Arbuscular mycorrhizal fungi enhance yield and photosynthesis of switchgrass (Panicum virgatum L.) under extreme drought and alters the biomass composition of the host plant

Drought is one of the major environmental stressors that affect plant growth and performance. To cope with such stresses most plants utilize belowground association with arbuscular mycorrhizal fungi (AMF). The AMF provide plants with nutrients and water in exchange for carbon fixed by their hosts. Very little is known about how this mutualism affect plant growth and physiology under extreme drought and in the context of lignocellulosic crops such as switchgrass. We investigated how AMF association affects switchgrass growth and physiology when under extreme drought (droughted to just above the permanent wilting point) in a factorial greenhouse experiment. This extreme drought was imposed at a level slightly above the permanent wilting point of switchgrass, which we measured. Although drought significantly depressed mycorrhizal colonization by ∼37%, the aboveground biomass in mycorrhizal plants increased by more than 3-fold under drought as compared to non-mycorrhizal plants. The increase in biomass was accompanied by a significant increase in net CO2 assimilation, stomatal conductance, and an enhanced intrinsic Water Use Efficiency. Drought increased aboveground lignin, starch, and sucrose by 9%, 151% and 16% respectively whereas AMF decreased total starch by 43%. Drought also decreased total cellulose by 10% as compared to the no-drought plants. AMF and drought also altered the composition of matrix polysaccharides. In this study, we found evidence that AMF are mostly beneficial to switchgrass under extreme drought and both the factors affect switchgrass biomass quality.