REMOTE SENS. ENVIRON. 47:29-35 (1994)
Remote Sensing of Chlorophyll a Fluorescence
of Vegetation Canopies: 2. Physiological
Significance of Fluorescence Signal In
Response to Environmental Stresses*
R. Valentini, t G. Cecchi,* P. Mazzinghi, § G. Scarascia Mugnozza, t
G. Agati, § M. Bazzani, ~ P. De Angelis, t F. Fusi, § G. Matteucci, t
and V. Raimondi ~
M e a s u r e m e n t s of laser-induced chlorophyll fluorescence of living leaves were compared to ecophysiological
parameters, both in near and far field conditions. Near
field measurements were carried out with a two-wavelength portable fluorometer, both in the laboratory and
in the field. Results show significant changes of the 690 nm
and 730 nm chlorophyll fluorescence bands in different
environmental conditions. Water stress and carboxylation
limitations also affect the fluorescence spectra. Far field
measurements from a ground-operated fluorescence LIDAR system confirm those results. The F 6 9 0 / F 7 3 0 fluorescence ratio is then demonstrated as a good index for
vegetation remote sensing.
INTRODUCTION
Remote sensing of laser-induced fluorescence in green
terrestrial plants shows good potentials for species identification, green biomass estimation, leaf area index,
and canopy structure (Chapelle et al., 1984). Airborne
systems for laser induced fluorescence measurements
are today available (O'Neal et al., 1980, Hoge et al.,
*Research supported by National Research Council of Italy,
Special Project RAISA,Subproject 2.4.6 UR 2.36, Paper n. 1129.
t University of Tuscia, Department of Forest Resources (DISAFRI), Viterbo, Italy
*Research Institute on Electromagnetic Waves (IROE) of the
National Research Council of Italy (CNR), Firenze, Italy
~Institute of Quantum Electronics (IEQ) of the National Research Council of Italy (CNR), Firenze, Italy
Address correspondenceto GiovannaCecchi, Research Inst. on
Electromagnetic Waves (IROE) of the National Research Council of
Italy (CNR), Via Panciatichi64, 1-50127 Firenze, Italy.
Received 1 October 1992; revised 1 May 1993.
0034-4257 / 94 / $6.00
©Elsevier Science Inc., 1994
655 Avenue of the Americas, New York, NY 10010
1983; Diebel-Langohr et al., 1985; Pantani and Cecchi,
1992). In the future, according to the development
of specific research projects, such as the European
EUREKA Project LASFLEUR (EU380), a larger database of airborne fluorescence data will be collected on
terrestrial ecosystems. Nevertheless, the feasibility of
remote sensing techniques to the identification of physiological processes and to the assessment of the impact
of environmental stresses on plant physiology is still
under discussion.
Chlorophyll fluorescence has been used for many
years as a powerful tool in plant physiology to understand the primary events of photosynthesis and the
stress development affecting photochemistry. In particular, the quantum efficiency of the Photosystem II (PSII)
and the electron transport to carbon metabolism are
currently investigated by pulse-modulated fluorometers
and saturating light pulses on intact leaves under natural
conditions (Schreiber et al., 1986). The analysis of the
fluorescence signal quenching components has provided
useful information on the action of different environmental stresses (temperature, water, high light, etc.)
(Snel and van Kooten, 1990). The developed indexes,
such as the ratio of the variable fluorescence and the
maximum fluorescence (F~ / Fm), have been used to monitor stresses on intact leaves, both in laboratory and in
natural conditions (Demming and Winter, 1988).
These physiological indexes derived by pulse-modulated fluorometry are in practice undetectable by airborne remote sensing, especially when dark preadaptation of leaves is requested. However, since fluorescence
spectral shape can be measured with remote sensing
techniques, some spectral parameters have been developed for the detection of physiological stresses (Lichten-
29
30 Valentiniet al.
thaler and Rinderle, 1988). In particular, the ratio of
the fluorescence bands at 690 nm and 730 nm (F690/
F730) was considered by Lichtenthaler and Rinderle
(1988) as a possible index of plant stresses.
The physiological meaning of this spectral index
is still under debate. The fluorescence spectrum of a
chloroplast suspension shows a very low emission at 730
nm with respect to the 690 nm peak. The chlorophyll
reabsorption in the 690 nm band, however, changes the
ratio of the two peak intensities to values around 1 in
mature leaves. The fluorescence spectral shape of leaves
is therefore sensitive to the chlorophyll concentration
(Dahn et al., 1992). This factor is very important by
itself, since many kinds of damage lead to a reduction
in leaf chlorophyll content. Anyway, chlorophyll concentration is expected to change slowly in response to most
environmental changes. So the change in spectral shape
due to this effect is not very suitable as an early indicator
of stress conditions.
On the other hand, the variation of chlorophyll
concentration can be taken into account (Agati et al.,
1993) using the information provided by passive reflectance/transmittance spectra. Information concerning the physiological state must be extracted, in remote
sensing, by fluorescence spectra. In this work, the effect
of change in chlorophyll concentration was neglected,
since rapid changes in the chlorophyll fluorescence
spectra on selected targets were mainly measured, as
daily cycles on natural grown plants or artificially induced short term stresses.
In a remote sensing arrangement, as an airborne
fluorescence lidar, the fluorescence reabsorption must
be taken into account. Anyway, this first step is necessary to evaluate which fluorescence spectral parameters
are more sensitive to vegetation health state.
This article presents some results of laboratory and
field measurements, obtained with the near field systems
(LEAF and PAM fluorometers) and the far field system
(FLIDAR-3), described in Part 1. The aim of the present
work is a critical review of the relationships between
physiological processes and some indexes, like the
F690 / F730 ratio and the single fluorescence bands, to investigate their usefulness in vegetation remote sensing.
This work is not exhaustive of the subject since it
will require a big deal of common effort, as pointed out
in Part 1. However, the most important factors affecting
chlorophyll fluorescence were pointed out in some key
laboratory experiments and, whenever possible, compared with results of field and remote sensing experiments.
MATERIAL AND METHODS
The fluorescence spectra show peaks at wavelengths
that shift slightly in the range of 5 nm for the F685
band and even 20 nm for the /7730 band, depending
mainly on vegetation type and leaf structure. Thus,
the term red Fluorescence Ratio (RFR) is accepted to
replace the well-known F690/F730 ratio. LEAF measures the two bands at 6 8 5 + 5 nm and 7 3 0 + 5 nm,
while FLIDAR detects the complete spectrum from
500 nm to 800 nm (see Part 1).
Near Field Measurements
Near field measurements of fluorescence signal were
obtained with LEAF portable fluorometer (described in
Part 1). The first experiment was performed on leaves
of Populus alba L. and Quercus ilex L. seedlings in good
water conditions. Leaves were enclosed in a controlled
chamber where air temperature was maintained at
25 + 0.5°C and vapor pressure deficit at 0.6 + 0.05 KPa.
Light was provided with two 400 W HQI OSRAM lamps
and filtered with a water bath for thermal shielding.
Different light levels were obtained with plastic shields,
placed over the cuvette.
Diurnal cycle measurements of fluorescence with
the LEAF fluorometer clip-on probe (described on Part
1) were collected every minute, as an average of 15
measurements on an isolated 15-year-old tree (]uglans
regia L.). Photosynthetic photon flux density (PPFD)
was measured with a cosine corrected sensor (LICOR),
placed very close to the leaf and with the same orientation of the leaf. The sensor output was recorded by a
separate channel of the fluorometer itself, simultaneously to fuorescence measurement.
LEAF measurements on Fagus sylvatica L. were
collected in the forest on mature trees, in good water
conditions, averaging about 20 fluorescence data points
relative to different locations on each leaf surface. Photosynthesis measurements were carried out with a gas
exchange portable porometer (ADC).
Experiments on water stressed Populus alba L. seedlings were carried out inside greenhouse. Different water stress levels were induced on clones of the same
plant. The LEAF data are the average of about 20 points
on each leaf. The data were taken at two different times
of day.
Far Field Experiments
Remote sensing measurements were carried out on Fagus sylvatica L. and on Quercus pubescens Wild. young
trees with FLIDAR-3 (described in Part 1) at distances
ranging between 15 m and 60 m. Using a folding mirror
settled just over the tree to deflect the lidar beam, the
trees were irradiated also from the top, simulating an
airborne remote sensing experiment.
This experimental setup is suitable for young plants
grown at the border of the forest. Near field and physiological measurements were made simultaneously, directly on the same target leaf (or leaves), or on leaves
belonging to the same branch and close to the target
Fluorescence Response to Environmental Stresses 31
ones. The monitored leaves (see Part 1) were enclosed
in an environmental controlled cuvette (CMS, Walz) for
measurement of physiological parameters. The cuvette
temperature was maintained equal to the external air.
In addition, a stress was induced on the controlled
tree (Ouercus pubescens Wild., isolated tree). The tree
behavior from normal to stress conditions was monitored with the whole equipment. In particular, the
stress was produced simply by cutting the tree branch,
inducing mainly a water stress at first.
Juglans regia
2000
1.3
•
RFR
o
PPFD ]
1.2
~0
0
1500
"E
o
"" •
• ~,:~'~,~.~:.
1.1
o
1
•.
0.9
:0
,~ 1000
•~ o
..
*.
0 ".
. " O;
o .'.
5O0
©
,o
p
2
--t
4
o'~,~~
•
d~
018
0.7
o
~
I
I
I
I
I
I
6
8
10
12
14
16
18
20
0.6
24
22
time of day
RESULTS AND DISCUSSION
For the sake of clarity, the data presented here are split
into two parts, concerning the results carried out on
vegetation in the near and far field, respectively.
Near Field Detection of Fluorescence Signal
with LEAF Fluorometer
Figure 1 presents the RFR behaviour in comparison
with different steady-state light conditions. The data
show a linear decrease of RFR, increasing light, both
for Quercus ilex and Populus alba. The decrease of RFR
values in the range from 0 to 2000/tmol m -2 s-~ is
Figure 1. Variation of the RFRindex and of the total fluorescence Ft (F690 + F735) as a function of light (Quercus ilex
and eopulus alba).
Quercus flex
1.8
700
I -B~
1.8i r
RFR
Ft
8OO
"11
~
1,4
[]
500
4oo
SO0
0.8
~
O.(t
=
5OO
0
i
1000
8
~
~
200
i
1600
Figure 3. Changes of the relative contribution of the F690
and F730 bands to total fluorescence (Ft= F690 + F730)
(Quercus ilex).
Populus alba
1.2
700
Quercus flex
70
RFR
[]
[]
•
Ft
80O
~
•
~
[]- 400
O.8
300
0.7
200
F6851Ft
"n
F720IFt
I
60©
500
O:
25% for Quercus ilex and 18% for Populus alba. The
total fluorescence (Ft) shows a similar behavior and
decreases increasing light levels. The decrease is more
pronounced than that of RFR and the variations are
about 50% for both Quercus ilex and Populus alba.
The same behavior was confirmed by a daily cycle
of RFR taken in the field on a leaf of Juglans regia L.
(Fig. 2). The fluorescence ratio decreases during the
day and reaches its minimum value at the highest value
of PPFD. RFR ranges from about 1.1 during night
to about 0.75 during day, in full sunlight conditions
(PPFD = 1800/tmol m -2 s-t).
The decrease of RFR with increasing light level was
found in all the experiments to be determined by a
relative higher decrease of the 685 nm band with respect
to the 720 nm one. Figure 3 presents an example of
this behavior, where the two bands, normalized to total
fluorescence (Ft), are plotted as a function of PPFD.
RFR behavior was also investigated as a function of
photosynthetic rates, at constant PPFD levels. This was
analyzed exposing leaves of Quercus ilex and Populus
alba to different levels of CO2 concentration.
100
2000
PPFD (umol m's')
1.1
Figure 2. Diurnal behavior of the RFR index and of the
photosynthetic photon flux density (PPFD). The PPFD was
measured close to the fluorescence detection point.
~
N
*~ 50
u."
0
40
O.e
0
400
800
1200
PPFD (~nol m=s~)
1000
100
2O00
30
0
i
500
i
1000
PPFD ( ~ o I m's')
i
1500
2000
32
Valentini et al.
Figure 4 presents the result; RFR is plotted as a
function of intercellular CO2 concentration (Ci) at two
different light levels (80 and 800/~mol m -2 s -1) for
Quercus ilex and three light levels (100/~mol m -2 s -;,
500/~mol m -2 s -1, and 1400 pmol m -2 s -;) for Populus
alba. Increasing Ci, the RFR index increases for both
the species. Since the photosynthetic rate increases with
increasing CO2 concentrations, this behavior is opposite
to that of Figure 1, where RFR decreases with increasing
PPFD. Also, Populus alba shows higher RFR variations
(30%) with respect to Quercus ilex (11%), opposite to
the variation with PPFD.
On the contrary, the total fluorescence decreases
increasing Ci, showing a similar behavior to that of
Figure 1. Nevertheless, the two bands behave differently
with respect to Figure 3: The 690 nm band decreases
slower than the 730 nm one (data not shown).
RFR measurements as function of the maximum
photosynthetic rates (Amax)in Fagus sylvatica leaves are
shown in Figure 5. Although leaves were exposed to a
constant light level, RFR decreases when Am~xincreases.
Figure 4. Variation of the RFR values with intercellular
CO2 concentration under different light conditions (Quercus
ilex and Populus alba).
Ouercus
ilex
1.8
1.76
rr
Fagus sylvatica
1.1
2500
.
li
*
i
i
i
2000
0.9
i[]
n-
i
[]
0.8
0.7
0.6
.
--
.
.
.
~
i
0.5
1
:~
i
1.5
1500
0
1000
3~
500
i
i
I
2
2.5
3
3.5
i
i
4
4.5
A.,,,(0mol CO2 m's')
Figure 5. Relationship between net photosynthesis (Amax)
and the RFR values for Fagus sylvatica leaves in natural conditions.
In this experiment, the chlorophyll content of the sampled leaves was not found to be correlated with the
RFR values, probably due to the small variations of
chlorophyll concentrations among beech leaves.
The effect of water stress on RFR was monitored
on clones of Populus alba L. grown in greenhouse. In
Figure 6, RFR values, taken at two different times of
day, are plotted as a function of water potential. The data
reveal an appreciable difference between the stressed
and unstressed conditions. Actually, the stressed plants
show higher values of RFR than the unstressed ones.
This RFR variation is more pronounced for the data
taken in the afternoon, when the lack of water enhances
the difference between stressed and unstressed plants.
1.7
[J.
rr
Far Field Detection of Fluorescence Signal
with the FLIDAR System
Far field measurements on different plant species were
carried out with the FLIDAR system. The plants were
monitored simultaneously with the near field equipment, including physiological measurements. The data
1.65
1.6
I -b PPFD = 800 lamol m ' g ' i
1.55
0
I
I
I
I
100
200
300
400
600
CI (ppm)
Populus
Figure 6. Variation of the RFR ratio for water-stressed Populus alba seedlings. The water stress condition is expressed
by the levels of predawn water potential.
alba
1.4
Populus alba
1.2
.......
+ .......
~
. . . . . . . .
,
ni,
rr
-~
..........
+
.
~.
+
O.65
. . . . . . . . .
•
Not
+
stressed
Water
stressed
0.6
1
[]
r~
~: 0.55
0.8
.
.
.
.
.
.
.
.
0.5
•
0.6
i
i
100
200
300
CI (ppm)
PPFD = 1400 I~mol re's"
i
i
400
600
0.45
000
0
~
[]
D a t a s e t 1 (H 12:30)
s []
~
D a t a s e t 2 (H 16:20)/
i
0.6
i
1
P r e d a w n W a t e r Potential (Mpa)
] []
[]
I
1.5
2
Fluorescence Response to Environmental Stresses 33
presented here refer to Fagus sylvatica L. and Ouercus
pubescens Wild. species.
Figure 7a shows RFR values, obtained in vivo on a
leaf of Fagus sylvatica. The measurements were taken
in a beech wood, on the Appenines mountains in the
central part of Italy. The beech tree under control was
a healthy young plant, at the border of the forest. The
fluorescence signal was monitored for several hours,
observing the night-to-day transition. The relative PPFD
values together with net photosynthesis measurements
are shown in Figure 7b.
RFR shows an opposite behavior compared to that
of PPFD. Actually, high RFR values correspond to low
PPFD, and PPFD variations, mainly due to meteorological conditions (wind and clouds) induce variations on
RFR. RFR decreases from 0.95 to 0.6 during the nightto-day transition and at high light condition PPFD
reaches a value of 1200/~mol m -z s -]. The near field
measurements, obtained with LEAF fluorometer, show
the same RFR behavior confirming FLIDAR results.
Photosynthetic rates range from a minimum of - 0.5
/~mol m- 2 s-~ during the dark respiration to a maximum
of 8/~mol m-2 s-1. Figure 8 presents two fluorescence
spectra detected during dark and light conditions, respectively. RFR and total fluorescence decrease in sunlight, confirming near field measurements. The total
Figure 7. Diurnal behavior of far field RFR measurements
on Fagus sylvatica: a) RFR values during the night-day transition; b) net photosynthesis and PPFD values of the target.
Fagus sylvatica
a)
1.1
1
0.9
r,i,
r,-
Fagus sylvatica
600
500
...... d a r k
--
:.
,,
sunlight
',. . . . . .
/
'.
400
....
.
-.'.....
..
". . . . .
300
200
100
0600
.~
6~o
~oo
T~o
wavelength ( n m )
Figure 8. Laser-induced fluorescence spectra of Fagus sylvatica in dark and light conditions.
fluorescence decrease is related to quenching processes
involved in photosynthesis, but for far field measurements this decrease is much lower than that expected
and also measured by LEAF and PAM fluorometers.
This is probably due to other mechanisms, concerning
the different excitation (wavelength and pulse duration).
However, from a remote sensing point of view,
the total fluorescence intensity has a lower interest.
Actually, these absolute measurements require the
knowledge of all those parameters, such as geometrical
factors, and atmospheric transmission, which do not
play a role in differential measurements.
Fluorescence spectra were also teledetected with
FLIDAR-3 on unstressed and water stressed Quercus
pubescens tree. As shown in Figure 9 for the unstressed
target, the red fluorescence ratio changes from 0.98 at
6:00 a.m. (dark conditions) to 0.76 at 1:00 p.m. (full
sunlight).
0.8
0.7
Figure 9. Diurnal behavior of the RFR values on Quercus
pubescens tree under unstressed and water stressed condi-
0.6
tions.
0.5
~
~
~
~
1;)
1~
Quercus pubescens
12
1,1
time of day
Fagus sylvatica
b)
1
2000
I ~A
--~--PPFD 1
~
~
6
0,9
1600
I,
"E
n-
o
..~ 1OO0
o
".
~ .......•
" ~ 3 /
0,8
2
0,7
6OO
....... - ......
0
0
4
6
6
7
8
time of day
9
10
11
-2
12
•
0,6 4
6
8
unstressed
[
stressed
]
10
" ~
12
time of day
14
16
18
34 Valentini et al.
Quercus pubescens
350
300
...... u n s t r e s s e d
......- ~
/.
,~.
-.i
250
--
stressed
...
...'
". .
...:.
"-..
200
f-
o
1so
100
/
°oo
650
wavelength (nm)
Figure 10. Fluorescence spectra of Quercus pubescens un-
der unstressed and water stressed conditions, showing the
peak at 713 nm.
A similar behavior, but with lower RFR index values, is shown for the water-stressed target: The values
vary from 0.94 at 8:00 a.m. to 0.67 at 3:00 p.m. The
spectral analysis reveals (Fig. 10) that under stress conditions the peak at 690 nm is strongly decreased. Moreover, a small peak, placed by a gaussian fit at 713 nm,
appears and remains in all the spectra obtained during
the afternoon. This band is probably related to changes
in pigment composition. Also in this experiment LEAF
measurements confirmed the RFR behavior detected
by FLIDAR-3.
CONCLUSIONS
This work shows that chlorophyll fluorescence spectra
are a sensitive tool for vegetation remote sensing. The
connection between plant physiology parameters and
RFR was demonstrated by several laboratory and field
key experiments, However, since many parameters
affect RFR, the extraction of a vegetation stress index
from remote sensing spectra is not easy.
The ratio F690/F730 has been already considered
by Lichtenthaler and Rinderle (1988) as a useful indicator of stress conditions in plants. Its usefulness for
chlorophyll change determination was shown for several
species and different growth conditions (Rinderle et
al., 1991). The application of this spectral index of
fluorescence showed significant changes varying environmental and physiological conditions. Also temperature showed a significant effect on the F690 / F'/30 ratio
(Lipucci di Paola et al., 1992), with a variation of about
35% for a temperature change of 16°C.
The first factor to be considered using the F690/
F730 ratio is its relative dependence on the optical
properties of leaves. In particular, the F690 band is
strongly affected by reabsorption of chlorophyll. The
relative concentration of chlorophyll was shown as a
primary factor determining the RFR value (D'Ambrosio
et al., 1992; Dahn et al., 1992). This effect can be taken
into account with a simple model using the combined
information delivered by refectance and transmission
spectra (Agati et al., 1993). So remote fluorescence
spectra must be measured in close connection with
passive reflectance spectra. The combination of the two
measurements can deliver reliable information about
chlorophyll concentration of the target, which is useful
by itself.
Beyond this nearly static factor, RFR was found
dependent on environmental conditions, especially
PPFD, showing a daily, cyclic variation that cannot be
attributed to variations in chlorophyll concentration.
This was also confirmed in laboratory experiments, with
a direct relationship between RFR and light level (in
addition, carboxylation rates and water stress modify
RFR value).
RFR variations, due to increasing light levels, are
essentially determined by the relative decrease of the
690 nm band with respect to the 730 nm band. The
fluorescence intensity is anyway decreasing for both
bands in these conditions. Reasons for this behavior
could be attributed to changes of the optical properties
of leaves, due to chloroplast movements or structural
modifications of the leaf. This could explain the observed
changes in RFR in water stress conditions. However,
this is not likely to be the only factor affecting the RFR
value. Actually, a variation of RFR without any change
in leaf absorption during induction kinetics was recently
demonstrated (Agati et al., 1992). The energy transfer
processes, taking place at the onset of photosynthesis,
can differently quench the two fluorescence peaks,
showing a direct connection between RFR and the
photosynthetic process.
This was also demonstrated by experiments with
increasing carboxylation rates at a constant light level.
RFR variations were produced by a decrease of the 730
nm band, while the 690 nm one remains almost constant. Intercellular CO2 concentration (see Fig. 4) and
photosynthetic rates (see Fig. 5) induce changes in RFR,
showing that the F730 fluorescence band is quenched
by photochemistry.
Remote RFR measurements carried out in the far
field with fluorescence lidar techniques were successful
on different trees and different environmental conditions. The RFR variations showed the same behavior of
the laboratory experiments. The variations were also
similar to the field measurements carried out on single
leaves with LEAF fluorometer.
The main difference between near field and far
field measurements was found in the total fluorescence
intensity variation with PPFD, during daily cycles. The
FLIDAR measured a nearly constant fluorescence intensity, while the LEAF (and PAM) fluorometer showed
the usual fluorescence quenching induced by the actinic
Fluorescence Response to Environmental Stresses
light. The fluorescence signal is generally quenched
under light conditions by photochemical
and nonphotochemical quenching (Shreiber et al., 1986). Therefore, lower fluorescence quantum yield is expected at
increasing light levels.
A possible explanation of the observed contrasting
behavior is that the FLIDAR fluorescence
signal is
induced by an excitation pulse, whose duration is about
10 ns. This is much shorter than the pulse used by the
LEAF fluorometer that is longer than 1 ms. Under dark
conditions, when reaction centres are open, a short
pulse is equivalent to a measurement of Fo since most
of excitation is trapped in the antenna chlorophylls.
During high light conditions most of the reaction centres
are closed and the excitation gives a higher fluorescence
signal, since the photochemical quenching has a longer
time scale. It is not clear why the nonphotochemical
quenching seems not to affect laser-induced fluorescence signal under full sunlight conditions. Further
research work is needed to understand the physiological
processes that are at the basis of the observed discrepancies. In particular the different duration of the excitation
pulses should be taken into account to analyse photochemical processes.
The authors wish to thank Professor Luca Pantani for the
useful discussions and Dr. llaria Ambrosini, Mrs. Maria Grazia
Baldecchi, Mr. Faust0 Meiners, Mr. Bruno Radicati, Mr. Daniele
Tire& Mr. Massimo Trambusti, and Mr. Giancarlo Valmori
for their support during the laboratory and field experiments.
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September
1992, (M.
Murata, Ed.), Kluwer Academic, Dordrecht,
pp. 639-642.
Agati, G., Fusi, F., Mazzinghi, P., and Lipucci di Paola, M.
(1993) A simple approach to evaluate the reabsorption
effect of chlorophyll
fluorescence
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J. E., III, and
Newcomb,
W. W. (1984) Laser-induced
fluorescence
of
green plants. 1: A technique
for the remote detection
of
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35
Dahn, H. G., Gunther, K. P., and Ltideker, W. (1992) Characterisation of drought stress of maize and wheat canopies
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H. K. (1992),
Increase of the chlorophyll fluorescence
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B., and Winter, K. (1988). Characterization
of
three components
of non photochemical
quenching
and
their response to photoinhibition,
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G., and Pantani, L. (1985) Measuring oil at sea by means
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