KERNEL GROWTH OF IN-VITRO CULTURED WHEAT SPIKES OF CULTIVARS WITH DIVERGENT SOURCE-SINK LIMITATIONS

A 50% reduction at anthesis in the main stem reproductive sink size of soft red winter wheat (Triticum aestivum L. em. Thell) elicits differ ential kernel size (KS) responses among cultivars. In this study, sour ce levels were manipulated to evaluate cultivar source and sink limita tions to kernel growth. Detached spikes of 'Arthur' (non-responsive, K S unaffected by 50% spikelet removal), 'Caldwell', 'FL 302' and 'Adena ' (responsive, KS increases with 50% spikelet removal) cultivars were cultured in vitro from 14 days after anthesis (during endosperm cell e xpansion stage) to maturity with sucrose levels of 25, 50, 100 and 200 mM. Relative differences in KS among cultivars for each sucrose level were similar to those of field-grown plants at maturity. The KS of al l in vitro cultured cultivars reached maximum at sucrose levels of 50 mM or greater; however, these kernels were 11-21% smaller than those o f field-grown plants. On average, kernels on spikes cultured in 100 mM sucrose grew at the same rate as kernels on field-grown plants, but f or a significantly Shorter duration (22.8 vs. 31.7 days). Sucrose conc entrations of peduncle and chaff tissues increased as sucrose levels i ncreased. Compared to field-grown plants, peduncle and chaff tissues o f cultured spikes averaged over all sucrose levels and cultivars had 4 .6-fold greater water soluble carbohydrate concentration and 59% more tissue dry weight at maturity. Smaller kernels but abundant accumulati on of water soluble carbohydrates in peduncle and chaff indicate that factors other than carbohydrate supply limited kernel growth of spikes cultured in vitro. When cultured in vitro, the non-responsive cultiva r Arthur did not reach maximum KS at a lower sucrose concentration tha n the responsive cultivars. Thus, the apparent kernel growth limitatio n hue to sink activity observed after partial spikelet removal of fiel d-grown Arthur was not observed when spikes of this cultivar were cult ured in vitro.