Kuecke, M. & Schnug, E. (1996) Advantages of nitrate determination by use of E. coli cells in different matrices. Comm. Soil Sci. Plant Anal. 25, 997-991.
I suggest you to go through the following article for finding a rapid colorimetric method for determination of nitrate in plant tissue. For your use, I am also attaching a PDF copy of the same article herewith.
D.A. Cataldo, H. Haroon, L.E. Schrader and V.L. Youngs (1975) Rapid Colorimetric Determination of Nitrate in Plant-Tissue by Nitration of Salicylic-Acid. Communications in Soil Science and Plant Analysis 6(1):71-80.