Abstract:  Plant factory is the most advanced production facility in the field of facility horticulture. It could make the best use of water, fertilizer, electric energy, and other resources to meet the needs of plant growth and realize high-yield and high-quality leaf vegetable production. In theory, plant factories could produce vegetable products with rich nutrient elements according to people's wishes. It was particularly important to improve the yield and quality of vegetables by regulating the light environment under certain conditions of nutrition supply. It was found that the yield and quality of lettuce could be significantly improved by short-term red and blue LED light before harvest. In a plant factory with the controllable environment, the red and blue LED light of different light intensity was used to light continuously (CL) 72 hours before harvest, and "Italian bolting resistant" lettuce (Lactuca sativa L.) was used as the experimental material. The effects of irradiation on the contents of C, N, P, K, Ca, Mg, Fe, Mg, Cu, and Zn in lettuce leaves were studied. After the lettuce was planted, it was continuously cultivated for 17 days under the light period of 6：00-22：00. The red and blue light intensity in the culture stage were all 150μmol·m-2·s-1. CL treatment with different light intensities for 72h was started from the 18th day after transplantation. The light intensity of CL was 100, 150, 200, 300 and 500μmol·m-2·s-1 (represented by CL100, CL150, CL200, CL300 and CL500). And set a growth light intensity of 150μmol·m-2·s-1 during the whole growth period and a light period of 6：00-22：00 as control treatment (CK). In the experiment, samples were taken at the beginning and the end of concentrated continuous irradiation and 4 lettuce plants were randomly selected as repeated samples in each treatment, then dried and ground into powder. The contents of K, P, Ca, Mg, Fe, Mn, Cu and Zn were determined by atomic absorption spectrophotometer and inductively coupled plasma mass spectrometer, and the contents of N and C were determined by combustion isotope analysis. The results showed that the fresh weight and dry weight of lettuce increased with the increase of CL light intensity, and reached the maximum value respectively at CL500. The content of P, K, Ca and Mg in the dry matter of lettuce was the highest under the treatment of CK. After continuous irradiation with LED red and blue light for the last 72 hours before harvest, the content of C in lettuce dry matter increased with the increase of CL light intensity. The total amount of C, N, Fe and Zn in lettuce dry matter increased with the increase of CL light intensity, which was the highest under CL500 treatment. The total amount of P, K, Mg and Cu showed a trend of increasing first and then decreasing with the increase of CL light intensity. Under the treatment of CL200, the total amount of P and K was the highest; under the treatment of CL300, the total amount of Mg and Cu was the highest. The light intensity of CL has no significant effect on the total amount of Ca and Mg. Therefore, continuous red and blue LED light could be applied before harvesting for improving the quality of lettuce or producing functional vegetables rich in some nutrients.

Key words:  Pre-harvest continuous light, LED red and blue light, Plant factory, Hydroponic lettuce, Mineral elements