Evolution and functional ecology of the embryo to seed size ratio.

Abstract

• Continuous variation exists in the ratio of embryo to nutritive tissues of Angiosperms. Embryo to seed size ratio ranges from seeds dispersed with an embryo consisting of a clump of cells embedded in copious endosperm, e.g. several Ranunculaceae, to seeds containing only an embryo and no nutritive tissue, e.g. many Fabaceae. Within the group of species with small embryos, two different categories have been discerned: (1) seeds with an embryo that has not yet completely differentiated at the moment of dispersal, and (2) seeds with completely differentiated but small embryos relative to the size of the seeds. The requirement of growth of the embryo inside the seed prior to germination is commonly regarded as a dormancy mechanism, known as morphological dormancy, which delays germination until conditions are optimal for seedling establishment and survival. Although growth of the embryo inside the seed is undeniably related to a delay in germination, it can be questioned whether it is the main mechanism regulating germination timing. In some early angiosperms, for example, the root and the shoot only starts developing after germination. We will present the results of a literature survey of embryo growth inside the seed in function of temperature and time in over 100 species. In addition, the analysis of the relationships between the embryo to seed size ratio of about 1500 angiosperms and other plant functional traits and habitat while taking phylogeny into account provide hints about the relevance of storing food reserves outside the embryo.