Effects of using LED supplementary lighting to improve photosynthesis on growth and yield of strawberry forcing culture

Most strawberry fruits in Japan are produced under forced-production culture, using June-bearing cultivars. Strawberry plants are transplanted in a heated greenhouse in autumn and the normal harvest period is from late November to June. Insufficient solar radiation for fruit loading during the winter sometimes causes a decline in growth and yield. Therefore, we investigated the effects of supplementing natural light with artificial lights using light-emitting diodes (LEDs). The experiment was conducted in a greenhouse (2400 m(2) in area) located in Yamamoto town, Miyagi prefecture, Japan (37.961°N, 140.877°E). Strawberry plants were transplanted in an elevated bed. We ran two experiments: experiment 1 was on light quality [photosynthetic photon flux density (PPFD) ratio of red light to blue light], while experiment 2 examined various lamp positions relative to the plants. In experiment 1, strawberry plants were exposed to three different qualities of light (red-to-blue ratios of 2:1, 3:1, and 4:1) with LED lamps (peak blue wavelength 450 nm, peak red wavelength 660 nm, SFL 120 BR-ERx). In experiment 2, LED lamps of two light qualities (red-to-blue ratios of 3:1 and 4:1) were installed in three different positions (above, in the middle, and at the bottom of treatment containers). Yields increased in almost all supplementary-light treatments relative to controls in both experiments. In experiment 1, the yield was higher when the PPFD ratio was 3:1 or 4:1 than when it was when 2:1. In experiment 2, strawberry yield increased by about 10% relative to the control, but no significant difference was observed among light positions or light quality. In addition, no increase in total dry weight was observed as a result of supplemental lighting, but the fruit distribution ratio (fruit dry matter/total dry matter) increased. It is possible that morphological changes caused by supplemental LED lighting led to the increase in fruit yield.