Kernel growth of in vitro cultured wheat spikes of cultivars with divergent source-sink limitations

Abstract

A 50% reduction at anthesis in the main stem reproductive sink size of soft red winter wheat ( Triticum aestivum L. em. Thell) elicits differential kernel size (KS) responses among cultivars. In this study, source levels were manipulated to evaluate cultivar source and sink limitations to kernel growth. Detached spikes of ‘Arthur’ (non-responsive, KS 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 expansion 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 all in vitro cultured cultivars reached maximum at sucrose levels of 50 mM or greater; however, these kernels were 11–21% smaller than those of 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 for a significantly shorter duration (22.8 vs. 31.7 days). Sucrose concentrations of peduncle and chaff tissues increased as sucrose levels increased. Compared to field-grown plants, peduncle and chaff tissues of 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 accumulation 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 cultivar Arthur did not reach maximum KS at a lower sucrose concentration than the responsive cultivars. Thus, the apparent kernel growth limitation due to sink activity observed after partial spikelet removal of field-grown Arthur was not observed when spikes of this cultivar were cultured in vitro.