Abstract:

The phenomenon of “more ovules and less seeds” at the fruit level is a common problem in seed production， in which the process of sexual reproduction is crucial. It is important to study the dynamics of ovule development and seed formation before and after fertilization to reveal the mechanism of the formation of seed yield. In this study， we explored ovule development before and after fertilization， the fertilization process， and the seed-setting pattern of Medicago sativa cv. Xinmu No. 4 by observations of paraffin sections stained with fluorescent dyes. We also conducted artificial controlled experiments to explore the factors restricting the formation of seed yield from the viewpoint of sexual reproduction. The main results of our study are as follows： 1） Before fertilization， there were no abnormalities in ovule development， and there was no location effect on the distribution of the vascular bundle on the ventral sutures. However， during fertilization， 19.6% of mature embryo sacs were coated with callus and could not be fertilized normally. 2） The pollen germination rate and pollen tube growth rate were higher after cross-pollination than after self-pollination. After entering the ovary， the pollen tubes preferentially reached the middle and base of the ovary， and fertilized the ovule in these positions. 3） Under natural conditions， on the second day after pollination， the middle and base parts of the ovule expanded， and on the third to sixth day after pollination， the proportion of the enlarged parts of the ovule had further increased. Subsequently， the basal part of the ovule became further enlarged and the middle to upper parts of the ovary began to abort. Finally， the seed setting pattern was formed with high ovule abortion rates at both ends of the pod and a low ovule abortion rate in the middle. 4） Resource restriction not only affected the number of ovules that formed before fertilization， but also increased the abortion rate of enlarged ovules after fertilization. In summary， many factors affect the development of ovules and the formation of seeds in M. sativa cv. Xinmu No. 4， including callose deposition， the fertilization sequence， resource allocation， and location effects. The low fertilization rate caused by callose deposition and high abortion rate of ovules after fertilization M. sativa cv. Xinmu No. 4 were identified as the main factors affecting seed yield.

Key words: Medicago sativa cv. Xinmu No.4, ovule development, seed formation, seed set pattern, influencing factors