Optical spectroscopic techniques measure how matter interacts with electromagnetic radiation. It determines the transmission, emission, and absorption of electromagnetic radiation by light. Spectroscopy measures the interactions of electrons, protons, and ions in a material, based on collision energy. It is a popularly used non-destructive analytical tool in plant science and agriculture research, which helps determine the presence of microbial infection, pests, toxins, chemical composition, and adulterants in agricultural produce.
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Every element or compound has its unique spectral signature, i.e., it responds specifically to a particular wavelength, depending on its composition. In biological science, spectroscopy is widely used for qualitative and quantitative analysis. In plant science and agriculture, different types of optical spectroscopic techniques are used, including:
- Nuclear Magnetic Resonance (NMR)
- Ultraviolet-visible (UV-VIS)
- Fluorescence Spectroscopy
Infrared (IR) and ultraviolet-visual spectroscopy are mostly used in agriculture. This article discusses some of the current and future optical spectroscopic techniques associated with plant science and agricultural research.
Ultraviolet-Visible (UV-Vis) Spectroscopy
The UV spectrum used for spectroscopy lies within the UV (100 nm to 380 nm) and visible (380 nm to 750 nm) range of wavelengths. This spectroscopy is used for detecting bruises and diseases in plants, which are analyzed by studying the external damage of the sample. This technique is also used for quantitative analysis, for example, the determination of nutrient content of agricultural produce. Spectroscopy is also popularly used for quality control of edible oils, in terms of their fat oxidation and color.
Infrared (IR) Spectroscopy
IR spectroscopy operates within the IR band that stretches from 780 nm to 1 mm. This band stretch has been further divided into three sub-divisions, namely, infrared (30 µm to 1 mm), mid-infrared (5 µm to 30 µm), and near-infrared (780 nm to 5 µm).
This spectroscopic technique is widely used for quality control of pulses and vegetables.
- Mid-Infrared Spectroscopy (MIR): This is used to determine specific bonds and functional groups. It is also used for qualitative and quantitative analysis of carbon, nitrogen, and lignin. Typically, MIR has been used to study soil composition, its organic content, and other properties. MIR can be used as attenuated total reflectance (ATR), combined with the Fourier transform process (FT) and diffuse reflectance infrared Fourier transform (DRIFT) process. Researchers use ATR to determine the organic content of the soil, while DRFIT is used to analyze the chemical composition of soil and humus. MIR is used to determine the lignin, carbohydrates, cellulose, fats, and protein content in plants. Importantly, MIR is utilized to diagnose fungal disease in plants, as well as to measure levels of mycotoxins in cereals during the farming, processing, and storing stages.
- Near-infrared spectroscopy (NIRS): This has been used particularly in agriculture. This analytical tool is used to determine if vegetables and fruits are mature enough for harvest. Dry matter estimation ensures that fruits and vegetables are harvested at the right time to guarantee proper ripening. Typically, NIR spectroscopy is an essential tool for processing centers, which help monitor and select the right cereals, fruits, mushrooms, and vegetables for processing. NIR is also used to detect fungal disease, microbial contamination, and estimate the levels of mycotoxin present in the whole supply chain for cereals and grains. This analytical tool is also used for drought management as it identifies crops suffering from water stress and, thereby, helps farmers manage irrigation appropriately. Importantly, plant growth tracked by leaf spectrophotometer helps farmers and scientists to monitor the crop development for optimal nutrient (fertilizer) application.
The light emitted by a fluorescent molecule or fluorophore is called fluorescence. Typically, fluorescence spectroscopy is a sensitive technique that is used for quantitative analysis of small concentrations of compounds. This tool is used to detect contaminants, such as pathogens like Salmonella, mycotoxins, and additives such as aspartame. It is also used for structural analysis, for instance, the determination of minor changes in the structure of proteins, carbohydrates, and lipids in oils.
Raman Spectroscopy (RS) is another form of vibrational spectroscopy technique. RS provides spectral information of a sample based on the Raman effect, where incoming photons interact with electrons in a compound. Depending on the levels of vibrational energy in the atoms of the compounds, photons can lose or gain energy. RS is used for quality control, for example, it can detect the presence of adulterants in oils. It also assesses the chemical composition of food ingredients and products.
Other Spectroscopic Methods
Some other spectroscopic techniques used in agriculture and plant science are nuclear magnetic resonance (NMR) spectroscopy and atomic emission spectroscopy. NMR is based on the spectral information generated depending on the magnetic properties of atoms in a compound. This technique is used for soil analysis and the study of plant tissues.
Scientists use this technique to monitor the ripening, drying, and adulteration of food and agricultural products. Atomic Emission (AE) spectroscopy is popularly applied for the qualitative and quantitative detection of chemicals, especially, elements. AE spectroscopy coupled with inductively coupled plasma is used to detect the presence of trace elements.
Spectroscopic techniques are incorporated in small handheld tools so that they can be used by farmers as well.
Researchers have stated that spectroscopic techniques could play a major role in the future of smart agriculture. As climate change has heavily affected agriculture, smart agriculture that uses various optical sensors based on spectroscopy could be used to detect biotic and abiotic stresses of the plants early. Hence, farmers could implement appropriate measures immediately and protect their agricultural produce from massive loss.
Light is one of the most important environmental factors that affect plant physiology, which in turn impacts the yield and quality of plant-based produce. Plants' response to different light spectrums has been associated with growth and the production of hormones and secondary metabolites. This strand of research is heavily dependent on numerous spectroscopic techniques, used in situ or remotely. In the future, advanced inexpensive spectroscopic instruments must be available to all farmers.
References and Future Reading
Cavaco, A. M., et al. (2022) Making Sense of Light: The Use of Optical Spectroscopy Techniques in Plant Sciences and Agriculture. Applied Sciences. 12(3). pp. 997. https://doi.org/10.3390/app12030997
Avantes. (2021) Spectroscopy Applications in Precision Agriculture. [Online] Available at: https://www.avantes.com/content/uploads/2021/03/Precision-Agriculture-eBook-by-Avantes210208v2.5.pdf
Galletti, P. et al. (2020) Integrating Optical Imaging Tools for Rapid and Non-invasive Characterization of Seed Quality: Tomato (Solanum lycopersicum L.) and Carrot (Daucus carota L.) as Study Cases. Frontiers in Plant Science. 11. https://doi.org/10.3389/fpls.2020.577851
Farber,C. et al. (2019) Advanced spectroscopic techniques for plant disease diagnostics. A review. TrAC Trends in Analytical Chemistry. 118. pp. 43-49. https://doi.org/10.1016/j.trac.2019.05.022
García-Sánchez, F. et al. (2017). Using Near-Infrared Spectroscopy in Agricultural Systems. In K. G. Kyprianidis, & J. Skvaril (Eds.) Developments in Near-Infrared Spectroscopy. https://doi.org/10.5772/67236