Agriculture Reference
In-Depth Information
time span and the higher the temperature within limits is, the more growth can
be expected. Following this logic, Raun et al. (
2001
) concluded that the sum of the
signals from two reflectance sensing dates should be divided by the cumulative
growing degree days
between the readings. For these calculations that were based
on winter wheat in Oklahoma, USA, a growing degree day was defined as the sum
of the daily maximum- and minimum temperatures minus 4.4 in °C. It is obvious
that details of this procedure for estimating the yield potential must be adapted to
respective crops and to local conditions. Raun et al.
2001
used the described method
for site-specific sensing with the normalized difference vegetation index (NDVI)
during the second half of the tillering time span and obtained an accuracy of 83 %.
For high yielding crops, a red edge index instead of the NVDI is recommended (see
Fig.
6.7
and text to it).
Though yield estimating via reflectance is not yet state of the art, it probably will
become an important method for getting logic to site-specific control algorithms
and thus for providing more efficiency to several farming operations.
6.4
Fluorescence Sensing
The denotation “fluorescence” indicates a flowing or a flux of radiation that is emitted.
The emitter can be plants or also dead material. However, the latter case is left out here.
Contrary to reflectance, the radiation does not originate from irradiance that is
simply thrown back from the canopy. Instead, fluorescent radiation can be traced
back to photons that did enter an absorption process in plants. However, the photons
that enter such processes in plants and induce fluorescence and those that leave the
canopy as fluorescent light are different. The light that excites plants to fluoresce
always has shorter wavelengths than the fluorescence that finally results from it. The
development of fluorescent light implicates a
prolongation of wavelengths
.
Common ranges for plant fluorescence are either the
blue to green region
extending from about 400 to 600 nm or the
red to far-red region
from approxi-
mately 650 to 770 nm wavelength. The blue-green fluorescence is induced by ultra-
violet light, thus by light that is not used by photosynthesis. It is assumed that the
blue-green fluorescence develops within phenolic materials in the cell walls of
plants (Buschmann and Lichtenthaler
1998
). The red to far-red fluorescence can
also result from ultraviolet radiation, yet in addition it can come from light that
entered a photosynthetic process and hence chlorophyll molecules. Consequently, it
is named
chlorophyll fluorescence
. The visible wavelengths that induce the chloro-
phyll fluorescence can range from the blue to the red region. But the exciting wave-
lengths always are below those of the final fluorescence.
The fact that the fluorescence always has higher wavelengths than the respective
exciting light means that - in terms of energy per photon - the fluorescent light is
of photosynthesis
because it is a means of getting rid of surplus energy. A question
is, however, why are plants wasting energy?
