A simple model for predicting sunburn on Satsuma mandarin fruit

Abstract

Fruit sunburn, which is caused by high temperatures and strong sunlight, will increase under climate change due to warmer temperatures. In this study, to facilitate effective sunburn management, the occurrence of sunburn was predicted. The study included an investigation of sunburn-inducing conditions and the estimation of fruit skin temperatures in early maturing Satsuma mandarin. The fruit skin temperature fluctuated with the ambient air temperature and solar radiation, and the estimation function developed for these two factors by multiple regression was significant, with an R2 of 0.884 (P < 0.001). A specific sunburn-inducing air temperature could not be established due to the constant changes in solar radiation. At the maximum solar radiation, fruit sunburn was induced by air temperatures of 32–35 °C under strong sunlight between 11 am and 3 pm. The risk of sunburn occurrence was classified into 4 levels, normal, caution, alert, and serious, by a discriminant analysis that reflected the fruit skin temperatures over 5 days. Finally, the change in the number of sunburn risk days in recent decades was traced by applying the sunburn occurrence prediction method and risk classification to meteorological data from the last 30 years. The number of sunburn risk days (caution level and higher) in the 2010s (14 days) was significantly higher than that in previous decades (less than 6 days). Specifically, the ratio of the number of alert and serious days to the total number of sunburn risk days clearly increased over time, from 5.8% in the 1990s to 15.0% in the 2000s and 19.2% in the 2010s.