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

With the continuous increase of light emitting diodes (LEDs) energy-to-photon conversion efficiencies, LEDs have become a viable light source for the production of transplants under closed-type conditions. The objective of this experiment was to evaluate different blue and red photon flux (PF) ratios using LEDs for the production of cucumber seedlings (cucumis sativus). Cucumber seedlings (cv. Cumlaude) 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 blue-red (B-R) PF ratios of 0B-100R, 10B-90R, 30B-70R, 50B-50R, 75B-25R, 100B-0R, and one blue-green-red PF ratio of 20B-28G-52R.  Peak wavelengths were 455 nm and 661 nm for the blue and red LEDs, respectively in the B-R ratio treatments and 473 nm, 532 nm, 660 nm for the blue, green, and red LEDs, respectively in the B-G-R treatment. Seeds were germinated under darkness and transferred to treatments after radicle emergence (24h). Hypocotyl length decreased with the increase of blue light percent up to 75B-25R; however, 100B-0R showed the longest hypocotyl of all the treatments. Hypocotyl length was 70 % greater in the 0B-100R treatment and 80 % greater in the 100B-0R treatment compared to the 75B-25B treatment. Chlorophyll concentration increased with the increase of the percentage of blue light except for 100B-0R, which had significantly lower chlorophyll concentration than 50B-50R, and 75B-25R. Cucumber shoot fresh and dry mass decreased with the increase of the percentage of blue light except for the two monochromatic treatments; 0B-100R had the smallest and 100B-0R had the greatest shoot dry and fresh mas of all the treatments. Net photosynthetic rate (Pn) was not different between 10B-90R, 30B-70R, 50B-50R, 75B-25R, and 100B-0R; however, Pn was significantly lower in 0B-100R and 20B-28G-52R than 10B-90R, 30B-70R, and 100B-0R. The optimal spectrum for the growth of cucumber transplants under artificial light will need to be selected based on grower preferences. For example, the 100B-0R treatment had the greatest growth rate of all the treatments; however, the long hypocotyl is not preferred by growers. The most compact cucumber transplant were in 75B-25R, but they had the smallest shoot dry mass. An adequate compromise may be 10B-90R since it had a good growth rate and an acceptable plant height. More research is needed to find the optimal spectrum for the growth and development of cucumber transplants under artificial lighting.