Agriculture Reference
In-Depth Information
photosynthetic process is completely blocked, the emitted chlorophyll fluorescence
is six times as high. The heat dissipation as well goes up to the six-fold if no photo-
synthesis occurs. However, the absolute level of energy dissipation via heat is very
much higher than via fluorescence. Under the same conditions, about seven times as
much energy is lost via heat than via fluorescence (Table 6.1 ).
A prime objective in precision farming is obtaining site-specific information
from the crop itself about the energetic efficiency with which the photosynthetic
process is going on. Such information would supply essential knowledge for a
logically controlled site-specific application of e.g. water and farm chemicals.
Theoretically, the dissipation either of heat or of chlorophyll fluorescence would
be candidates for respective signals about the status of the photosynthetic process.
From the amount of energy that is involved, it might be assumed that the heat
dissipation could provide the best signals for this (Table 6.1 ). However, up to now,
detecting the heat emission from plants in an accurate way is possible only in labo-
ratories. For doing this with crop canopies in fields by remote or proximal sensing,
no precise techniques exist. This has to do with the fact that the steadily changing
weather affects the physical measuring conditions. Contrary to this for fluorescent
radiation, remote sensing from aerial platforms and proximal sensing from farm
machines in an online and on-the-go manner is state of the art.
6.4.1
Fluorescence Sensing in a Steady State Mode
The methods for fluorescence sensing operate either in a steady state - or in a non-
steady state mode . This refers to the radiation that induces the fluorescence and
which either is temporally constant (= steady) or for which a change is programmed
during the time of sensing. Whenever passive sensing based on natural light occurs,
a steady state mode exists. Because practically within the sensing time for a signal,
the natural light is constant.
But in case active sensing takes place since the induction is based on artificial
radiation ( e.g. by laser light), this can be done either in a steady state- or also in a
non-steady state mode. The latter aims at detecting how the fluorescence behaves
when the illumination changes and therefore often in denoted as a “kinetic” sens-
ing method . It is explained in detail in the next section. This section deals with
steady state methods.
There are many factors that influence the course of the fluorescence spectra such
as the exciting radiation and plant species. Yet an important feature are the high
levels or peaks in the red (F680) and far red (F735) wavelength region. With a low
chlorophyll concentration, the peak in the red range dominates. But with a high
chlorophyll content, the peak shifts to the far-red region (Fig. 6.13 ). This phenom-
enon is explained by re-absorption of photons from the fluorescent light
(Lichtenthaler 1996 ; Buschmann and Lichtenthaler 1998 ). If the wavelength is
below 700 nm, the fluorescent photons coming from internal leaf cells can be
re-absorbed when hitting adjacent chlorophyll molecules. With a wavelength above
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