The Chlorophyll Fluorescence Ratio F690/F735 as a Possible Stress Indicator

Abstract

The chlorophyll fluorescence emission spectra, as excited by blue and red light including laser light, were determined depending on the chlorophyll content and the photosynthetic activity of leaves and needles. The fluorescence-emission spectra exhibit two maxima or a shoulder in the 690 nm and the 735 nm region. The corresponding fluorescence ratio F690/F735 for green leaves is higher (values of 0.8 to 1.1) when excited with blue light (range 400 to 500 nm) than excited with light in the yellow to red wavelength region (525 to 633 nm), which only yields values for F690/F735 of 0.5 to 0.7. The values for the ratio F690/F735 are drastically increased with decreasing chlorophyll content of leaves and to a lower degree also by a decline of photosynthetic activity (e.g. herbicide treatment, needles of damaged forest trees) no matter whether the fluorescence is excited by red or blue light. Since stress induces a lower chlorophyll content as well as lower rates of photosynthesis, the ratio F690/F735 can be taken as indicator of stress to plants. The application of the ratio F690/F735 in detecting stress to terrestrial vegetation via remote sensing of the chlorophyll fluorescence is discussed.