Nitrogen and Dry Matter Accumulation by Kernels Formed at Specific Florets in Spikelets of Spring Wheat1

Kernels formed at specific florets in central spikelets of the spring wheat (Triticum aestivum L.) cultivars ‘Era’ and ‘Kitt’ were compared in 1975 for N percentage, N content (mg), and dry weight (mg) per kernel. There was a lower N percentage in the third and fourth kernels of Era (numbered from the base of the spikelet) than in the first two. The third and fourth kernels of Kitt did not show this lower N percentage, however. In both genotypes the fourth kernel was noticeably smaller and contained much less N than any of the first three. When a fifth kernel was present, as with the genotype designated ‘8037’, that kernel, in turn, was substantially smaller and contained less N than the fourth. In 1976 kernel number per spike was systematically reduced in two ways for the genotypes Era and 8037, resulting in increased assimilate supply to the remaining kernels. Nitrogen percentage, N content, and dry weight of the remaining kernels from altered spikes were substantially greater than in corresponding kernels of intact spikes. The fourth and fifth kernels from the base of a spikelet in the altered spikes, however, were still much smaller and contained less N than the first three. Examination of the vascular connections in the rachilla for Era and 8037 revealed that the fourth and fifth kernels in a spikelet were served by much smaller diameter vascular bundles than the first three. In addition, all bundles that served the fourth and fifth kernels were ultimately connected to bundles that also served one or more of the first three. In contrast, some of the bundles that served each of the first three were independently linked to the rachis. Our results suggest that kernels formed in the fourth and more distal florets of central spikelets may be limited in their capacity to accumulate assimilate compared with the first three. This maybe a result of their position relative to the rachis and, possibly, dif. ferences in their growth potential.