Effect of Pulsed Supplemental Lighting on Growth Rate of Lettuce (Lactuca sativa L.) Seedings

Abstract

Young lettuce seedlings were exposed to 10 -?20 klx of light from a metal halide lamps for 10 or 12 hr photoperiod to simulate low light intensity in winter (control). As treatments, see- dlings were given 1 to 3 klx of (a) pulsed or (b) continuous supplementary light from glow lamps. Pulsed or continuous supplemental light treatments were applied only during the light period.
1. Under 10 klx main light conditions for a 10 hr photoperiod, dry matter production was the highest with 2 klx pulsed for 6 min every 30 min. It was 2.4 times as high as that without supplemental illumination and 1.6 times with 2 klx continuous supplemental illumination. Un- der 20 klx main light conditions, the treatment with 2 Mx pulsed 1 min every 5 min and 6 min every 30 min was more effective than that without supplemental light or with 2 klx continuous light. On the other hand, 2 klx pulsed supplemental illumination applied for 6 sec every 30 sec was least effective.
2. When the pulsed supplemental light was turned on, the temperature of the leaves and their photosynthetic rate increased rapidly. When pulsed supplemental light was turned off, leaf temperature fell immediately, but the leaves maintained higher photosynthetic rates for a few minutes compared with the leaves that was not exposed to supplemental illumination.
3. At the early growth stages, active leaf elongation was not recognized during the latter half of the 12 hr photoperiod due to water deficiency. But under pulsed supplemental illu- mination, the period of active leaf elongation extended to the latter half of the 12 hr pho- toperiod due to the adequate supply of water and acceleration of photosynthesis.
4. On the basis of these results, pulsed supplemental illumination (for 6 min every 30 min) contributed to the acceleration of the early growth rate of lettuce under limited light con- ditions.