Response of Greenhouse Tomato Photosynthesis to Different Spectra of Intra-canopy LEDs Under Different Ratios of Red-Blue-White Overhead LEDs

Different light spectra trigger different plant growth and physiological processes. Therefore, the optimum spectral compositions for different growth processes or plants at different growth stages may be different. Greenhouse tomato (Solanum lycopersicum) is a tall crop; most of the growth in early growth stage is leaf/vegetative growth while both leaf and fruit (generative) growth occurs in mid to late growth stage once the plant reaching full canopy. The leaf growth mostly occurs in the top canopy while fruit growth in the middle and bottom canopy. Therefore, optimized seasonal and vertical spectral distribution profiles could be developed for improving both vegetative and fruit growth. in this study, we investigated the effects of seasonal variations of overhead LED spectra (placed above crop canopy) and different spectra of intra-canopy LEDs (placed inside the canopy near the developing cluster of fruit) on leaf photosynthesis of greenhouse tomato from October 2015 to February 2016 in a large greenhouse (200m²). The greenhouse was divided into 4 sections (50m²/section). Two seasonal overhead LED spectral treatments with red-blue-white ratios ranging from 10:0:0 (full red) to 6:2:1 were applied right after planting (2sections/treatment). Four intra-canopy LED treatments (blue, red, white and UV-A) were applied to the 4 plots (24 stems/plot) inside each section when a full crop canopy had developed. The two overhead LED treatments provided 140µmol m^-2 s^-1 while the blue, red and white intra-canopy LEDs provided 10µmol m^-2 s^-1 of light. The UV-A LEDs provided 8µmol m^-2 s^-1 of light. The photosynthesis of the 5^th fully expanded leaf was measured with a portable photosynthesis measurement system. Adding blue light or high blue to red overhead light ratios increased leaf photosynthesis over 100% of red light. The blue intra-canopy LED treatment had the highest photosynthesis, while the white LED treatment had the lowest photosynthesis. The leaves grown with more blue light also were thicker and had higher leaf chlorophyll than those grown under red or white LEDs. Therefore, adding blue light inside crop canopy could also increase plant photosynthesis and productivity, in addition to the beneficial effects on fruit quality discovered in our early study.