TRANSPORT OF RECENTLY ASSIMILATED N-15 NITROGEN TO INDIVIDUAL SPIKELETS IN SPRING WHEAT GROWN IN CULTURE SOLUTION

Two cultivars of spring wheat (Triticum aestivum L.), Sport (high prot ein) 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 cult ivars received the same total amount of N during development. At weekl y intervals from anthesis to maturity the daily nitrate dose was, for selected groups of plants, labelled with N-15. After the labelling per iod the selected plants were harvested and analysed. The cultivar WL4 produced more biomass than Sport, as well as more spikelets and more g rains 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 het erogenous: mean grain weight was highest in middle spikelets, which al so contained more grains; the N content followed the pattern of dry we ight 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 N-15 showed that the main stem ear ma tured much earlier than tiller ears. The results of this nitrogen-labe lling experiment show that, late in development, substantial amounts o f recently-absorbed N were immediately assimilated and transported to the ears. Transport of N-15 decreased earlier to the top spikelets tha n to the bottom spikelets. As both cultivars were grown under identica l conditions and both received the same amount of N it was concluded t hat the difference in grain N concentration was not caused by differen ces in the capacity of N assimilation and translocation but rather by different rates of accumulation of non-nitrogenous dry matter in the g rains. (C) 1996 Annals of Botany Company