Osmotic stress and melatonin in Pfaffia glomerata: biochemical responses and 20-hydroxyecdysone modulation

Osmotic stress, driven by factors such as soil drying and salinization, poses a significant challenge to plant growth and metabolism. Melatonin has shown promising results in alleviating abiotic stress by inducing antioxidant defenses in several species, which is particularly interesting when considering medicinal plants. Thus, this study investigates the physiological and biochemical responses of Brazilian-ginseng (Pfaffia glomerata) to PEG 4000-induced osmotic stress and the mitigating effects of exogenous melatonin. Under osmotic stress, P. glomerata exhibited reduced growth, diminished photosynthetic pigments, and increased reactive oxygen species (ROS) levels, highlighting the detrimental impact of water deficit on plant health. Melatonin, in turn, differentially affected leaves and roots, failing to restore shoot growth while promoting root elongation under osmotic stress. In addition, melatonin increased the activity of antioxidant enzymes, particularly peroxidase (POD), reducing ROS production and membrane damage in the leaves. In roots, PEG only increased catalase (CAT) activity. Osmotic adjustment following melatonin application was also evident through elevated sucrose and proline levels, supporting cell turgor and stress adaptation. Interestingly, osmotic stress increased 20-hydroxyecdysone (20-E) levels in roots; however, this increase occurred independently of melatonin. Therefore, despite the induction of osmotic adjustments and antioxidant defenses, melatonin was unable to reverse the growth restraints caused by osmotic stress in P. glomerata. Furthermore, our findings reveal a complex interplay between osmotic stress, antioxidant defenses, and secondary metabolite production. The insights gained offer potential applications for improving stress resilience and secondary metabolite synthesis in medicinal plants, with implications for sustainable agriculture.