The relationship between plant height and sugar accumulation in the stems of sweet sorghum (Sorghum bicolor (L.) Moench)

Abstract

Sweet sorghum (Sorghum bicolor (L.) Moench) is a tall, seed-propagated C4 grass with stems that contain saccharine juice. The sugars in the juice can be easily extracted with a press, which generates large amounts of bagasse. Sweet sorghum has potential as a multi-purpose crop whereby depending on the available infrastructure and market demands, all fermentable sugars from juice and biomass can be converted to renewable fuels and chemicals, or the juice can be processed to syrup, fuels, or chemicals, while the bagasse is either burned or used as fodder. Large-scale industrial production of sweet sorghum requires large amounts of seed, but due to their height, sweet sorghums are not compatible with existing combine harvesters. In addition, grain yield is often limited. The availability of hybrid seed that can be produced on short seed parents would enable combine harvesting and offer greater seed yield. This requires the availability of short, sweet inbred lines. All known sweet sorghums, however, are tall, and prior research identified a positive correlation between height and sugar accumulation. Since the physiological mechanisms underlying sugar accumulation in sweet sorghum are not well understood, it is unknown whether the apparent association between sugar accumulation in the stem and plant height is the result of selection, or dictated by physiological or genetic constraints. Three experiments were conducted to examine this relationship. First, the role of shading on sugar accumulation was examined in breeding populations with contrasting heights. Second, the sugar concentration was compared between short plants harboring the unstable dwarf3 (dw3) allele and their tall Dw3 revertants. Third, tall photoperiod-sensitive lines were compared with their matching short, photoperiod-insensitive lines. The results from these three experiments indicated that high sugar concentration in sweet sorghum is not conditional on the plants being tall, making the development of short, sweet inbred lines feasible. This information will also significantly benefit studies aimed at identifying QTL for sugar yield in sweet sorghum.