Variety Comparison of Effect of Supplemental Lighting with LED on Growth and Yield in Forcing Culture of Strawberry

Production areas and levels are continuously declining for Japanese strawberry production. Solving these problems, there is an increasing trend towards strawberry productions in large-scale industrial facilities. Techniques to obtain consistently high yield are required in large-scale greenhouse. This requires the development of environment control techniques (e.g., light, air temperature, CO2 concentration, humidity, wind velocity) to allow plants to realize their full photosynthetic potential. Miyoshi et al. (2013) provides an example of environment control in forcing culture of strawberry, reporting on an energy-saving control of ambient air temperature using a constant soil temperature layer. Furthermore, Hidaka et al. (2012) reported that controlling the light environment directly influences leaf photosynthesis and fruit yield; this is required because variable light environments that are dependent on factors such as cropping season and cultivation location frequently lead to inadequate light levels for leaf photosynthesis, plant growth and fruit yield, resulting in declining productivity in greenhouse production. Consequently, the development of a supplementary lighting technique, independent of cropping season or cultivation location, is needed for consistently high strawberry production. Hidaka et al. (2013) examined the effects of 12 h of supplemental lighting (6:00 18:00) from two different commercial light sources, high-irradiance LEDs and fluorescent lamps. In that study, we found high-irradiance LEDs significantly enhanced leaf photosynthesis compared with fluorescent lamps. This led to improved fruit quality and a significant increase in marketable yield of strawberries in forcing culture. Hidaka et al. (2014) further examined the optimum photoperiod of supplemental lighting with LEDs to the June bearing strawberry (Fragaria ananassa Duch. cv. Fukuoka S6). The best fruit yield in this cultivar was found under 12-h illumination when four different photoperiods (12-h, 14-h, 16-h and 24-h illumination) were compared. Darrow (1966) classified June bearing strawberry Fragaria ananassa Duch.) as a facultative short day plant. Ito and Saito (1962) and Taylor (2002) clarified that each cultivar has a respective critical day-length that is needed to induce flower bud differentiation. Photoperiods exceeding a critical day-length may inhibit flower bud differentiation, and subsequently bring decreasing in yield. Therefore, a sufficient effect of supplemental lighting with 12-h photoperiods, which was seen when using the cultivar known as ‘Fukuoka S6’ (Hidaka et al., 2014), may not always be obtained using any other cultivars. Understanding the varietal differences in the supplemental lighting effect is required for developing a technique of supplemental lighting that can be broadly and successfully applied. The mechanism that produces an increase in yield based on supplemental lighting may also vary with each cultivar having