A Comparison of Rapid Potentiometric and Colorimetric Methods for Measuring Tissue Nitrate Concentrations in Leafy Green Vegetables

Leafy green vegetables such as lettuce (Lactuca sativa), Asian greens (Brassica spp.) and spinach (Spinacia oleracea) have a tendency to accumulate high concentrations of potentially harmful nitrate-nitrogen (NO3-N). It would be advantageous for growers to have rapid and inexpensive methods to accurately measure plant tissue NO3-N to make fertility and harvest management decisions for these crops. This study compared fresh sap expressed from whole leaves and analyzed with a Cardy meter with the analysis of dry leaf tissue extracts analyzed with a benchtop ion selective electrode (ISE) and an automated colorimetric method for determining NO3-N concentration. Results from ISE and colorimetric analysis of the same dry leaf tissue extracts had a strong relationship (r(2) = 0.92). The ISE was relatively easy to operate and affordable, suggesting it is an adequate substitute for automated colorimetric analysis of dry plant tissue extracts. Results of fresh whole leaf sap analyzed with the Cardy meter showed a poor relationship with dry leaf tissue extracted and analyzed using the ISE (r(2) = 0.25) or with colorimetric analysis (r(2) = 0.21). When fresh whole leaf sap was diluted 1:1 with aluminum sulfate [Al-2(SO4)(3)] to adjust for potential matrix effects, there was still a relatively poor relationship (r(2) = 0.41) between the diluted sap samples analyzed with a Cardy meter and the dry leaf tissue extracted and analyzed with the ISE. When the same dry leaf tissue extracts were analyzed with the Cardy meter and the ISE, the results related well (r(2) = 0.96). As a result of tissue processing and/or instrument differences, Cardy meter analysis of sap expressed from whole leaves was not comparable to ISE or colorimetric analyses of dry leaf tissue extracts for leafy green vegetables.