Organogenic Regeneration of an Elite Cultivar of Chinese Jujube (Zizyphus jujuba Mill)
Abstract
An efficient and relatively simple regeneration system was developed for an elite cultivar of Chinese Jujube, a perennial tree, by culturing young twig segments as explants from 8-15 year old trees. The twig segments were disinfected by submerging them in 1% sodium hypochlorite (NaOCl) for 15 min with 3 min vacuum. Calli developed from both ends of the twig segments on half-strength MS medium supplemented with sucrose and BA or BA and NAA in combination. The frequency of shoot formation from calli was higher than 80% when the explants were placed on the half - strength MS medium supplemented with BA (2.581 μM) and NAA (2.685 μM). Roots were produced from adventitious buds for 90% of the regenerated shoots when they were placed on the MS medium supplemented with 4.920 μM IBA and 5.708 μM IAA. After transplanting to soil, 82% of the regenerated seedlings survived when they were covered with glass containers to maintain humidity. The results suggest that Chinese jujube can be reproduced and multiplied using organogenesis with the appropriate explant and culture medium.
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Introduction
Chinese jujube (Zizyphus jujuba Mill) is a fruit tree grown in vast areas of China because of its wide adaptation to various climates and high degree of tolerance to barren soil (Mengjun, 2003). The species are rich in sugars and vitamins, and this fact has made Ziziphus species important fruit trees for many centuries, including China, India, Persia, Armenia, the Mediterranean and in the dry regions of the southwestern US (Sweet, 1985; Reich, 1991; Gilman and Watson, 1994; Hache and Xu, 1995; Outlaw et al., 2002).
The fruit has been used medicinally for millennia by many cultures. The total jujube growing area in China is about 66.7 million ha with annual fruit production of 780,000 tons (Qu and Wang, 1993). However, jujube production is limited by damages caused by the peach fruit borer (Carpasina niponesis Wds) and the witch-broom disease incited by Mycoplasma - like organisms. Evidence indicates that approximately 7,500,000 kg of jujube are lost annually in a production base in the Taihang Mountain area in China owing to infestation by the pest (Liu et al., 1997). The fruit is rich in vitamin C (500 mg/g), flavone, and 18 amino acids (Chen et al., 1996; Romero Rodriguez et al., 1992; Arndt et al., 2001). The commercial importance of jujube has increased in recent years because of the recognition of the nutritional value of the fruit. However, some nutrients are degraded during the drying and processing of the fruit for market. Consequently, methodology needs to be developed to increase shelf life of fresh jujube to bypass the drying and processing required for marketing.
With the advent of biotechnology, incorporation of desirable genes from alien species using gene transfer techniques has resulted in the development of Roundup - Ready soybean, Bt corn and Bt cotton (Engel et al., 2002). Likewise, transferring insect and disease resistance genes as well as genes that can prolong fruit shelf life into superior cultivars of jujube could increase quality and expand production of this fruit crop.
Most jujube cultivars produce fruit without cross-pollination, but seeds from such self-pollination are usually not viable (Reich, 1991). Most Chinese cultivars are vegetatively propagated by grafting or budding onto a thorny rootstalk which produces many suckers from the roots. There is evidence that jujube cultivars will root on hard or soft wood cuttings. However, successes have been limited to date with this process of plant reproduction. Micropropagation has been practiced in the past to multiply the Chinese jujube in large quantities that is more efficient than conventional multiplication by soft cutting (Cheong and Kim, 1984; Cheong et al., 1987; Mitrofanova et al., 1994; Gu et al., 2008; Soliman and Ghada Hegazi, 2013; Shi et al., 2015). To date, successful plant regeneration has been reported in Z. jujuba from stem (Mathur et al., 1995) and leaf (Gu et al., 2005). However, the regeneration frequency was low. Thus an alternative approach must be developed to improve efficiency of regeneration so that it could be applied for large - scale commercial production.
The purpose of this study was to test the feasibility of using young twigs from mature trees growing in the field as explants for developing a simple and reliable regeneration system for Chinese jujube.
Conclusion
The current study describes methodology which provides an 80% efficiency of plantlet regeneration and survival for Chinese jujube. Young twigs appeared to be a much better source of explants than the leaf discs reported before (Chen et al., 1996). BA promoted bud differentiation, and the additions of IBA and IAA were essential for root induction and shoot elongation. This was the first report using young twigs as explants for Chinese jujube fruit tress, and the improved culture system is relatively simple but effective in producing high frequencies of calli that in turn regenerate well.
