Integrated assessment of photosynthesis, water use efficiency and yield in cotton under differential irrigation regimes

Water scarcity poses a significant constraint to cotton productivity in semi-arid regions. This study evaluated the physiological responses and yield performance of two cotton genotypes (Gossypium hirsutum L.), GSHV-97/162 (V1) and LRA-5166 (V2), under four irrigation regimes: 100% (M1), 75% (M2), 50% (M3) of field capacity (FC) and rainfed (M4). The experiment was conducted using a split-plot design with stage-specific drought stress imposition. Key physiological parameters photosynthetic rate, stomatal conductance, transpiration, water use efficiency (WUE), light absorption and leaf water potential were measured at 60, 90 and 120 days after sowing (DAS). Results indicated a substantial decline in seed cotton yield from 1603.05 kg ha⁻¹ in M1 to 1405.88 kg ha⁻¹ in M4. Photosynthetic rate at 90 DAS dropped from 28.7 µmol CO₂ m⁻² s⁻¹ (M1) to 24.1 µmol CO₂ m⁻² s⁻¹ (M4), while transpiration decreased from 6.12 to 3.95 mmol H₂O m⁻² s⁻¹. WUE peaked under moderate stress (M2) at 0.385 µmol CO₂ mmol⁻¹ H₂O, but declined under severe stress (M4). Light absorption at 120 DAS reduced from 42.7% (M1) to 26.4% (M4) and leaf water potential declined from -18.44 bars to -23.08 bars. Genotype V1 consistently outperformed V2 under stress, with higher photosynthetic activity (26.60 vs. 23.92 µmol CO₂ m⁻² s⁻¹), more bolls (17.55 vs. 9.55 per plant) and greater boll weight (4.15 vs. 2.83 g) under full irrigation. Heatmap visualization revealed strong trait interdependence and confirmed the detrimental impact of moisture deficit on gas exchange, water balance and yield. The study highlights the value of integrating physiological and agronomic traits to identify drought-resilient genotypes and optimize irrigation strategies for sustainable cotton production.