Growth and Photosynthesis in the First Leaf of Barley The Effect of Time of Application of Nitrogen

Using Proctor barley grown in sand under controlled-environment conditions it was shown that when application of nitrogen, as nitrate, was delayed beyond day 4, at which time the first leaf was beginning to unfold, absolute and relative growth-rates of seedlings were reduced so that the young plants were significantly smaller. Delay in nitrate application led to reduction in length, breadth, area, and dry weight of the first leaf, and also to a lower photosynthetic activity on day 8, as measured by infra-red gas analysis. Measurement of the uptake of 14CO2 by first leaves showed that application of nitrate on days 2 or 4 led to high rates of fixation of carbon over the period days 8–12, whereas application on days 6 or 8 led to a substantially lower maximum rate of fixation which was maintained for a shorter period. When nitrate was applied on day 8 total fixation of carbon over the period days 7–14 was only 60 per cent of that for leaves on plants for which nitrate was supplied on day 2. When amounts of carbon fixed were compared on a leaf dry-weight basis, maximum values were found to be similar for all treatments, suggesting that the differences in fixation per leaf result mainly from the effect of treatment on leaf area. For all treatments it was confirmed that a decline in photosynthetic activity occurred by day 14. This was not correlated with photosynthetic activity in the second leaf, nor with emergence of the third leaf. Analysis of different parts of the lamina of the first leaf showed all to be affected by the timing of the supply, both in growth and in photosynthetic activity. Leaf dry weight did not increase after day 8 for any treatment, yet when nitrate was applied on day 8 plants showed a fourfold increase in photosynthetic rate. The significance of this in relation to carboxylation and other resistances in photosynthesis is discussed.