Evaluation of drought and heat stressed grain sorghum (Sorghum bicolor) for ethanol production

Sorghum is one of the most drought-tolerant grain crops and is used in biofuel production. Since sorghum is often exposed to drought and high temperature (heat) stress, this study investigated the effect of stress applied at different phenological stages of crop development on the glucose levels in grain and subsequent ethanol production. Short season sorghum hybrid DK-28E grown under controlled environment was exposed to drought and heat stress at five different stages of growth, namely: (1) pre-flowering (boot leaf emergence) to flowering, (2) flower to seed-set, (3) seed-set to early seed-fill, (4) early seed-fill to mid seed-fill, and (5) mid seed-fill to late seed-fill stage. Drought stress at any of the growth stages did not statistically affect either the glucose content or the ethanol production compared to the control (337 mg/g), although the ethanol yield increased up to 4.5% (352 mg/g) in flowering to seed-set stage. Heat stress, on the other hand, significantly reduced the glucose release and ethanol yield compared to the control (322 mg/g). Marginal ethanol yield reduction by 9% (293 mg/g) and 8.3% (295 mg/g) was seen in plants stressed during early seed-fill to mid seed-fill and mid seed-fill to late seed-fill stages, respectively. The results suggest that the phenological stage when sorghum is exposed to stress affects the ethanol yield. Overall, from the point of ethanol yield, it appears that grain sorghum cultivated in semi-arid regions where heat and drought stress are prevalent can be used for biofuel production. However, economic viability of ethanol production, especially of grain sorghum from high temperature stress needs to be investigated.

Research highlights
► Sorghum in semi arid regions is often exposed to heat and/or drought stress.
► Effects of heat and drought stress on sugar and ethanol yield was quantified.
► Drought stress did not affect sugar or ethanol yield.
► Heat stress during seed filling stages decreased sugar and ethanol yield.
► Grain produced under drought or heat stress can be used for biofuel production.