Background

Nitrogen is an important macronutrient required by plants for normal growth and development. Usually most plants absorb nitrogen mainly in the form of nitrate grown under aerobic conditions (Xu et al., 2016). To determine the nitrate accumulation in plants, we need to test the nitrate content in different tissues of plants. There are some methods for determination of nitrate, for example, potentiometric method (Carlson and Keeney, 1971), phenoldisulfonic acid method (Bremner, 1965), Cadium reduction (Huffman and Barbarick, 1981) and other methods. These methods have some disadvantages, such as lower sensitivity, interferences, technician exposure to carcinogenic chemicals (Cataldo et al., 1975; Vendrell and Zupancic, 1990)

Here, we provide the salicylic acid method that is free of interferences, reliable and stable. Nitrosalicylic acid is formed by the reaction of nitrate and salicylic acid under highly acidic conditions. The complex is yellow under basic (pH > 12) condition with maximal absorption at 410 nm. The absorbance is directly proportional to nitrate content. Therefore the nitrate content in tissues can be calculated based on their absorbances. This method is suitable for determination of nitrate concentration in plants.