Scientia Horticulturae 134 (2012) 1–6 Contents lists available at SciVerse ScienceDirect Scientia Horticulturae journa l h o me page: www.elsevier.com/locate/scihorti Review A critical evaluation of on-farm rapid tests for measuring nitrate in leafy vegetables Sophie E. Parks a,∗ , Donald E. Irving b , Paul J. Milham c a New South Wales Department of Primary Industries, Locked Bag 26, Gosford, New South Wales 2250, Australia b New South Wales Department of Primary Industries, Yanco, New South Wales 2703, Australia c New South Wales Department of Primary Industries, Richmond, New South Wales 2753, Australia a r t i c l e i n f o Article history: Received 1 June 2011 Received in revised form 5 October 2011 Accepted 16 October 2011 Keywords: Nitrate accumulation Plant analysis Chlorophyll meter Nitrate test strip Nitrate-selective electrode Plant sap a b s t r a c t Nitrogen (N) supply to crops impacts on the synthesis of the green photosynthetic pigment, chlorophyll, and on plant nitrate concentration. Insufficient N supply decreases nitrate concentrations, chlorophyll synthesis and plant productivity. Excess N supply causes undesirably high nitrate concentrations in leafy vegetables and does not increase chlorophyll concentration. Consequently, analysis tools have been developed to determine the nitrogen status of crops. This paper reviews three tools that have been rec- ommended for on-farm monitoring of crop nitrogen status: nitrate-selective electrodes, nitrate-sensitive test strips, and chlorophyll meters. The electrodes and test strips analyse plant sap, which in addition to nitrate, also contains other substances, including chloride. These substances decrease the accuracy of nitrate measurements made using ion selective electrodes more than those made using test strips. Chlorophyll meters indicate whether sufficient N has been assimilated to maximise plant production, but provide no information about current supply or whether excessive nitrate has accumulated in the leaves. Consequently, chlorophyll meters may be useful for on-farm monitoring of crop N status; how- ever, for managing both the sufficiency of N supply and nitrate levels, only nitrate test strips can be recommended. Greater general awareness of the weaknesses of field N testing methods would lead to better guidelines for their use, and perhaps, improvements in the technologies. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Nitrate-selective electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Nitrate-sensitive test strips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Chlorophyll meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5. Comparison of rapid nitrate tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Accuracy of the methods in plant production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.2. Cost and suitability of methods for testing plant samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Development of nitrate test methods for leafy vegetables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction Many leafy vegetables accumulate nitrate. Examples include Swiss chard (Beta vulgaris L.) (Parks et al., 2008), rape (Brassica campestris L.), Chinese cabbage (Brassica chinensis L.), spinach (Spinacia oleracea L.) (Chen et al., 2004) and lettuce (Lactuca ∗ Corresponding author. Tel.: +61 2 4348 1914; fax: +61 2 4348 1910. E-mail address: sophie.parks@industry.nsw.gov.au (S.E. Parks). sativa L.) (Huett and Dettmann, 1992). Nitrate supply has a major impact on the concentration of accumulated nitrate, but tempera- ture (Van der Boon et al., 1990; Cantliffe, 1972a) and light intensity (Cantliffe, 1972b; Proietti et al., 2004) during growth, plus a range of other factors such as genotype, diurnal variation, molybdenum deficiency, soil type, production system, and irrigation regime can all contribute to the concentrations found in leaves. It is undesirable to produce vegetables with accumulated nitrate, as vegetable quality may be compromised. Examples include reduced vitamin C concentration in lettuce (Poulsen et al., 0304-4238/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2011.10.015