Ratio of Red to Far–Red Light-emitting Diodes Affects Growth and Cell Cycle of Lettuce Plants

As a popular artificial lighting source, light-emitting diodes (LEDs) with a short wavelength range are easily used to control light quality in plant factories. In this study, we investigated the effect of various combined ratios of red and far-red LEDs on growth and cell cycle of red leaf lettuce (Lactuca sativa L. ‘Sunmang’). Sixteen-days old lettuce seedlings were transferred into a growth chamber and cultivated under various combinations of red (R) and far-red (FR) LEDs (R/FR=0.7, 1.2, 4.1 and 8.6, PPF 131.71 ± 7 µmol∙m^-2∙s^-1, 12 h photoperiod) for 22 d. Red LEDs 100% (R100) and fluorescent lamps (control) were also irradiated to compare with R/FR treatments. Growth characteristics such as leaf length, leaf width, leaf area, fresh and dry weights of shoots and roots and SPAD value were measured at 11 d and 22 d of treatment. In addition, cell cycle was analysed every 2 d or 3 d until 20 d. As a result, fresh and dry weights of shoots and roots and leaf area of all R/FR treatments were higher than those of control at harvest. In particular, R/FR 1.2 showed the highest values among R/FR treatments. The number of leaves appeared to increase as R/FR increased. R/FR treatments including R100 significantly decreased specific leaf weight compared with control at 11 d of treatment but the specific leaf weight in R/FR 0.7, 1.2 and 8.6 was similar with that in control at harvest. All R/FR treatments led to longer leaf shape than control although no significant difference was observed among the treatments. In contrast, SPAD value in all R/FR treatments was lower than that in control. Cell cycle of lettuce leaves subjected to R/FR treatments appeared to increase from 4 d of treatment suggesting that R/FR LED treatments stimulate cell division of lettuce which was consistent with the growth results. In conclusion, this study suggested that R/FR 1.2 was the best ratio for lettuce growth among the treatments and R/FR should be considered when designing artificial lighting system in closed-type plant factories because it affects vegetative growth of leafy vegetables such as lettuce.