The complexity of sound quantification of specialized metabolite biosynthesis: The stress related impact on the alkaloid content of Catharanthus roseus.

Medicinal plants grown under stress conditions reveal higher concentrations of relevant specialized metabolites than well-watered plants, putatively due to an enhanced biosynthesis. Yet, stress also reduced the biomass gain. Accordingly, the concentration increase in comparison to control plants could also be due to lesser biomass employed as the reference value, whereas the rate of biosynthesis may remain unchanged. For an unequivocal proof that stress indeed enhances the biosynthesis, the total amount of the substances per plant has to be determined. In this study, we investigated the stress-induced impact on the alkaloids accumulated in Catharanthus roseus and quantified both, the changes in concentration and in the entire amount of alkaloids. At any time, all Catharanthus roseus plants grown under drought stress exhibited a markedly higher alkaloid concentration compared to the well-watered controls. However, by calculating the entire alkaloid content per plant, a corresponding increment occurred only within the first two weeks of drought stress. Thereafter, no significant differences among drought treatments and control were detected. Finally, within the last week, the alkaloid content per plant decreased markedly, although there was a meaningfully higher concentration of alkaloids in the drought-stressed plants. In contrast, when plants had been exposed to high salt concentrations, the alkaloid concentrations were quite the same in stressed and control plants. The related total contents were significantly lower in plants exposed to salt stress. These results display that both phenomena, an increased rate of biosynthesis and lesser reference values, i.e., the biomass, contribute to the stress-related increase in the concentration of natural product. Moreover, it has to be considered that the enhancement of biosynthesis could be due to either an "active" up-regulation of biosynthetic capacity or a "passive" shift caused by the over-reduced status as a result of the stress-induced stomatal closure.