Field observations of clear summer day fruit surface temperatures in apple and satsuma mandarin

　To enhance the study of sunburn and fruit surface temperature (FST) prediction in domestic fruit trees, we examined the spatiotemporal characteristics of FST and the predominant factors influencing FST on clear summer days by measuring the FST of detached apple and satsuma mandarin fruits, and monitoring ambient summer meteorological condition. With respect to the diurnal variation in FST, we found that the maximum FST of fruit (FST_max) occurred on the surface facing southwest to west, approximately 2 h post-meridian transit time, thereby indicating that compared with other orientations, the surface of fruit facing these directions is at higher risk of sunburn. Our observations also revealed that under clear conditions, the FST_max of apple and satsuma mandarin fruits can be at least 15°C above the daily maximum air temperature (AT_max). We also established that in addition to AT_max, fruit size plays a key role in determining FST. Furthermore, comparative black globes measurements also revealed that under clear conditions, a large black globe warmed to a greater extent than a similar smaller globe. Our findings in this study indicate that FST appears to be influenced by both fruit size and meteorological factors, using the data of which will enable precise FST predictions.