Analysis Of The Main Factors And Threshold Temperature On The Sunburn In Vitis C.V. Red Globe

The experiments were conducted in an 8 years old grape orchard, grown with Vitis cultivar'Red Globe'to study the effect of sunlight, air temperature, wind speed and other factors on grape sunburn. The results showed that the higher temperature on fruit surface was the direct factor inducing grape sunburn. The threshold temperature induced sunburn was from 42.0℃to 42.8℃(average temperature was 42.3℃) in fruit fast growth stage. The maximum fruit surface temperature appeared around 14:00 pm. It would reach to maximum after 12 minutes if the fruits were suddenly exposed to the sunlight from shadiness. The fierce sunlight increased both air temperature and fruit surface temperature. Therefore, strong sunlight was the main factor for producing grape sunburn. The illumination per 10000 Lx could add about 1℃on fruit surface at noon of a sunshiny and windless day in summer. The fruit surface temperature was significantly higher on the days without wind than when the wind speed over 1.5m/s. When wind speed was over 3.0m/s, the fruit surface temperature dropped greatly within 6 minutes. The higher relative humidity increased grape sunburn under the same light and temperature conditions. So, the grape berries would be easier sunburned under the weather conditions of strong light, windless and high air temperature. It would get worse when the fruit is close to the ground or grown in sandy soil without cover plants, as well as exposed indirectly to the sunlight from 10:00 to 15:00 in summer days.