2021 Volume 33 Issue 1 Pages 1-6
High temperature in summer can inhibit the growth of wasabi (Eutrema japonicum (Miq.) Kiudz.) and degrade the quality of its rhizome. Several materials for shading sunlight have been conventionally used for avoiding this negative effect during summer season, but adequate shading materials have not been selected based on scientific data. In the present study, we investigated effects of near-infrared ray cutting materials on growth of wasabi. Six types of the prototype nets, which effectively absorbed near-infrared ray in shortwave radiation, were used for the cultivation experiment. They had different shading ratios of 27–58% on the photosynthetically active radiation basis. Commercially available white-colored net with 39% shading ratio, which can reflect near-infrared ray effectively, and conventionally used black-colored net with 79% shading ratio were also applied. Five months later, the rhizome weight of wasabi was the highest under the prototype net with 38–46% shading ratio. The content of allyl isothiocyanate in wasabi rhizome was the highest under the prototype net with 38% shading ratio. These values were comparable to those under white-colored net and significantly higher than those under black-colored net. Surface temperature of black-painted brass boards placed horizontally was 1–2°C lower under the prototype net with 38% shading ratio than under white-colored net with 39% shading ratio during daytime. This may be attributed to the fact that downward shortwave radiation was significantly smaller under the prototype net than white-colored net, suggesting that the prototype net can cut near-infrared ray more efficiently. Our results suggest that the prototype nets with 38% shading ratio reduce negative temperature effect during summer season and promote growth of wasabi.
