Fig. 4

Principal component analysis biplot of the DSI of seven physiological traits of 49 switchgrass genotypes under well-watered (control) and drought stress conditions after 30 days of treatment. Arrows represent physiological traits with various length based on the impact of each trait on the separation of genotypes. The 49 switchgrass genotypes were clustered into three major groups. Group I include switchgrass genotypes that have the best performance based on DSI of the physiological parameters, while group III genotypes have the worst performance, and group II include the intermediate genotypes. The proportion of variance for principal component analysis based on the DSI of seven physiological traits is shown in Additional file 10: Figure S9, where it suggested the top two PCs can explain 75.31 % of total variation. RWC relative water content, EL electrolyte leakage, Pn photosynthetic rate, g _s stomatal conductance, Tr transpiration rate, Ci intercellular CO₂ concentration, WUE water use efficiency