Effects of Supplemental Red and Far-Red Light at Different Growth Stages on the Growth and Nutritional Properties of Lettuce

The understanding of the supplementation scheme of red (R) and far-red (FR) light in the cultivation of leafy vegetables in plant factories with artificial lighting (PFALs) is still limited. This study investigated the effects of supplemental R and FR light at different plant growth stages on the morphology, growth characteristics, and nutritional properties of lettuce. Supplemented R + FR throughout the entire growth stages was beneficial for the growth of lettuce, for which the total fresh weight was increased by 53.76%, and it also enhanced the content of soluble sugars by 39.98% and vitamin E by 34.21%. The pre-supplementation of FR light followed by supplementation of R light at various growth stages not only increase the total fresh weight of lettuce by 26.10% but also ensured that most nutritional indices did not decrease, and it even increased the content of soluble sugars by 35.24% while decreasing the nitrate content by 31.52%. The scheme of pre-supplementation of R light followed by the supplementation of FR light promoted a more upright plant architecture in lettuce, which was advantageous for improving the cultivation density of lettuce in plant factories with artificial lighting, thereby enhancing the yield/m². Moreover, it could increase the vitamin E content of lettuce. The growth and nutritional properties of lettuce exhibit significant effects under different supplementation methods of R and FR light. In PFALs, the selection of different light supplement schemes also requires a careful balance between yield and quality. From an energy-saving perspective, the pre-supplementation of FR light followed by supplementation of R light at various plant growth stages is beneficial for lettuce production in PFALs.