Light and electron microscopy of embryo development in an annual × perennial Medicago species cross

Abstract

Using light and transmission electron microscopy, we studied embryo development from 3 to 14 days after pollination in an interspecific Medicago cross (perennial M. sativa L., 2n = 4x = 32 × annual M. scutellata (L.) Mill, 2n = 4x = 32) to determine factors contributing to abortion in wide crosses. An intraspecific M. sativa cross and an M. sativa self-mating were used for control embryos. Nucellus and integumentary tapetum at the base of the embryo sac appear to be important sources of nutrition during the first 2 weeks of embryo development. Micrographic evidence supports the conclusions that maternal and embryonic tissue of interspecific ovules fail to carry out a timely sequence of metabolism involving lipid, starch, and nucellar crystals. Delayed breakdown of starch and lipid in the integumentary tapetum and nucellus is a probable factor in reduced development of coenocytic endosperm. At the late heart stage of embryo development, relative inactivity of dictyosomes and endoplasmic reticulum in hybrid ovules suggests failure of nutrient metabolism and transport in all nutritive tissue including the nucellus, integumentary tapetum, endosperm, and suspensor of the embryo. Early embryo development in Medicago appears to be a complex phenomenon requiring a high degree of coordination between anabolism and catabolism in both maternal and embryonic tissue.