933 
luorescence 
s, measured 
about 500 
■ation effect may 
the intensity of 
7hl-a LIF while 
f the observed 
nm ( F690) in 
¡orption of F690 
F690/F735 ratio 
it in leaves, as 
tus (see e.g. 
tio on intensity 
unt for correct 
Fmax ( c,osed RC ) 
pen RC) 
II I-» 
60 80 100 
NTENSITY (a.u.) 
5S of F690/F735 
robing laser 
of initially 
I RCs. 
Since the origin of blue, green and red fluorescence from plant leaves 
and, thus, the mechanisms of their saturation are different, one can expect 
dependence of other fluorescence ratios, e.g. the blue/red F440/F690, 
green/red ones F530/F690 on the intensity of laser pulses. 
It is apparent, that for accurate estimates of biological characteristics 
of leaves and algae it is necessary to use the value of appropriate 
’unsaturated’ parameters, e.g. rj = lim r)(I), ( F530/F690) = lim F530/F690( I) 
I->0 I->0 
which are independent on laser intensity I. The solution of a problem may be 
limitation of I down to the level providing measurements in "near-unsaturated" 
mode. According to our experiments the photon flux density of probing laser 
pulses must not exceed lCr 2 cm’ 2 s _1 to meet this requirement. This estimate 
has been obtained experimentally for the case of laser excitation with the 
second harmonic of Nd:YAG laser (532 nm). 
Thus, the fluorescence saturation may distort the results of lidar remote 
sensing, that should be taken account when data processing. On the other hand, 
this phenomena opens up possibilities for obtaining new information from 
analysis of nonlinear fluorescent characteristics (Chekalyuk et al., 1992b), 
that can be applied for study of primary photosynthesis processes and for 
lidar biomonitoring of unfavorable environmental influences on photosynthetic 
apparatus. 
5. - IMPACT OF ENVIRONMENTAL FACTORS ON CHLOROPHYLL 
FLUORESCENCE YIELD 
One of the key problems related to interpretation of Chl-a LIF measurements 
(as well as any other fluorescent data) is high variability (more than 3-5 
times) of in vivo Chl-a fluorescence yield. To a large extent this variability 
is caused by changes in physiological state of photosynthetic apparatus, in 
particular - by variations in functional state of PSII RCs. 
Because of such changes, the actual Chl-a fluorescence yield (<1>) for 
dark-adapted cells may vary between its minimum ($ ) and maximum ($> ) 
levels. The ratio 4> /$> ■ 3 ± 0.3 for algae and it is about 5 for plant 
max c 
leaves, indicating week dependence upon species examined. Since the functional 
state of PS II RCs is controlled by the influence of varied environmental 
factors (nutrient availability, light, presence of toxic substances, etc.), it 
leads to the corresponding changes in in vivo Chl-a fluorescence yield. For 
correct interpretation of in situ fluorescent measurements it is necessary to 
take into account this source of variability. 
On the other hand, relying on this dependence, in principle it is 
possible to develop the approaches to solution of inverse problems, e.g. the 
assessment of the environmental impacts from measuring the LIF fluorescence of 
Chl-a in vivo „ as well as lidar monitoring of on-going photosynthesis of 
phytoplankton and vegetation (see e.g. Chekalyuk and Gorbunov, 1994b,c; 
Gorbunov and Chekalyuk, 1994). 
6 - - DIURNAL RHYTHM OF IN VIVO CHLOROPHYLL FLUORESCENCE 
The bright manifestation of such variations is diurnal rhythm of Chl-a 
fluorescence yield of phytoplankton and vegetation caused by changes in solar 
illumination. According to field measurements, the fluorescence yield may vary 
1 to 3-5 times during a day (Kiefer, 1973; Chekalyuk and Gorbunov, 1992a; 
Gorbunov and Chekalyuk, 1992,1993), depending on physiological state of 
photosynthetic apparatus and solar irradiance conditions (Gorbunov and 
Chekalyuk, 1992). Usually, chlorophyll-a fluorescence in maximum at night and 
reduces to its minimal value at the noon (Fig.5).