Influence of irrigation and harvest age on the physicochemical properties and phytochemical composition of Pelargonium sidoides DC

Abstract

• Variation in plant available water (75%–25%) did not significantly alter the phytochemical composition of dried P. sidoides roots across 6-, 12-, and 18-month harvest ages. • Root colour attributes were darker and shifted towards red–orange hues at 12 and 18 months, suggesting ontogenetic accumulation of pigment-related phytochemicals. • Multivariate analysis indicated that harvest age exerted a stronger influence on phytochemical accumulation than water availability. • Total phenolic and flavonoid contents were positively correlated with Umckalin, Umckalin-7-sulphate, and Fraxinol-8-sulphate. The sustainable production of commercially valuable medicinal plants such as Pelargonium sidoides requires optimised cultivation practices. Despite its value, research on the combined effect of irrigation levels and harvest age on phytochemical composition of cultivated P. sidoides roots remains limited. This study examined colour properties, phytochemical content, antioxidant capacity and physicochemical properties of dried P. sidoides root samples from plants that were cultivated under 75% (well-watered), 50% (moderate water deficit), and 25% (severe water deficit) plant available water (PAW) and harvested at 6, 12, and 18 months. Lightness and hue colour attributes of the root samples decreased at 12 and 18 months, suggesting ontogenetic biosynthesis of colour-related phytochemicals. Phytochemical analysis revealed no significant variation in total phenolic content (TPC = 58.04–81.70 mg GAE/g) or total flavonoid content (TFC = 34.55–46.49 mg QE/g) across treatments. 2,2-diphenyl-1-picrylhydrazyl (DPPH) antioxidant capacity was significantly lower at 12 months (p = 0.0023) and 2,2-Azinobis-(3-ethylbenzthiazolin-6-sulfonic acid) (ABTS) was significantly affected by harvest age (p = 0.0417), while ferric reducing antioxidant power (FRAP) was unaffected. High performance liquid chromatography quantification revealed a significant effect of harvest age on umckalin (p = 0.0395), with pairwise differences observed for umckalin-7-sulphate and fraxinol-8-sulphate under specific treatment combinations. Positive correlations between coumarins, TPC, and TFC (R 2 ≥0.36) and a strong association between TPC, TFC, and FRAP (R 2 =0.95), indicated that phenolics and flavonoids drive the ferric reducing power. Principal component analysis separated the 6-month harvest from the 12- and 18-month harvests, with no distinction among irrigation levels. Fourier transform infrared spectroscopy confirmed functional groups consistent with phenols, flavonoids and coumarins. These findings demonstrate that P. sidoides can be cultivated under reduced water conditions while preserving bioactive compounds and supporting a sustainable and resource-efficient production approach.