Pulsed Light at Lower Duty Ratios with Lower Frequencies is Less Advantageous than Continuous Light for CO2 Uptake in Cos Lettuce

Abstract

The CO2 uptake rate (CUR) of the aerial part of cos lettuce (Lactuca sativa) was measured in pulsed and continuous light at an average photosynthetic (wavelength range: 400–700 nm) photon flux density (PPFD) of 100 μmol m-2 s-1 from white light-emitting diodes (LEDs). Three duty ratios of 25, 50, and 75%, combined variously with eight frequencies of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 kHz were used for pulsed light irradiation. The CURs were significantly lower at a duty ratio of 50% for frequencies of 0.1–0.8 kHz and at a duty ratio of 25% for all frequencies than those in continuous light although the average PPFD was unchanged. These results indicate that pulsed light at lower duty ratios, combined with lower frequencies, makes CUR of cos lettuce lower than that attained in continuous light. We infer, based on a PPFD-CUR curve obtained with cos lettuce plant used in the experiment, that when greater PPFD is provided, pulsed light at lower duty ratios with lower frequencies is less advantageous than continuous light for photosynthesis.