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Phenolic stress related increases in the concentrations

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Phenolic
compounds such as phenolic acids and tannins are secondary metabolites. Although the increment in the amount
of these compounds under drought stress reduces plant growth, it acts as a
defense mechanism against different stresses. Under water stress, plant growth
is typically reduced before photosynthesis (Herms and Mattson, 1992), resulting
in the synthesis of secondary compounds under these conditions is as a pathway
to waste of surplus energy by plants that limit production of  reactive oxygen species. Increasing irrigation
intervals increased the leaf tannin content. The increasing tannin content under
water stress has also been reported in other plants and tree species (Anuraga
et al., 1993; Kouki and Manetas, 2002; Tharayil et al., 2011; Zhang et al.,
2012; Umebese and Falana, 2013). The increment
in phenolic compounds under drought stress is due to their antioxidant capacity
(Kim et al., 1997). In Hypericum
brasiliense, not only the concentration but also the total content of the
phenolic compounds is drastically enhanced under drought stress in comparison
to the control plants (de Abreu and Mazzafera, 2005).
Similarly,
in Labisia pumila, not only the concentration but also the overall
production of total phenolics and flavonoids per plant is enhanced in plants
suffering from drought stress (Jaafar et al., 2012).

In sage (Salvia officinalis),
drought stress induces a massive increase of monoterpenes, which
over-compensates the reduction in biomass (Nowak et al., 2010). As a
result, the total amount of monoterpenes accumulated in sage is significantly
higher in plants suffering from moderate drought stress as compared to well
watered controls. In contrast, in catmint and lemon balm plants the slight
drought stress related increases in the concentrations of monoterpenes do not
compensate the stress related detriment of growth. Consequently, the overall
content of terpenoids is lower in the drought stressed plants of Melissa
officinalis and Nepeta cataria than in the corresponding controls
(Manukyan, 2011). Azhar et al. (2011) reported that Trachyspermum ammi L.
plants under dry conditions decrease vegetative biomass accumulation but
enhance their phenolic contents.

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 Mucilages are insoluble hydrocarbons that are produced after
decomposition of energy. When the plants need to more energy for their metabolic
activity, produces more mucilage.

Karami et al. (2011) also stated that the amount of mucilaginous
brassica leaves grows under drought stress. The effect of drought stress on the
increase of secondary metabolites (essential oil of aromatic plants and
mucilage) by Salehi Arjmand (2005) has also been reported.

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