Genomewide Association for Sugar Yield in Sweet Sorghum

Sweet sorghum [Sorghum bicolor (L.) Moench] is characterized by juicy stems with high concentrations of fermentable sugars. The genetic basis of these phenotypes is not understood. A sweet sorghum diversity panel (n = 252), consisting of sweet sorghum cultivars (n = 80) and diverse landraces selected for matching plant height and maturity (n = 172), was genotyped and phenotyped in three environments over 2 yr for sugar-yield-related traits. Sugar yield is the product of juice volume and sugar concentration (Brix). Juice volume but not Brix was significantly higher in sweet sorghum cultivars than in diverse landraces. Most diverse landraces had white midribs, whereas most sweet cultivars had green midribs. The presence of green midribs was strongly correlated with increased sugar yield, juice volume, and moisture but was not significantly correlated with dry biomass. Genomewide association identified a major quantitative trait locus for midrib color, sugar yield, juice volume, and moisture at similar to 51.8 Mb on chromosome 6, a genomic region previously reported to contain the Dry midrib (D) locus. Midrib color itself was more highly predictive of sugar yield than any significant single nucleotide polymorphism (SNP) in this region, suggesting that either the causal mutation at the D locus is not in high linkage disequilibrium with any SNP in the dataset or that multiple mutations affect midrib color in sorghum. Significant negative correlations between Brix and grain harvest index indicate the existence of trade-offs between grain and sugar yields.