Effects of molybdenum on nitrate metabolism of winter wheat

Hoagland's solution with nitrate N as only N source, in which 0, 0.78, and 2.74 μmol/L Mo being separately added, was used to culture winter wheat seedlings, and NO~-_3-N, NH~+_4-N concentration in plant leaves, total uptake N in plants and nitrate-reductase activity (NRA) were measured at every 2 days for investigating the effects of Mo on nitrate N metabolism. Results show that NRA, as regulated by metabolism pool and storage pool, varied continuously during the cultural period. However, at any case, Mo had an obvious effect on it: at the onset of 4 days, the treatment with suitable Mo concentration was the highest in NRA, followed by that treated with high Mo concentration, and that deficient in Mo was the lowest. Thereafter, due to nitrate accumulation in high amount in plants, NRA in the Mo deficient treatment thus became the highest. Nitrate N was negatively correlated with ammonium N, and the sum of nitrate and ammonium concentration was almost stable at any measurement. When nitrate N concentration was higher, ammonium N would become lower, or in visa versa. Such a balance was closely related with Mo supply: at adequate concentration of Mo, the high concentration of ammonium N was opposite to low nitrate N at the onset of 4 days, since then, both concentrations trended to be the same level. The treatment of Mo deficient was similar to the adequate, but nitrate N was no great change and the sum of ammonium and nitrate N concentrations was higher. In contrast, ammonium N concentration in the high Mo treatment was higher than the nitrate concentration throughout the whole period of the culture. Crop uptake nitrogen, which agreed well with nitrate N reduction in the solution, was the highest in the treatment with suitable Mo concentration, followed by that with high Mo treatment while that deficient in Mo was the lowest. Considering the lower total concentration of nitrate and ammonium N in plants and the highest uptake of N by plants, it was clear that adequate Mo supply did not only promote nitrate uptake and transformation into ammonium, but also favored ammonium transformation into organic nitrogen and therefore played an important role in the nitrate N metabolism.