Abstract
Nitrate is an important component of the nitrogen cycle and is therefore present in all plants. However, excessive nitrogen fertilization results in a high nitrate content in vegetables, which is unhealthy for humans. Understanding the spatial distribution of nitrate in leaves is beneficial for improving nitrogen assimilation efficiency and reducing its content in vegetables. In this study, near-infrared (NIR) hyperspectral imaging was used for the non-destructive and effective evaluation of nitrate content in spinach (Spinacia oleracea L.) leaves. Leaf samples with different nitrate contents were collected under various fertilization conditions, and reference data were obtained using reflectometer apparatus RQflex 10. Partial least squares regression analysis revealed that there was a high correlation between the reference data and NIR spectra (r2 = 0.74, root mean squared error of cross-validation = 710.16 mg/kg). Furthermore, the nitrate content in spinach leaves was successfully mapped at a high spatial resolution, clearly displaying its distribution in the petiole, vein, and blade. Finally, the mapping results demonstrated dynamic changes in the nitrate content in intact leaf samples under different storage conditions, showing the value of this non-destructive tool for future analyses of the nitrate content in vegetables.
Highlights
Nitrate is a salt of nitric acid that occurs naturally in the environment
Spinach (Spinacia oleracea L.) plants were cultivated using a mixture of fertilizer and hydroponic treatments to provide a broad range of nitrate contents
Reference nitrate content values of 2,000–4,000 mg/kg were usually obtained from spinach leaves grown under normal cultivation conditions, while values of 4,000–6,000 mg/kg were detected in leaves grown under high fertilization conditions
Summary
Nitrate is a salt of nitric acid that occurs naturally in the environment. It is found in the soil and is taken up by all plants to be used as a primary nitrogen source. High levels of nitrate fertilization contribute to elevated nitrate contents in the harvested plants. Leafy vegetables that are grown in the soil accumulate high concentrations of nitrates in their leaves and stems, which are consumed by humans as a rich source of inorganic nitrate. Many countries have set regulations to control the maximum levels of nitrate in Mapping Nitrate Content in Spinach foodstuffs, and a reduction in the nitrate concentration in vegetables has been considered at production sites
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