Growth and morphological response of cucumber seedlings to supplemental red and blue photon flux ratios under varied solar daily light integrals

• Supplemental LED lighting improved growth and morphology of cucumber seedlings.
• Under high DLI the increased of blue photon flux had no effect on the seedlings.
• Under low DLI the increased of blue photon flux decreased seedling's growth rate.
• The decreased in growth rate was caused by the decreased of leaf area under low DLI.
• Red LEDs are preferred as supplemental lighting for cucumber seedlings.

High intensity light-emitting diodes (LEDs) have the potential to be used as supplemental lighting technology in greenhouses. However, LED light quality requirements of greenhouse crops grown when supplementing the solar spectrum are unknown. In this study, to find the requirements, cucumber (Cucumis sativus L. cv. Cumlaude) seedlings were grown in a greenhouse with and without supplemental LED lighting (PPF: 54 ± 1.1 μmol m−2 s−1) at varied blue (400–500 nm with the peak at 455 nm) and red (600–700 nm with the peak at 661 nm) photon flux (PF) ratios (B:R ratios) under different solar daily light integrals (DLI). The treatments were 0B:100R% (54 μmol m−2 s−1 red PF), 4B:96R% (2.3 and 52 μmol m−2 s−1 blue and red PF, respectively), 16B:84R% (8.5 and 46.2 μmol m−2 s−1 blue and red PF, respectively), and a control without supplemental lighting. The solar DLIs during the experiment were 5.2 ± 1.2 and 16.2 ± 5.3 mol m−2 d−1 created inside a greenhouse using shade screen. Regardless of B:R ratio, morphological and growth parameters of the seedlings were all improved under supplemental LED lighting compared to the no-supplemental-light control. Under high DLI conditions, no significant differences were found for any parameters between the different B:R ratios. Under low DLI, chlorophyll concentration increased with increasing B:R ratio (i.e., increasing blue PF without increasing photosynthetic photon flux, PPF) of the supplemental lighting. Dry mass, leaf number, and leaf area decreased with increasing B:R ratio under low DLI conditions. The reduction in dry mass and leaf number were attributed to the reduction in leaf area. Leaf net photosynthetic rate measured under ambient CO2, ambient temperature, and 1000 μmol m−2 s−1 PPF (light source: tungsten halogen lamp) also showed no difference among treatments of B:R ratios, indicating that B:R ratio treatments did not cause any changes in plant photosynthetic apparatus. When used for supplemental lighting in the greenhouse, use of 100% red LED is preferred for cucumber seedlings, and additional blue LED was not beneficial.