EFFECTS OF BLUE/RED RATIO AND LIGHT INTENSITY ON PHOTOMORPHOGENESIS AND PHOTOSYNTHESIS OF RED LEAF LETTUCE

One reason why the effect of light quality on plant growth remains unclear is that light quality affects photosynthesis and photomorphogenesis during long-term cultivation, and in some cases, photomorphogenesis may affect the plant growth rate. Thus, in this study, we evaluated the effect of blue/red (B/R) ratio and light intensity (photosynthetic photon flux [PPF]) on photomorphogenesis by separating the effects of light quality and PPF on photosynthesis. Red leaf lettuce was cultivated under light-emitting diodes (LEDs). Nine test treatments were established at 11 days after sowing (DAS) by using a combination of PPF values of 100, 200 and 300 µmol•m^-2•s^-1 and B/R ratios of 0.18, 0.23 and 0.33 with a mixture of blue (450 nm) and red (660 nm) LEDs. Under 100 µmol•m^-2•s^-1 PPF, the total dry weight was not different among the B/R ratios. Under 200 and 300 µmol•m^-2•s^-1 PPF, the total dry weight at 21 and 25 DAS was the highest at B/R ratios of 0.23 and 0.33, respectively. The total dry weight was the highest at these B/R ratios mostly because of the high net photosynthetic rate and large amount of light received by the leaves. The change in leaf shape changed to the amount of light received by the leaves. It was shown that the difference in growth rate was due to a strong change in leaf morphogenesis under different B/R ratios, because the change in leaf shape changed the amount of light received by the leaves.