Leafy green vegetables can accumulate high concentrations of nitrate–nitrogen (NO3-N) that are potentially harmful if consumed by humans (Blom-Zandstra, 1989; Corre and Breimer, 1979; Maynard et al., 1976; Roorda van Eysinga, 1984). Nitrate-N concentration in leafy greens can fluctuate diurnally because it is often inversely related to light intensity (Muramoto, 1999; Reinink, 1991; Steingrover et al., 1986; Steingrover and Ratering, 1986). As a result of rapid NO3-N fluctuation, it would be convenient for growers to be able to quickly monitor concentrations to optimize fertilization and harvest timing. Although conventional means of measuring plant tissue NO3-N are accurate, well accepted, and reliable, they often require sophisticated equipment and trained technicians and can be time-consuming, expensive, and impractical outside of a laboratory setting.
Many laboratories are interested in analytical methodology capable of overcoming the inconveniences and high costs of conventional methods such as sequential or flow injection analysis (Lin et al., 2005). Potentiometric methods involve the use of ion selective electrodes (ISEs) to measure ion concentrations in plant tissue and soil extracts and are relatively inexpensive, easy to operate, and rapid (Watson and Isaac, 1990). These advantages of potentiometry have led to the development of NO3-N ISEs that expedite analytical procedures (Lin et al., 2005). An Orion 93-07 NO3-N ion-selective electrode and Orion 90-02 double-junction reference electrode (Thermo Fisher Scientific, Waltham, MA) gave accurate and precise NO3-N measurements of vegetable tissue extracts (Consalter et al., 1992) and in situ measurements of soil NO3-N concentration (Thottan et al., 1994). Lin et al. (2005) analyzed solutions of mineral water and vegetable extracts with two ISEs and a spectrophotometric method and found that the potentiometric methods were both precise and accurate with no significant difference between the two methods at the 95% confidence level.
Relatively inexpensive and portable ISE nutrient monitoring devices such as the Cardy NO3-N meter (Horiba Insruments, Irvine, CA, and Spectrum Technologies, Plainfield, IL) are capable of measuring fresh plant sap NO3-N levels directly. Several authors have shown that measurements made with the Cardy meter using nondiluted plant sap were well correlated to conventional methods of NO3-N analysis (Altland et al., 2002, 2003; Hartz et al., 1993; Westcott et al., 1998). However, others found Cardy meter NO3-N values were consistently higher than values derived from conventional analysis methods unless sap was diluted to account for ionic strength effects and specific ion interferences (Davenport and Jabro, 2001; Errebhi et al., 1998; Hochmuth, 1994; Kubota et al., 1996; Rosen et al., 1996; Westcott et al., 1993). Although several of these authors suggest that dilution may be necessary to improve accuracy, most still found adequate relationships between conventional methods of tissue analysis and fresh sap analysis with the Cardy meter (Errebhi et al., 1998; Kubota et al., 1996; Rosen et al., 1996; Westcott et al., 1993, 1998).
The objective of this study was to compare the analysis of fresh leaf sap using a Cardy portable NO3 meter with dry leaf tissue extracts analyzed using ISE and colorimetric methods to determine if rapid, less expensive tissue processing and analysis methods can substitute for more laborious procedures requiring expensive instrumentation for quality assessment in leafy green vegetables.
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