Jan. 14, 2021
Jan. 11, 2021
Spectroscopy is a precise and non-destructive technique that can tell us about several processes going on in plants and trees. After being integrated into small handheld devices, it can provide instant and accurate results in the field, forests, and laboratories. It replaces several conventional methods, which were time-consuming and often needed weeks of estimation in expensive laboratories. By reducing time and costs, spectroscopy has found a place in precision farming. It is worth learning about some of its common applications in agriculture, as well as forestry.
Spectroscopy is a technique that measures the different spectra of light that are absorbed or reflected by any matter. Plant tissues, including leaves, are made up of numerous bio-compounds - such as carbohydrates, sugars, starch, amino acids, proteins, pigments, minerals, etc. - all of which have a different spectral signature.
Since spectroscopy is a very sensitive process, it can detect even molecular or atomic differences in compounds.
Spectroscopy has a wide range of use on different scales. At large scales, it is used in remote sensing to study vegetation types, deforestation, changes in land-use, etc
At the individual plant scale, there are also numerous uses of leaf spectroscopy.
The spectral signature of leaves is not constant. They vary with species, age, plant condition, and environmental factors since these factors affect the many processes occurring in plants. Some of the important processes spectroscopy sheds light on are covered below.
Of all the compounds in a leaf, the pigments absorb the maximum amount of light. Therefore, pigments have a strong spectral signature. Depending on the colour of the light absorbed and those reflected, it is easy to analyze the pigment makeup of leaves.
Though the colour of the leaves is influenced by the predominant pigment at any given time, other pigments are also present in a leaf; hence, leaves are green when chlorophyll concentrations are at their highest. During leaf senescence (leaf death), due to the age of a crop or during autumn in trees, chlorophyll is broken down and other pigments are expressed when their concentrations are higher than the concentration of chlorophyll; so leaves appear yellow, orange, or brown when they are dying (See Figure 1).
Various indices available in spectrometers can be used to calculate the levels of pigments in leaves, such as
Figure 1: Leaf spectroscopy of Sweetgum Leaves (Liquidambar styraciflua L.) at different ages of the crop. From Jensen 2007. (Image credits http://www.gers.uprm.edu/geol6225/pdfs/08_rs_vegetation.pdf)
Chlorophyll a and chlorophyll b absorb light that is vital to the process of photosynthesis.
Spectroscopy is a non-destructive method to measure the amount of chlorophyll in the leaf canopy of annuals and perennials to predict trends in photosynthesis. Since all compounds produced in the plants start with products of photosynthesis, chlorophyll is very important for plants (See Figure 2).
Figure 2: Chlorophyll absorbs light at ~420 and ~650 nm, i.e., blue and red light; water-content in plants absorbs light in the shortwave infrared spectrum. From NASA (Image credits: Eric Brown de Colstoun)
It is possible to get a snapshot of the nutrients that plants have absorbed until the day of testing by analysing the plant mineral content through leaf spectroscopy. This method is, in general, better than testing soil nutrient content to detect nutritional imbalances and correct it with proper fertilisation. Then there are other cases where spectroscopy of leaves is particularly suitable as described below.
Water content is important for plants, as it is one of the raw ingredients of photosynthesis and is necessary to maintain biomass. Water content in plants can decline due to decreasing soil water content, as well as high temperature, competition among plants, etc.
Therefore, the analysis of plant water content through leaf spectroscopy has many applications besides determining plant productivity (See Figure 3).
It is being used to design water saving strategies and to fight climate change.
Figure 3: The reflectance changes as the water content in Magnolia (Magnolia grandiflora) decreases. (Image credits: http://cstars.metro.ucdavis.edu/files/3613/4419/0702/Lecture_3-Leaves__Plants.pdf)
Plant diseases are a major cause of crop and profit loss for growers. Despite having resistant varieties, it is difficult to eradicate diseases.
Spectroscopy is handy here, as infected leaves have a different spectral image than healthy leaves. The symptoms and stress caused by the pathogens change pigmentation, alter surface temperature, and reduce chlorophyll content.
These changes sometimes begin weeks ahead of the appearance of visual symptoms.
Hence, by monitoring and measuring the change in the spectral image of leaves, early detection of many diseases is possible. The progression of diseases, which is usually accompanied by an increase in severity and development of further symptoms like leaf area loss or lesions, can be arrested with timely and early treatment to reduce crop loss.
Air pollutants such as sulfuric acid (think acid rain) and ozone cause damage to the leaf surface and roots of plants when they soak into the ground. On the leaves, the pollutants attack and destroy the wax and affect the leaf membrane. The membrane contains chloroplasts, and thylakoids, which, as well as the leaf wax, are bio-chemicals produced by plants.
Alterations in their structure due to pollutants can be detected by spectroscopy to tell us the extent of damage to plants. Depending on the type of damage, it is also possible to identify the pollutant that has caused the leaf damage.
Near Infrared (NIR) spectroscopy is one of the most common techniques used in leaf spectrometers. The CI-710 Miniature Leaf Spectrometer, designed and developed by CID Bio-Science, uses NIR spectroscopy to measure transmission, absorption, and reflection of light. It is a small device that is suitable for testing leaves of various sizes and thickness. Its precise and quick results can be immediately used in a wide choice of indices the supporting software provides, making it more versatile.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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