| Nitrate, one kind of agro-chemical, poses serious potential hazards to human health, and is popularly highly accumulated in vegetables especially in leaf vegetables, which may bring potential harm to people who eat them. The main way to reduce human diet ingested nitrate is lowing nitrate level in vegetable by agronomic measures. In this study, the"Pre-harvest Short-term Continuous Light (PSCL)", which means exposing plants to continuous light for a short-term period before harvest, is raised. The changes of nitrate, soluble sugar and Vc concentration in hydroponic lettuce under such light were investigated, then effects of light quality and light intensity were also studied, the main results and conclusions are as follow:1. Nitrate concentration in hydroponic lettuce decreased substantially and soluble sugar as well as Vc concentration increased rapidly in short-term continuous light. After 72 h continuous fluorescent light, the nitrate concentration was reduced 2378.1 mg·kg-1 in blade and 1569.7 mg·kg-1 in petiole. The soluble sugar and Vc concentrations were increased 9.70 times and 8.18 times in blade respectively, and 5.30 times and 7.38 times in petiole respectively.2. Nitrate concentration decreased at various rates in continuous light process, while soluble sugar and Vc concentration increased at a roughly constant rate. Decrease rate of nitrate concentration fell gradually in 72 h continuous fluorescent light, resulting in decrease in the first 24 h accounting for over 50% of that in 72 h, while increase rates of soluble sugar and Vc concentration were roughly steady, showing little trend of slowing down. From economic considerations, 48 h is enough to reduce nitrate concentration in hydroponic lettuce, but longer light could be applied if more increase in soluble sugar and Vc concentration was expected.3. Nitrate, soluble sugar and Vc concentration were significantly regulated by light quality of short-term continuous light. Compared with red monochromatic light, mixed red and blue light is more conducive to lower nitrate concentration as well as increase soluble sugar concentration, under continuous light with R/B of 4, the nitrate concentration was lowest and the soluble sugar concentration was highest, such differences among different R/B treatments were consist during the entire light period. Increasing blue light helped improve Vc concentration, and it got more pronounced with illumination time. On balance, the optimal R/B of continuous light for hydroponic lettuce is 4.4. Nitrate, soluble sugar and Vc concentration were significantly regulated by light intensity of short-term continuous light. Decrease in nitrate concentration and increase in soluble sugar as well as Vc concentration increased gradually with light intensity increasing from 50μmol·m-2·s-1 to 200μmol·m-2·s-1, however, the marginal benefit of increase in light intensity to lettuce quality became smaller and smaller when light intensity exceeded 100μmol·m-2·s-1, so from economic considerations, 100μmol·m-2·s-1-50μmol·m-2·s-1 is appropriate.To conclude, the results reported here showed that short-term exposure of lettuce plants to continuous illumination before harvest is suited to produce hydroponic lettuces with low nitrate accumulation and high soluble sugar content and this process is significantly affected by light quality and light intensity. The optimal R/B of continuous light for hydroponic lettuce is 4 and the corresponding light intensity is between 100μmol·m-2·s-1 and 150μmol·m-2·s-1. Further study is needed to investigate the effect of PSCL on quality of other vegetables.
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