Differences in absorption and utilization of nitrate nitrogen and ammonium nitrogen in the seedling of Rosa roxburghii

Abstract：【Objective】Rosa roxburghii Tratt. is one of the most important fruit trees in southwestern China. The content of vitamin C in 100 g of R. roxburghii can be higher than 2 000 mg, which attracts the attention of fruit industry and academia. The production of R. roxburghii in the southwest region of China is expanding. So far, the absorption and utilization characteristics of different forms of nitrogen in the crop have not been well defined, and therefore there is a lack of information about nitrogen nutrition management for the crop. In this study, we investigated the absorption kinetics, the uptake efficiency and nitrogen utilization efficiency of NO₃^-and NH₄⁺. The differences in the uptake and utilization efficiency of NO₃^-and NH₄⁺were studied under different nitrogen concentrations and temperatures, and the effects of supply of different proportions of nitrate and ammonium nitrogen were studied. The purpose of this study was to provide the theoretical guidance for the selection of nitrogen fertilizer in the cultivation of R. roxburghii.【Methods】In this study, the seedlings of R. roxburghii Tratt.‘Guinong 5'were used as the material. TheHoagland and Arnon nutrient solution formula and sand culture were used in the nitrogen treatments.Five nitrogen treatments were set with different ratios of NO₃^-to NH₄⁺, i.e. 1∶0, 1∶3, 1∶1, 3∶1 and 0∶1, and seedlings biomass, nitrogen absorption and utilization efficiency were investigated after 60 d of the treatments. Depletion method were adopted to measure the affinity, the maximum absorption rate (Imax) and the flow velocity (α) of NO₃^-and NH₄⁺in root under different concentrations and proportions of NO₃^-and NH₄⁺. This study also analyzed the effects of different temperatures on absorption kinetics of NO₃^-and NH₄⁺ions after supplied with NO₃^-or NH₄⁺at the concentration of 2.0 mmol·L^-1.【Results】The absorption pattern of nitrate nitrogen, ammonium nitrogen and total nitrogen followed the Michaelis-Menten kinetics equation. The preference of nitrate nitrogen absorption did not change due to the changes in NO₃^-and NH₄⁺ions or temperature in the medium. The results provided important scientific basis for selection of nitrogen fertilizer. When NO₃^--N or NH₄⁺-N was supplied alone, the maximum absorption rate, affinity and flow velocity of NO₃^-were higher than those of NH₄⁺, and this characteristic was irrespective with the changes in the concentrations of NO₃^-and NH₄⁺ions. The Imaxand α values of NO₃^-and NH₄⁺were significantly decreased by extreme temperatures in the medium, but the values of Imaxand α of NO₃^-were always higher than those of NH₄⁺. 30 ℃ resulted in the lowest maximum root absorption, affinity and flow rate of NH₄⁺, and 5 ℃ had the lowest flow rate of NO₃^-. The maximum root absorption rate of NO₃^-and NH₄⁺was maximum at 25 ℃ and 20 ℃, respectively; the affinity of NO₃^-and NH₄⁺reached the maximum value at 25 ℃ and 15 ℃, respectively; and the flow rates of NO₃^-and NH₄⁺in roots was highest at25 ℃ and 15 ℃, respectively. The Imaxvalues and α values of NO₃^-and NH₄⁺increased with the increase in the ratio of nitrate nitrogen to ammonium nitrogen. When the ratio was 3∶1, the total nitrogen content of the roots was the highest, suggesting this ratio was favorable for nitrogen absorption. In the sand culture experiment, the nitrogen use efficiency as well as the biomass and N uptake were all highest in the treatment with 3∶1 ratio of nitrate nitrogen to ammonium nitrogen.【Conclusion】R. roxburghii has nitrate preference, and the maximum root absorption, the affinity and the flow rate of NO₃^-are always larger than those of the NH₄⁺. Changes in nitrogen concentration and temperature do not affect the priority in absorption of NO₃^-. The maximum absorption of nitrogen occurs under the temperature of 20-25 ℃. The biomass, N uptake, and thus the nitrogen use efficiency can be further improved with the ratio of NO₃^--N:NH₄⁺-N of 3∶1.