Spice and medicinal plants grown under water deficiency conditions reveal much higher concentrations of relevant natural products compared with identical plants of the same species cultivated with an ample water supply. For the first time, experimental data related to this well-known phenomenon have been collected and a putative mechanistic concept considering general plant physiological and biochemical aspects is presented. Water shortage induces drought stress-related metabolic responses and, due to stomatal closure, the uptake of CO2 decreases significantly. As a result, the consumption of reduction equivalents (NADPH + H(+)) for CO2 fixation via the Calvin cycle declines considerably, generating a large oxidative stress and an oversupply of reduction equivalents. As a consequence, metabolic processes are shifted towards biosynthetic activities that consume reduction equivalents. Accordingly, the synthesis of reduced compounds, such as isoprenoids, phenols or alkaloids, is enhanced.

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Drought stress is generally considered as a negative factor in agriculture, being responsible for severe yield losses. However, medicinal plants grown under semi-arid conditions usually produce higher concentrations of active substances than same species grown under moderate climates. Currently, only limited information is available on the physiological background of this well-known phenomenon. To elucidate this issue, we review here the existing findings to unveil the underlying metabolic mechanisms. Indeed, we found that the drought stress-related metabolic changes are responsible for the accumulation of active substances in semi-arid regions. In particular, the biochemical coherences are as follows: due to limited water supply and much higher light intensities, the plants suffer from drought stress. Then, due to water shortage, stomata are closed and the uptake of CO 2 decreases markedly. As a result, CO 2 fixation via Calvin cycle decreases. This decline of CO 2 fixation leads to a massive decrease in the consumption of reduction equivalents (NADPH+H + ), generating in turn a massive oversupply of NADPH+H + . As a consequence, metabolic processes are pushed towards the synthesis of highly reduced compounds, such as isoprenoids, phenols, or alkaloids. Our findings open the path to design practical approaches for enhancing the product quality of spice and medicinal plants. Indeed, by deliberately applying moderate drought stress during their cultivation, the quality of spice and medicinal plants can be enhanced significantly.

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