1.5. Fluorescence of Green Plants. 
Fluorescence involves the absorption of photons having sufficient energy to excite the molecules to a higher energy 
state. As the excited molecules return to the ground state, the excitation energy is lost by internal conversion to heat 
or by emission as light (fluorescence) at a longer wavelength than the excitation wavelength. In plants, 
measurements of chlorophyll fluorescence provide information on photosynthetic efficiency and the response of plants 
to stress (Foyer, 1993). There is a strong inverse relationship, in general, between chlorophyll fluorescence and the 
level of photosynthetic activity of plants. Chlorophyll fluorescence occurs in the red (685 nm) and near infrared (740 
nm) regions of the spectrum when the plant is irradiated at wavelengths extending from 320 to 650 nm. Another 
fluorescence emission exists in the blue region when plants are excited with ultraviolet (UV) radiation. The origin 
of the blue emission is not completely understood, but there is increasing evidence that it is associated with both non 
photosynthetic molecules as well as molecules involved in the electron transport mechanism of the plant. These 
emissions are highly specific for plant canopies and contain complementary information. Their determination by 
remote sensing techniques is a promising tool for monitoring plant canopies (Chappelle and Williams 1987; Guyot, 
1993). 
The chlorophyll fluorescence yield of chlorophyll is greatest (approximately 1CT 1 ) at excitation wavelengths 
of 435 and 670 nm (Chappelle and Williams, 1987). In photosynthesis, the two photosystems drive a complex 
oxidation-reduction chain, along which electrons are transported to C0 2 . The energy required to transfer an electron 
to the acceptor corresponds to that of a red photon at 675 nm. The extra energy of shorter wavelength photons is 
converted to heat. Chlorophyll a has two partly overlapping florescence bands with maxima at 690 and 735 
(Lichtenthaler and Rinderle, 1988). Chlorophyl 1 fluorescence at 690 nm comes primarily from photosystem 2, while 
photosystem 1 contributes only to fluorescence at 735 nm. However, photosystem 1 absorbs at 680 nm, so 
fluorescence of photosystem 2 may be reabsorbed by photosystem 1. Thus at the leaf level, measured fluorescence 
depends, not only on the photosynthetic activity, but also on chlorophyll content (Lichtenthaler and Rinderle, 1988). 
At the canopy level, measured fluorescence depends on leaf fluorescence, canopy geometry, and the reflectance and 
fluorescence of the soil background (Guyot, 1993; Rosema et al., 1991). In a simulation study, Olioso et al. (1989) 
concluded that fluorescence at 690 nm was more suitable than fluorescence at 740 nm for monitoring plant canopies 
because it depends predominantly on fluorescence yield and canopy geometry. 
When excited by ultraviolet radiation, many plant species exhibit blue-green fluorescence with maxima 
around 440 nm (Banninger and Chappelle, 1991; Chappelle and Williams, 1987; Chappelle et al., 1984; Lichtenthaler 
et al., 1991). The fluorescence spectra differ among plant types. Chappelle and Williams (1987) used ultraviolet 
laser to excite leaves of plants and observed fluorescence maxima at approximately 440, 525, 685, and 740 nm. 
Using this information, they were able to identify five major plant groups on the basis of their fluorescence spectra. 
Table 1 shows the relative magnitudes of fluorescence maxima observed when leaves were illuminated at 337 nm. 
The ratio of blue/red fluorescence provides additional useful information for discriminating among plant types. The 
dicots and monocots have distinct fluorescence maxim a at 440 nm, 685 nm, and 740 nm, and a minor maximum or 
shoulder at 525 nm. The monocots have a much higher 440/685 florescence ratio than the dicots. The difference 
in the blue-green fluorescence between monocots and dicots may be due to leaf morphology and leaf vein 
arrangements (Stober and Lichtenthaler, 1993). Hardwoods and conifers have a strong fluorescent maximum at 525 
nm. Conifers have no maximum at 685 nm; the absence of this max imum may be due to the rapid transfer of the 
fluorescence energy at 685 nm to photosystem 1. 
Table 3. Relative fluorescence intensities at four wavelengths of five plant types (after Chappelle and Williams, 
1987). 
Plant Type 
440 nm 
525 nm 
685 nm 
740 nm 
440/685 
Monocots 
972 
0 
164 
180 
5.40 
Herbaceous Dicots 
276 
0 
141 
141 
1.96 
Hardwoods 
90 
69 
164 
181 
0.53 
Conifers 
46 
40 
12 
25 
3.83 
Algae 
149 
0 
1696 
378 
0.09