Effects Of Molybdenum Application On Nitrate Uptake,Accumulation And Metabolism Of Chinese Cabbage Under Different Ratios Of NO₃^-to NH₄⁺

A solution culture experiment was conducted to investigate the effects of molybdenum (Mo) on nitrate absorption, nitrate accumulation, nitrate metabolism and growth and quality of two Chinese cabbage cultivars (low nitrate accumulator L18and high accumulator H96) under different ratios of N03--N to NH4+-N. The main results are follows:1. A solution culture experiment was conducted to determine the effects of Mo application on growth and quality of two Chinese cabbage cultivars under different ratios of NO3--N to NH4+-N. The results indicated that fresh weight, Mo content and nitrate content were significant decreased with a decreasing ratio of NO3--N to NH4+-N. At the same time, the contents of vitamin C, soluble sugar, total free amino acids and soluble protein in Chinese cabbage increased. Mo application increased the fresh weight of Chinese cabbage under each ratio of NO3--N to NH4+-N.When the ratio of NO3--N to NH4+-N was100/0and50/50, Mo increased the fresh weight of Chinese cabbage significantly. Mo application also increased the contents of vitamin C, soluble sugar, total free amino acids and soluble protein in Chinese cabbage, especially under single nitrate nutrition. One exception was soluble protein of L18cultivar. Mo application decreased the nitrate content in Chinese cabbage significantly when the ratio of NO3--N to NH4+-N was100/0. To a certain extent, Mo can modify the yield and quality in Chinese cabbage under each ratio of NO3--N to NH4+-N.2. A solution culture experiment was conducted to investigate the effects of Mo application on nitrate and ammonium absorption of two Chinese cabbage cultivars under different ratios of NO3--N to NH4+-N. The results showed that Mo can significantly decrease the nitrate uptake rate of both Chinese cabbage cultivars, when the Mo application was long-term and at high concentrations. This indicates that Mo application mainly affects the low-affinity nitrate system. The effects of Mo application on ammonium uptake rate are similar to those for nitrate. Ammonium can decrease the Vmax and Km of nitrate uptake, especially Vmax. The effect of nitrate on the Vmax and Km of ammonium uptake is similar.3. A solution culture experiment was conducted to determine the effects of Mo application on nitrate metabolism under different ratios of NO3--N to NH4+-N.The results showed that nitrate contents and NRA (nitrate reductase activity) were significantly decreased with a decreasing ratio of NO3--N to NH4+-N. At the same time, the contents of ammonium and total free amino acids in both Chinese cabbage cultivars increased. At a ratio of NO3--N to NH4+-N of100/0, Mo application promoted greater reduction of nitrate. Mo application promoted the conversion of ammonium nitrogen to organic nitrogen when the ratio of NO3--N to NH4+-N was50/50. At a ratio of NO3--N to NH4+-N of10/90, Mo application inhibited the absorption of ammonium by Chinese cabbage and stimulated the conversion of ammonium to organic nitrogen, which could alleviate the toxicity of ammonium to a certain extent. It is hypothesized that the external sources of nitrogen forms were the main external factors affecting nitrogen metabolism and Mo application could stimulate nitrogen metabolism of both Chinese cabbage cultivars under each ratio of NO3--N to NH4+-N.4. A solution culture experiment was conducted to determine the effects of Mo application on Molybdenum (Mo), phosphorus (P) and potassium (K) accumulation of two Chinese cabbage cultivars under different ratios of NO3--N to NH4+-N. The results showed that Mo content decrease with a decreasing ratio of NO3--N to NH4+-N. This indicates that ammonium could inhibit the translocation of Mo from roots to shoots in Chinese cabbage. In addition, Mo application can inhibit the translocation of P from petiole to leaf. K content and K accumulation decreased with a decreasing ratio of NO3--N to NH4+-N. When the ratio of NO3--N to NH4+-N was50/50, Mo application could improve the potassium content. This indicates that ammonium could inhibit the uptake of K, and Mo could alleviate the effects to some extent.