Agriculture Reference
In-Depth Information
F740 max.
F685 max.
maxim. fluorescence
steady state fluoresc.
F740
F685
Fig. 6.15
Spectra of chlorophyll-fluorescence that were acquired from maize leaves after 5 min of
dark adaptation in the laboratory. The illumination with a 300 W xenon arc lamp began at time 0
and was to simulate constant solar radiation following the dark adaptation. The initial rise of the
fluorescence according to the Kautsky effect cannot be seen. But the gradual decrease to the final
steady state fluorescence is apparent. The graph also points out the maxima of the chlorophyll fluo-
rescence - as explained in Fig.
6.13
- at 685 and 740 nm wavelength (From Entcheva Campbell
et al.
2008
, altered)
6.4.3
Fluorescence or Reflectance
Principally, both active as well as passive sensing of chlorophyll fluorescence can be
done on a site-specific basis. However, passive sensing in Fraunhofer lines is not yet
used in farming operations whereas active sensing of the chlorophyll content via the
red/far red fluorescence ratio is state of the art. An advantage of fluorescence sens-
ing - when compared to chlorophyll sensing via reflectance - is that radiation from
the soil does not result in errors. Because the soil does not emit fluorescent radiation.
So contrary to chlorophyll sensing by reflectance, a not
closed crop canopy
is not
a problem.
But whereas reflectance allows to sense the chlorophyll content within the leaves
as well as the leaf-area-index, this is not
per se
possible by fluorescence. Even if
fluorescence sensing is done in combination with a device that scans the complete
field surface, it is the canopy surface that supplies the signals and leaves below it are
disregarded. Hence fluorescence cannot as well provide information about the chlo-
rophyll content per unit of field area and thus about the
longterm yield
potential of
crops - as suitable reflectance indices can (see Sect.
6.3
). However, a combined use
of fluorescence- and reflectance sensing may be reasonable.
