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Medicinal plants grown under semi-arid conditions reveal much higher concentrations of relevant natural products than the equivalent plants, but cultivated in moderate climates. This phenomenon can be explained on the basis of general plant physiological and biochemical considerations: in semi-arid regions - due to limited water supply and much higher light intensities - the plants are exposed to a higher level of drought stress than the plants grown in moderate climates. Up to now, only limited experimental data on these coherences are available. In this section, a comprehensive survey on the relevant literature is presented. In most of the studies available, the content of secondary plant products indeed is enhanced in plants suffering drought stress.

The metabolic background of the stress-induced enhancement of natural products can be explained as follows: because of the stomata closure due to the incipient water deficiency, the uptake of CO2 markedly decreases. As result, the consumption of reduction equivalents (NADPH+H+.) for the CO2-fixation via Calvin cycle declines considerably, generating a massive oversupply of NADPH+H+. As consequence, metabolic processes are pushed toward the synthesis of highly reduced compounds, like isoprenoids, phenols or alkaloids.

Using sage (Salvia officinalis) as model plant, the interaction of drought stress induced metabolic changes and synthesis and accumulation of secondary plant products have been investigated. When the plants had been cultivated under moderate drought stress, the overall content as well as the concentration of total monoterpenes was markedly higher than in the corresponding, well-watered control plants. When these plants had been cultivated in parallel under elevated CO2 (700 ppm), the drought stress related increase in monoterpene synthesis was totally compensated. Obviously, despite the stress related stomata closure, still large amounts of CO2 could enter the leaves. Based on these considerations, putative applications to increase the quality of medicinal plants by deliberately applying drought stress are delineated. (C) 2012 Elsevier B.V. All rights reserved.