The genetics and ecology of variation in secondary plant substances

Abstract

There is an enormous amount of variation in secondary plant metabolites. The variation, both quantitative and qualitative, can be exploited to study the genetics and the ecological functions of these substances. The present review discusses some selected groups of secondary metabolites, viz. those that have been studied both from the genetical and the ecological point of view. Variation in secondary metabolism may be caused by genetical differences or by the environment. The control and expression of two groups of secondary plant substances: cyanoglucosids and flavonoids are discussed as examples. The different functions of secondary metabolites in the ecology of the plant can be grouped as follows: Functions related to the primary metabolism, functions related to the abiotic environment and functions related to the biotic environment. One secondary metabolite can have different functions, belonging to different function groups.One function of secondary metabolites that has received considerable attention in the past is protection against herbivores. There is certainly evidence that some secondary metabolites give protection against some herbivores. However, only a few combinations of plants and herbivores have been studied in detail. Less than a score of these studies have made use of variation in secondary plant substances with a known genetical base. Even if we can prove the defensive function of a particular secondary metabolite, this does not, in itself, constitute a proof of adaptedness. Only if and so far as the benefit of chemical defense exceeds the cost can we speak of adaptedness. A review of the literature on costs and benefits of chemical defense shows that both costs and benefits have been defined too narrowly in the past. Discussion of the ecological functions of cyanogenesis in white clover shows that both costs and benefits of cyanogenesis can be large, but that these effects are prominent in different stages of the life cycle of the plant. So in order to study the cost and benefit of a chemical defense system, the life history of the plant has to be taken into account.KeywordsWhite CloverChemical DefenseSecondary PlantPyrrolizidine AlkaloidLotus CorniculatusThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.