Study On Aril And Seed Coat Development Of Four Genera In Celastraceae

The remarkable characteristic on reproductive structures of Celastraceae is that most of taxa have various kinds of arils or similar seed structures. The previous studies mainly focus on the fleshy and completely covering arils and there is no detailed seed coat observations in Celastraceae so far. In this paper, we chose four Celastraceae plants, namely, Euonymus przewalskii and Maytenus hookeri with fleshy but partially covering arils, Microtropis thyrsiflora with sarcotesta, and Tripterygium wilfordii with neither aril nor sarcotesta, to reveal the development characteristics of the seed coat and arils and their similarities and differences. We observed the structure characteristics and chemical composition of seed coat and arils of the four species, in the different development stages from flower buds to mature fruit, with optical microscope, scanning electron microscopy, paraffin section and histochemistry staining techniques. In addition, we discussed their ecological functions and evolutionary implications.The main results were as follows:(1) The arils of Euonymus przewalskii and Maytenus hookeri both originated from the raphe and the outer integument cells at the micropylar after fertilization. Histochemical staining showed that the arils didn’t contain saccharides and proteins; (2) The seed of Microtropis thyrsiflora has red fleshy seed coat-sarcotesta which formed from the outer integument. The outer epidermic cells of sarcotesta are cutinized with saccharides and proteins. And there were vascular bundles in the inner parenchymatous layers; (3) The seed of Tripterygium wilfordii has neither the aril nor sarcotesta. The mature testa was thick and strongly cutinized with saccharides, and the endotesta was thin and contains protein.The comparative analysis showed that the fleshy aril and sarcotesta were significantly different in development and chemical compositions. However, they have similar structures which are similar to the fruit pulp. They are analogous structures and may be the important factors to attract animals to spread their seeds. The geological age of different taxa in Celastraceae showed that Microtropis occurred very early, indicating that the sarcotesta originated earlier and was a primitive trait. Then it evolved into different types of fleshy aril from earlier tiny and basally covering to later half and whole covering their seeds.