The main cultivation method of strawberry (Fragaria ×ananassa Duch.) in Japan is forcing culture using June-bearing cultivars with flower initiation that is transplanted at the end of September and kept warm from mid October to develop flowers, and then the fruit is harvested from December to the following May. Two kinds of light treatments are carried out in forcing culture; to prevent dormancy with low light intensity and to promote leaf photosynthetic rate with high light intensity. Therefore, day extension and light–break treatments are applied during this cultivation period to prevent strawberry plantlets from dwarfing by low temperatures and a short daylength (Fujishige, 2006). These light treatments are different from the supplemental lightning used to increase photosynthesis. In addition, in this forcing culture, a phenomenon called plant dwarfing, and a decreased yield, occur midwinter at the end of January to mid February. The causes are decreasing assimilation products by low temperatures, short daylength, and low light intensity during midwinter (Shigeno et al., 2001), and growth depression of leaves by preferential assimilate partitioning to fruit (Nishizawa and Hori, 1989).
Based on reports about the effect of supplemental lighting on growth and yield of strawberry (Hidaka et al., 2013; Inada and Matsuno, 1985; Shishido et al., 1995) and photosynthetic characteristics of strawberry leaf blades (Ogiwara et al., 2003), the manner of enhancing leaf photosynthesis by supplemental lightning may be effective in addressing the issues of plant dwarfing and decreased yield during midwinter. However, currently, few strawberry growers in Japan use supplemental lightning for increasing photosynthesis.
Plants adapt to changes in the light environment by controlling their morphology using receptors such as phytochromes, cryptochromes, and phototropin (Ballare and Casal, 2000). Recently, LEDs came to the forefront as a light source in greenhouses because LEDs require less power than existing light sources, they are long-lived, and they are able to emit almost monochromatic light (Massa et al., 2008). Research on the relationship between light wavelength and plant growth are promoted by continuous irradiance of specific wavelength to plantlet with LEDs (Li et al., 2012; Lu et al., 2012; Mizuno et al., 2011). Light requirements are clearly different among plant species (Olle and Virsile, 2013). Plant growth differs based on light quality. Blue light promoted stem elongation of eggplant and sunflower seedlings, and suppressed stem elongation of leaf lettuce (Hirai et al., 2006) and tomato (Li et al., 2017). Green LEDs increased the root dry weight of red leaf lettuce (Johkan et al., 2012), and red LEDs probably prevented strawberry seedlings from excessive stem elongation in a low-temperature treatment in darkness (Fushihara and Mitsui, 1996). In addition, LEDs can irradiate at a close distance from objects because they do not emit energy in the infrared range. Therefore, LEDs are suitable for use to promote plant growth in a controlled environment (Yeh and Chung, 2009). Okamoto and Yoshizawa (1996) reported that diffused reflection by reflective mulch increased the yield of strawberry because light conditions were improved from fruit set to harvest. These results indicate that optimizing a combination of wavelengths and the manner of irradiance (such as light intensity and irradiated site) with LEDs could be a strategy to increase yield. In the basic knowledge of photosynthesis, plants absorb red and blue light better than green light, and the characteristic absorption wavelength did not involve temperature and ambient CO2 concentration (McCree, 1971–72). Moreover, Pn of higher plants was the greatest under red, blue, and green, and there were linear relationships between Pn and light intensity (Inada, 1976). Recently, Terashima et al. (2009) reported that a combination of green light and white light was more effective for Pn of sunflower leaves than red light. The effect of photosynthesis is different based on light quality, but there are few reports about the photosynthetic response to different wavelengths in strawberry using LEDs to promote growth. In this study, we clarified the characteristics of the light–photosynthetic response by using LEDs (blue, green, and red) at different wavelengths to promote growth and development of strawberries.
The aim of this study was to establish efficient supplemental lightning based on photosynthetic characteristics of strawberry leaf blades. First, we investigated the photosynthetic characteristics of the adaxial and abaxial sides of mature and young leaves under blue, green, and red LED irradiation for a short time using ‘Tochiotome’. Second, the photosynthetic characteristics of the adaxial side of young leaves were compared under red and blue LEDs among three strawberry cultivars: Tochiotome, Sachinoka, and Eran.
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