Abstract:【Objective】Sunburn occurs in the vineyard as the result of a combination of high-light intensities, high temperature and UV radiation. It is a physiological disease that widely occurs on grape berries, which is closely related to light, temperature, grape berry structure and physiological and biochemical changes within grapes. Grape sunburn causes brown spots on the surface of the fruit, and the fruit shrinks and wilts, which greatly reduces the quality and yield of the fruit and affects the economic income. Under the climatic conditions of high temperature and strong light in Guangxi, some grape varieties under the two-harvest-a-year cultivation mode have serious sunburn phenomenons. Here, in order to enrich the law and mechanism of grape sunburn and provide theoretical basis for prevention and control of sunburn in production. We explored the differences in sunburn sensitivity and sunburn process between summer and winter fruits of Jumeigui grape under two-harvest cultivation mode. 【Methods】From April to December 2022, in the grape planting base of Guangxi Zhencheng Agriculture Co., Ltd., Nanning, Guangxi, the berries of Jumeigui grape in different phenological periods of summer fruit and winter fruit under two-harvest-a-year cultivation were used as test materials. The sunburn of Jumeigui grapes in each phenological period was artificially induced by high-light and high-temperature heating lamp. The sunburn induction was stopped when the corresponding sunburn traits occurred on the berries. For each sampling phenological period, the threshold temperature on the surface of Jumeigui grapes was determined when the sunburn occurred and the incidence of sunburn berries was calculated after induction for a certain period of time. By comparing the incidence of sunburn berries and the threshold temperature of sunburn, the typical sunburn sensitive and insensitive phenological periods of Jumeigui were determined. In the typical sunburn sensitive and insensitive phenological periods of the summer and winter fruits of Jumeigui, the paraffin section method was used to make sections and hematoxylin staining was used to study the anatomical structure of the berries during different sunburn process, namely “Normal berries”, “Berries under sunburn induction for 6 min”, “Light sunburn berries” and “Severe sunburn berries”. Meanwhile, the content of total phenols in fruits was determined by Folin-Ciocalteau method, and the results were expressed as gallic acid equivalent (GAE), the content of N in fruit was determined by flow analyzer, and the content of P, K, Ca and Mg was determined by inductively coupled plasma emission spectrometer. The difference significance analysis and basic trait discussion were carried out for the above indicators. 【Results】The results of this study showed that the summer fruit and winter fruit of Jumeigui grape under two-harvest-a-year cultivation were consistent with the more sensitive phenological period of sunburn, both of which were E-L 31-32 period, and the threshold temperature of sunburn was about 41.1 °C. The incidence of sunburn berries in winter fruit was significantly lower than that in summer fruit after the same artificial sunburn induction time. In the anatomical structure of the fruit, compared with the sunburn sensitive period (E-L 31-32), the fruit in the sunburn insensitive period (E-L 36) had larger epidermal cells, smoother cuticle, more closely arranged cells, and less starch granules. Compared with the sunburn sensitive phenological period, the total phenol content of the fruit in the sunburn insensitive phenological period decreased significantly by 36.11%, and the contents of K, Ca and Mg of the fruit in the sunburn insensitive phenological period increased significantly by 22.97%, 51.58% and 56.22% respectively. Compared with summer fruits, the contents of K and Ca in winter fruits increased significantly by 23.66% and 31.68% respectively, while the contents of N and P decreased significantly by 364.14% and 24.99%. In addition, the contents of N, P, K, Ca and Mg in severely sunburned fruits were significantly higher than those in normal fruits during the sunburn process of artificially induced fruits. 【Conclusion】Under the two-harvest-a-year cultivation of Jumeigui, the sunburn sensitive period of both summer fruit and winter fruit was E-L 31-32, and the resistance of winter fruit to sunburn was better than that of summer fruit to a certain extent. The sensitivity of Jumeigui grape to sunburn at different phenological stages was also different, which was related to the characteristics of wax layer and epidermal cell wall in fruit anatomical structure and total phenol content in fruit. At the same time, sunburn sensitivity was also negatively correlated with K and Ca contents in fruit, and positively correlated with N and P contents. Therefore, in order to prevent the sunburn of Jumeigui grape in production, it can be considered to apply K and Ca fertilizers before its sunburn sensitive phenological period ( E-L 31-32 ).
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