Effects of drought on root growth
The wide distribution of S. breviflora in the desert steppe is closely related to its high level of drought tolerance (Lv et al. 2016; Ren et al. 2017). Root systems are the main organs to uptake water from soil and directly affect the water availability in plants, which are sensitive to drought stress by modifying their morphological traits or biomass as usual (Chen et al. 2013; Huang 2001; Zhang et al. 2018; Zhou et al. 2018).In this study, root biomass and morphological traits such as total root length, root surface area, root volume of S. breviflora at the 25 % FMC were significantly lower than at the 75 % FMC. These results are consistent with previous findings that drought caused a significant reduction on root biomass for grass species (Chen et al. 2013; de Vries et al. 2016; Zhang et al. 2018). In addition, Huang (2001) found that drought caused a significant decrease in the total root length of three tall fescue cultivars. A meta-analysis also showed that drought significantly decreased not only root biomass, but also total root length, root volume and root surface area (Zhou et al. 2018).