Interactive Effects of Tillage Systems and Fertilization Strategies on Physiological and Biochemical Defense Mechanisms of Sesame (Sesamum indicum L.) under Water Deficit Stress

Water deficit stress is a major constraint to sesame (Sesamum indicum L.) productivity in semi-arid regions. This two-year field experiment (2022–2023) evaluated the interactive effects of three tillage systems (conventional tillage, CT; minimum tillage, MT; and no-tillage, NT), four fertilization treatments (control, urea, vermicompost, and vermicompost + urea, V + U), and two irrigation regimes (optimal irrigation, OI; and water deficit stress, WDS) on 24 physiological, biochemical, and yield-related traits using a split–split plot design. Under WDS, NT combined with V + U most effectively mitigated drought-induced physiological damage, maintaining higher relative water content (72.4–74.1%), chlorophyll stability index (78.3–80.6%), and maximum quantum yield of PSII (0.74–0.76). This treatment also enhanced proline (18.7–21.3 µmol/g FW), glycine betaine (42.6–48.3 µmol/g FW), superoxide dismutase (285–312 U/mg protein), and catalase (48.2–53.7 U/mg protein), indicating improved osmotic adjustment and antioxidant defense. Structural equation modeling showed that tillage × fertilization effects on seed yield under WDS were mediated primarily through membrane stability, photosynthetic efficiency, and antioxidant capacity. Principal component analysis and hierarchical clustering further demonstrated clear treatment separation and coordinated associations among stress-response traits. Under WDS, NT × (V + U) increased seed yield by 38.4–42.7% relative to CT × control. Overall, no-tillage combined with integrated vermicompost and urea improved physiological protection and yield stability, providing an effective soil–nutrient management strategy for enhancing sesame drought resilience in semi-arid cropping systems.