natural-products-in-plant-pest-management

50 J.C. Pretorius and E. van der WattMonoterpenes, consisting of two isoprene units (C 10), form the maincomponent of essential oils and are therefore economically important due totheir aroma (perfume), e.g. menthol, camphor, geraniol and pinene (Fig. 3.4).Their application potential in agriculture lies in the fact that specific monoterpenesare growth inhibitors of higher plants (e.g. menthol and pinene) andtherefore have the potential to be developed as natural herbicides.Sesquiterpenes consist of three isoprene units (C 15) (Fig. 3.5). One of thebest known sesquiterpenes is absissic acid (ABA), which has a hormonalfunction. Plants showing natural resistance towards drought stress synthesizemore ABA than drought-sensitive crops. Theoretically, the externalapplication of ABA to crops has the potential to manipulate drought resistancein crops. Other sesquiterpenes have medicinal value e.g. as diureticsand as antimicrobials (e.g. kadinene from juniper berry).Diterpenes consist of four isoprene units (C 20) and are the rarest types ofterpenes in plants. Of the most important in this group is the hormone gibberellicacid (GA; Fig. 3.6). Although natural hormones are used in the horticulturaland agricultural industries on a small scale as growth stimulants, itis expensive and not economically justifiable on a large scale.OHCOOHIso-amyl alcoholAngelic acidFig. 3.3. Hemiterpenes found in the form of alcohols or acids.OHOMenthol Camphor Geraniol PineneFig. 3.4. Examples of monoterpenes.OHH 3 COH CHCH 33HOHH COOHCH 3HABAKadineneFig. 3.5. Examples of sesquiterpenes.