Comparison of Lettuce Growth under Continuous and Pulsed Irradiation Using Light-Emitting Diodes

Abstract

We determined the effects of various frequencies of pulsed light-emitting diode (LED) irradiation on the growth characteristics of Lactuca sativa L. ‘Sunmang’. Seedlings were grown in a 20°C growth chamber with photosynthetic photon flux density (PPFD) of 253.67 μmol·m-2·s-1 and a 12-h photoperiod for 18 days. The seedlings were then transplanted into pots containing a growing medium, followed by placement in growth chambers equipped with a combination of red (R), white (W), and blue (B) LEDs (R:W:B = 7:2:1) that provided pulsed irradiation at various frequencies (0.3, 1, 3, 10, and 30 kHz) at 75% duty ratio (PPFD 190 μmol·m-2·s-1). The control (continuous irradiation) was compared with the treatments 4 weeks after transplanting. Most growth parameters such as shoot fresh weight and leaf area under pulsed LEDs were similar to those of the control. Treatments with lower frequencies (0.3, 1, and 3 kHz) were more effective for growth even though the light intensity was lower than that in the control. No significant difference was observed in the maximum quantum yield of PSII and photosynthetic rates between the treatments. Macronutrient (K, Ca, and Mg) levels were significantly higher under all treatments compared to the control. Light use efficiency was the highest under irradiation with 1 kHz pulsed LEDs. In conclusion, pulsed LEDs with 75% duty ratio and low frequencies did not show significant inhibition on plant growth, suggesting that pulsed LED irradiation technology has a potential to save energy consumption for producing crops in plant factories.