Secondary metabolites and phenylpropanoid pathway enzymes as influenced under supplemental ultraviolet-B radiation in Withania somnifera Dunal, an indigenous medicinal plant

• s-UV-B increases secondary metabolite concentrations in plant parts of W. somnifera.
• Withaferin A and withanolide A concentrations are altered in response to s-UV-B.
• Higher secondary metabolite-concentrations were found in roots compared to leaves.
• Phenylpropanoid pathway enzymes’ activities increase under s-UV-B.

The present study aims to investigate the effects of supplemental ultraviolet B (3.6 kJ m−2 day−1 above ambient) radiation on secondary metabolites and phenylpropanoid pathway enzymes of Withania somnifera under field conditions at 40, 70, and 100 days after transplantation. Secondary metabolites’ (alkaloids, anthocyanins, carotenoids, flavonoids, lignin, phytosterols, saponins, and tannins) concentrations were analysed at the end of the treatments. Activities of phenylalanine ammonia lyase (PAL), cinnamyl alcohol dehydrogenase (CAD), 4-coumarate-CoA ligase (4CL), chalcone–flavanone isomerase (CHI), and dihydroflavonol reductase (DFR) were also determined. In treated plants, secondary metabolite-concentrations generally increased (higher concentrations being recorded in roots compared to leaves). Anomalies were recorded for lycopene in roots and phytosterols in leaves (all sampling ages); β-carotene declined in leaves at third sampling age. s-UV-B-treated plants depicted decrease in withanolide A content with concomitant increase in withaferin A (two major alkaloids analysed by HPLC) compared to their respective controls. Phenylpropanoid pathway enzyme-activities increased in leaves and roots under s-UV-B treatment, the latter showing greater increase. The study concludes that s-UV-B is a potent factor in increasing the concentrations of secondary metabolites and their biosynthetic pathway enzymes in W. somnifera.