The Science    

Switchgrass is a promising perennial bioenergy crop that grows well on low-productivity farmland without fertilizer or irrigation. While switchgrass can survive droughts, scientists don’t fully understand how drought affects the plant’s growth or downstream biofuel production. In this study, scientists evaluated how switchgrass responded to drought stress at distinct stages of development — vegetative growth, flowering, and when leaves and shoots die off in the fall (senescence) — and the effects on fermentation of plant sugars. The findings suggest that the timing of drought stress has little impact on plant size but does change fermentation rates. Drought during the growing stage can make plants more hardy at the expense of biofuel production, while late-season drought enhances ethanol yields. 

The Impact

Accumulation of certain compounds in switchgrass biomass can inhibit fermentation, resulting in lower biofuel yields. Understanding how varying drought conditions affect plant metabolism is crucial for improving biofuel production processes as droughts are expected to become more frequent and severe as the climate warms.

Summary

Using a custom-built, programmable irrigation system, scientists with the Great Lakes Bioenergy Research Center restricted water to switchgrass plants during vegetative growth, flowering, and senescence to understand the metabolic response to drought stress, whether those responses are dependent on the developmental stage, how the metabolic shifts affect fermentation, and which metabolites most strongly inhibit fermentation.

Findings showed that drought at each stage reduced the amount of carbon dioxide the plants took in but didn’t affect biomass yield, suggesting switchgrass has effective mechanisms for withstanding short-term severe drought without compromising growth.

Drought stress during the vegetative stage caused plants to accumulate sugars (glucose and fructose) and other chemicals, while drought during later stages resulted in limited metabolic changes. No single metabolite consistently responded to drought across stages, suggesting that metabolic drought response is developmentally specific. Treated biomass from vegetative-stage drought showed elevated levels of saponins, natural compounds that help protect the plant, which positively correlated with slower fermentation of the hydrolysate. Senescence-stage drought lowered saponin levels in hydrolysates and enhanced ethanol yields.