Impact of drought stress signals on growth and secondary metabolites (SMs) in medicinal plants

Abstract

Medicinal plants having diversified phytochemical compounds like secondary plant metabolites (alkaloids, terpenoids, phenols, steroids, flavanoids, tannins, cyanogenic glycosides and glucosinolates, essential oils and aromatic compounds etc) are subject to abiotic stress like drought. Drought, one of the major ecologically limiting factors has significant impact on growth and secondary metabolic process of several medicinal plants. Water stress causes a reduction in plant size, density, reduces plant leaf area, and decrease in whole biomass, and not only alters the plant structurally and anatomically but also leads to fluctuation of their secondary chemical constituents. Secondary plant metabolites (SPMs) are useful to assess the quality and quantity of the therapeutic ingredients and such metabolites synthesized by the plant helps to cope up towards the negative effects of stress for adaptation and defence. A large number of studies manifested from the relevant review that drought influences on SPMs production and accumulation from plant parts like roots, stems, leaves, flower, fruits, seeds etc and causes an increase or decrease in their solute concentration by up to 50%. Studies showed that a medicinal plant produces different concentration of a particular metabolite grown under stress and non-stress environment. Generally, drought stress accumulates a higher concentration of active phytochemicals like alkaloids, tannins, terpenoids etc whereas concentration of phenols, flavanoids and saponins etc decreases under drought. In most cases as a whole, it may have to be concluding from comparative analysis that medicinal plants grown under drought exhibits higher content of secondary plant products than grown under optimal conditions. In addition, all secondary products may not increase in equal proportions under stress and it depends on the intensity of the drought as well as species of medicinal plants. Thus, moderate drought stress significantly enhanced the quality as well as quantity of secondary active substances in medicinal plants. However, for better understanding indepth further research is utmost essential at molecular level using new techniques viz. Proteomics, metabolomics, transcriptomes and genomics etc.