Growth and Morphology of Greenhouse Cucumbers Seedlings Grown under Varied Daily Solar Light Integrals with Different Supplemental LED Blue:Red Ratios

Light Emitting Diodes (LEDs) are a promising addition to current supplemental greenhouse lighting technology. To promote their adoption, supplemental LED light quality requirements for cucumber (Cucumis sativus L.) seedlings were investigated. Cucumber is a widely used model species for photomorphogenic studies and it is commonly grown in commercial greenhouses where supplemental lighting is necessary. Previous studies using LED as a sole source of light to grow cucumber plants have revealed the benefits of increasing the blue:red photon flux ratio (Hogewoning et al., 2010) but optimum blue photon flux for supplemental lighting in greenhouse is not known. In our present experiment, cucumber seedlings (cv. Cumlaude) were grown in a greenhouse to reach the second true leaf stage with and without supplemental red and blue LED lighting (PPF: 55 μmol·m^-2·s^-1). Varied levels of blue photon flux at the same PPF created different blue:red photon flux ratios examined in this experiment. The treatments were: 1) 0%Blue:100%Red (55 μmol·m^-2·s^-1 red PPF); 2) 4%Blue:96%Red (2.2 blue and 52.8 red μmol·m^-2·s^-1 PPF), (3) 16%Blue:84%Red (8.8 blue and 46.2 red μmol m^-2 s^-1 PPF), and a control without supplemental lighting. Treatments were examined under low or high daily solar light integrals (DLI) (6.1 ± 0.5 and 21.3 ± 2.0 mol·m^-2·d^-1, respectively) using different shade screens deployed in the greenhouse. The supplemental lighting provided an 18-hour photoperiod (2:00–20:00 hours). Blue and red LEDs had peak wavelengths of 455 nm and 661 nm (FWHM: 20 nm and 15 nm) respectively. Growth parameters such as dry and fresh shoot mass and morphological parameters such as leaf count, stem diameter, hypocotyl length, and chlorophyll concentration showed that the supplemental lighting improved the growth and transplant quality of cucumber seedlings, compared with the control. However, there were no significant differences among treatments with different blue:red ratios regardless of DLI. In addition to growth and morphological parameters, leaf photosynthetic capacity measured under ambient CO₂, ambient temperature and 1000 μmol·m^-2·s^-1 PPF also showed no difference among treatments of blue:red ratios. In conclusion, under LED supplemental lighting, no additional benefits for cucumber (cv. Cumlaude) were obtained by increasing the blue:red photon flux ratio in neither high nor low DLI.