Favorable Impacts of Drought Stress on the Quality of Medicinal Plants: Improvement of Composition and Content of Their Natural Products

Abstract

AbstractThis review deals with the well-known phenomenon that spice and medicinal plants grown under semi-arid conditions generally reveal significantly higher concentrations of relevant natural products than identical plants, grown and cultivated in moderate climates. Basic biochemical reflections display that drought stress and the related metabolic changes are responsible for the higher natural product accumulation in plants grown in semi-arid regions (Selmar et al., Environmental challenges and medicinal plants. Springer, New York, 2017). Related data from the literature on the effect of drought on the concentration of natural products are compiled, and the relevant aspects are also outlined.A thorough reflection on this issue emphasizes the necessity to differentiate decidedly between “concentration” and “content” of natural products (Paulsen and Selmar, J Appl Bot Food Qual 89:287–289, 2016). Next, basic plant physiological coherences expound that in principle there are three causes for the observed changes in the concentration of natural products (Yahyazadeh et al., Phytochemistry 152:204–212, 2018). Changes in the reference values: Drought stress-induced enhancement of the natural products’ concentration is due to a reduced production of biomass in the stressed plants. Passive shift: Under drought stress, the stomata are closed. The shortage of CO2 evokes a strongly elevated level of NADPH+H+. Accordingly, all processes consuming NADPH+H+ are boosted, although enzyme activities were not changed. Active upregulation: The activity of enzymes responsible for the biosynthesis of natural products is enhanced by increased gene expression. Apart from these quantitative changes, in some cases, also the spectrum of specialized metabolites is altered in response to the stress situation (Abouzeid et al., J Nat Prod 80:2905–2909, 2017).Based on these physiological considerations, practical aspects for the application of drought stress to deliberately improve the quality of spice and medicinal plants are displayed (Kleinwächter and Selmar, Physiological mechanisms and adaptation strategies in plants under changing environment. Springer, New York, 2014; Kleinwächter and Selmar, Agron Sustain Dev 35:121–131, 2015).KeywordsDrought stressSpecialized metabolitesOver-reduced stateSpice plantsMedicinal plantsPhytomodificines