Multitrait Evaluation of Drought Tolerance at the Seedling Stage in Advanced Backcross Germplasm Derived from Sugarcane × Tripidium arundinaceum

Drought is a major abiotic constraint on sugarcane production, but systematic multitrait evaluation of drought tolerance remains limited in genetically complex advanced backcross progenies derived from sugarcane × Tripidium arundinaceum. In this study, 100 genotypes, including 93 BC4F1 or BC5F1 progenies and seven parental or control genotypes, were evaluated under a 20 d progressive drought treatment in pots at the seven-leaf stage. Sixteen traits related to root architecture, leaf water status, gas exchange, PSII photochemistry, antioxidant activity, and lipid peroxidation were measured. Drought significantly reduced TRL, RSA, RV, RWC, Pn, Gs, Tr, ΦPSII, and ETR and significantly increased SOD activity and MDA content. Twelve traits with relatively clear and biologically interpretable relationships with drought tolerance were integrated using membership functions and trait weights to calculate an integrated D value. D values ranged from 0.078 to 0.516, and 19, 45, and 36 genotypes were classified into groups with relatively high, moderate, and low drought tolerance within this germplasm collection, respectively. Genotypes 55-109, YCE07-65, 55-43, 211-172, and 91-74 had the highest integrated D values. Within the integrated evaluation framework, higher D values were most strongly associated with RSA, RV, TRL, Pn, ΦPSII, and ETR, while PC1 was strongly correlated with the D value (r = 0.893). Maintenance of root growth and photosynthetic function therefore represented major phenotypic features associated with overall drought tolerance under the present experimental conditions. These features may provide useful criteria for screening drought-tolerant germplasm, but their utility requires validation in independent drought experiments and field environments.