ABSTRACT
Animal life on earth is primarily sustained by green plants with photosynthetic capacity to convert carbon dioxide and water into basic macronutrients, i.e., carbohydrates, protein, and fat. In fact, on a global basis over 65% of food protein and over 80% of food energy is supplied by plants, and in terms of gross tonnage, plant products directly contribute about 82% of the total world food harvest (Deshpande, 1992). The photosynthetic process of plants, however, is not confined to the production of basic macronutrients. It also includes the biosynthesis of a variety of organic compounds. Traditionally, the processes generating plant compounds have been categorized as either primary or secondary metabolism. Research in plant physiological characteristics since the 1980s, however, has clearly shown that such a distinction between primary and secondary metabolites is at best arbitrary. The once-popular view of a secondary metabolite as one that does not play an indispensable role in plant life at the cellular level is no longer valid. It is now widely recognized that plants do not haphazardly produce a large number of chemical compounds; rather, each metabolite is biosynthesized for a definite purpose, and all products are interrelated according to a complex process that conserves energy and scarce organic nutrients. Antinutritional or toxic compounds that occur naturally in many plants can be considered secondary metabolites. Most secondary metabolites are now known to be essential to plant life; many of them provide a defense mechanism against bacterial, viral, and fungal attack analogous to the immune system of animals. Many are also produced in large amounts as a direct result of some adverse environmental condition.
