Moderate gamma irradiation enhances the biostimulant activity of humic acid in tomato: Integrated effects on growth, physiology, yield, and fruit quality

Humic acid is widely used as a plant biostimulant, but its effectiveness depends strongly on its physicochemical characteristics and application rate. Gamma irradiation may modify the structure and biological activity of humic substances, although its effects on horticultural crops remain insufficiently characterized. This study evaluated the effects of non-irradiated and gamma-irradiated humic acid on the growth, physiological performance, mineral nutrition, antioxidant status, yield, and fruit quality of tomato ( Solanum lycopersicum L.). The humic-acid treatments were arranged factorially using three irradiation doses—0, 2, and 5 kGy—and two application rates—2.5 and 5.0 kg ha −1 —together with an untreated control. Gamma irradiation dose and humic acid application rate significantly affected most measured traits, whereas their interaction was significant only for total marketable yield. Humic acid irradiated at 2 kGy and applied at 5.0 kg ha −1 produced the greatest plant height, leaf area, root dry weight, chlorophyll index, net photosynthetic rate, leaf nitrogen, phosphorus and potassium concentrations, antioxidant activity, fruit number, marketable yield, total soluble solids, vitamin C, and lycopene content. Increasing the humic acid rate from 2.5 to 5.0 kg ha −1 significantly increased marketable yield when the humic acid was non-irradiated or irradiated at 2 kGy, but not when irradiated at 5 kGy. Treatment-adjusted correlation analysis linked canopy development and photosynthetic performance with fruit production and marketable yield, while days to first harvest was negatively associated with major growth and productivity traits. Hierarchical clustering and principal component analysis separated the treatment comprising humic acid irradiated at 2 kGy and applied at 5.0 kg ha −1 from the remaining treatments because of its consistently favorable multivariate profile. An ensemble multi-criteria decision-making framework integrating TOPSIS, VIKOR, WASPAS, and EDAS also ranked this treatment first under alternative criterion-weighting scenarios. These results indicate that moderate gamma irradiation can enhance humic acid biostimulant activity, whereas a higher irradiation dose may reduce the benefits obtained from increasing the application rate.