Seed micromorphology and ex vitro germination of Dendrobium orchids

Abstract

Dendrobium species are adapted to disperse and germinate in various habitats. Whilst seed dispersal in orchids is known to be highly dependent on seed size and air space, little is known, generally, about seed micromorphology in Dendrobium and how this trait varies between species. In other plant families, small embryos may be used as an indicator of the presence of morphophysiological dormancy. However, the presence of seed dormancy in orchids is difficult to assess as germination is usually stimulated in vitro. We investigated the mature seed characters of nine Dendrobium species and assessed relations with seed germination. Seeds were found to be c. 0.2-0.6 mm long and 0.05-0.09 mm wide, containing embryos c. 120-240 µm long and 50-80 µm wide. The estimated seed air space around the embryo was 13 to 36% of the seed volume. Seed germination ex vitro on plain agar medium, in the absence of dormancy-breaking chemicals, progressed to the spherical protocorm stage at a broadly similar level to that observed in vitro on ½ Murashige and Skoog medium. We conclude that Dendrobium seeds with small air spaces and relatively large embryos can be non-dormant and this adaptation may allow seed germination in nature in moist, epiphytic microenvironments.