Divergent Regulatory Roles of Supplemental Far-Red or UV-A Radiation in Modulating Pak Choi (Brassica rapa L. ssp. chinensis) Growth and Quality Under Different Light Levels

This study investigated the effects of far-red (FR, 740 nm) and ultraviolet-A (UV-A, 370 nm) supplementation on the growth and nutritional quality of pak choi (Brassica rapa ssp. chinensis ‘Xiawang’) under different light intensities. Plants were grown under a white LED background light (6500 K) at two light intensities (100 and 400 μmol·m−2·s−1) either without (T100 and T400) or with an additional 40 μmol·m−2·s−1 of FR or UV-A radiation (T100FR, T100UV-A, T400FR, and T400UV-A). The results demonstrated a significant interactive effect between light intensity and FR/UV-A supplementation. Under low light intensity (T100), both FR and UV-A significantly enhanced biomass accumulation, the net photosynthetic rate, and the content of soluble proteins, soluble sugars, and cellulose. Under a high light intensity (T400), FR and UV-A primarily promoted morphological development (leaf expansion and thickening), photosynthetic performance, and carbohydrate accumulation. Specifically, FR notably promoted petiole elongation while reducing photosynthetic pigment content, whereas UV-A markedly increased the vitamin C content while decreasing nitrate accumulation. In conclusion, both FR and UV-A supplementation effectively improve the growth and quality of pak choi, but, through light-intensity-dependent mechanisms. FR acts mainly via morphological and photosynthetic optimization, whereas UV-A acts as a physiological and nutritional regulator. These findings highlight the complementary roles of FR and UV-A, providing practical guidance for optimizing spectral strategies in controlled environment agriculture (CEA).