Blue Light Dose-response of Growth and Morphology of Tomato Seedlings under Different Blue and Red Photon Flux Ratios Using LEDs

Light-emitting diodes (LEDs) are a potential light source for the production of transplants because of their increasing energy-to-photon conversion efficiencies and capability to customize the light spectrum. However, more research is needed in order to find optimal light recipes for different horticultural plant species. The objective of this experiment was to evaluate different blue and red photon flux (PF) ratios using LEDs for seedling production of tomato (Solanum lycopersicum) and their rootstock (S. lycopersicum x S. habrochaites).  ‘Komeett’ tomato and ‘Beaufort’ rootstock were grown in a growth chamber until the second true leaf stage with 100 μmol·m^-2·s^-1 photosynthetic photon flux of LED lighting, 18-hour photoperiod, 25 °C temperature, and ambient CO₂. The treatments consisted of six blue-red (B-R) percent PF ratios of 0B-100R, 10B-90R, 30B-70R, 50B-50R, 75B-25R, and 100B-0R, one blue-green-red percent PF ratio of 20B-28G-52R, and a cool-white fluorescent (CWF) control.  Peak wavelengths were 455 nm and 661 nm for the B and R LEDs in the B-R ratio treatments and 473 nm, 532 nm, 660 nm for the B, G, and R LEDs in the B-G-R treatment. For ‘Komeett’, hypocotyl length decreased with the increase of percent blue PF up to the 75B-25R. Plants under CWF showed the shortest hypocotyl of all treatments and plants under 100B-0R showed as high hypocotyl length as the ones in 10B-90R treatment. Chlorophyll concentration per leaf area was no significantly different in plants grown under the 10B-90R, 20B-28G-52R, CWF, 30B-70R, 50B-50R, 75B-25R treatments, but it was lower in the 0B-100R and 100B-0R treatments. Stem diameter, leaf number, fresh mass, and dry mass increased with the increase of the percent B PF from 0B-100R up to 30B -70R and 50B-50R, and then it decreased from 50B-50R to 100B-0R. Plants under the 30B-70R and 50B-50R showed 66% and 59%, respectively, greater dry mass and plants under the 0B-100R treatment showed 65% lower dry mass than plants under the CWF control. For ‘Beaufort’ all the measured parameters were negatively affected by severe intumescence symptoms. Plant dry mass was reduced by an average of 178% compared to the CWF control (no intumescences in the control). Intumescence development in leaves decreased with the increase of percent B PF. The 0R-100B treatment did not show any intumescence symptoms but leaf yellowing/bleaching affected plant growth. More research is needed to find the optimal spectrum for tomato transplants production grown under sole-source lighting.