Preliminary Studies On Fruit Set And Fruit-Shaped Development Of Fingered Citron

Citrus medica L.var.sarcodactylis Swingle, Rutaceae, Citrus, is a variant of Citrus medica without seeds. Because of peculiar finger-shaped fruit, fingered citron has highly ornamental value and is popular among people of all ages. Due to the domestic and foreign researches are few about fingered citron, there are lack of basic theoretical research on developments of seedless and fruit shape. In this paper, we use three years old C. medica cv. Qingpi and citron with flowers (buds) or small fruit as test materials, researching on the manner of fruit set and the differentiation processes of flower buds, to reveal the formation of fruit and the morphogenesis mechanism of fruit shape in fingered citron.1、By emasculation and bagging of fingered citrons before flowering, we compared the emasculated fruit with the natural control and found the fruit growth (transverse and longitudinal diameter multiplication) did not change significantly (p>0.05) between them, showing that fingered citron without pollination or other the stimulation can fructify, which is natural parthenocarpy. Using real-time PCR technology to evaluate the IAA9gene expression levels in fingered citron and citron fruit around their flowering, we discovered the target gene performing down regulation significantly in the bottom of fingered citron on3days post anthesis(3DPA), implying that high content of endogenous auxin in the fingered citron may cause its automously fruit setting. Moreover, the results indicated the fertilization of citrus occurs in about3DPA The IAA9gene exhibited a significantly different expression level between the finger part and bottom part of fingered citron on3DPA (p<0.01), while no much difference on other time spots. It reflects the bottom and the finger of fingered citron may come from different organizations, the ovary and style respectively.2、We collected the fingered citron pollen on the blossom day and put that into a in-vitro-culture medium, which contained5%sucrose,0.05%boric acid,0.01%calcium nitrate, and finally the maximum germination rate was30.57%. This result indicated a normal staminal fertility, meaning the seedless fruit has nothing to do with stamens. The investigation of the receptivity of pinky (style) in fingered citron fruit on the flowering day and after Id,2d,3d,4d,5d,6d showed the stigma (finger) within a few days after flowering had normal enzyme activity. Selected citron as control, we compared the germination of pollen in the stigma of fingered citron by artificial pollination on day and after Id,2d,3d,4d,5d,6d. Pollen tubes of citron can grow into ovary on3or4DPA, The amount of pollens on the style of fingered citron were less than that in citron, and only some pollens germinated and extended. No pollen tubes grew into the base (ovary) as time going in fingered citron, which indicated fingered citron was unfertilization. Through the observation of paraffin crosscut of fingered citron and citron fruit on flowering days, we found fingered citron ovules were degenerated or abnormal, probably resulting the abnormal fertilization and parthenocarpy.3、Utilizing polar auxin transport inhibitor (NPA) to treat the fruit of fingered citron to illustrates the correlation between auxin (IAA) and fruit shaping in fingered citron. Studies have shown NPA had a certain effect on the pattern fromation of the fruit in fingered citron, and the effect negatively correlated with the fruit maturity. When the fingered citron rippened, the synthesis activity of IAA on the top was stronger than that at the bottom, consistent with the results of fruit setting rate. NPA or IAA application either alone can make fingers of fingered citron grow uneven, indicating that IAA plays an important role in the pattern formation of fruit shape in fingered citron.4、Paraffin technology was used in observing the continuous differentiation from bud to small fruit of fingered citron. After safranin and fast green double staining, sections were observed that flower buds initiated from the primordia differentiation and then grew to inflorescences. The top flowers of the inflorescence center would grow into fruits. This differentiation process is similar with most plants, which can be divided into sepals period, petal period, stamen period and pistil period. After the staminal primordium development, the pistil centre gradually bulged, then the top differentiates to be little fingers, which finally grew into the early fruit. On the basis of the analysis of slice, immunofluorescence technique was take to describe the distribution of endogenous auxin in fingered citron during fruit development. We found that the regions of IAA distribution coincided with the finger edge. Therefore, during the fruit development in fingered citron, its peculiar shape have a close relationship to the IAA distribution.All in all, although the degenerated or the abnormality of ovule resulted the fingered citron unfertilization, the parthenocarpy fruit could set well and be seedless with its endogenesis auxin. With the fruit development in fingered citron, the fruit shaped gradually to be finger-like, which might be related to the distribution of endogenous IAA.