natural-products-in-plant-pest-management

78 J.C. Pretorius and E. van der Watt3.6 Natural Compounds from Plants with Bio-stimulatoryPotentialAllelochemicals found in plants are probably all secondary metabolites thatare distinctive from primary metabolites in that they are generally nonessentialfor the basic metabolic processes such as respiration and photosynthesis(Richard, 2001). They are numerous and widespread, especially inhigher plants (Pillmoor, 1993), and often present in small quantities (1–5%) ascompared to primary metabolites (carbohydrates, proteins and lipids).Approximately 88,000–100,000 secondary metabolites have been identifiedin all plant forms, showing both structural and activity diversity (Verpoorte,1998). Ecologically, these chemicals play essential roles in attracting pollinators,as adaptations to environmental stresses and serve as chemical defencesagainst insects and higher predators as well as microorganisms (Rechcigland Rechcigl, 2000). Although the purpose of the production of secondarymetabolites in plants has long been argued among researchers, it is now universallyaccepted that they are produced as a result of abiotic (Beart et al.,1985) and biotic stresses (Bourgaud, et al., 2001), probably as part of a plantdefence arsenal. Besides the role secondary metabolites play in plant metabolism,the growth promotion or inhibitory properties of certain naturalcompounds from plants have been extensively researched (Wu et al., 2002).Current status of plant products with bio-stimulatory potentialAlready, at the end of the millennium, two successful natural products developedin Moldavia (formerly part of the Soviet Union) are Moldstim andPavstim, extracted from hot peppers (Capsicum annum L.) and leaves ofDigitalis purpurea L., respectively (Waller, 1999). Both products have beenused on a large scale as plant-growth regulators and for disease control.These developments are excellent examples of how natural plant resourcescan be exploited and applied in agriculture.However, from an agricultural perspective, plant extracts containinggrowth-promoting substances have always been of interest to the researchcommunity in terms of the role they could play in addressing future foodsecurity issues. The ideal breakthrough would be to identify a plant or plantsthat contain bio-stimulatory substances promoting growth and resistance topathogens, as well as yields in agricultural and horticultural crops. At thispoint it seems appropriate to consider recent discoveries of bio-stimulatorycompounds from plants that have the potential to adhere to the ‘ideal breakthrough’criterion in terms of their application potential as natural productsin the agricultural industry.Extracts from numerous plant species, with bio-stimulatory properties,were identified and evaluated for their commercial potential. Channal et al.(2002b) reported on seed germination as well as seedling growth enhancementof sunflower and soybean by leaf extracts from three tree species