Nitrate Absorption by Barley

The absorption of NO3by barley (Hordeum vulgare L.) was investigated by following the disappearance of NO3-. The absorption was related to several parameters: NO3and Ca"+ concentrations, pH, and the presence of various anions. Absorption rate increased with increasing Ca"+ concentration, reaching a maximum at approximately 5 mM Ca2", and was considerably inhibited by NH4'. Absorption was influenced markedly by pH, and little or not at all by anions (Cl-, Br-, S042-), and was decreased by respiratory and oxidative phosphorylation inhibitors. Nitrate is the most common form of nitrogen absorbed by plants growing in the field. Of the numerous reports on the use of nitrogen by plants, few deal with the kinetic quantitative parameters that describe the mechanisms of NO3absorption. Previous reports indicate that NO3is absorbed very rapidly by plant roots at rates comparable to those of K+, Rb+, Cland H2PO4(14, 18, 20). Principles developed from research with other ions are probably applicable to the study of NO3absorption by plants. This study was done primarily to investigate the influence on NO3absorption of various factors such as NO3concentration, pH, presence of other ions, and metabolic inhibition. MATERIALS AND METHODS Preparation of Seedlings. Seeds of barley (Hordeum vulgare L. var. Arivat) were allowed to germinate at 25 C in aerated deionized-distilled H20 in the dark. The water was renewed once, 20 hr after germination began. After 24 hr the germinating seeds were rinsed with distilled H20 and spread on two layers of cheesecloth supported on a stainless steel screen. This screen was suspended about 1 cm above the surface of 4 liters of aerated 0.2 mm CaSO4 solution in a Pyrex beaker, and placed in the dark at 25 C. This procedure has been detailed by Epstein (5). Experimental Procedures. Six days later the seedlings were transferred to smaller screens for absorption studies under various treatment conditions. Seedlings damaged during transfer were discarded. Smaller screens with seedlings were placed on beakers of various capacities containing treatment solutions. The ratio of tissue to solution volume was adjusted in each experiment so that the initial concentration of NO3 in the treatment solution was depleted only 15% or less at the end of the absorption period. The solutions were aerated to provide ample 02 and to maintain the solution concentration at the root surfaces. When the effect of anaerobiosis was studied, N2 gas was bubbled through the solution. Research was supported in part by Kearney Research Foundation. 55 University of California, Davis, California 95616 Transpiration was found to have a minimal effect on NO3absorption. When transpiration was minimized by varying the humidity of the atmosphere around the shoots there was no apparent effect on NO3absorption. Relative humidity in the laboratory ranged from 60 to 70%. For chemical analysis of NO3-, two samples were taken from each treatment solution. The absorbance values of the duplicate samples were almost identical each time and the mean did not vary more than + 2% within replicates. A separate standard curve was prepared for each experiment by taking appropriate dilutions from the treatment solution. Thus, any possible error due to changes in absorbance values of the solutions of the same concentration at different times was eliminated. Though the absolute rates of absorption may vary between different batches of seedlings, all the treatment effects were reproducible within 10%. Measurement of Absorption Rates. Absorption rates were determined by the disappearance of NO3from the external solution, which represents a net flux into the roots. This flux was expressed as ,umoles NO3absorbed g-' fresh weight root. Nitrate levels were determined by the phenoldisulfonic acid method (15). Color development was read at 420 nm on either a Hitachi or Spectronic-20 spectrophotometer.