Introduction
Drought stress is a major constraint to sustainable agricultural production, particularly in high-value and water-sensitive crops such as sweet basil (Ocimum basilicum L.). The use of natural biostimulants, including humic acid (HA), has emerged as a promising approach for enhancing plant resilience to water deficit. This study comprehensively evaluated the effectiveness of HA fertigation in mitigating drought-induced stress in greenhouse-grown green basil by examining the individual and interactive effects of irrigation regime and HA concentration on growth, physiological performance, and biochemical responses.
Materials and Methods
A factorial experiment was conducted using a randomized complete block design (RCBD) with three replications during the 2025 growing season at the Greenhouse and Controlled Environments Research Center. Treatments consisted of three irrigation regimes based on field capacity (FC): 100% FC (well-watered control), 75% FC (moderate drought), and 50% FC (severe drought), combined with four HA concentrations applied via fertigation (0, 2, 4, and 6 g L⁻¹). Humic acid was applied at three critical developmental stages: the first true-leaf stage, the third true-leaf stage, and the onset of flowering. Drought stress was imposed after seedling establishment and maintained gravimetrically throughout the experiment. At harvest, proline, total phenolics, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), photosynthetic pigments, Rubisco activity, net photosynthetic rate (Pn), leaf relative water content (RWC), and biomass were measured. Data were analyzed using analysis of variance (ANOVA), and treatment means were compared using Duncan's multiple range test at p≤0.05p\le0.05p≤0.05.
Results
Both drought stress and HA significantly affected all measured traits (p≤0.01p\le0.01p≤0.01), and their interaction was significant for most variables, indicating that the effectiveness of HA depended on the plant water status. Severe drought markedly increased oxidative stress, raising MDA and H₂O₂ concentrations by 58% and 41%, respectively, compared with the well-watered control. Application of 6 g L⁻¹ HA reduced these oxidative markers by 20% and 16%, respectively, relative to untreated drought-stressed plants, suggesting enhanced antioxidant protection. Drought increased proline accumulation by 22%, reflecting osmotic adjustment, while HA further promoted proline accumulation and increased total phenolic content by 9% under severe stress. Water deficit also reduced chlorophyll content, Rubisco activity, and net photosynthetic rate by 15%, 29%, and 30%, respectively, whereas plants receiving 6 g L⁻¹ HA maintained significantly higher values for all three parameters. Notably, the net photosynthetic rate of plants treated with 6 g L⁻¹ HA under 50% FC was comparable to that of untreated plants grown under 75% FC. Severe drought without HA reduced shoot fresh weight, shoot dry weight, and RWC by 32%, 36%, and 24%, respectively, whereas HA application alleviated these reductions by maintaining 17% greater shoot dry weight and 8% higher RWC under severe drought. These improvements are likely associated with enhanced root development, improved water uptake, and increased soil water retention.
Conclusion
Humic acid fertigation, particularly at 6 g L⁻¹, substantially improved drought tolerance in greenhouse-grown basil by enhancing water status, preserving photosynthetic capacity, strengthening antioxidant defenses, and reducing oxidative damage. The ability of HA-treated plants under severe drought to achieve physiological performance comparable to untreated plants under moderate drought highlights its potential as a sustainable strategy for reducing irrigation requirements while maintaining crop productivity