الفهرس 
Effect of drought stress and salicylic acid on the secondary metabolites of thymus vulgaris L. and eriocephalus africanus L
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Abstract
Polyphenolic compounds are considered valuable secondary plant metabolites owing to the myriad of biological activities they exert. Drought is among the most common agricultural stresses that significantly influence both growth and metabolic activities of plants.Aim of the work: This study aimed to investigate the combined effect of drought-stress, induced through decreasing amounts of irrigation water (or Field Capacity FC), and application of increasing concentrations of a plant growth regulator, salicylic acid (SA) on the major secondary metabolites of Thymus vulgaris L. and Eriocephalus africanus L. and related bioactivities. In this respect, several parameters were monitored in plants from different treatments and compared to those from control group including; total polyphenolic and flavonoid yield and composition, essential oil yield and composition, as well as antioxidant and cytotoxic activity of the alcoholic extracts. Results: Drought alone significantly (p<0.05) increased the total flavonoid and phenolic contents (TFC and TPC), essential oil (EO) yields and antioxidant potential in both plants. TFC in plants irrigated at 25% FC (drought stressed), and sprayed with 3 mM SA was increased from 6.1±0.3 to 32.1±0.1 in T. vulgaris, and from 109.6±0.4 to 167.9±0.2 mg rutin equivalent /gm dried extract in E. africanus. This was associated with a rise in TPC from 8.5±0.3 to 68.5±1.2 and 181.4±0.7 to 243.4±0.7 mg gallic acid equivalent/gm in the two plants, respectivelys