Screening and Identification of Drought-Sensitive and Drought-Tolerant Poplar Germplasm Based on Short-Term Physiological and Biochemical Differences

Drought responses in poplar are genotype-dependent, yet standardized metrics for quantifying drought tolerance remain scarce. Here, we employed logistic modeling of relative electrolyte leakage (REC) for the first time in poplar to derive clone-specific semi-lethal polyethylene glycol (PEG) concentrations (LC50), transforming a traditional descriptive assay into a quantitative, high-throughput drought-injury metric. Six elite Populus cultivars were exposed to increasing PEG concentrations, and their REC curves were fitted using a logistic function (R2 = 0.885−0.981). The derived semi-lethal PEG concentration (LC50) ranged from 7.99% in ‘PZ1’ (drought-sensitive, SS) to 13.44% in ‘YX2’ (drought-tolerant, ST), enabling clear classification. Under 10% PEG stress, ST maintained leaf water content (LWC) at 73%, while SS dropped to 63%. Malondialdehyde (MDA) content doubled in SS (44.7 nmol·g−1 FW) but increased by only 25% in ST (33.5 nmol·g−1 FW). Gas exchange analysis revealed that SS exhibited approximately twice the reduction in net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) compared to ST, with intercellular CO2 accumulation (Ci) occurring only in SS—indicating both stomatal and non-stomatal limitations. Osmolyte profiling showed that SS accumulated large amounts of soluble sugars (Ss) (+128%) and proline (Pro) (+230%), whereas ST maintained stable soluble protein (Sp) levels and only moderately increased proline (+120%). Antioxidant capacity differed markedly: catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activities increased by 5.6-, 1.8-, and 2.0-fold in ST, respectively, compared to 3.4-, 1.3-, and 1.7-fold in SS. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) of ten physiological traits explained 89% of the total variance (R2X = 0.954, Q2 = 0.973), identifying POD, SOD, CAT, and Pro as the most discriminative variables (VIP > 1). This four-marker signature converts a conventional dose–response assay into a rapid, low-cost screening module that can be deployed in robotic phenotyping platforms. Specifically, the high-ranking genotype ‘YX2’ is recommended for immediate use in water-limited plantations and as a robust parent for next-generation dryland breeding programs.