Contrasting Growth and Physiological Response Profiles of Three Native Alpine Elymus Forage Grasses Under Combined Cadmium and PEG-6000-Induced Osmotic Stress

The co-occurrence of soil drought and cadmium (Cd) pollution represents an important combined-stress scenario in degraded alpine grasslands, yet comparative responses of native Elymus forage grasses remain poorly resolved. Seedlings of E. sibiricus cv. Chuancao, E. breviaristatus, and E. nutans cv. Gannan were exposed to a full-factorial hydroponic matrix of four nominal Cd concentrations (0, 5, 10, and 20 mg·L−1) and four PEG-6000 concentrations of 0, 5, 7.5, and 10% (w/v). Growth, biomass allocation, tissue water status, malondialdehyde (MDA), soluble sugars, antioxidant enzyme activities, and photosynthetic pigments were measured, and stress tolerance coefficient, membership function analysis, and principal component analysis were used for integrated evaluation. Cd × PEG interactions affected growth, biomass, MDA, soluble sugars, peroxidase, catalase (CAT), and total chlorophyll, demonstrating trait- and species-dependent non-additive responses. Under the most severe combined treatment, E. breviaristatus maintained comparatively better shoot growth, lower absolute MDA, and relatively stable CAT and pigment traits; E. nutans maintained CAT and chlorophyll concentration but showed severe membrane lipid peroxidation, whereas E. sibiricus was more sensitive in shoot growth and pigment maintenance. Multi-method evaluation ranked E. breviaristatus first for relative multi-trait performance. Because Cd availability, tissue Cd concentrations, and root-to-shoot translocation were not measured, this ranking reflects phenotype-based performance under common external treatments rather than intrinsic Cd tolerance. E. breviaristatus should therefore be regarded as a candidate for subsequent Cd-accumulation, soil-pot, and field validation.