Decision support strategy for preventing sunburn: Precise prediction of sunburn occurrence reflecting both biotic and abiotic factors in late-maturing citrus orchards

Abstract

This study emphasized not only the basic concept that the sunburn of fruit was caused by fruit skin temperature rising above its tolerable threshold but also that the threshold was variable, namely, fruit became more vulnerable to sunburn on its own as the growth period progressed. These main results were derived from two major functions: first, multiple linear regression to estimate the fruit skin temperature utilizing solar radiation and ambient air temperature, and second, nonlinear regression with a sigmoid function, which represents the variability in the fruit skin temperature threshold inducing sunburn. The specific major results are as follows. Both individual and multiple regressions showed that the increase in fruit skin temperature was more affected by solar radiation than by ambient air temperature. Considering the entire period from July to September, sunburn was related to fruit skin temperatures above 46 °C. However, the specific investigation dividing the entire period into 10-day periods indicated that the fruit skin temperature threshold for inducing sunburn decreased over time. In the discriminant analysis for predicting the occurrence of sunburned fruits, the prediction accuracy based on the variable threshold was 81.9%, which was higher than 67.0% based on the fixed threshold of 46 °C. The reference ambient air temperature for the occurrence of sunburn decreased with seasonal progress in the field from higher than 35 °C in early August to 28 °C in mid-September and in the greenhouse from higher than 39 °C to 31.7 °C. The variability of thresholds related to the internal tolerance of fruit for sunburn presented the reason why sunburned fruits could occur or increase despite the weakening of abiotic factors inducing sunburn, such as solar radiation and ambient air temperature, as the season progresses from summer to autumn.