Adaptive Patterns in Alkaloid Physiology

Abstract

Alkaloid transport and storage are reviewed, with emphasis on problems associated with presence of toxic chemicals in living plants. Many patterns in the physiology of alkaloids and other defense compounds are shaped by the twin requirements that these compounds must be inactive in the plant and yet active in the presence of herbivores. The distribution of alkaloids in the plant and changes in distribution during the plant's life are also reviewed. Within individual plants, alkaloids are generally concentrated in those parts upon which herbivore attack would have the greatest effect on the plant's fitness. The relative defense requirements of different parts, based on their contribution to fitness and their vulnerability to herbivores, shift during the plant's life. These shifts are mirrored by corresponding shifts in alkaloid concentration. Ovules, seeds, and immature fruits are often the sites of highest alkaloid concentration. Allocation of defensive chemicals between mature and immature foliage should follow different patterns in trees and in herbs. In trees, the "phenological protection" given to synchronously produced flushes of new leaves and the need for protection of mature leaves until a new flush of young leaves can be produced have selected for greater toxin accumulation in mature leaves. In herbs, young leaves lack both mechanical and phenological defense, so there is strong selection for providing them with chemical defenses.