Effect of molybdenum on absorption, transformation and distribution of nitrate-nitrogen of apple rootstock Malus hupehensis Rehd. seedlings

【Objective】 The low nitrogen fertilizer use efficiency restricts sustainable development of fruit industry. Excessive application of nitrogen fertilizer and improper agricultural measures lead to soil acidification with different degrees in current apple orchards in China, and soil acidification can cause low content of soil available molybdenum, which decreases nitrogen fertilizer use efficiency. The aim of the study was to explore appropriate molybdenum(Mo)level to enhance nitrogen fertilizer use efficiency of Malus hupehensis Rehd. seedlings, and provide scientific data for utilization of nitrogen and molybdenum fertilizer in apple industry. 【Methods】 Malus hupehensis Rehd. seedlings were used as experiment materials and the Hoagland solution with nitrate N was used as only N source in the study. The experiment included five different Mo levels: control(CK, no Mo in solution), M1(0.25 mol/L molybdic acid in solution), M2(0.5 mol/L molybdic acid in solution), M3(1.5 mol/L molybdic acid in solution) and M4(spraying 0.04% molybdic acid,no Mo in solution). Effects of Mo on absorption, transformation, and distribution of nitrate-nitrogen in the seedlings were detected using the 15N-labeled tracer method, meanwhile effects of Mo on nitrate reductase(NR) activity of leaves and roots in Malus hupehensis Rehd. seedlings were analysed in the study. 【Results】 Mo can enhance root activity of the seedlings with different degrees and the treatment with 0.5 mol/L Mo in solution has the highest effect in the study. The NR activity in roots of the treatment with 0.5 mol/L Mo is higher than that of the other treatments on the 8 d, and there is significant difference between the treatment with 0.5 mol/L Mo and the control. The leaf NR activities in all the treatments reach the maxima on the 4 d and then decrease. The dry weight amounts of M1, M2, M3 and M4 treatments are higher than that of the control and are increased by 3.88%, 21.12%, 12.38% and 19.90%, respectively. The 15N absorption of M2 treatment is the largest and as well as the total N. The 15N utilization ratios of all Mo treatments are higher than that of the control, and the treatment with 0.5 mol/L Mo has the highest ratio of 13.97%, which is 48.92% higher than that of the control. Mo also has obvious effect on the 15N distribution ratios in the seedlings. The 15N ratios in stems and leaves of M2 treatment and M4 treatment are significantly higher than those of the control, while the highest 15N distribution ratio in roots is found in the control. The Ndff values of leaves, stems and roots of M2 treatment are the highest among all the treatments, which are 1.59-, 1.56-and 1.33-fold compared with the same part in the control respectively. These results show that appropriate Mo supply can enhance the 15N absorption, 15N distribution and 15N utilization of the seedlings.【Conclusions】 Mo can enhance biomass, root activity, 15N absorption, the total N and the 15N utilization ratio of Malus hupehensis Rehd.seedlings. In short, the treatment with 0.5 mol/L Mo is regarded as the most appropriate content to the growth, absorption, transformation, distribution and utilization of nitrate-nitrogen in Malus hupehensis Rehd. seedlings.