Supplemental inter-lighting with additional far-red to red and blue light increases the growth and yield of greenhouse sweet peppers (Capsicum annuum L.) in winter

Far-red light has been included in horticultural light sources due to its physiological activity related to phytochrome-mediated responses. Recent studies have shown that adding far-red light to red and blue inter-lighting LEDs improves the growth and production of greenhouse tomatoes. However, in long-term cultivation, the effects of inter-lighting with additional far-red light may vary according to the seasonal light environment. This study aims to investigate the effect of adding far-red light to red and blue inter-lighting on growth and yield of sweet pepper in winter and summer. Sweet pepper (Capsicum annuum L. cv. Kori) plants were grown in greenhouses for five months under average daily light integrals of 16.6 and 32.2 mol m− 2 day− 1 in winter and summer, respectively. The inter-lighting with adding 55 µmol m− 2 s− 1 of far-red light to 71 µmol m− 2 s− 1 of red and blue light (red:blue = 8:2). The shoot dry weight and fruit yields were 20% and 17% higher, respectively, with additional far-red in the winter, but no significant differences in growth, yield, or morphology were observed in the summer. In summer, stem length and leaf area of plants increased rapidly due to sufficient sun irradiance, resulting in a higher far-red fraction at the bottom canopy. Thus, the far-red fraction at the middle and bottom canopies would have been sufficient with sunlight alone. Moreover, additional far-red inter-lighting promotes a decrease in leaf photosynthetic capacity in the bottom canopies. Therefore, plant growth and morphology were not influenced by adding far-red to inter-lighting in the summer at our latitude. We concluded that the additional far-red could contribute to growth and yield of sweet peppers in insufficient light conditions, but this effect disappears in conditions with ample sunlight.