Effects of Continuous Light Before Harvest on Nutrient Element Contents of Hydroponic Lettuce Cultivated Supplied With Three Nitrogen Levels and Two LED Red and Blue Light Qualities

Red light and blue light are the main active spectra for plant photosynthesis and photomorphogenesis, also red and blue light LEDs have been the dominant light sources for hydroponic leafy vegetable production in plant factory. It is suggested that continuous light (CL) of red and blue light before harvest can improve the yield and quality of hydroponic leafy vegetables. In this study, effects of nitrate levels supplied and 72 h continuous light before harvest with the red and blue light on shoot dry weight and nutrient element content as well as accumulation were conducted using hydroponic method and ICP-AES analysis technology. Three nitrate levels (8, 10 and 12 mmol . L-1, N8, N10 and N12) and two light qualities (2R : 1B and 4R 1B) of 150 mu mol . m(-2) . s(-1) were designed. The results showed that shoot dry weight increased with supplied nitrate level, and CL also improved shoot dry weight. Nitrate level did not alter N, C and P contents in lettuce shoot. However, Ca and Mg contents were improved with nitrate level increment, also K, Fe, Mn, Cu and Zn contents were decreased with the increase of supplied nitrate level. Moreover, nitrate level did not alter C, K, P and Fe accumulation in lettuce shoot. However, N, Ca and Mg accumulation were improved with nitrate level increment, also Mn, Cu and Zn accumulation were decreased with the increase of supplied nitrate level. Except for C element, contents and accumulation of other nutrient elements were subjected to nitrate levels and CL light qualities. Under N10 and N12 conditions, CL with 4R : 1B is easier to obtain relatively high nutrient element content and accumulation. Through CL, C content in lettuce shoot was improved, but K, P and Fe contents were decreased, and N, Ca, Mg, Mn, Cu and Zn contents did not change significantly. However, CL promoted N, C, K, P, Ca and Mg accumulation in lettuce shoot, but did not alter microelement accumulation. In summary, high nitrate level and CL before harvest with 4R : 1B spectrum facilitates high dry shoot yield of hydroponic lettuce and high contents and accumulation of most major and medium nutrient elements.Y