| Vegetables are the main food in hunman’s daily life, but they are fond of nitrate nitrogen and are high-nitrate-accumulated plants at the same time. According to the statistics,70%-80% of nitrate in human’s body is from vegetables. It has been demonstrated that nitrate can become poisonous nitrite in human’s body, which can lead to methemoglobinemia. Besides, nitrite and nitrogen compounds in stomach can produce nitrosamine, which can cause cancer in digestive system. Chinese cabbage (Brassica chinensis L.) is one of the most important green leafy vegetables. In agricultural production, a large quantity of chemical fertilizer especially nitrogen fertilizer was applied to promote the yield of vegetables and this made nitrate accumulate in vegetables extensively. As our life improves, people are paying attention to this problem. So, how to reduce the accumulation of nitrate in vegetables especially in leaf vegetables is one of the problems to be solved in Chinese vegetable industry, and it has also become a hot spot in current researches. This research studied the effects of different NO37NH4+ ratio, light intensity, temperature and air humidity on the growth of Chinese cabbages and nitrate content in them by hydroponic and pot experiment. We also studied the correlation between nitrate content in Chinese vegetables and photosynthetic characteristics, nitrogen metabolism enzymes in leaves and soil, NO3--N in soil. This paper proposed the genotypic difference and molecular mechanism of nitrate accumulation to provide a theoretical basis for the selection of varieties and reasonable fertilization and scientific management. The main results are summarized as follows:1. In the hydroponic experiment:(1) NO3-/NH4+ had a significant effect on the biomass of Chinese cabbages. In addition to number 1,2 and 6, with the increasing ratio of NO37NH4+, other varieties of cabbages increased their biomass. Compared with the biomass in 50/50, when NO37NH4+ increased to 75/25 and 100/0, the biomass increased by 0.8%～82.2% and 7.8%～103.3%, respectively. With the increasing ratio of NO3-/NH4+, the biomass of number 1,2 and 6 increased first and then decreased. Compared with the biomass in 50/50, the biomass of number 1,2 and 6 increased by 30%,7.5% and 15.1% respectively when NO3-/NH4+ increased to 75/25, and then decreased by 9.9%,3.1% and 5.1% respectively when NO3-/NH4+ increased to 100/0.(2) NO3-/NH4+had a significant effect on the nitrate contents of Chinese cabbages. The order of nitrate contents in Chinese cabbages was leafstalk>leaf>root. In addition to number 1 and 7, the nitrate contents in leaves increased with the increasing ratio of NO3-/NH4+. The nitrate contents in leaves increased by 1.0%～65.7% and 7.9%～78.0% respectively when NO3-/NH4+ increased to 75/25 and 100/0. However, in addition to number 1,4,5 and 6, with the increasing ratio of NO3-/NH4+, the nitrate contents in leafstalks increased first and then decreased. The nitrate contents in leafstalks increased by 3.0%～20.4% and 0.3%～17.1% respectively when NO3-/NH4+increased to 75/25 and 100/0. In addition to number 1,3,4 and 5, with the increasing ratio of NO3-/NH4+, the nitrate contents in roots increased. And the nitrate contents in toots increased by 9.8%～59.9% and 42.3%～196.9%respectively when NO3-/NH4+ increased to 75/25 and 100/0.(3) Photosynthesis had a significant effect on the nitrate contents in leaves. With the increasing of Pn and Ci, the nitrate contents in leaves decreased. And with the increasing of Tr, the nitrate contents in leaves increased.(4) Nitrate reductase (NR), nitrite reductase (NiR) and glutamine enzyme (GS) had a significant effect on the nitrate contents in leaves. The nitrate contents in leaves had a significant negative correlation with NR and a significant positive correlation with NiR and GS.(5)The expression levels of NRT1 varied in leaves of number 9 and 10. NRT1 expressed in both leaves. However, the expression in number 10 (high nitrate accumulated variety) was significantly higher than that of number 9 (low nitrate accumulated variety).In the pot experiment:(1) Light intensity, temperature and air humidity had a significant effect on the biomass of the two varieties of Chinese cabbages (number 9 and number 10). With the increasing of light intensity and air humidity, the biomass increased, and the optimum light intensity and air humidity is 120001x and 90%. The biomass of the two Chinese cabbages increased firstly and then decreased with the increasing of temperature and the optimum temperature was 15℃. In any condition, the biomass of number 10 was always higher than that of number 9.(2) Light intensity, temperature and air humidity had a significant effect on the nitrate contents in leaves and light intensity also had a significant effect on nitrate contents in leafstalks. With the increasing of light intensity, the nitrate content in leaves decreased significantly. When light intensity increased to 60001x and 120001x, the nitrate contents in leaves of number 9 and 10 decreased by 22.3% and 36.7%,20.3% and 43.3% respectively than that of 30001x and the nitrate contents in leafstalk increased 8.4% and 22.3%,8.2% and 31.4% at the same time. With the increasing of temperature, the nitrate content in leaves increased significantly. When the temperature increased to 15℃ and 25℃, the nitrate contents in leaves of number 9 and 10 increased by 15.0% and 18.3%,6.7% and 16.7% respectively than that of 5℃. With the increasing of air humidity, the nitrate contents in leaves increased first and then decreased. When the air humidity increased to 70%, the nitrate contents in leaves of number 9 and 10 increased by 9.6% and 5.9% respectively and when he air humidity increased to 70%, the nitrate contents in leaves of number 9 and 10 decreased by 4.5% and 2.3% respectively than that of 50%. In any condition, the nitrate contents of 9 were always higher than that of 10.(3) The nitrate contents in leaves had a significant negative correlation with Pn. And increasing the light intensity can improve Pn significantly.(4) The nitrate contents in leaves had a significant negative correlation with NR. And increasing the light intensity can increase the activity of NR significantly.(5) The nitrate contents in the edible parts of Chinese cabbages was significantly positive correlated with the NO3--N content in soil. And the NO3-N content in soil decreased significantly with the increasing of the activity of nitrate reductase in soil. |
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