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the fluorescence spectra of april cucumber (Fig. 2,
a
, curves
1
. . .
3
—
spectra during different measurements), fluorescence spectra of watercress
(Fig. 2,
b
, curves
1
. . .
4
— spectra during different measurements) and grass
from Decor Aros turf lawn mixture (Fig. 2,
c
, curves
1. . . 3
— spectra
during different measurements).
In Fig. 2,
a. . . c
it can be clearly seen that plants’ fluorescence spectra
in the normal state have two maxima at about 680 nm (for some plants
the maxima are weakly expressed — see. Fig. 2,
c
) and at about 740 nm.
For most plants in normal states, the ratio of fluorescence intensities
R
680
/
740
at the wavelengths of 680 and 740 nm is less than 0.8 (at the
fluorescence excitation wavelength in either green or blue-green regions of
the spectrum) [15].
When a plant is under stress, its fluorescence spectrum changes.
Fig. 3. . .5 show specific examples of plants’ measured fluorescence
spectra under various stress conditions.
Fig. 3 shows the spectra of laser-induced grass fluorescence (grown
from a mixture of Decor Aros lawn mixture) under normal (curves
1
,
2
)
and stress (curve
3
) conditions caused by introduction to the soil copper
sulphate CuSO
4
(5 g, diluted in 200 ml of water for 3 pots (
9
×
9
×
10
cm)
with grass).
Curve
1
corresponds to the measurement of the laser-induced fluorescence
spectrum made in a month after the first grass shoots, and curve
2
— two
weeks later, just before the introduction of the soil pollutant. Curve
3
corresponds to the fluorescence spectrum of grass under stress, measured
two weeks after introducing copper sulphate to the soil.
In Fig. 3 it is seen that the influence of the stress factor (in this
case caused by introducing copper sulphate) may result in changing the
fluorescence level. The shape of the fluorescence spectrum changes little.
This effect is clear, as the first phase of the plant stress is the primary
inductive stress response [24]. This stage is characterized by a decrease in
the rate of photosynthesis, which is accompanied by a significant increase
in the chlorophyll fluorescence intensity. In this case, the increase in
fluorescence quantum yield is due to a decrease in the efficiency of primary
processes of photosynthesis. The absorbed light energy is not used in
photosynthesis, so the fluorescence intensity increases.
Fig. 4 illustrates a different change pattern in the fluorescence spectra of
a stressed plant. Fig. 4 shows the spectra of the laser-induced fluorescence
of watercress under normal (curves
1. . .3
) and stress (curves
4. . .7
)
conditions caused by mechanical damage to plants, i.e. salad trampling.
Different curves correspond to different measurements in time (before
the mechanical damage and in the range of 20 to 40 minutes after the
mechanical damage).
ISSN 0236-3933. HERALD of the BMSTU. Series “Instrument Engineering”. 2015. No. 2 75