Effect of Organic and Chemical fertilizers on growth and active constituents of Dodonaea viscosa

Dodonaea viscosa , a common plant in northern Iraq, is recognized for its ornamental and medicinal value. This study was conducted to evaluate the effects of two types of fertilizers: organic (humic acid, HA) at four levels (0, 0.75, 1.5, and 2 mL·L⁻¹) and chemical (NPK 20:20:20) at three levels (0, 0.5, and 1 g per pot). Growth parameters, including plant height, leaf number, branch number, stem diameter, fresh and dry weight, total chlorophyll, nitrogen (N), phosphorus (P), potassium (K) content, protein, and total phenolic acids, were assessed. Vegetative growth traits were significantly improved by foliar application of HA at 2 mL·L⁻¹ combined with NPK at 0.5 g per pot, except for nutrient content (NPK) and protein, which increased at NPK 1 g per pot. The study also aimed to quantify total phenolic compounds and evaluate their antioxidant activity using the DPPH assay, revealing that D. viscosa leaves contain high levels of phenolics and act as potent natural antioxidants. The highest antioxidant activity (85.5 µmol·L⁻¹) was observed with the ethyl acetate extract at 200 µmol·L⁻¹. Moreover, the interaction between HA and NPK enhanced the content of specific phenolic acids, including quercetin, gallic acid, and cinnamic acid. Gas chromatography–mass spectrometry (GC-MS) analysis identified eleven compounds in the organic layer of the extract, with myrcene (43.431%), eugenol (20.122%), and α-terpineol (15.501%) being the predominant constituents. Overall, the results indicate that the optimal combination of HA and NPK not only improves growth and biomass of D. viscosa but also enhances the quantity and quality of secondary metabolites. However, these effects are dependent on concentration and environmental conditions, highlighting the need for further research to optimize applications in this species.