Agriculture Reference
In-Depth Information
The right inverted Gaussian curve (Fig. 6.14 ) stands for the radiance band that
the plants emit, hence for reflected plus fluorescent radiation . This radiation that
is directed away from the canopy is compared to the irradiance (left).
The purpose of recording radiation lateral to the Fraunhofer line is to obtain an
estimate of the reflectance R without any contribution of fluorescence. The quotient
c/a provides this reflectance R, which can be regarded as a coefficient of reflection
(see Sect. 3.1 ). Since the solar irradiance hits a reference panel that is non-
fluorescent, its component b in the Fraunhofer line cannot contain energy that is
converted to fluorescence. This means that the product of reflectance R and compo-
nent b provides the absolute reflection part precisely within the Fraunhofer line.
And because the plant radiance within the Fraunhofer line d contains reflection as
well as fluorescence f in absolute values, subtracting the product R × b from it pro-
vides finally the fluorescence.
So fluorescence f = d − R b = d − c b/a.
Moya et al. ( 2004 ) and Liu et al. ( 2005 ) have shown that this method provides
good results. However, this method inherently does not allow to sense vegetation
stress via the F680/F735 ratio that can be used with active sensing.
6.4.2
Fluorescence Sensing in a Non-Steady State Mode
This method goes back to Kautsky and Hirsch ( 1931 ), who watched the fluores-
cence intensity of leaves that were held in the dark and then suddenly were illumi-
nated. It showed up that starting from a very low level, the fluorescence intensity
rose steeply to a maximum. While the illumination continued, the fluorescence then
fell gradually. This descent of fluorescence took several seconds and sometimes
included smaller intermediate maxima. Finally - with the illumination still on - the
fluorescence got to a steady state level (Fig. 6.15 ). This reaction of fluorescence to
varying illumination often is called the Kautsky effect .
The physiological background of this phenomenon is that the photochemical fac-
tory of a plant needs adjustments and time to get to the steady state operational
mode. The initial rise of the fluorescence intensity with the start of the illumination
is attributed to progressive saturation of the photochemical system. And the slow
decrease of the fluorescence intensity after having attained the maximum is most
likely due to protection mechanisms since the plant has to avoid adverse effects of
an excess of light.
The significance of the Kautsky fluorescence curve is that it allows to get information
about the most important crop property - its photochemical devices - under fairly con-
trolled conditions. Because the light that enters the process is artificially programmed.
And simultaneously, the waste energy that the crop sheds as fluorescence is recorded.
Temporal optical indices that represent variations in the rise, in the maximum or in the
decay of the Kautsky fluorescence have been developed and can assist in assessing the
photochemical situation (Buschmann and Lichtenthaler 1998 ; Thiessen 2002 ).
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