Investigation On Seedless Characters And Preliminary Studies On The Seedless Mechanism Of Litchi Cv. Seedless

Litchi cv. Seedless is highly appreciated by consumers for its big fruit, good quality and high seedless rate. Litchi cv. Seedless had been selected and used as a cultivar for more than 20 years, and was also introduced and planted in Guangdong and Guangxi province. In recent years, ‘Seedless’ litchi had been planted commercially in large scale in Hainan Island. ‘A4 seedless’ and ‘Nandao seedless’ are the main two lines of litchi cv. Seedless. In this study, we investigated the fruit and seed development characters among different production area and different lines. ‘A4 seedless’ was chosen as main material to uncover the parthenocarpy type. And the seedless mechanisms involved in pollen germination, development of embryo sac and Ca content in the ovule were investigated. The main results were as follows:1. Non-pollinated fruit(emasculation and bagging) of ‘A4 seedless’ was able to bear fruits, although average fruit-setting number and the fruit setting rate per panicle were slightly lower with no statistically significant difference compared to pollination with normal pollen and inactivated pollen.These results indicated that seedless fruit of ‘A4 seedless’ belonged to nutritional(spontaneous) parthenocarpy, and that fruit setting and development did not rely on pollination and fertilization but the stimulation of pollen might increase fruit setting percentage to a certain extent.2. Study on seedless characters of ‘Seedless’ litchi from different production area and different lines showed that seedless rate of ‘A4 seedless’ can be above 75%, and that the seedless rate of different production area displayed no obvious difference but different lines had remarkable difference in seedless rate. For example, the seedless rate of ‘Nandao seedless’ was only 37.7%, but ‘A4 seedless’ in the same production area was nearly twice that of ‘Nandao seedless’.3. The use of high performance liquid chromatography(HPLC) method enabled IV us to detect the soluble sugars, malic acid and vitamin C in the aril of different seed development types of ‘A4 seedless’. The aril of different seed development types had the same sugar composition, while sucrose and total soluble sugar contents in the aril of normal-seeded fruit were significantly lower than the aril of seedless fruit. It seems that the content of sucrose was inversely correlated with the size of seed. In addition, malic acid and vitamin C contents in the arils of different seed development types showed significant difference. The aril of aborted-seeded fruit had the highest content and was significantly higher than those in the seedless fruit.4. After comparing the fruit cracking incidence among the three different seed development fruit type, we found that chicken tongue fruit displayed much higher fruit cracking rate than normal seed and seedless fruits. The possible reason was discussed.5. The cause of seedless of ‘A4 seedless’ was not due to the pollen sterility. That pollen tube did not guide to the micropyle was likely to be a reason for the incomplete fertilization. Pollen viability of ‘A4 seedless’ was normal. Its germination rate was even higher than cultivars with normal seeds. Two hours to four hours after self-pollination, pollen grain successively underwent germination, penetrated into stigmas and growing towards ovules. In one to two days after pollination, pollen tubes went through the ovary wall but became twisted for getting lost, and in five days after pollination the pollen tube became obvious self-twisted and could not enter micropyle.6. Embryo sac sterility of ‘A4 seedless’ was the reason for failing to bear seeds normally. Compared with ‘Heiye’ cultivar, in two days before blooming embryo sac of ‘A4 seedless’ showed a delay in development and an obviously small embryo sac cavities. One day after blooming, when egg cell and polar nuclei can be find in embryo sac of ‘Heiye’, embryo sac of ‘A4 seedless’ was empty and began to collapse and deform. Developing abnormally embryo sac of ‘A4 seedless’ could not produce normal signal to guide pollen tube to grow directionally toward micropyle. This might be the reason for pollen tube getting lost. Therefore, without completing fertilization due to abnormal embryo sac was the main reason for seedless characteristic of ‘A4 seedless’.