Plant Structure and Light Interactions
One of the primary functions that helps a plant grow is photosynthesis. During photosynthesis a plant absorbs light energy and converts it to chemical energy. Light absorption happens in a molecule called chlorophyll. Figure 5 shows the percentage of visible light that is absorbed by chlorophyll (two pigment molecules) that would be found in a healthy plant leaf that is supporting an actively growing plant and by β-carotene, the pigment made in bright orange pumpkins, fruits, or carrot roots. Chlorophyll a (depicted by the solid line) and chlorophyll b (dashed line), absorb mostly blue and red wavelengths and reflect most of the green light, causing plants to look green. What we cannot detect with our eyes is that healthy plants reflect much more NIR wavelengths than the visible green. When a plant becomes unhealthy, it will reflect less NIR wavelengths and more of the blue and red wavelengths.
We can use Fig. 6 below to compare the information generated from a sensor to information we are accustomed to obtaining from our eyes and brain. Assume each line is the reflectance of light data obtained by a sensor collecting data from three different fields.
Figure 5. The absorption spectra of chloroplast pigments. A peak on the graph indicates a high absorption of light at the corresponding wavelength. A dip on the graph indicates little or no absorption of light at the corresponding wavelength.
Adapted from Biology 2e (Figure 8.14), 2018, OpenStax. CC BY 4.0.
Figure 6. Absorption/Reflectance of three fields. Each field is represented by a different line type (dotted, dashed, or solid). A peak on the graph indicates a high reflectance at the corresponding wavelength. Likewise, a dip on the graph indicates a low reflectance.
Image created using Microsoft Powerpoint by C. Mick, 2022
Figure 7. Aerial image of fields. Fields such as A, B, and C can be scanned by a sensor to generate reflectance data. The reflectance data for these three fields is in Fig. 6.
Reprinted from Green Field, by R. G. Angel, n.d., StockSnap.io. CC0 1.0.
See if you can answer this question: Which field in Fig. 7 is the solid line, which field is the dashed line and which field is the dotted line in Fig. 6?
The visual data from Fig. 7 we routinely interpret tells us that field A is green and healthy, field B is brown soil and field C is yellow and less healthy. The first way we connect the data from Fig. 7 and Fig. 6 is to recognize that we should be looking at the visible light wavelengths (450-650 nm) in addition to the NIR wavelengths (700-900 nm). Figure 6 shows us the spectral reflectance for the three different fields. The dotted line is uniformly reflecting light at all those wavelengths increasing from the visible to the NIR region. This spectral signature is indicative of soil and is useful when distinguishing the difference between soil and vegetative spectral signatures. Thus, the dotted line would be the soil (Area B).
The smooth line on the graph has two peaks – one in the green band and one in the NIR band. This indicates the field providing data for the smooth line is reflecting the highest amount of light in the green and NIR wavelengths. Using this information and the information you now know about plant light reflectance for healthy vs. unhealthy vegetation to match the solid and dashed lines with the plants that are reflecting different amounts of visible and NIR wavelengths. This activity illustrates how sensors provide data that our eyes and brains cannot. We should be familiar with the choices of sensors available to collect this reflectance data.