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

The distribution of photosynthate labelled with ¹⁴C was studiedin spring wheat grown with different amounts of nitrogen fertilizerin the three years 1972–4, after exposing the flag leafor the leaf below the flag leaf to ¹⁴CO₂ at 6–10 or 19–26days after anthesis. The movement of ¹⁴C to ears was unaffectedby 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 theend of the day was unaffected by nitrogen in 1973, but at 22days after anthesis in 1974 the concentration of sucrose inthe glumes and rachis, and in the flag leaf lamina was increasedby nitrogen. Loss of sugar by translocation and respirationduring the night may explain why this increase in concentrationwas not reflected in the ¹⁴C distribution 24 h after supplying¹⁴C. The proportion of the total ¹⁴C content of the shoot that wasin the ear at maturity ranged from 68 to 95 per cent dependingon when and to which leaf the ¹⁴CO₂ was supplied. Less than5 per cent remained in the leaf exposed to ¹⁴CO₂. The proportionof the final ear weight contributed by the leaf below the flagleaf 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 lostby maturity, presumably by respiration. Most loss occurred inthe first 24 h.