Spectral Transmission Characteristics of a Prototype Light-quality Control Film and its Effects on the Growth of Spinach

To promote growth spinach, we developed a prototype of light-quality control film (LQCF) that suppresses blue light transmission, thus relatively enhancing the proportion of red light that constitutes photosynthetically active radiation (PAR). From 2014 to 2018, we conducted 19 cycles of cultivation experiments to evaluate its effects on spinach growth.The developed LQCF had a 75 % PAR transmission rate with an increased proportion of red light by approximately 4 % and decreased proportion of blue light by approximately 3 %, as compared with outdoor insolation. After 1.5 years of outdoor use, no changes were recorded in its spectral transmission characteristics (STCs). After approximately 4 years of outdoor use, the proportion of red light was still approximately 3 % higher than in outdoor insolation. This result demonstrates that STCs of the LQCF have been maintained for 4 years.In each spinach cultivation experiment, the films were replaced with new ones. Consequently, batches of spinach grown under LQCF had greater fresh weight (48 % maximum increase), dry matter weight (37 % maximum increase), and leaf area (41 % maximum increase) than those grown under an UV-absorbing film, regardless of the spinach variety, except for spring and autumn cropping types. LQCF absorbed a portion of sunlight, thus creating partial shading conditions, which may have contributed to a poorer growth of these cropping types. The LQCF developed in this study was not able to maintain growth promotion effects throughout the year. Nevertheless, as far as the varieties examined in this study were concerned, this prototype film contributed to leaf area expansion and efficient photosynthesis when spinach was cultivated outside the range of optimum growth temperatures. The observed growth promotion effects increased dry matter production.