Occurrence of the Waxy Alleles wxa and wxb in Waxy Sorghum Plant Introductions and Their Effect on Starch Thermal Properties

The existence of two waxy alleles, wxa associated with no detectable granule bound starch synthase (GBSS) and wxb associated with apparently inactive GBSS, was recently reported in sorghum [Sorghum bicolor (L.) Moench]. In this paper, the occurrence of the wxa and wxb alleles in the USDA‐ARS photoperiod‐insensitive sorghum collection was determined, and the effects of the wxa and wxb alleles on thermal properties of sorghum starch (gelatinization temperatures and energy requirements) measured by differential scanning calorimetry. Of the 51 purported waxy accessions examined, 14 tested positive for presence of amylose by iodine staining and were considered to be previously misclassified wild‐type lines. Nine accessions were mixed for presence or absence of amylose. Twenty‐four of the 28 accessions confirmed to be waxy by negative iodine staining for amylose had no detectable GBSS using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) (wxa), and four were show to contain GBSS (wxb). Mean gelatinization onset, peak, and end temperatures were significantly lower for wild‐type than either of the two waxy genotypes. Mean gelatinization onset temperature was slightly higher for waxy‐GBSS+ genotypes than waxy‐GBSS− genotypes. Mean gelatinization end temperature was slightly higher for waxy‐GBSS− genotypes than waxy‐GBSS+ genotypes. Significant genetic variation was observed within genotypic classes, suggesting influence of additional modifier genes affecting sorghum starch structure or micro‐environmental effects.