Partitioning of dry matter and nitrogen to the spike throughout the spike growth period in wheat crops subjected to nitrogen deficiency

Experiments were conducted over four seasons, in the field, with five cultivars. Nitrogen (N) fertilizer was applied to crops at various dates and rates to achieve various N deficiencies. Crop N status and the accumulation of dry matter (DM) and N in the vegetative parts and in the spike was assessed regularly throughout the spike growth period. Changes in partitioning during the spike growth period were studied using cv. Soissons. For crops subjected to continuous N deficiencies starting on various dates, the proportion of aerial DM in the spike was lower and the proportion of aerial N in the spike was higher than in well-fertilized crops. The earlier the deficiency, the larger were the differences in partitioning. Some crops were subjected to temporary nitrogen deficiencies. In these cases, a nitrogen deficiency starting on a particular date was brought to an end by applying N fertilizer. Crops exposed to temporary N deficiencies displayed a higher proportion of aerial DM and a lower proportion of aerial N in the spike than did crops subjected to continuous deficiencies. Differences in DM and N partitioning became significant at the beginning of the spike growth period if crop N status had been affected by deficiency as early as 800 or 700 degree days before anthesis. At anthesis, the relative proportion of aerial N in the spike (proportion of aerial N in the spike of a crop divided by the proportion of aerial N in the spike of its well-fertilized control) was linearly related to nitrogen nutrition index (NNI), an indicator of crop N status. This relationship was determined for Soissons and four other cultivars. It did not differ significantly between cultivars or between crops subjected to temporary or continuous deficiency. Relative aerial N content and relative spike N content were also related to NNI at anthesis.