Embryology of two threatened species of Pachycladon (Brassicaceae)

Abstract

Ovule development and embryogenesis are described for Pachycladon exilis and P. cheesemanii using thin sectioning of resin-embedded tissue. The structures of megasporogenesis and embryogenesis in these two species of Pachycladon were similar to those described for Arabidopsis thaliana. There was, however, a significant difference in the timing of events, with an extended delay apparent in Pachycladon between anther dehiscence and the first cell divisions of the embryo and endosperm. Initial endosperm development in the two Pachycladon species was of the free nuclear type, followed by cellularisation in a manner similar to that seen in Arabidopsis. Within one day of anthesis a chalazal proliferating tissue mass developed adjacent to the chalazal pole of the embryo sac. However, free nuclear division of endosperm nuclei was not observed until 7 days after anther dehiscence. The stages of embryogenesis for the two Pachycladon species were approximately correspondent in time, but the final silique length and rate of silique elongation of P. cheesemanii was greater than for P. exilis. The siliques of P. cheesemanii also took at least an extra 7 days to desiccate at maturity. © 2003 Taylor & Francis Group, LLC.