It is important to enhance crop productivity to meet the rising demand for food from the ever-increasing global population. Precision farming deploys smart farming systems such as sensors to enhance agricultural productivity and profitability without causing harm to the environment. An effective sensor could sense nitrogen concentrations, which is one of the main limiting factors for crop growth and agricultural productivity. This article discusses the use of Raman spectroscopy to detect nitrogen deficiency in plants.
Image Credit: Olya Maximenko/Shutterstock.com
Plants experience several biotic and abiotic stresses that impact their normal cellular functions. Nutritional deficiencies of plants are determined using chromatography- and colorimetry-based methods, and atomic absorption spectrophotometry (ICP). All these methods are invasive and destroy the sample during analysis. Therefore, more non-invasive methods are required for quick and effective determination of the nutritional content of plants.
Role of Nitrogen for Plant Growth
Nitrogen is essential for plant growth and helps determine agricultural productivity. Nitrogen content in a plant is associated with its chlorophyll content and photosynthesis efficiency.
When a plant is low in nitrogen concentration, it undergoes senescence, and this results in the lowering of plants’ biomass. Nutritional deficiencies are often taken care of by adding fertilizers in the agricultural field. However, in the case of applying more fertilizers than a plant needs, the excess is removed by runoff or infiltration into the water table. This pollutes the aquatic ecosystem, resulting in eutrophication.
Precision agriculture can prevent such pollution by sensing the exact nutritional requirement of a plant.
Utilization of Sensors for Early Detection of Nitrogen Deficiency in Plants
Optical sensors play a vital role in providing valuable information at the right time via spatially-resolved measurements of biophysical parameters.
This information helps farmers and scientists formulate appropriate management strategies to resolve the issues associated with plant growth and development. Sensors also enable the application of fertilizers in accordance with the plants’ nutritional requirements.
One of the optical sensors, i.e., near-infrared hyperspectral imaging used to determine nitrogen concentration in cucumber plants has been developed based on an assessment of chlorophyll content via foliage reflectance and transmittance, which has been associated with nitrogen deficiency. One limitation of this method is that the spectral reflectance due to nitrogen deficiency could overlap with the spectral transmission, owing to other nutrient deficiencies. Another limitation is associated with the late detection of nitrogen deficiency in plants.
Raman Spectroscopy to Detect Nitrogen Deficiency in Plants
To overcome the above-stated shortcomings, scientists have recently developed a Raman spectroscopy (RS) sensor.
RS is a non-invasive and non-destructive technique based on measuring inelastic light scattering induced by molecules that are excited to higher vibrational or rotational states. RS has been previously utilized to measure chlorophyll, β-carotene, and storage lipid contents in microalgae, under nitrogen-deficient conditions.
Researchers have recently discovered that Raman peaks associated with nitrogen can be measured in leaves; thereby, it could serve as an early evaluator of nitrogen content in a plant.
RS can be used to analyze a plant’s health in a non-invasive manner. Researchers identified a 1046 cm–1 Raman peak to be the signature of nitrogen status in Arabidopsis sp. This Raman peak for nitrogen was confirmed by using standard chemicals, including calcium nitrate, ammonium nitrate, and potassium nitrate.
Importantly, this Raman peak is specific to nitrogen stress and does not correlate with other common nutritional stresses of the plant, such as potassium and phosphorus. Scientists further analyzed the nitrogen status of Pak Choi (Brassica rapa chinensis) and Choy Sum (Brassica rapa var parachinensis) using the RS method.
The main significance of the above-mentioned study was the early detection of nitrogen deficiency in Arabidopsis and other vegetable crops, i.e., within three days of nitrogen starvation. This is before the onset of symptomatic manifestation of deficiency in plants. Early detection could enable farmers and scientists to formulate effective means to ameliorate stress. RS is a label-free, non-invasive, and non-destructive technique and these are some of the key advantages of using RS to detect nitrogen content in plants.
Advantages and Development of Portable Hand-Held Raman Spectrometer
Scientists have recently developed a portable Raman leaf-clip sensor for quick evaluation of plant stress, which includes nitrogen deficiency in plants. Handheld Raman spectrometers have been used in precision agriculture for the early diagnosis of plant stress. Scientists stated that this tool offers rapid and in vivo measurements of the nutritional content of the plant in farms.
The Raman leaf-clip sensor contains a Raman fiber probe (anodized aluminium) that is attached to a portable Raman instrument using an 830 nm excitation laser. This tool gently holds the leaf to suppress relative beam shift over the leaf surface. It also blocks ambient light and inhibits transmitted laser radiation, making the device safe for the eyes of the user. This analytical tool was tested on Arabidopsis thaliana to determine its ability to detect nitrogen deficiency in vivo and in situ, and the results were comparable to a benchtop Raman spectrometer.
The main advantages of this analytical tool are its simplicity, accuracy, and speed. It can also be used easily by farmers in the field and scientists in a laboratory. This tool can prevent agricultural loss due to nitrogen deficiency or premature leaf deterioration. It can also detect plant stress levels caused by drought, heat and cold, saline, and light.
Professor Nam-Hai Chua, co-Lead Principal Investigator, who was associated with the development of the Raman leaf-clip sensor, stated that this tool will positively aid farmers to maximize crop yield. It will also minimize aquatic pollution by reducing nitrogen runoff and infiltration into the water table.
References and Further Readings
Weng, S. et al. (2021) Advanced Application of Raman Spectroscopy and Surface-Enhanced Raman Spectroscopy in Plant Disease Diagnostics: A Review. Journal of Agriculture and Food Chemistry. 69(10). pp. 2950–2964. https://doi.org/10.1021/acs.jafc.0c07205
Huang, H. C. et al. Early Diagnosis and Management of Nitrogen Deficiency in Plants Utilizing Raman Spectroscopy. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2020.00663
Sanchez, L. et al. (2020) Raman Spectroscopy Enables Non-invasive and Confirmatory Diagnostics of Salinity Stresses, Nitrogen, Phosphorus, and Potassium Deficiencies in Rice. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2020.573321
Gupta, S. et al. (2020) Portable Raman leaf-clip sensor for rapid detection of plant stress. Scientific Reports. 10, 20206. https://doi.org/10.1038/s41598-020-76485-5
Singapore-MIT Alliance for Research and Technology (SMART). (2020). SMART researchers design portable device for fast detection of plant stress: Raman leaf-clip sensor would allow rapid diagnosis of nutrition deficiency in plants, enabling farmers to maximise crop yield in a sustainable way. ScienceDaily. [Online] Available at: www.sciencedaily.com/releases/2020/12/201208111422.htm