Relationships between Seed Coat Development and Curved Embryo in Papilionoideae (Leguminosae)

Abstract

Leguminosae can be classified into two subgroups based on the embryo shape: the one is Papilionoideae, having a curved embryo in seeds, and the other is composed of the remaining five subfamilies, having a straight embryo. To identify the factors that induce the curved embryo of Papilionoideae, I observed the development of the inner structures of seeds of five species of the subfamily in comparison with three species with straight embryos of other two subfamilies, Cercidoideae and Caesalpinioideae. As a result, I recognized that radicles grow with their apexes facing the micropyle, and cotyledons grow with their apexes facing the chalaza in the seeds. For this reason, when the distance between the micropyle and the chalaza on the raphe-side (i.e., raphe side lengths of the seed coat) is significantly shorter than that on the anti-raphe side, as commonly observed in Papilionoideae seeds, the embryo axis curves and the angle between the radicle and cotyledon axes becomes acute. The present results suggested that restraint of growth of the seed coat at the raphe side is a preceding factor of a curved embryo.