Appropriate Nitrogen form Ratio and UV-A Supplementation Increased Quality and Production in Purple Lettuce (Lactuca sativa L.)

Purple lettuce (Lactuca sativa L. cv. Zhongshu Purple Lettuce) was chosen as the trial material, and LED intelligent light control consoles were used as the light sources. The purpose was to increase the yield and quality of purple lettuce while lowering its nitrate level. By adding various ratios of NO3−-N and NH4+-N to the nutrient solution and 20 µmol m−2 s−1 UV-A based on white, red, and blue light (130, 120, 30 µmol m−2 s−1), the effects of different NO3−-N/NH4+-N ratios (NO3−-N, NO3−-N/NH4+-N = 3/1, NH4+-N) and UV-A interaction on yield, quality, photosynthetic characteristics, anthocyanins, and nitrogen assimilation of purple lettuce were studied. In order to produce purple lettuce hydroponically under controlled environmental conditions, a theoretical foundation and technological specifications were developed, taking into account an appropriate UV-A dose and NO3−-N/NH4+-N ratio. Results demonstrate that adding a 20 µmol m−2 s−1 UV-A, and a NO3−-N/NH4+-N treatment of 3/1, significantly reduced the nitrate level while increasing the growth, photosynthetic rate, chlorophyll, carotenoid, and anthocyanin content of purple lettuce. The purple leaf lettuce leaves have an enhanced capacity to absorb nitrogen. Furthermore, plants have an acceleration of nitrogen metabolism, which raises the concentration of free amino acids and soluble proteins and promotes biomass synthesis. Thus, based on the NO3−-N/NH4+-N (3/1) treatment, adding 20 µmol m−2 s−1 UV-A will be helpful in boosting purple lettuce production and decreasing its nitrate content.