A Combination of Downward Lighting and Supplemental Upward Lighting Improves Plant Growth in a Closed Plant Factory with Artificial Lighting

Abstract

<jats:p>“Plant factory with artificial lighting” (PFAL) refers to a plant production facility that can achieve mass production of vegetables year round in a controlled environment. However, the high-density planting pattern in PFALs causes low light conditions in the lower canopy, leading to leaf senescence in the outer leaves and thus to reductions in plant yields. In the present study, the effect of supplemental upward lighting underneath the plants on photosynthetic characteristics and plant yield was examined in lettuce, in comparison with supplemental downward lighting from above the plants at the same light intensity. Supplemental upward lighting increased the curvature factor of the photosynthetic response to light from above the plants. Moreover, supplemental upward lighting significantly enhanced the lettuce yield by retarding the senescence of the outer leaves. Here, we propose a novel cultivation system with a combination of downward lighting and supplemental upward lighting that can effectively increase plant growth and yield in PFALs.</jats:p>

Continuous Irradiation with Alternating Red and Blue Light Enhances Plant Growth While Keeping Nutritional Quality in Lettuce

“…Although the development of LEDs has brought new opportunities for plant factories with artificial lighting, the cost of electricity for lighting is still high. Because there is great potential to reduce costs by designing more efficient lighting systems, many studies have investigated the effects of light intensity and wavelength on plant growth (Hogewoning et al, 2010;Joshi et al, 2017;Li et al, 2013;Li and Kubota, 2009;Lin et al, 2013;Zhang et al, 2015). There has been controversy in the recent literature regarding the effect of alternating red and blue lighting on the growth of lettuce: Kuno et al (2017) showed that alternating irradiation promoted growth compared with simultaneous irradiation of red and blue light at the same DLI and day length at 30 DAS, whereas Jishi et al (2016) showed no significant difference between the two conditions at 21 DAS.…”

Supplemental LED inter-lighting compensates for a shortage of light for plant growth and yield under the lack of sunshine

“…Frantz et al [ 25 ] found that supplemental light within a cowpea canopy significantly delayed senescence of the interior leaves. In addition, supplying upward lighting from underneath retarded the senescence of outer leaves of lettuce and increased photosynthetic rate, improving total plant growth [ 26 , 27 ]. Many studies have shown that supplemental lighting above or within the canopy enhanced the yield of tomato plants when sunlight is limited, mainly in winter [ 28 – 33 ].…”

Plant Production in Controlled Environments

“…For indoor vertical farming, it is imperative to determine the lowest possible light intensity or daily light integral (DLI), with corresponding lowest electricity costs, but with little compromise on yield and quality [5]. Researchers have attempted to manipulate light intensity, photoperiod, light spectrum, lighting direction (downward, sideways, or intra-canopy), and lighting with specific wavelengths delivered at specific timings [6][7][8][9]. In addition, others have tried different combinations of varying wavelengths of LEDs, which can be turned on simultaneously, alternating or with partial overlap of different LEDs [10].…”