Study on the biological characteristics of cross-pollination in kumquat

Abstract：【Objective】Pollen viability and stigma pollinability are important indicators for assessing the quality of parental gametes. Understanding the changing pattern of kumquat pollen viability and stigma pollination rate and finding the best pollination period can effectively improve the quality and success rate of hybrid breeding. Simultaneously, the growth of kumquat pollen tubes under self-pollination and heterogametic pollination conditions was studied to find out the biological characteristics and mechanism of kumquat heterogametic pollination to provide a theoretical basis for kumquat promiscuity. By studying the fruiting rate under different treatment conditions, we can provide some practical basis for kumquat promiscuity.【Methods】Embryo types of 23 Fortunella germplasm resources were statistically analyzed. Materials with different embryo numbers were selected to analyze pollen viability and stigma receptivity at four different stages (Small bud stage, Big bud stage, Early florescence stage and Full bloom stage) by using TTC staining and benzidine- peroxidase methods, respectively. Subsequently, three treatments, including emasculation without pollination, artificial self-pollination, and hybrid pollination were conducted. The growth of pollen tubes was compared under both artificial self-pollination and hybrid pollination. Finally, fruit setting rates after three treatments were statistically analyzed and compared.【Results】Fortunella japonica (Thunb) Swingle had the fewest embryos, making them suitable as female parents, followed by F. hindsii (Champ) Swingle, F. crassifolia Swingle and F. margarita (Lour.) Swingle. F. obovata Tanaka had the most embryos. Pollen viability of all tested Kumquat materials at all four stages followed the same trend of“increasing first and then decreasing”andthe peak reached at the early florescence stage. The highest pollen viability was found in the Lanshanjingan (F. crassifolia Swingle), which was significantly different from other species, and Liuyangjingan CQ (F. crassifolia Swingle) had the lowest pollen viability. Stigma receptivity followed an increasingdecreasing trend at all four stages, with maximum receptivity observed at the early florescence stage. Lanshanjingan exhibited the highest pistil receptivity, while Liuyangjingan CQ showed the lowest. There were no significant differences in pistil receptivity among other kumquat varieties, and therefore they can all serve as female parents for hybridization. Analysis of pollen activity and stigma receptivity revealed that kumquats exhibited synchronous maturation of male and female reproductive organs, reaching peak values at the early florescence stage. Pollen tubes of both self-pollination and hybridization treatments began germination approximately 2 hours after pollination. Hybrid pollen tubes showed a faster elongation rate compared with self-pollination pollen tubes. After 12 to 24 hours of pollination, most hybrid pollen tubes reached the base of the style as a result of fast elongation, whereas some selfpollination pollen tubes had already ceased elongation at 2/3 of the style length. After 48 hours of pollination, all hybrid pollen tubes and a few self- pollination pollen tubes penetrated the base of the style and entered the ovary. Under the condition of emasculation without pollination, all 7 kumquat germplasm obtained fruits, and Lanshanjingan had the highest fruiting rate and showed significant difference from other kumquat germplasm. The kumquat flowers can develop into fruits without pollination, demonstrating the characteristic of parthenocarpy. Under conditions of artificial self-pollination, Lanshanjingan had the highest fruit setting rate, showing significant differences from other kumquat germplasms. Under the condition of hybridization pollination treatment, the fruiting rate of Liuyangjingan CQ was the highest, followed by Lanshanjingan. However, there was no significant difference in fruit set rate between these two kumquat varieties, but there was a significant difference between them and among the other 5 kumquat varieties. Except for Ningboluowen CS [F. japonica (Thunb) Swingle], the fruit setting rates of the other 6 kumquat from cross- pollination treatments were higher than those from artificial self-pollination; among the 7 kumquat varieties, only Ningbojindan (F. crassifolia Swingle) showed no significant difference in fruit setting rates under both artificial self- pollination and cross- pollination treatments. The difference in fruit set rate indicated that the cross-compatibility of kumquats was stronger than self- compatibility, which was more conducive to the hybrid breeding of kumquats. With the same hybrid paternal parent, the higher the stigma pollinability of the female parent, the higher the fruit setting rate. The fruit setting rate of intraspecific hybridization was higher than that of interspecific hybridization, and there was a significant difference.【Conclusion】The female parent with single embryo should be selected in kumquat hybridization; kumquat belonged to hermaphroditism, and the best time for pollen collection and pollination was at the early florescence stage; kumquat cross- compatibility was stronger than self-compatibility, which was beneficial to kumquat cross breeding.