Join us to learn more about this project!
In science, it is always helpful to ask a lot of questions. It is important to know why and how scientists get answers. It is also important to understand the results of experiments. Here are some pictures that were obtained by remote sensing in Biosphere 2. The scene of desert vegetation is shown in both visible and infrared images. Visible is considered "natural" color, or how you and I see the world every day. The infrared pictures are false color composites that show the same plants at different wavelengths that are not in our visible region. What differences and similarities do you see in these pictures?
(Vis-IR comparison of Desert scene)
To begin to understand the images above, it will be helpful to learn about basic leaf anatomy and how it affects the way plants that reflect light in the visible and near infrared spectrums. First, we will discuss the basic structure of a leaf.
Leaf Anatomy (Figure from Introduction to Remote Sensing, Campbell 1996)
The typical plant leaf is made up of many tissues and cells. The top layer is called the upper epidermis and is covered by a waxy cuticle, which prevents moisture loss. On the underside of a leaf, the lower epidermis includes stomata, which are openings to the inner leaf that allow air to enter. The stomata are each opened and closed by a pair of guard cells, which control the amount of CO2 (used in photosynthesis), and moisture let into the leaf (Campbell 1996). Below the upper epidermis are elongated cells including chloroplasts, which contain chlorophyll and other photosynthetic pigments. These cells form the palisade layer. Between the palisade tissue and the lower epidermis lies tissue made up of non-uniform cells separated by air spaces. This tissue is called spongy mesophyll where the cells absorb carbon dioxide and expel oxygen into the air spaces in the processes of photosynthesis and respiration. Although the structure of leaves contain these basic components, all leaves are not the same which allows them to respond differently to like circumstances such as exposure to identical levels of light.
How do plants reflect light at the visible and infrared wavelenghts? In visible light, humans observe plants to be primarily green in color. This is due to the fact that chlorophyll molecules in plants absorb most visible (blue, red, and green light). The red and blue light is important for the photosynthesis process. However, more of the green light that comes into contact with vegetation is reflected than blue and red light, causing green to be the prominent color of vegetation according to the human eye.
This graph shows how chlorophyll will absorb blue and red light, but does not absorb as much green light. Instead, the green light is reflected causing a leaf to look green. (Figure from Introduction to Remote Sensing, Campbell 1996)
Unlike the visible spectrum, near infrared reflectance depends on the spongy mesophyll tissue of the plant. This is because most visible light is absorbed in upper layers (by chlorophyll) while infrared radiation passes through to the mesophyll. The more spongy the mesophyll layer is, the more the leaf reflect will reflect light in the near infrared. It is the air spaces between the cells that makes the mesophyll spongy, so those plants with many air spaces between their mesophyll cells will reflect the most infrared light. There are significant structural differences in the mesophyll in plants, causing them to reflect varying amounts of light in the near infrared spectrum. The mesophyll cells and air spaces strongly reflect (goes up) and transmit (goes down) incoming radiation. Reflectivity in the near infrared will vary more between species than in the visible spectrum, allowing people to classify healthy vegetation using infrared light quite efficiently.
This picture shows red, green, blue, and infrared light interacting with a leaf. (Figure
from Introduction to Remote Sensing, Campbell 1996)
The figure below shows how red (R), green (G), blue (B), and infrared (IR) light is reflected by a leaf. The palisade tissue containing chloroplasts absorbs the red and blue light, while the green light is slightly absorbed but mostly reflected by the same tissue. The infrared radiation passes through the cuticle and palisade tissue and comes into contact with the mesophyll tissue where it is scattered by the air spaces in between the mesophyll cells and then either reflected or transmitted. This produces the rise in reflectance seen in the following figure.
This graph shows how a leaf reflects light in the visible and near infrared spectrums.There is a high reflectance of green light and a very high reflectance of near infrared light. (from Introduction to Remote Sensing, Campbell 1996)
When comparing young to mature leaves of healthy vegetation, the young leaves have a lower reflectivity than the mature leaves. This is because young leaves are still developing structurally and are more compact with less air space in the mesophyll tissue than the mature leaves (Gausman 1974). The pictures below show the cellular structure of a young, compact citrus leaf (top) and a mature citrus leaf (bottom). Do you see how the mature leaf has spaces between cells? Those are the air spaces within its mesopyll that help scatter (reflect and transmit) infrared radiation. Therefore, the mature leaf reflects more near infrared radiation than the young leaf.
Shown above are the cellular structures of a young (top) and a mature (bottom) citrus leaf. Notice the high amount of spaces between cells in the mature leaf. (Figure from Gausman 1974)
The following graph shows the reflectance of the young and mature leaves. As you can see, the mature leaf has a higher reflectance of near infrared light (at wavelength 750- to 1350 nanometers).
When vegetation becomes stressed, reflectivity is lower than that of healthy vegetation. Stress can include disease, lack of moisture, insect damage, and/or maturation. When vegetation is stressed, reflectivity in the near infrared is influenced either by the collapsing of the intercellular spaces in the mesophyll layer or by cell wall degeneration (Gausman 1974, Campbell 1996). The destruction of the intercellular air spaces may reduce light scattering within the leaf causing reflectance to decrease.
It was mentioned before that different species of plants have different mesophyll structures that cause the plants to vary in the amount of near infrared light they reflect. In the pictures below, there are two transections of leaves. One is a corn leaf (left) and the other is an oleander leaf (right). Try to figure out where the different tissue layers and cells are. You may be able to see that the oleander has more air spaces in the mesophyll tissue than the corn does. Accordingly, the oleander’s reflectivity in the near infrared is higher. You will be able to see the percent reflectance of near infrared radiation for corn and oleander, along with eucalyptus and hyacinth leaves in the graph below the pictures.
The pictures show the cellular structure of a corn leaf (left) and an oleander leaf (right). (Figure from Gausman, 1974)
The graph shows the percent reflectance of radiation for corn, oleander, eucalyptus, and hyacinth species. (Figure from Gausman, 1974)
References
Gausman H.W. 1974. Leaf Reflectance of Near-Infrared. Photogrammetric Engineering, Vol. 40, pp. 183-191.
Campbell, James B. 1996. Introduction to Remote Sensing, Second Edition. New York: The Guilford Press, pp. 444-476.
Now that you're an expert on plant reflectance...