The red laser-induced chlorophyll-fluorescence induction kinetics of predarkened leaf samples were registered simultaneously in the 690 and 730 nm regions i.e., in the region of the two chlorophyll fluorescence emission maxima. From the induction kinetics the chlorophyll fluorescence ratio F690/F730 was calculated. The ratio F690/F730 shows to be dependent on the chlorophyll content of leaves. It is significantly higher in needles of damaged spruces (values of 0.45-0.9) than in normal green needles of healthy trees (values of 0.35-0.5). During development and greening of maple leaves the ratio F690/F730 decreases with increasing chlorophyll content. Determination of the ratio F690/F730 can be a suitable method of monitoring changes in chlorophyll content in a non-destructive way in the same leaves during development or the yellowish-green discolouration of needles of damaged spruces in the Black Forest with the typical tree decline symptoms.

Using the two-wavelength chlorophyll fluorometer the fluorescence induction kinetics (Kautsky effect) were measured simultaneously in the 690 nm and 730 nm region for ten common tree species during the greening period (April to July). The chlorophyll-fluorescence ratio F690/F730 (i.e. ratio of fluorescence intensity at the two maxima near 690 and 730 nm) was calculated from the laser-induced induction kinetics (He/Ne-laser 632.8 nm) at the fluorescence maximum and the steady state. The ratio F690/F730 decreases with increasing chlorophyll content of developing leaves. Its dependence on the chlorophyll content can be fairly well expressed by a power function which has a general validity for leaves, pigment extracts and chloroplast suspensions. The ratio F690/F730 is somewhat higher at maximum (fm) than at steady-state fluorescence (fs), but there is a very good correlation between both values. The ratio F690/F730 is a good indicator of the chlorophyll content and can be used as a non-destructive measure of the chlorophyll content of leaves. It also appears to be a suitable fluorescence parameter in the future remote sensing of the physiological state of the vegetation by laser-equipped airborne systems.

The 1 fl r n of lants as excited by UV-lasers has r e ! ! o s e c f as a possibility to remotely sense stress effects in terrestrial plants. This blue fluorescence with a maximum near 440, has nothing to do with the red chlorophyll fluorescence. Several secondary plant products such as chlorogenic acid, trans-stilbenes, sinapic acid and possibly other phenolics of the epidermis la er are candidates for emitting blue fluorescence. ðer the blue fluorescence is increased or decreased by stress has yet to be determined as must the nature and composition of blue fluorescing substances in different lant species. The chl r h 11-fl resc nce is characterized gy two maxima .ea?% ind E O A, respectively. The ratio of the two maxima F690/F730 is mainly determined by the chloro hyll concentration of the leaf and to a lower degree also l ! y the photosynthetic activity of plants. The increase of the fluorescence ratio F690/F730 due to shortterm or long-term stress to vegetation is obtained with red or blue laser-light excitation of the chlorophyll fluorescence. The chlorophyll fluorescence ratio F690/F730 is therefore a very suitable parameter for the laser-induced remote sensing of the state of health of terrestrial vegetation.

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