Effects of intermittent light exposure with red and blue light emitting diodes on growth and carbohydrate accumulation of lettuce

In this study, effects of continuous and intermittent irradiation of mixed red (R) and blue (B) light (RB) provided by LEDs on the growth and carbohydrate accumulations of lettuce were examined. Combined RB was provided 16 h per day (24 h) in all the six treatments, among which, continuous RB with only one light/dark (L/D) cycle of 16 h/8 h in treatment L/D(1) was regarded as control. Five intermittent RB treatments that were respectively L/D(2) with two L/D cycles of 8 h/4 h over a 24-h period, L/D(3) with three L/D cycles of 6 h/3 h or 4 h/2 h, L/D(4) with four L/D cycles of 4 h/2 h, L/D(6) with six L/D cycles of 3 h/1.5 h or 1 h/0.5 h, and L/D(8) with eight L/D cycles of 2 h/1 h were set up. Results indicated that shoot biomass of lettuce was increased by intermittent RB treatment L/D(2), L/D(4), L/D(6) and L/D(8) to different extents, while decreased significantly by L/D(3) compared with the control. Significantly higher hexose/sucrose ratio (in comparison with any other treatment) accompanied with the strongest combined activities of sucrose degrading enzymes as well as the weakest combined activities of sucrose synthesizing enzymes was observed in plants treated with L/D(2) treatment. Significantly higher content of soluble sugar especially fructose as well as lower contents of crude fiber and starch were observed in lettuce exposed to L/D(2) and L/D(3) compared with any other treatment. All intermittent treatments significantly enhanced the glucose content in lettuce compared with the control. Both L/D(2) and L/D(3) treatments promoted lettuce taste via improving the sweetness and crispness, whereas only L/D(2) simultaneously increased the shoot biomass (in terms of dry weight) compared with the control.