Physiological and photosynthetic responses of cotton to integrated soil fertility management under legume-based systems in northwestern Tanzania

Cotton production in Tanzania is constrained by declining soil fertility, which could limit nutrient availability, photosynthetic functioning, and productivity. However, physiological and photosynthetic responses of cotton to legume-based soil fertility management across growth stages and contrasting environments remain insufficiently characterized. This study determined how preceding cowpea, lablab, and Mucuna residues, farmyard manure (FYM), and mineral fertilizer influenced cotton physiological and photosynthetic functioning across growth stages and agroecological conditions. Field experiments were conducted at Mwamala and Ukiriguru from September 2025 to March 2026 using a randomized complete block design with four replications and six soil fertility treatments. Cotton responses were assessed at vegetative, flowering, and boll development stages using a MultispeQ V2.0 sensor and PhotosynQ RIDES 2.0 protocol to measure relative chlorophyll content, photosynthetically active radiation (PAR), chlorophyll fluorescence, photosynthetic electron transport, PSI redox state, and proton-motive-force-related traits. Treatment responses were trait-, stage-, and site-specific. At Ukiriguru, vegetative-stage PAR ranged from 256.7 µmol m⁻² s⁻¹ in the control to 457.0 µmol m⁻² s⁻¹ under FYM ( p = 0.003). At Mwamala, flowering-stage SPAD ranged from 40.21 under lablab to 45.78 under Mucuna ( p = 0.024), while boll-stage PSI oxidized centres ranged from 0.11 under FYM to 0.37 under Mucuna ( p = 0.010). Boll-stage Fm′ ranged from 3456 under cowpea to 4513 under lablab ( p = 0.040). These responses covered leaf light environment, chlorophyll status, PSI redox behaviour, and chlorophyll fluorescence, providing physiological indicators of soil fertility management practices that could support cotton growth and productivity under legume-residue-based systems in northwestern Tanzania.