Potential of salt and drought stress to increase pharmaceutical significant secondary compounds in plants

It is well known that spices derived from plants grown under mediterranean or semi-arid climate conditions, respectively, are much more pronounced in taste and are more aroma intensive than those obtained from equivalent plants, but cultivated in moderate climate, e.g. in Central Europe. Analogous quality differences are observed with regard to medicinal plants, i.e. the content of the relevant secondary plant products in general is less in plants grown in a moderate Atlantic climate than in those derived from semi-arid regions. Based on plant physiological considerations, this phenomenon can be explained easily: in semi arid regions - due to limited water supply and much higher light intensities - the plants are exposed to a significant higher drought stress than the plants grown in Central Europe. Unfortunately, in the literature only very limited data on these obvious coherences are available. This review is aimed to compile the corresponding data published so far. In most cases, comparative analyses revealed that the content of secondary plant products indeed is higher in plants that suffer drought stress than in those cultivated under optimal conditions. Under stress, a strong oversupply of reduction equivalents is generated. In order to prevent damage by oxygen radicals, NADPH + H+ is reoxidized by photorespiration or violaxanthine cycle. Yet, the high concentration of reduction equivalents also leads to a stronger rate of synthesis of highly reduced compounds, i.e. isoprenoids, phenols or alkaloids. Unfortunately, these coherences have not been considered adequately neither in general plant physiology nor in reflection of medicinal or spice plants. In this review, a basis for further considerations of the physiology and biochemistry of secondary plant metabolism is presented. Special emphasis is put on aspects of quality improvement by increasing the concentration of secondary compounds in spices and medicinal plants by deliberately applying drought stress.