Multi-Stage Phenotyping Identifies Promising Drought-Responsive Upland Rice Germplasm for Tropical Breeding Programs

Global climate change severely challenges crop production in tropical and subtropical ecosystems, driving the need for germplasm evaluation. In this study, a sequential evaluation approach was used to identify drought-resilient Thai indigenous upland rice (Oryza sativa L.) accessions. A large-scale screening of 336 accessions at the seedling stage (2022 and 2023) using leaf rolling (LR) scores yielded 23 promising accessions, which were further evaluated across seedling, active tillering, and reproductive flowering stages under severe water-deficit stress. At the seedling stage, accessions ULR 014, ULR 018, and ULR 092 demonstrated consistently outstanding performance, with low leaf rolling scores (LR). At the tillering stage, accessions ULR 086, ULR 014, and ULR 397 exhibited superior post-drought vegetative plasticity, achieving exceptionally high tiller regeneration rates (31.5–36.9%) after re-watering, which serves as a vital recovery mechanism for rainfed direct-seeded systems. Under terminal drought at the flowering stage, accessions ULR 026 (14.8 g/plant; 3.27% yield reduction), ULR 130 (14.3 g/plant; 0.00% yield reduction), ULR 129 (13.0 g/plant; 8.45% yield reduction), and ULR 014 (11.4 g/plant; 12.31% yield reduction) maintained high grain yield potential and yield stability, while ULR 014, ULR 254, and ULR 397 achieved high proportions of 100% completely filled grains under severe stress. Hierarchical cluster analysis classified the selected germplasm into four groups (G1, G2, G3, and G4) based on stage-specific traits. Genotypes in Group 4 (ULR 014, ULR 254, ULR 032, ULR 346, and IR62266) demonstrated comprehensive multi-stage drought resilience with high 100% seed filling (36.9%) and total filled grain (66.5%), while Group 2 (ULR 026, ULR 130, ULR 129, ULR 075, ULR 071, and ULR 003) exhibited the lowest yield reduction under drought (13.3%). The phenotypic variations observed across developmental stages underscore the strong stage dependence of drought response in these local landraces, demonstrating that early-stage resilience does not guarantee reproductive tolerance. The candidate parental sources identified in this study, particularly ULR 014 (high yield maintenance and 100% filled grain under flowering stress), ULR 026 and ULR 130 (yield stability and minimal yield loss), and ULR 397 (strong tiller recovery), offer valuable breeding materials for developing further breeding program.