Main stem sink manipulation in wheat : effects on nitrogen allocation to tillers

The role of main stem (MS) sink size on N use by field-grown soft red winter wheat (Triticum aestivum L. cv Hart) was determined. At Feeke's growth stage 8 (last leaf just visible), 100 micromoles of 99 atom% (15)N-ammonium was injected into the lower MS. At anthesis, MS sink size was adjusted by removal of 0, 33, 66, and 100% of the MS spikelets; tiller spikes were left intact. The MS and tiller average kernel size was unaffected by MS sink manipulations. The MS kernel N concentration increased when MS spikelets were removed. Tiller kernel N concentrations were unaffected except when the entire MS reproductive sink was removed, which caused an increase in tiller kernel N concentration. Net losses of MS vegetative N during grain fill were similar for all treatments except for plants lacking MS spikelets, which mobilized 30% less N from the MS. Labeled N was predominately (>90%) associated with the insoluble reduced N fraction of plant tissues at anthesis. Allocation of labeled N to tillers was not proportional to reduction in MS sink size. These results indicate that the reproductive sink on an individual culm has first priority for vegetative N mobilized during grain fill even when sink demand is reduced substantially.