natural-products-in-plant-pest-management

Natural Products from Plants 55Phenolic aromatic compoundsPhenols are the most abundant aromatic and naturally occurring componentsof plants. The phenols are classified in different ways, depending onauthors. We will only briefly discuss the simple phenols, phenyl propanoids,flavonoids, tannins and kinones.Simple phenols are all monomeric (consisting of only one aromaticring; Fig. 3.16). However, the most general simple phenols with growthinhibitingalle lopathic properties that have the potential to be developed asnatural herbicides are vanillin, vanillic acid (a benzoic acid) and hydrokinone(Fig. 3.17). p-Hydroxy-benzoic acid and vanillic acid are the most commongrowth- inhibiting benzoic acids involved with allelopathy and arefound in maize, wheat, sorghum and barley.Phenol is probably the precursor of all other phenolic compounds foundin plants. Hydrokinone, resorsinol and catechol are found in low concentrationsin plants and their functions are not fully understood. They are mostlysecreted by insects as a defence mechanism against other insects and animals.Salicylic acid, on the other hand, possesses anaesthetic properties andis the active ingredient of Aspirin ® . In plants a number of functions havebeen identified for salicylic acid, namely induction of flowering as well as theinduction of ‘pathogenesis related’ (PR) proteins (peroxidase, chitinase andβ-1,3-glucanase) that increases a plant’s resistance to fungal infections. Whensecreted by plants, it also has an allelopathic growth inhibiting effect on otherplants in the environment.Phenyl propanoids are synthesized from the aromatic amino acid phenylalanine and contained in their structure is a 3-carbon side chain coupled to aphenol, e.g. coumaric acid and caffeic acid (Fig. 3.18).OHOHHOOHOHOHOHCOOHOHPhenol Hydrokinone Resorsinol Catechol Salicylic acidFig. 3.16. Simple phenols.OHO–CH 3HOHydrokinoneRR = -COH=vanillinR = -COOH=vanillic acidFig. 3.17. Simple phenols with great potential to be developed as natural products.