, compared with 5.1 on day 28 for control plants. Increases in both nodule d.w. and numbers per plant were inhibited after day seven at all concentrations of NO 3. The severity of inhibition of N2 fixation increased with increasing NO 3 concentration and with time. The total amounts of N2 fixed per plant between days 0-7 after supplying 10,100 and 1000 mmol m -3 NO 3, respectively, were 37-39, 28-30 and 0-13%, of the total N acquired. Between days 7-28 the proportional contributions of N2 fixation to total N acquisition declined to 3, 0. 5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N2 fixation between days 0-7 were, respectively, 5.4, 3.2, and 2.0 mmol N d-l g-l nodule d.w., compared with 7.9 mmol N d -1 g-i nodule d.w. for zero NO 3 plants. There was no evidence of a transitory increase in N2 fixation following the addition of NO 3, even at the lowest supply concentration.

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