Understanding The Structure And Development In An Apocarpous Plant, Sagittaria Trifolia(Alismataceae) By Geometric Morphometric Tools

Gynoecium is unique angiosperm reproductive organs, and its structure has a high diversity. In the early stages of angiosperms evolution, the structure of gynoecium has undergone a large change. Generally the primitive gynoecia are considered to have some apocarpy with incomplete closure; while the relatively evolved gynoecia are syncarpous, and the carpel within it is entirely closed. Previous studies have indicated that the original gynoecium has a lot of disadvantages compared to those subsequent gynoecia, and these disadvantages would affect the quantity and quality of offspring of angiosperms, as well as the dispersal of these offspring. But those original gynoecia have not been eliminated, which appear in many plant groups, especially in the basal angiosperms. Some species with the original gynoecium have adapted to the external environment very well, and their individuals distribute throughout the world. Why do these primitive gynoecia still have so adaptable? To answer this question, we take a basal monocots Sagittaria trifolia as the material to observe the structure of the gynoecium at different levels in the various stages, and through several modern geometric tools, the structure, development and the role in the reproductive process of S. trifolia were analyzed. We understand that the mophology and structure of the apocarpous gynoecium is very important to the reproduction of those basal angiosperms. Particular experiments and researches are carried out in the following three aspects:1, Carpel closure of S. trifolia. Through using semi-thin sections and Cryo-SEM techniques, we observed carefully the inside and outside structure of the carpel, and found the carpel closure of S. trifolia was not complete, which had different topological form from those carpel closure in previous studies. In the carpel with this special closure, pollen tube could enter into or leave from the carpel by two passages. This type of carpel closure occurred as a result of the fusion of the middle parts of two carpel margins in the development of the gynoecium.2, The extra-gynoecial pollen-tube growth and its impact on fruit set in S. trifolia. The pollen tube could grow outside the gynoecium on account of the incomplete closure of the capel. To test the impact of this extra-gynoecial pollen-tube growth on fruit set, we observed the pollen deposition in the gynoecium and the fruit set of S. trifolia in the conditions of natural pollination, and simulated the extra-gynoecial pollen-tube growth in the gynoecium of S. trifolia. We proved that in the gynoecium of S. trifolia, extra-gynoecial pollen-tube growth could help reduce the impact of uneven pollination on the reproductive process of S. trifolia, and this improvement was promoted significantly with the increasement of total amount of pollen load in the gynoecium. In addition, the results of simulation analysis also showed that the arrangement of carpels on the receptacle and the maximal elongated length of pollen tube could effect on the function of extra-gynoecial pollen-tube growth in the gynoecium.3, Fruit formation and seed germination of S. trifolia. The gynoecium of S. trifolia would change into aggregate fruits after pollination. The small achenes in the aggregate fruit could spread on the water when they leaving from the fruit stalk. Through the micro-observation and geometric morphometric analysis to the variation of the shape and structure of carpel during fruit development, we found that the deformation of the carpel was towards a certain direction, and this deformation made the carpel in mature gynoecium of the flower benefit to the pollination process, and achene in the fruit was able to be dispersed by water very well. In addition, we observe the seed germination and analyze the internal structure of the achene, we drew a conclusion:the ventral base became the most fragile region of achenes on account of existence of an opening at the carpel base, which allowed the hypocotyl of the seed to break the pericarp.Combining with above results, we understand the entire development process of the gynoecium from the floral primordia to mature fruits, the morphology and structure of the apocarpous gynoecium has been proved important in the generation and propagation of the offspring in basal angiosperms, at least in Sagittaria. In addition, through the application of several modern geometric tools, we found that these tools can help us more effectively extract more morphological information from the reproductive organs of the plant, and help us understand the relationship between the reproductive structure and the reproductive process.