Leaf colour pattern varies depending on leaf age, pathogen infection, environmental, and nutritional stresses. Thus, these patterns are widely used to diagnose plant health statuses in agricultural fields. Vis-NIR handheld sensor measures the spectral
pattern of leaf reflectance from ∼400 nm to ∼900 nm that cover absorption peaks of all phytopigments and water. With a
higher resolution in wavelength space, the spectral information allows us to accurately estimate the concentration of those small
compounds in a non-destructive manner. Most handheld sensors specialized for plant diagnostics provide limited information of individual plant constituents such as anthocyanin and chlorophyll while general handheld spectrometers measure and save raw reflectance intensities from which users need to extract the information of interest.
Investigating anthocyanin content is a continuing interest within the agriculture research community due to its high an- tioxidant properties, which are known for myriad of health benefits1. Consumption of anthocyanin rich foods has been shown to reduce the risk of cardiovascular disease2, type 2 diabetes3, and inflammation4. In plants, the amount of anthocyanin determines the color and appearance of the leaves, which serve as one of the markers of the quality5. Currently, the conventional form of anthocyanin measurement utilizes either chemical or biochemical techniques. The chemical approach involves harvesting the leaves followed by multiple extraction steps6. Such a method is thus, time-consuming, and expensive. Another quantifying method is biochemical processes where leaves are analyzed using one of the following techniques: gas chromatography (GC), high performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), liquid chromatography- mass spectrometry (LC-MS) etc.7. The aforementioned techniques are laborious, expensive, require special equipment, and regrettably destructive.
Optical spectroscopy is a rapid, non-destructive analytical technique that uses the concept of light reflectance to evalu- ate leaf structures and their biochemical properties. The intensity of the reflected light at various wavelengths depends on the absorption of light by the pigments present in the leaf. This allows for the quantitative measurement of the functional properties of plants such as carotenoids and anthocyanin. Optical spectroscopy has been widely adopted for plant health monitoring8–10.
Presently, several expensive industrial systems are available, such as the SpectraVue Leaf Spectrometer11 and SPAD 502 Plus Chlorophyll Meter12 that measure leaf pigments and hence, specific nutrients level. However, these systems are either bulky or limited in the type of pigments detectable. With this in mind, we had developed a portable, cost effective, and easy to use handheld Vis-NIR spectrometer to quantify the plant pigments in a non-invasive manner.
The capability of the spectrometer is also expandable to include chlorophyll quantification, which is crucial in determin- ing senescence in plants. Unlike senescence in animal cells, where the cell cycle arrest in damaged or aged cells13, plant senescence refers to the essential redistribution of nutrients to ensure species propagation14,15. Leaf yellowing is usually the tell-tale sign of chlorophyll degradation, which indicates the initiation of senescence. Yet even before yellowing becomes visible, chlorophyll levels have already started to decrease16,17. Therefore, the timing of crop harvest is pivotal as premature senescence can reduce final crop yield while overdue harvesting can compromise nutrient quality of the crop18,19.
In this study, we have newly designed and developed an affordable handheld device that collects leaf reflectance spectra, wirelessly transfers the spectral data through Bluetooth, and provides both raw spectral data and processed information (estimated chlorophyll and anthocyanin contents) to users. We begin by explaining the intuition behind the design and construction of the spectrometer. Next, using anthocyanin and chlorophyll content as case studies, we successfully showed that our device is capable of accurately quantifying these phytopigments. Data obtained from the spectrometer significantly correlated with existing conventional established methods of quantitating anthocyanins and chlorophyll. These findings strongly suggest that our developed portable handheld Vis-NIR system has utility as a non-invasive and cost-effective method of monitoring plant pigment levels.