Abstract
Water availability is a key environmental factor influencing the cultivation of medicinal plants, as it affects both biomass yield and the biosynthesis of bioactive compounds, thereby modulating their pharmacological properties. Solidago chilensis Meyen (Brazilian arnica) was subjected to an agronomic trial aimed at assessing fresh and dry biomass production, flavonoid content, and foam index (as an indirect measure of saponin concentration) under distinct irrigation regimes. The experiment followed a completely randomized design (CRD), with treatments combining irrigation frequencies (daily, every 3.5 days, and every 7 days) and volumes (12%, 6%, and 3% of the soil’s water retention capacity). While no qualitative changes in phytochemical composition were observed, water availability significantly influenced productivity and secondary metabolite accumulation. Flavonoid levels were highest under frequent irrigation, whereas the foam index increased under less frequent watering, suggesting that water stress may enhance saponin biosynthesis. Overall, irrigation volume had a comparatively minor effect on the parameters assessed, with irrigation frequency being the primary driver of biomass production and phytochemical modulation. These findings indicate that frequent irrigation promotes biomass accumulation and flavonoid synthesis, whereas reduced irrigation frequency may trigger adaptive responses leading to increased saponin content in S. chilensis.