Influence of rhizosphere and soil properties on antioxidants in selected Western Ghats medicinal plants

Soil physicochemical properties and rhizosphere microbial attributes may be associated with plant growth, metabolic activity, and the accumulation of bioactive compounds in medicinal plants. The phytochemical profiles of Sida rhombifolia, Phyllanthus rheedei, Hellenia speciosa and Arisaema tortuosum were assessed, and rhizosphere microbial activities and soil physicochemical properties were compared to explore possible associations among plant–site combinations. The differences observed here likely result from a combination of soil type, location, and plant species. Because species and sampling sites are confounded, these results indicate broad associations rather than effects unique to specific plant species. Endophytic Achromobacter, Cupriavidus, Acinetobacter, and Sphingomonas strains isolated from these plants were found to be positive for most of the plant growth-promoting traits. Alkaloid concentrations of the studied medicinal plants ranged from 0.3 to 0.6 g kg⁻¹ and flavonoid levels ranged from 1.2 to 1.7 g kg⁻¹. Differences in ABTS and DPPH radical scavenging capabilities were noted among the plant and location combinations that were sampled. The highest ABTS radical scavenging activity was observed in P. rheedei and was statistically comparable to H. speciosa. Lower DPPH IC₅₀ values were observed in A. tortuosum and H. speciosa, indicating comparatively higher DPPH radical scavenging capacity in these extracts. Important bioactive components such as 2-methoxy-4-vinylphenol, 9,12-octadecadienoic acid, and 2-heptanol were detected by GC–MS analysis. Although principal component analysis (PCA) showed associations among soil characteristics, microbial activity, and phytochemical accumulation, these findings should be considered exploratory rather than conclusive.