Stigma morphological changes and early embryogenesis development of Yingqiuhong Chinese jujube (Ziziphus jujuba Mill.)

Abstract:【Objective】Fruit set in Chinese Jujube (Zizyphus jujuba Mill.) is usually only about 1% of the total number of flowers because of serious flower and fruit drop. Moreover, severe embryo abortion makes it difficult to obtain seedlings and select new varieties to meet the market demand. The fruit of Chinese jujube has various origins, from parthenocarpy, closed pollination or open pollination, and the fruit set can be improved by providing appropriate pollinizer trees. The complexity and diversity of the fruit-bearing and fertility characteristics make Chinese jujube breeding very difficult. Therefore, it is necessary to study the reproductive characteristics of Chinese Jujube in order to improve the breeding efficiency and overcome the problems of severe flower and fruit drop and early embryo abortion. At present, most of studies focus on different artificial hybrids, while there are few studies on the different fruiting characteristics of Chinese Jujube. Embryo observation of cultivars with parthenocarpy and characteristics of pollen tube growth of self-incompatible cultivars were previously reported. Yingqiuhong is an early-maturing table Jujube variety, which has self-fruiting characteristics and serious problem of flower and fruit drop. This study provides reference for overcoming the problems of severe flower and fruit drop and early embryo abortion in Chinese Jujube.【Methods】The flowering dynamics of Ying-qiuhong jujube were observed in the field, and pollen germination, pollination, fertilization, early em-bryonic development and stigma changes were observed microscopically.【Results】The results showed that the Yingqiuhong jujube flower was a day-blooming type, and mucilage was observed on the stigma surface during opening period, indicating that Yingqiuhong is a wet-stigma jujube. The shape of the style changed as the flowering progressed, so were the nectar volume (increased first and then decreased) and honey plate color (from yellow to light yellow, and finally white). The opening time for a single flower was relatively short (lasting about 26 h). The best pollination period was from petal expan-sion to stamen extension stage, which lasted only 4.5 h. Especially in the stamen extension stage, a large amount of secretion covered the stigma surface, which provided substrate for pollen germination. Intensive pollen germination was observed, and the germination rate was higher than that in the petal ex-pansion stage. The stigma had the strongest adhesion to pollen and the strongest pollination ability during petal expansion period and stamen spreading period, and a small amount of pollen germinated instigma 2 h after pollination. The pollen tubes continued to grow along the stylopodium 8 h after pollina-tion and a few pollen tubes grew to 1/3 of the style, while most pollen tubes grew to 2/3 of the style 36 h after pollination, and reached the base of the style 48 h after pollination. No callosal block was observed at the end of the style. The pollen tube stopped growing when it reached the synergids and re-leased sperms in the area between the egg and the central cell. The released spermatids moved to the egg cell and the central cell, and the polar nuclei in the center of the embryo sac began to fuse to form secondary nuclei. One sperm cell fused with the secondary nucleus of the embryo sac to form the prima-ry endosperm nucleus 2 d after pollination. The other sperm cell fused with the egg cell to form the zy-gote 3 d after pollination; the zygote began its first division 5 d after pollination, which was transverse and formed two cells, and the basal cell was near the micropyle and the apical cell was at the distal micropyle. The basal cell divided laterally to form a three-cell proembryo, and the apical cell of the three cells divided longitudinally to form two unequal cells, then a six-cell proembryo. However, on the 6th day after pollination, the integument began to atrophy and the development of proembryo stopped. The ovary became yellow, stopped expansion, and then fell off. The normal developing ovules showed obvious expansion and growth. The proembryo developed into globular embryo 12 d after pollination.【Conclusion】Stigma morphological changes, pollination, fertilization and early embryogenesis development of Yingqiuhong Chinese jujube were revealed, and the dehiscence of integuments caused the early abor-tion of Jujube on the 6th day after pollination. The abortion of the early embryo of Yingqiuhong jujube may be related to the severe flower and fruit drop.