The Role of Chlorophyll Fluorescence as a Bioassay for Assessment of Toxicity in Plants

The typical green terrestrial plant is well adapted to sensing and reporting significant changes in its environment. This allows native plants growing in natural settings to be used to assess changes that might be toxic to plant or animal tissue. The basis of this bioassay is the chlorophyll molecule, which serves as an intrinsic fluorescent probe of the performance and capacity of photosynthesis. Under normal conditions, 97% of the light energy absorbed by chlorophyll is converted to biochemical forms of energy in photosynthesis. Stress conditions can reduce the rate of photosynthesis, disturb the pigment-protein apparatus, or block the light-driven photosynthetic electron transport in the chloroplast. This results in an increased loss of absorbed light energy of 6 to 10% via chlorophyll fluorescence with a peak in emission at 683 nm at physiological temperatures. The inverse relationship between in vivo chlorophyll fluorescence and photosynthesis has long been known as the Kautsky Effect.

Light-induced chlorophyll fluorescence from dark-adapted leaves can be recorded with portable, sensitive instruments using intact leaves. This nondestructive method essentially monitors the physiological well-being of the plant. Any stress including disease, nutritional stress, water, temperature, radiation, and chemical stress can be quickly and accurately recorded. The overall photosynthetic process can be thought of as a series of sensitive sites connected to the fluorescent photosynthetic reaction center, which respond to a large number of different insults and report these effects as a change in fluorescence. Chlorophyll fluorescence in intact native plants can be used to assess toxicity in the environment or in a laboratory bioassay.