Asian Journal of Research in Botany

Medicinal-plant quality is often treated as a stable property of a species or cultivar, yet the concentration, profile and total yield of bioactive constituents are highly plastic. Environmental stress can redirect carbon, energy and reducing power from growth towards defence-associated specialised metabolism, but the resulting quality response is neither universally positive nor reliably predicted by stress intensity alone. This critical narrative review evaluates how drought, salinity, light and ultraviolet radiation, temperature, mineral imbalance, atmospheric change, toxic elements and biotic elicitation modify medicinal quality. It integrates physiological, redox, hormonal, transcriptional and metabolic mechanisms with evidence from field, greenhouse, controlled-environment and in vitro studies. The synthesis shows that moderate, developmentally timed stress can increase selected phenolics, terpenoids, alkaloids or saponins, particularly when signalling is activated before irreversible damage. Severe or prolonged stress commonly reduces biomass, disrupts source–sink relations, alters compound ratios and may lower total recoverable yield despite an apparent rise in tissue concentration. Quality therefore cannot be inferred from concentration alone; active-constituent yield per plant or unit area, compositional consistency, safety and post-stress recovery are equally important. The evidence is strongest for drought- and light-mediated modulation of phenylpropanoid and essential-oil pathways, while responses to combined stresses, field variability and climate extremes remain poorly resolved. Mechanistic studies increasingly identify reactive oxygen species, jasmonate and abscisic-acid signalling, transcription-factor networks and chromatin-level regulation as points of control, but causal validation in medicinal crops is still limited. Agronomic opportunity lies in calibrated eustress, deficit irrigation, spectral lighting, elicitors and recovery-aware harvest scheduling, implemented within a quality-by-design framework rather than as indiscriminate stress imposition. Progress requires dose–time response experiments, multi-omics linked to validated chemical assays, genotype-by-environment analysis, safety surveillance and multi-location trials that report both concentration and total metabolite yield.