Effects And Mechanism Of Inorganic Sulfur On Nitrate Reducing In The Leaves Of Non-heading Chinese Cabbage

At present, reduction and regulation measures of nitrate accumulation in edible parts of vegetable crops became an important topic from inhibiting nitrogen uptake in soil, which included formulated fertilization, nitrification inhibitors and so on. Recently, from the perspective of nitrate assimilation, we have explored a positive measure for nitrate reduction basing the N-C metabolic coupling mechanism. However, there was a limiting knowledge on reduction and regulation measures of nitrate through the N-C-S metabolic coupling mechanism.Thus, the present work aimed to study the effect and mechanism of inorganic sulfur on nitrate reduction in the leaves of non-heading Chinese cabbage. The dynamic change of plant growth and nitrate content were determined by dealing with different inorganic sulfur types. The key technologies were formed to significant reduce nitrate accumulation, such as perfect inorganic sulfur and its applicative concentration, period, pattern, frequency. On this above basis, the nitrate content, enzyme activities, intermediate products and free amino acids content for nitrogen and sulfur metabolism and photosynthetic characteristics in leaves of non-heading Chinese cabbage were also investigated to explore the physiological and biochemical regulatory mechanism of inorganic sulfur on promoting the assimilation of nitrate, formation free amino acids and soluble protein. The major findings were summarized as follows:1. The regulation effect of inorganic sulfur on nitrate reduction was in the order of SO₃2->S2O₃2->SO42->S, which showed that the unsaturated valence state of sulfur had better reduced effect for nitrate content. In addition, the NaHSO₃ with spraying measure presented the most significant effect on reducing accumulation of nitrate in leaves of non-heading Chinese cabbage.2. The most significant effect on reducing nitrate accumulation was found on the 12th d when the 10 mmol·L^-1 NaHSO₃ was sprayed on the leaves of non-heading Chinese cabbage grown in middle nitrogen soil, which not only did the nitrate content in the leaves and petioles of non-heading Chinese cabbage decrease by 33.25% and 15.91%, but also the NR activity distinctly increase by 54.27% and 64.59% respectively.3. Interval 8 days after our first 10 mmol·L^-1 NaHSO₃ treatment the second same NaHSO₃ was sprayed, then the nitrate content was further improved after 4 days in the leaves of non-heading Chinese cabbage grown in middle nitrogen soil. Compared with CK, the nitrate content in the leaves and petioles of non-heading Chinese cabbage decrease by 56.77% and 32.58% respectively, and the NR activity distinctly increase by 80.69% and 89.13%.4. The results showed that the key enzymes of NR、GS/GOGAT、GPT activities in the nitrogen metabolic processes were dramatically increased after 12th d of foliar spray by 10 mmol·L^-1 NaHSO₃, and the key enzymes of OAS-TL activities in the metabolic processes of the sulfur also increased significantly. Moreover, the NaHSO₃ dramatically increased the content of total nitrogen and total sulfur of plants, soluble protein, soluble sugar, Vc, free amino acids and amino acid compositions, and sulfur-containing substance of glutathione and methionine.5. Furthermore, the NaHSO₃ treatment improved the CE, Vcmax and carboxylase activity of Rubisco by inhibiting photorespiration of cabbage leaves. At the same time, the Q, ETRmax and electron transport capacity of PSII were also increased significantly, which provided adequate supplies of energy for NO₃^- assimilation.According to the above results, the physiological and biochemical mechanism of NaHSO₃ on promoting nitrate assimilation may be attributed to the dual regulatory pathways. On the one hand, NaHSO₃ stimulated NR activity firstly, then drove assimilation of nitrate by transamination. Meanwhile the metabolic pathways of sulfur indirectly affected the process of nitrogen metabolism to reduce nitrate accumulation. On the other hand, NaHSO₃ promoted photosynthetic carbon assimilation efficiency and triggered anabolic metabolism of nitrogen indirectly in carbon skeleton and energy supplies, by restraining photorespiration and improving PSⅡelectronic transmission ability and increasing carboxylation reaction.