Jujube (Ziziphus jujuba Mill.), a member of the Rhamnaceae family, a native fruit tree of China, which has been cultivated for thousands of years [1-3]. It is one of the world’s major fruit crops grown in India, Russia, America, the Middle East, southern Europe, South Korea, Japan, and China [4-6]. Z. jujuba tree is characterized with barren-tolerance, wide adaptability. Besides, the high amounts of vitamin A, C and B complexes, and minerals in jujube fruits [7]. It is also a very important source of income for farmers in mountainous regions [8, 9].

Unlike apples or peaches, jujubes do not have big, showy flowers [10]. Z. jujuba floral buds rapidly differentiate and bloom in one growing season, which is unique and different from most fruit trees [8]. Floral bud differentiation is critical to the production of Z. jujuba because it directly influences the number of floral buds and fruits, and its subsequent yield [11].

Z. jujuba ‘Jinsi No.4’ was originated in the Yuncheng County, ShanXi Province, China. As a high-value, well known, and first grade product and rare species, the fruit of Z. jujuba has a sugar content of 22.75%, an acid content of 0.368%, soluble solids content of 26.5%, Vc (Vitamin C) content of 292.5 mg/100 g, and the edible percentage is as high as 97%. Since 1980s, Z. jujuba ‘Jinsi No.4’ has been gradually introduced into over 10 provinces and cities across China. Z. jujuba was highly valued because of its early and large fruit with thin peel, thick pulp, sweet taste, as well as special flavor [12]. In our previous study, Zhiqiang Han et al. [13] found that ‘Jinsi No.4’ displayed a large number of flowers, but with low yields in the productivity. Deyi Yuan et al. [14] also indicated the highest percentage of the flowers fallen, which was 76.89% in Z. jujuba ‘Hunanjidanzao’. Feng Zou et al. [12] described male and female gametophytes in Z. Jujuba ‘Jinsi No.4’. Studies on flower differentiation processes have both a basic and applied interest. Therefore, understanding the floral initiation and differentiation of Z. jujuba is crucial for promoting yield. However, little is known regarding flower bud differentiation of jujube ‘Jinsi No.4’. Thus, the main objective of this paper was to determine the time of flower bud differentiation and developmental stages of the flower organs in ‘Jinsi No.4’ jujube in Hunan province of Southern China.

The experiment was carried out in the Z. jujuba germplasm resource pool, Hengyang, Hunan Province, China. The experimental plot was located on a hill with red soil located at longitude of eastern 112°18′54″, latitude of northern 26°54′48″, and altitude of 70 m [12].

Three plants of eight year-old Z. jujuba ‘Jinsi No.4’ with normal flowering and fruiting were used. From April to June of 2010 and 2011, three fruit-bearing shoots of jujube were cut every day from the middle section of the tree crown while fruiting sections were germinated until the shoots of jujube were visible, and cutting three per day when the length of fruit bearing shoots of jujube was greater than 3 cm. The third and fourth nodes of floral buds were picked as the samples. In order to record the detail of the progresses of the flower development, ten of the materials were observed for external morphological features and photographed with a SZX-16 stereomicroscope (Olympus, Japan). The other ten of the samples were fixed in FAA solution (5% formalin, 5% acetic acid, 90% alcohol, by volume), and stored at 4 °C [15]. Paraffin sectioning followed the methods introduced by Feng Zou et al. [16], dehydrated through an ethyl-alcohol series, embedded in paraffin with a 58-60 °C melting point, and sectioned at a thickness of 10 µm by the microtome. The sections were stained with hematoxylin-eosin Y. The sections were observed and photographed by a BX-51 microscope (Olympus, Tokyo, Japan) [3].

The floral differentiation of fruit trees involves the transformation of leaf buds on branches into a floral bud [17]. Bud differentiation was observed to be more complex than the differentiation of vegetative organs [18, 19]. Many studies of Z. jujuba floral differentiation have been conducted. Qu Ze-zhou et al. [8] observed the biological features of Z. jujuba, dividing floral differentiation of Z. jujuba cv. Mayazao into six stages. Chen Yi-jin [20] also reported floral differentiation of Z. jujuba cv. Huizao. Meng Yu-ping et al. [19] and Wang Li-hong et al. [21] divided the floral differentiation of Z. jujuba cv. Hupingzao into five stages, including the floral primordium differentiation, sepal differentiation, petal differentiation, stamen differentiation, and pistil differentiation. Noemi and Bert [22] described flower development of Z. jujuba mauritiana in detail from floral primordium differentiation to blossom, and divided the process into eleven stages by the SEM and paraffin sections. Michal Meir et al. [23] reported the developmental stages of fruit bearing shoots of Z. Jujuba cv. Ben-Li. According to our observations, the floral bud differentiation process of ‘Jinsi No.4’ could be divided into six stages, i.e., pre-differentiation, initial differentiation, sepal differentiation, petal differentiation, stamen differentiation, and pistil differentiation periods. Similar results were reported in ‘Mayazao’ [8], ‘Huizao’ [20], and ‘Xinlezao’ [11]. During the flower development process, we divided flower development of ‘Jinsi No.4’ into seven stages, i.e., alabastrum, alabastrum break, sepal flattening, petal flattening, stamen flattening, filament withering, and ovule swelling. Different from ‘Jinsi No.4’ occurred during petal flattening stage, the stigma bi-split, floral plate nectar secretion, and pollination of Z. jujuba cv. Qiyuexian occurred during sepal flattening stage [24].

Fig. (2). Flower development stages of ‘Jinsi No.4’ jujube. A-C, Jujube flower showing the bud period with f0, f1 and f2. D-E, Jujube flower showing the period of bud split with five sepals and petals. F-G, Jujube flower showing the period of sepal spread with one stigma, five petals located between the sepals and five stamens attached to the edge of the nectary disk. H, Jujube flower showing the period of petal spread with one stigma, and five filament and anthers. I, Jujube flower showing the period of stamen spread. J, Jujube flower showing the period of filament wilting. K, Jujube flower showing the period of ovary inflated. L, Jujube flower showing the flower dysplasia. an: anther; s: sepal; sti: stigma; p:petal; st: stamen; ov: ovary; fi: filament; fd: flower disc; fs: flower stalk. f0, f1 and f2 represent 0, 1 and 2 level flowers of inflorescence, respectively.

In our study, we observed that floral buds of ‘Jinsi No.4’ differentiated the current year in Hunan Province, Southern China. The time of a single flower differentiation was short, just 7 days. Niu Hui-ling et al. [24] reported that floral bud differentiation of Z. jujuba cv. Qiyuexian needed 11 days in Shanxi province of Northern China. This difference may be explained by differences in varieties or environmental conditions.

To our knowledge, the mechanism of floral bud differentiation has been concerned with overcoming alternate bearing, improving fruit quality, and advancing the fruiting of young trees. Jujube is known to produce erratic fruits which lead to lower yield [7]. There are a number of reasons for erratic fruit production in jujube including: flower organ abortion [3, 14], poor pollination and fertilization [25-27] and insufficient nutrients [11, 19, 21]. Feng Zou et al. [3] studied floral biology of jujube and found 25.80% aborted ovules, which were likely the cause of the low seed set in Jujube ‘Lizao’ during reproductive organ development. As we know, quality of the reproductive organs depends on the factors and conditions for flower bud formation, which in turn influences the quantity and quality of fruit production. Z. jujuba floral bud differentiated with the growth of shoots and branches, with a long period of differentiation and blossom, which also consumed many nutrients. Floral bud differentiation of Jujube was closely linked to nutrient content [11, 19, 21]. In our experiment, we observed that some floral organs of ‘Jinsi No.4’ had poor development, which may attribute to low yields. Therefore, future research is needed to determine whether the dysplastic flowers were caused by nutrients limitations. By means of monitoring the development and quality of the flower buds, it is possible to predict the potential yield and provide necessary agro-technical measures in the year preceding flowering. In the process of bud differentiation of ‘Jinsi No.4’, foliar spraying should be provided in April to avoid malnutrition.

Floral bud differentiation of ‘Jinsi No.4’ can be divided into six stages, i.e., pre-differentiation, initial differentiation, sepal differentiation, petal differentiation, stamen differentiation, and pistil differentiation. Flower development experienced seven stages, i.e., alabastrum, alabastrum break, sepal flattening, petal flattening, stamen flattening, filament withering, and ovule swelling. We observed that some of the dysplasia of floral organs in Z. jujuba ‘Jinsi No.4’, suggesting that the dysplasia of floral organs may be one of the main reasons for low yields. In the process of bud differentiation of ‘Jinsi No.4’, we should supply sufficient nutrients to avoid malnutrition in the April. Our results provided the details of flower bud development in ‘Jinsi No.4’ in Hunan, which will contribute to its production and management.