Trichoderma and organomineral fertilizer improve photosynthesis and growth of watermelon under salt stress

Salinity limits agricultural production in semi-arid regions, but the combined effects of bioinoculants and organomineral fertilizers on vegetable crops remain poorly understood. We investigated the isolated and combined effects of Trichoderma asperelloides and organomineral fertilizer on ‘Sugar Baby’ mini watermelon under salt stress, using a 4 × 5 factorial scheme (four mitigation strategies × five salinity levels: 0.4, 1.5, 3.0, 4.5, and 6.0 dS m⁻¹) over two independent cycles. High salinity reduced total dry mass by 46%, increased electrolyte leakage by 48%, and decreased relative water content by 10.6%, with declines in PSII quantum efficiency (Fv/Fm) and chlorophyll. The combined treatment increased primary root length (40%), number of leaves (13.2%), and leaf area (26.0%), reducing electrolyte leakage by up to 25% (22.5% in Cycle 1 and 25.2% in Cycle 2), and preserving the Fv/Fm ratio and chlorophyll under severe stress. Total carotenoids + 26.1% at 6.0 dS m⁻¹ exclusively in the combined treatment, a phenotype that warrants further investigation. Canonical correlation analysis revealed strong coupling between morphophysiological and photosynthetic variables (shared variance 87–89%). The combination of T. asperelloides with organomineral fertilizer is a promising strategy for increasing salinity tolerance in watermelon, but field validation and complementary biochemical analyses are recommended.