Analysis of Leaf Heat Balance Affected by Operation of a Frost Protective Fan in Tea Fields

Frosts in agricultural fields often bring serious damage to crops and cause huge economic losses in the crop production. Many crops can enhance their freezing tolerance through a phenomenon known as cold acclimation (Thomashow, 1999). Under the climate change with global warming, however, warmer winters are prone to bring insufficient cold acclimation of the crops, and furthermore, strong radiative cooling and extremely cold weather in spring cause serious frost damage in the crop production. Tea plants (Camellia sinensis (L.) O. Kuntze) also severely suffer such frost damage, which cannot be prevented enough by the conventional methods of frost protection such as frost protective fan (wind machine), heater and sprinkler. For example, the frost in 2010 caused huge economic losses in the tea production; i.e., 247 million dollars in China (Lou et al., 2013) and 45 million dollars in Japan (Matsuo et al., 2010). In order to prevent frost damage, operation of the frost protection methods can be expected to improve heat balance between tea leaves and their environments, which prevents leaf temperature from dropping below a fatal level under severe frost conditions. Therefore, the effectiveness of the frost protection based on the leaf heat balance must be evaluated, and it is keenly desired to improve frost protection methods based on the quantitative evaluation of their effects. However, many recent studies related to the frost protection (Doesken et al., 1989; Ribeiro et al., 2006; Araki et al., 2008; Battany, 2012) have been focused on only changes in temperatures of the ambient air and/or leaf because of difficulties in assessing complicated convection between leaves and the ambient air under fluctuated environments. In this study, the new method for the quantitative analysis of the heat balance of a single tea leaf was developed for the quantitative evaluation of the thermal effect brought by the frost protection during the frost night. The reliable application of the newly analysis method of leaf heat balance was demonstrated in the tea field under the operation of a frost protective fan which has been used successfully in tea fields to protect tea plants during the frost night.