The nitrate content of plants is extremely variable but may usually be detected in actively growing tissues receiving abundant nitrogen. Nitrate forms coloured compounds with several organic substances such as brucine, diphenylamine or phenoldisulphonic acid and most of these reactions have been proposed as a basis of a method of estimation. The phenoldisulphonic acid method is commonly used but shows interferences with nitrites and chlorides. A rapid colorimetric determination of nitrate in plant tissues was developed by Cataldo et al. (1975). It is free of interferences caused by the presence of ammonium, chlorides or nitrite.
The nitrates extracted from plant tissues are complexed with salicylic acid under highly acidic conditions to form a chromophore that absorbs at 410 nm in a basic solution. Absorbance is directly proportional to the amount of nitrate present and the colour is stable for at least 48 h.
• To 0.2 ml of nitrate containing extract, add 0.8 ml of 5% (w/v) salicylic acid prepared in concentrated sulphuric acid and mix thoroughly.
• After 20 min of incubation at 20 °C, gently add 19 ml of 2 n sodium hydroxide and homogenize. A yellow dye appears.
• After cooling, read the absorbance at 410 nm against a blank where salicylic acid is omitted.
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