Transport of Recently Assimilated15N Nitrogen to Individual Spikelets in Spring Wheat Grown in Culture Solution

Two cultivars of spring wheat ( Triticum aestivum L.), Sport (high protein) and WL4 (low protein), were grown to maturity in culture solution. Nitrogen in the form of nitrate was added in daily doses at stepwise-decreasing relative rates to ensure normal development, and both cultivars received the same total amount of N during development. At weekly intervals from anthesis to maturity the daily nitrate dose was, for selected groups of plants, labelled with 15 N. After the labelling period the selected plants were harvested and analysed. The cultivar WL4 produced more biomass than Sport, as well as more spikelets and more grains per ear, with a higher mean grain weight, such that grain yield of WL4 was 57% greater than Sport. The ears of both cultivars were heterogenous: mean grain weight was highest in middle spikelets, which also contained more grains; the N content followed the pattern of dry weight with more N in the middle spikelets; but the N concentration was practically the same in all spikelets (2.15% of d. wt in WL4 and 3.33% in Sport). The distribution of 15 N showed that the main stem ear matured much earlier than tiller ears. The results of this nitrogen-labelling experiment show that, late in development, substantial amounts of recently-absorbed N were immediately assimilated and transported to the ears. Transport of 15 N decreased earlier to the top spikelets than to the bottom spikelets. As both cultivars were grown under identical conditions and both received the same amount of N it was concluded that the difference in grain N concentration was not caused by differences in the capacity of N assimilation and translocation but rather by different rates of accumulation of non-nitrogenous dry matter in the grains.