Multivariate Phenotypic Diversity, Phenological Plasticity and Quantitative Drought Tolerance Indices in Sweet Sorghum (Sorghum Bicolor L. Moench) Under Contrasting Water Regimes

Water scarcity poses a major constraint to sweet sorghum productivity in semi-arid regions. In this study, 256 sweet sorghum genotypes were evaluated under well-watered (WW) and water-deficit (WD) conditions to assess agro-morphological diversity, phenological plasticity, and quantitative drought tolerance. Significant phenotypic variation was observed across all measured traits. Under WW, the average growth duration was 106.4 ± 1.07 days (range: 93–127 days), whereas drought stress slightly extended the growth cycle to 108.2 ±1.22 days under WD, accompanied by an increased coefficient of variation (18.01%). Plant height ranged from 94.5 to 362.1 cm under WW (population mean: 224.8 ± 2.16 cm) and declined to a maximum of 343.6 cm under WD (mean: 208.5 ± 2.24 cm). High broad-sense heritability (H2) coupled with high genetic advance as percent of mean (GAM > 20 %) was recorded for phenological duration and stem diameter, indicating strong genetic control despite drought-induced developmental plasticity. Multivariate ordination and plasticity trade-off analyses revealed that drought response is governed by two distinct functional axes: vegetative vigor paired with stress tolerance, and structural robustness aligned with juice quality, driven by an inverse coordination between vertical elongation and radial stem expansion. Quantitative drought evaluation using Drought Susceptibility Index (DSI) and Stress Tolerance Index (STI) effectively separated high absolute growth from true relative stress tolerance. Applying a multi-trait selection rule identified 14.8 % of the population as elite performers. Genotypes such as X012 and X066-1 exhibited superior drought tolerance (DSI < 0.50, STI > 1.25), maintaining structural stability and high sugar accumulation (Juice Brix > 16.5 %) under water stress. Overall, these findings demonstrate substantial phenotypic diversity and provide reliable selection metrics for breeding drought-adapted and bioenergy-efficient sweet sorghum germplasm.