Toward a general theory of plant. antiherbivore chemistry

TLDR: In this paper, it was concluded that since a definitive metabolic role could not be assigned to alkaloids, they best be considered as plant waste products, and more recent workers in the field of alkaloid biochemistry have concurred with this evaluation44,83 and the "waste product hypothesis" has been expanded to include plant secondary substances in general.

Abstract: Much research has been conducted in an attempt to fit the so-called plant secondary substances into the general framework of plant metabolism. Though some success has been achieved in this area, e.g., chlorogenic acid as a regulator of plant metabolic systems under stress21, most such studies have met with a conspicuous lack of success. Possible metabolic roles for the plant alkaloids as intermediate metabolites have probably received the greatest such attention and it was concluded at an early stage44 that, since a definitive metabolic role could not be assigned to alkaloids, they best be considered as plant waste products. More recent workers in the field of alkaloid biochemistry have concurred with this evaluation44,83 and the “waste product hypothesis” has been expanded, principally by Muller (1969, 1970), to include plant secondary substances in general.