Humic Substances Modulate the Seedling–Saline Substrate System in Papaya Cultivation

Salinity is a major abiotic constraint limiting papaya ( Carica papaya L.) production in arid and semi-arid regions, particularly during the seedling stage, when plants are highly sensitive to osmotic and ionic stress. Although humic substances are recognized as plant biostimulants, their interactive effects with saline irrigation water on papaya seedling physiology, substrate chemistry, and growth responses remain insufficiently understood, representing a critical knowledge gap in nursery management under water-limited conditions. This study evaluated the potential of humic substances to mitigate salt stress in papaya seedlings irrigated with saline water. The experiment was conducted under greenhouse conditions using a completely randomized design with a fractional factorial arrangement combining five levels of irrigation water electrical conductivity (0.4 to 6.0 dS m⁻¹) and five concentrations of humic substances (0 to 100 mL L⁻¹), plus a control treatment. Growth, gas exchange, chlorophyll fluorescence, pigment content, biomass accumulation, and substrate chemical attributes were assessed. Multivariate relationships were further examined using canonical correlation analysis. Results showed that humic substance application significantly improved physiological performance and growth under salinity. Increases were observed in membrane stability, chlorophyll indices, photosynthetic efficiency, intrinsic carboxylation efficiency, leaf area, and dry biomass. The Dickson Quality Index reached a maximum value of 0.75 under combined moderate salinity and humic substance application, indicating improved seedling quality. Humic substances also enhanced photochemical efficiency of photosystem II by reducing F₀ and increasing F _v /Fₘ and F _v /F₀. Gas exchange analysis indicated that salinity mainly imposed stomatal limitations, while humic substances improved water use and carboxylation efficiency. Additionally, improvements in substrate cation exchange capacity and nutrient availability, particularly Ca²⁺ and K⁺, were strongly associated with enhanced plant performance. Canonical correlation analysis confirmed a strong coupling between substrate chemical balance and seedling physiological responses. Overall, humic substances mitigated salt-induced damage by enhancing membrane integrity, stabilizing photosynthetic machinery, and improving nutrient dynamics in the root zone. These integrated effects promoted greater tolerance and growth under saline irrigation conditions. In conclusion, humic substances represent an effective and sustainable strategy to improve papaya seedling production under saline water use, offering practical implications for nursery systems in water-scarce regions. Future research should focus on elucidating molecular signaling pathways and optimizing application strategies under field conditions.