To clarify optimum temperature range for the growth of Wasabi (Eutrema japonicum (Miq.) Koidz.) seedlings, the seedlings of a cultivar Izuma and a selected strain Shizuoka A were grown in various temperature conditions. In experiment 1, daytime /nighttime temperatures were kept 15°C/15°C, 20°C/15°C, 25°C/20°C, 30°C/25°C and 30°C/20°C. Fresh and dry weights of Izuma were highest at 20°C/15°C and significantly lower at higher temperature treatments (25°C/20°C, 30°C/25°C and 30°C/20°C). Chlorophyll content of the identical leaves at 25°C/20°C, 30°C/25°C and 30°C/20°C decreased with days after the start of the experiment. Fresh and dry weights of Shizuoka A were highest at 15°C/15°C and significantly lower at higher temperature treatments (25°C/20°C, 30°C/25°C and 30°C/20°C). Chlorophyll content of the identical leaves at 30°C/25°C decreased with days. In experiment 2, nighttime temperature was kept 15°C and daytime temperature was set 15°C, 20°C, 25°C and 30°C. No significant differences in fresh and dry weights of Izuma were observed between 15/15, 20/15 and 25/15, and they were significantly higher than those at 30/15. This is also the case of Shizuoka A. Chlorophyll content of the identical leaves was kept almost constant during the experiment, suggesting that low nighttime temperature can inhibit chlorophyll degradation. By combining the two experimental results, the optimum temperature range for Izuma was determined to be 15°C nighttime temperature coupled with 15–30°C daytime temperature. The optimum temperature range for Shizuoka A was determined to include 25°C daytime and 20°C nighttime temperature, in addition to the above temperature range. The difference in the optimum temperature range between the two strains may be due to the difference of high temperature tolerance.

The most popular method for avoiding frost damage to tea leaves in Japanese tea-producing areas is the air stirring method using frost protection fans. To avoid unnecessary fan operation, this study constructed an air temperature sensing system using IoT and examined the stability of data transmission by Sigfox and Wi-Fi modules in an actual tea garden. In a wide range of area, the Wi-Fi router could not directly receive radio waves from the microcomputer connected to the temperature sensors. In addition, Wi-Fi modules experienced frequent data loss, and data acquisition was unstable even with the use of repeaters. On the other hand, Sigfox modules acquired the data from the sensors and sent the data to the cloud without communication failures, regardless of locations and weather conditions. Given the high spatial variation of the tea leaf temperature in the tea garden, this study introduced multi-point measurements by the IoT system. The operating time for the frost protection fans was evaluated using both existing conventional and proposed methods. In most cases, the fan operating time became shorter with the proposed methods than with the conventional methods. At the locations where the minimum air temperatures were lower than the temperature in the reference location, increases in the operating time were considered to avoid the risk of frost. This study confirmed that Sigfox was more suitable than Wi-Fi in the outdoor tea garden and concluded that the effects of multi-point measurements by the IoT system were useful for reducing unnecessary fan operation and avoiding frost risk.