Case study: The difficulty of correct reference values when evaluating the effects of drought stress: a case study with Thymus vulgaris

Medicinal and spice plants grown under semi-arid conditions frequently contain higher concentrations of relevant secondary metabolites than plants cultivated in moderate regions. It is well established that this phenomenon is due to the impact of drought stress. However, in principle, the increase in natural product concentration could be caused by two quite different effects. Firstly, it could be caused simply by a change of the reference values: typical stress-related reduced growth frequently results in a lower biomass of the stressed plants. In consequence − provided that the rate of natural product biosynthesis remains constant − this results in an enhanced concentration in the stressed plants. Secondly, there is a genuine stress-related increase in the rate of biosynthesis of secondary metabolites.In the first case, the total amount of secondary metabolites remains constant, while in the second case, it increases. Accordingly, a thorough differentiation requires a reliable quantification and comparison of the relevant factors. This especially accounts for the total biomass, which generally is far less in drought stressed plants than in the well-watered ones. Consequently, in all studies, where data on the total biomass are lacking, it is not feasible to evaluate reliably the total amounts of secondary metabolites only on the bases of the concentrations determined. Unfortunately, in most of the corresponding literature published so far, these data are missing, and thus, it is not possible to decide whether the drought stress-related increase of secondary metabolites is due to a genuine increase in biosynthesis or whether it is just due to a change of the relevant reference values, i.e., the total biomass. In this study, the relationships of these factors in well-watered and drought-stressed thyme plants are examined and discussed