Cucumber Seedlings Growth and Morphology under Supplemental Pulsed Lighting using Light-emitting Diodes

Emerging high intensity light emitting diodes (LEDs) are currently been studied for their potential as a supplemental lighting technology in greenhouses. In addition to the long durability, low diode operational temperatures, and spectral selectivity, LEDs unique features include the capability to turn ON and OFF at a rapid frequency (pulsed lighting). Previous research under sole source artificial light conditions has demonstrated that the use of pulsed lighting with optimal frequency and duty ratio could save energy consumption by increasing plant growth, while others demonstrated that there were rather reduction in photosynthesis observed under pulsed lighting with certain combinations of duty ratio and frequencies. However, to our knowledge, pulsed lighting has not been studied as supplemental lighting in greenhouses. In this study, greenhouse cucumber (Cucumis sativus cv. Cumlaude) was grown until the second true leaf stage under red supplemental LED light (661 nm peak wavelength, FWHM: 15 nm) for 18 hours (2:00–20:00) with an average intensity of 60 μmol·m^-2·s^-1 PPF. The treatments consisted of (1) no supplemental lighting (control), (2) continuous red-LED lighting, and (3) pulsed red-LED lighting at 50% duty ratio and 2500 Hz frequency. The solar PPF contribution for this experiment was 7.6 ± 0.7 mol·m^-2 per day. Plant height, hypocotyl length, epicotyl length, stem diameter, number of leaves, shoot fresh mass, shoot dry mass, leaf area, and chlorophyll concentration were evaluated. Supplemental lighting increased shoot dry mass (32 %) and plant height (55 %) compared to the control. No significant differences were observed in the growth parameters such as number of leaves, fresh mass, shoot dry mass, and leaf area between continues lighting and pulsed lighting treatments. However plant height and hypocotyl length were 6.2% and 7.5% respectively, greater in the pulsed light treatments. This study showed that supplemental pulsed lighting at 2500 Hz and 50% duty ratio did not increase cucumber plant growth and did increase seedling’s plant height, which is undesirable for cucumber propagators. Optimal pulsed lighting could be species specific, and in order to potentially increase growth and save energy, researchers have to develop recipes on the ideal frequency and duty ratio for greenhouse crops.