Introduction

Plants produce a large variety of secondary metabolites which are usually considered to function as defences against herbivores and pathogens, as many other contributions to this volume attest. Among the most characteristic features of these compounds are their vast number and enormous chemical diversity. Reports on plant secondary metabolites (PSMs) are replete with phrases describing their ‘tremendous array’ (Morrissey, 2009), ‘bewildering proliferation’ (Schoonhoven et al., 2005) or ‘extraordinary diversity’ (Howe & Jander, 2008). The diversity of secondary metabolites is apparent not only in their chemical structures, but also in their distribution in plants. The composition of secondary metabolites in plants varies at many levels of organisation, such as among different plant taxa (Wink, 2003), among different populations of the same taxon (Kliebenstein et al., 2001a) and between individuals of the same species (Pakeman et al., 2006). Within a plant, there is also variation among different organs (Brown et al., 2003), developmental stages (Lambdon et al., 2003) and environmental conditions (Engelen-Eigles et al., 2006), as well as the frequent presence of complex mixtures of secondary metabolites in individual organs. Most secondary metabolites, including alkaloids (Waffo et al., 2007), phenolics (Ashihara et al., 2010) and terpenes (Köllner et al., 2004), invariably occur in mixtures rather than as individual, isolated substances. The chemistry and distribution of secondary metabolites is so diverse that being able to explain the patterns of diversity seems an essential requirement for understanding their roles in plants.

This review will consider both the generation of secondary metabolite chemical diversity by the plant’s biosynthetic machinery and the functional importance of such diversity. We focus on the function of secondary metabolites in defence against herbivores, because this has received the most attention from researchers. Since there are so many levels of diversity in secondary metabolites, we will limit ourselves to just one: the occurrence of mixtures of a single class of compounds, such as alkaloids or terpenes, in individual organs.