Plants are essential sources of bioactive substances that promote health. For economic reasons, farmers usually focus on obtaining higher yields rather than crop nutraceutical quality. The application of non-essential elements (NEEs) is a technique used to increase secondary metabolites (SMs) in plants. This technique includes variations of the essential elements ratios in a nutrient solution or the inclusion of elicitors, such as salicylic acid or methyl jasmonate. Elicitor use is controversial because plants grow differently in inert substrates, in vitro and soil. Soil contains essential elements (EEs) and NEEs that can enhance SM synthesis and increase nutraceutical plant quality. However, any technique that modifies plant metabolism can decrease yields. Thus, developing techniques to increase both agricultural product yield and quality is necessary. This review aims to demonstrate the necessity for a new recipe or "cocktail" of plant nutrients based on EEs and NEEs, and elicitors apply to achieve both a high yield and crops nutraceutical quality.
Nitrogen is essential for the plant because it is used for the production of chlorophyll, proteins, nucleic acids, amino acids, and other cellular compounds; nitrogen is available in two forms: ammonium and nitrate. Several tools have been used to quantify nitrates in plants such as the Kjeldahl method and Dumas combustion digestion; however, they are destructive and long time-consuming methods. To solve these disadvantages, methods such as selective electrodes, optical sensors, reflectometers, and images based sensors have been developed; nonetheless, all these techniques show interference when carrying out measurements. Currently, biosensors based on genetic constructions, based on the response of promoter gene fused to Gene Fluorescent Protein (GFP), are gaining popularity, because they improve the accuracy of measurements of nitrate by avoiding the interference of carriers ion, high salt conditions, and other factors. The present review shows the different methods to quantify the nitrogen in plants; later, a biosensors perspective is presented, mainly focused on biosensors based on organism genetically modified. The review presents a list of promoter and reporter genes that could be used to develop different kind of sensors, and a perspective of sensors to measure quantitatively the nitrogen is presented.
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