Spectroscopy measures how electromagnetic radiation interacts with matter as a function of wavelength, with absorption or emission of radiant energy. Visible (Vis) and near-infrared (NIRS) spectroscopy are modern tools used for various applications in different areas of daily life (Kortum 2012), such as medicine, chemistry and industry; moreover, in recent decades, interest has increased with the development of sensors that can be integrated into agricultural tools and equipment (Sankaran et al. 2010). In modern agricultural systems, this technique has been used in early detection of plant stress (Zahir et al. 2022), soil studies (Wetterlind et al. 2013), food quality (with emphasis on fruit quality), nutrient quantification, measuring nutritional parameters in grains, forage quality and mineral contents in fertilizers and compost (García-Sánchez et al. 2017). Techniques based on hyperspectral data in the Vis/NIRS ranges are being incorporated more quickly in the agricultural productive sector than techniques based on spectral images because the latter have limitations in spectral resolution, their precision is often limited, and they cannot effectively detect biotic and/or abiotic stress (Lu et al. 2020).
Importantly, specific applications of this technique are linked to each range of the measured spectrum. For example, the Vis range is defined as the wavelengths between 380-750nm, and, specifically in plants, its absorption is mainly associated with the presence and concentration of the principal photosynthetic pigments (chlorophylls, carotenes, xanthophylls, anthocyanins and phycobilins) (Blackburn 2006). The spectral information of the Vis range is widely used to detect physiological changes caused by different types of biotic or abiotic stress since these kinds of changes lead to variation in the spectral signature in the Vis, caused by the breakdown of chlorophyll pigments and changes in the levels of carotenoids and other pigments through degradation of the cellular structure (Zubler 2020). Otherwise, the spectra in the NIRS are quantitatively and qualitatively related to the chemical composition because the energy absorbed in this region by the plant causes vibration in covalent bonds C-H, O-H and N-H, which are important components of organic matter (Beć and Huck 2019). The tissue presents a variable spectrum in the NIRS because of differences in the organic composition and spectral information for this range in terms of compounds and structure, which, along with their concentrations, must be studied in plants (Beć et al. 2020). Additionally, plants do not use the infrared spectrum in photosynthesis, so there is no overlap of the spectral signatures in these two ranges.
Spectral characterization is related to the description of frequency measurements in a range of the electromagnetic spectrum and the study of their interaction with matter, whether with percentages of absorption, reflection, or transmission. The main applications of Vis and/or NIR spectroscopy in potato crops (Solanum tuberosum L.) have been used to determine the internal components of tubers, such as the amount of dry matter, moisture content, carbohydrates (mainly starch), protein content, determination of minor components (fats and acrylamides), estimation of total and individual carotenoids (López et al. 2013) and disease detection (Zhou et al. 2015; Fernández et al. 2020). In addition, other authors have focused on relating spectral signatures to external properties of tubers or plants, which are important to production and quality, such as damage (Evans and Muir 1999), texture (Boeriu et al. 1998) and sprouting capacity (Jeong et al. 2008)
However, currently there are few reports focused on the spectral characterization of healthy potato plants at different phenological stages. This knowledge is necessary to generate a baseline that can be used as a basic input for subsequent applied research, where detailed information can be provided on the spectral changes that occur in plants subjected to different types of biotic or abiotic stress. Potato is the fourth most important crop in the world, after rice, wheat, and maize, and is an indispensable product for the food and nutritional security of the population (Thongam et al. 2017; Seminario-Cunva et al. 2018; Singh et al. 2019). Solanum tuberosum L., Solanum tuberosum L. subspecies andigena, and Solanum phureja are also cultivated within the Solanum genus (Beltrán et al. 2020). Phureja group (creoles) is a particularly important food in Andean countries such Colombia and Peru (Ghislain et al. 1999). In this study, five potato varieties developed from the Phureja group were selected as model organisms, which will help to understand the functioning of the spectral response in other similar varieties and species. On the other hand, in this crop, some diseases (e.g., Phytophthora infestans) and water stress are critical factors for harvest yield (Jacques et al. 2009; Vivelter et al. 2022), where spectral information can be used in future research projects. Specifically, the main objectives of this article were to characterize the spectral signatures of five potato varieties (at the leaf scale) in three of the main phenological stages of growth and to identify potato varieties using reflectance spectroscopy in the visible (Vis) and near-infrared (NIR) ranges.