Plant leaves absorb primarily red and blue light to make food (photosynthesis), and reflect greater amounts of green light–hence the leaves are green! Plant leaves that are stressed due to drought, flooding, ozone, excessive foot traffic, salt spray, acid rain, nutrient deficiencies, disease, pests, etc., are not as productive photosynthesizers, so they reflect more red and blue light. Using a filter that blocks out (absorbs) the green light, healthy plants will appear dark and stressed plant leaves will be a noticeably lighter color.
These filters do not tell you why the plant is stressed; they just make it easier see stressed parts of plants. You need to look for clues to find the cause of the plant stress:
Are all plants stressed or only particular species of plants stressed? Is there a common direction the plants are stressed? (Adjacent to road may indicate damage by road salt spray; if only the ocean-side leaves are burned, this indicates salt spray damage during storms, etc.) Look for presence of pests, blights. Is there damage only in low lying areas or atop hills or mountains?
Satellite imagery also does not show the causes of environmental stress–scientists must look for clues based on patterns in the imagery as well as do field work in the areas that appear stressed.
Dr. Barry Rock from the University of>
New Hampshire wearing the paper version of
the plant stress detection glasses during
a teacher workshop.