natural-products-in-plant-pest-management

58 J.C. Pretorius and E. van der Wattgroup and to evaluate the application potential of these compounds in theagricultural industry (Pretorius, 2003 and references therein).Tannins are generally found in the xylem and bark of vascular plants.Two types of tannins have been distinguished namely hydrolysable and condensedtannins. When hydrolysable tannins are broken down (hydrolysed),the well known gallic and digallic acids are formed (Fig. 3.23). Tannins combinewith proteins to form insoluble co-polymers and this technique is usedto tan animal skins. However, tannins are toxic to humans and animals andnot suitable for natural product development (Borris, 1996).Kinones are divided in three groups, benzokinones, naphtakinones andantrakinones (Fig. 3.24). Most kinones are coloured but do not contribute tocolour in plants as they are found in the inside tissue layers. The best knownexample of benzokinone is ubikinone (Fig. 3.25), also known as Co-enzymeQ, which acts as an electron carrier during the light reaction ofphotosynthesis. Juglone, the well known naphtakinone, has a strongHOHOHOC OHOCOOHHOHOCOOHHOGallic acidHODigallic acidFig. 3.23. Two hydrolysable tannins.OOOOBenzokinoneFig. 3.24. Three groups of kinones.OJuglone(a naphtakinone)OAntrakinoneOH 3 C-OH 3 C-OOUbikinoneHFig. 3.25. Ubikinone, the best known example of a benzokinone.