Effects of Nitrogen Fertilizer on the Distribution of Photosynthate during Grain Growth of Spring Wheat

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Abstract
The distribution of photosynthate labelled with 14C was studied in spring wheat grown with different amounts of nitrogen fertilizer in the three years 1972–4, after exposing the flag leaf or the leaf below the flag leaf to 14CO2 at 6–10 or 19–26 days after anthesis. The movement of 14C to ears was unaffected by nitrogen fertilizer except after early exposure in 1973, when nitrogen increased the retention of 14C in stems at maturity The concentration of sugar in the top part of the shoot at the end of the day was unaffected by nitrogen in 1973, but at 22 days after anthesis in 1974 the concentration of sucrose in the glumes and rachis, and in the flag leaf lamina was increased by nitrogen. Loss of sugar by translocation and respiration during the night may explain why this increase in concentration was not reflected in the 14C distribution 24 h after supplying 14C. The proportion of the total 14C content of the shoot that was in the ear at maturity ranged from 68 to 95 per cent depending on when and to which leaf the 14CO2 was supplied. Less than 5 per cent remained in the leaf exposed to 14CO2. The proportion of the final ear weight contributed by the leaf below the flag leaf was about half that contributed by the flag leaf. In 1974 about 24 per cent of the 14C absorbed by the flag leaf, and 56 per cent of that absorbed by the second leaf, was lost by maturity, presumably by respiration. Most loss occurred in the first 24 h.