Seed Dimorphism for Dispersal : Physiological, Genetic and Demographical Aspects
Résumé
Seed dimorphism for dispersal characteristics is widespread and essentially found in annual plant species. Each individual of such species is able to produce two types of seeds : some adapted for dispersal and others with no obvious dispersal properties. The morphological position of these dimorphic seeds is usually constant within the plant architecture. Using a few samples, it is shown that seed dimorphism for dispersal is often associated with physiological dimorphism with regard to dormancy and growth characteristics : dispersed seeds have no dormancy while non dispersed seeds may be dormant. Plants grown from dispersed seeds are more vigourous and produce more seeds than plants grown from non dispersed seeds, when environmental conditions are favorable. On the other hand, plants grown from non dispersed seeds are stress-tolerant : they can produce some seeds in extreme conditions while the others cannot. The proportion of dispersed seeds produced by a plant is variable, depending on the species, the population, and the individual. For some species, it is the product of both environmental conditions and genetic background. Moreover, for self-compatible species, seed dimorphism may be associated with different selfing rates : the non dispersed seeds being more likely to be inbred. The evolutionary significance of dimorphism for dispersal, and its possible association with other aspects of dimorphism, are discussed.