EFFECT OF OVERNIGHT SUPPLEMENTAL LIGHTING WITH DIFFERENT SPECTRAL LEDS ON THE GROWTH OF SOME LEAFY VEGETABLES

The use of light emitting diodes (LEDs) for supplemental lighting in greenhouses is a major interest. Overnight lighting with LEDs is expected to increase the crop yield and quality with less cost. Growth chamber experiments were carried out to investigate the effect of overnight supplemental lighting by LEDs with four different spectral qualities (blue, green, red, and far-red light) on the growth of three cultivars each of lettuce, garland chrysanthemum, Chinese mustard, and Welsh onion. Plants transplanted in 7.5 cm plastic pots were placed in the growth chambers at the 4-6 true-leaf stage and were grown under different spectral light treatments for three to five weeks. The originally installed cool white fluorescent lamps were used during the day (10 h) period. The photosynthetic photon flux (PPF, 400-700 nm) at the top of plants was adjusted to 100 µmol m^-2 s^-1. During the night (14 h) period, overnight lighting by LEDs was provided at a PPF level of 50 µmol m^-2 s^-1. The day and night air temperatures were maintained at 25 and 20°C, respectively. The relative humidity was maintained at 70% throughout the two periods. The results showed that the growth of most plant species was affected by the light quality of the overnight lighting. Particularly, the shoot fresh weights of lettuce under blue and red light were 22 and 38% greater, respectively than in the control without overnight lighting. Blue and red light showed a similar effect on the shoot fresh weight of garland chrysanthemum. Blue light promoted the elongation of stem and leaves of both crops.