Abstract

Jujube (Ziziphus jujuba Mill.) is an economically important crop in South Khorasan Province of Iran, making breeding research essential for its improvement. This study aimed to investigate the flowering biology of selected Iranian and Chinese jujube genotypes, identify barriers to controlled hybridization, and determine suitable maternal parents for breeding programs. The reproductive traits of five cultivars and one promising genotype from the jujube collection at the South Khorasan Agricultural and Natural Resources Research and Education Center were evaluated, with particular focus on pollination and crossing. Results revealed significant differences (P ≤ 0.01) among the studied cultivars and genotypes for most traits. Under open pollination, fruit set rates varied from 0% to 31.64%, with the highest percentage observed in the 'Li' cultivar. However, 'Li' failed to produce any viable embryos. In jujube fruit, there are usually a high percentage of empty seeds or seeds with damaged or aborted embryos. Among the tested samples, 'Lang' and Genotype 1 exhibited the highest rates of healthy embryo formation (22.9% and 16.51%, respectively). Controlled pollination experiments demonstrated the significant influence of the paternal parent on fruit set percentage. None of the cultivars ('Siojan', 'Majan', 'Lang', 'Li') or Genotype 1 were capable of self-fertilization at the single-flower level. This finding indicates that when these genotypes are used as maternal parents in breeding programs, emasculation is unnecessary for controlled crosses. Effective pollination control can be achieved simply by bagging flowers to prevent accidental pollen transfer by insect visits. Based on fruit set and embryo formation rates, 'Lang' and Genotype 1 are recommended as optimal maternal parents for jujube hybridization programs.





Extended Abstract



Introduction

Jujube (Ziziphus jujuba Mill.) is considered one of the strategic horticultural crops in South Khorasan Province, Iran, due to its high tolerance to environmental stresses and considerable medicinal and economic value. Despite its importance, breeding and hybridization in jujube face serious challenges, including high embryo abortion rates, low fruit set percentage, and the small size of its flowers. The present study aimed to evaluate reproductive traits, phenology, pollination behavior, and the rates of fruitset and embryo formation in Iranian and Chinese jujube cultivars in order to identify suitable maternal parents for use in hybridization breeding programs.

Materials and Methods

This study was conducted from 2021 to 2022 on five jujube cultivars and one genotype, including the Chinese cultivars 'Lang' and 'Li', the Iranian cultivars 'Siojan' and 'Mazhan', and Genotype 1 from the jujube collection of the Agricultural and Natural Resources Research and Education Center of South Khorasan. Traits such as flowering time, pollination type, fruit set percentage and embryo formation rate under various pollination treatments, were investigated. The experiment was laid out in a completely randomized design as part of a Master’s thesis at the University of Tehran. Data analysis was performed using Excel and SPSS softwares.



Results and Discussion

Significant differences (p < 0.01) were observed among the jujube samples for most evaluated traits. Jujube flowers exhibited a specialized pollination mechanism known as synchronous protandrous dichogamy, which prevents self-pollination. In this mechanism, anthers release pollen before the stigma becomes receptive, rendering each flower incapable of self- pollination. However, other flowers that have opened in the past days can receive the pollen grains of new flowers and thus there is a possibility of self-pollination on a same tree. Our findings confirmed that each flower alone on all studied genotypes was functionally self-incompatible due to protandry. Consequently, emasculation is unnecessary for controlled cross-pollination in these genotypes, but isolation is necessary for insects visit prevention. In jujube fruit, there are usually a high percentage of empty seeds or seeds with damaged or aborted embryos. Under open pollination, fruit set varied from 0 to 31.64%, with the highest rate observed in the 'Li' cultivar. However, this cultivar failed to produce any viable embryos and was deemed unsuitable for breeding purposes. In contrast, the 'Lang' cultivar and Genotype 1 showed high rates of viable embryo formation (22.9% and 16.51%, respectively) and were identified as promising maternal parents for hybridization. Additionally, results from controlled pollination experiments demonstrated that the paternal parent also significantly influenced fruit set percentage.



Conclusion

This study demonstrated that by selecting suitable protandrous maternal parents with high fruitset and healthy embryo formation rates, such as Genotype 1 and the 'Lang', jujube breeding and hybridization can be effectively performed without the need for emasculation. These findings facilitate the development of hybridization in jujube and open new pathways for its breeding.