Minimizing Nitrate Reduction during Kjeldahl Digestion of Plant Tissue Extracts and Stem Exudates

Abstract

MATERIALS AND METHODS ['4NJnitrate. Immediately after harvesting, the tissue was frozen with Dry Ice. The frozen tissue was then lyophilized and finally ground to a fine powder. Ten g ground shoot tissue were extracted with three successive 500-ml aliquots of MCW4 (13:4:3 by volume, see reference 12). The extract was separated into two phases, chloroform and methanol:water. The chloroform phase was discarded. The methanol:water phase was surface-aerated overnight at room temperature to remove methanol. The remaining liquid, which contained [14N]nitrate, organic nitrogen, and some dissolved methanol, was used for the subsequent procedural tests. Nitrate in the liquid was determined using the method of Cataldo et al. (2).Stem exudate was collected from 5-day-old, dark-grown com seedlings (DeKalb XL-45) which had been cultured on a minus-N nutrient solution (6) for 3 previous days. The resulting exudate was nitrate-free (verified by the method of Cataldo et al. (2), data not shown) and was used in the tests as collected.A salt solution of (NH4)2SO4 and KNO3 was also used in the various tests for comparison.Reduction of Nitrate during Digestion. The extent to which nitrate was reduced during Kjeldahl digestion was tested by adding selected quantities of KlNO3 to each of the three kinds of samples previously described. The final '5N enrichment of nitrate was approximately 50 atom %. Appearance of '6N in reduced N was used to indicate that reduction of nitrate had occurred during digestion of the sample. The atom per cent excess 15N observed in the reduced N fraction was used to calculate the total quantity of nitrate (14N + 15N) that had been reduced.The Kjeldahl procedure of McKenzie and Wallace (8) was followed, using 2.5 ml 36 N H2SO4, 1.5 g K2SO4, and 50 mg HgO as HgSO4. Samples were digested in a heating block for 1 h at 200°C to remove H20. The temperature was then increased and the samples were digested for an hour after the block reached 380°C (total time approximately 3 h). Ammonium was determined directly on the digestate (3).Nitrate Volatilization. The proposed procedure depends on the removal of nitrate by volatilization as nitric acid from concentrated H2SO4. The completeness ofnitrate volatilization was tested by heating selected quantities of umol)

Diffusion technique for 15N and inorganic N analysis of low‐N aqueous solutions and Kjeldahl digests

“…Originally this predigestion was developed to eliminate nitrate interference during Kjeldahl digestion of soil extracts and plant materials, 12 because during Kjeldahl digestion of organic matter, nitrate may be reduced to a variable extent resulting in poor reproducibility of N measurements. 21 Both water evaporation and nitrate exclusion brought about a substantial increase in the accuracy of the method; however, they do require 2 more days and also result in the N blank as mentioned before.…”

p-Fluorophenylalanine-Induced Restriction of Ion Uptake and Assimilation by Maize Roots

“…During the 12-h exposure period, the uptake solutions ( ( 14). Following treatment with H2SO4-H202 to remove nitrate (17), reduced nitrogen in the exudate was digested (15), and ammonium nitrogen in the digest was measured colorimetrically (2). Nitrogen in the tissue was partitioned into soluble and insoluble fractions (17).…”

“…Following treatment with H2SO4-H202 to remove nitrate (17), reduced nitrogen in the exudate was digested (15), and ammonium nitrogen in the digest was measured colorimetrically (2). Nitrogen in the tissue was partitioned into soluble and insoluble fractions (17). Nitrate and reduced nitrogen in the soluble fraction were determined by the same procedures used for the exudate.…”

Escarola (Cichorium endivia L.): sistemas de manejo y ciclos como factores de la incidencia de fisiopatías y la acumulación de nitratos.

“…Por otro lado, varios autores han señalado que en muestras con un elevado contenido en nitratos, el método Kjeldahl no es un método útil para estimar el contenido en nitrógeno total, ya que parte de los nitratos puede ser reducido durante la digestión (Pace et al, 1982;Jaroonchon et al, 2010), lo cual podría explicar la escasa correlación hallada entre el contenido de nitratos y el nitrógeno total en el caso de la escarola.…”

“…Para determinar el contenido de nitrógeno total en la planta existen diversas metodologías, dentro de las cuales la más utilizada es el método Kjeldahl (Pace et al, 1982;Rodríguez et al, 1998).…”