Seed heteromorphism and timing of germination : their mechanisms and adaptive significance

Abstract

Seedling emergence usually occurs asynchronously, spread over several weeks or months. In some species, two or more discrete cohorts emerge in different seasons of the year (e.g. in autumn and spring). Such phonological patterns may result from seed heteromorphism in relation to dormancy. The term heteromorphism was originally used with reference to morphological differences in seeds. However, it is now applied to physiological variation in dormancy characteristics, even without accompanying morphological dissimilarities (cryptic heteromorphism). The general tendency in heteromorphic plants is that dormant seeds are likely to be produced earlier and /or dispersed later than nondormant seeds. These patterns are closely related to the position of seeds within or between fruits in the same plant. Somatic heterochrony may well explain the evolution of seed heteromorphism. Three hypotheses are addressed in relation to the adaptive significance of seed heteromorphism: 1) risk spreading in a temporally variable environment, 2) avoidance of density-dependent mortality, and 3) escape from sibling competition. Although these are not mutually exclusive, risk spreading seems to be the most important, since the ratios of dormant: nondromant seeds vary with latitude or altitude depending on environmental harshness. Finally, I propose the idea that seed heteromorphism may have evolved even in perennial or iteroparious plants. I also point out that, in some cases, spreading germination over time could be realized even in species which do not show seed heteromorphism.