Agriculture Reference
In-Depth Information
Table 11.1
Types of plant fl uorescence and their applications
Fluorescence
Subtype and index
Properties
Trend when infection rises
Emission
Red: F680/F730
Chlorophyll
Long term falling
Blue green : F450/F730
Phenols
Short term rising
Excitation
F
UV
/F
VIS
Phenols
Short term rising
Kinetics
(F
M
− F
0
)/F
M
Photosynthesis
Decreased activity
Hypersensitive death of invaded cells is known as the most typical feature of rust
resistance (Heath
1982
), and this increases the concentration of phenols.
11.4.3
Fluorescence Indices Related to Infection
Indices are normally ratios of simultaneously measured radiation intensities. They
are rather insensitive to changes in measurement geometry, to sensor drifting or to
environmental conditions. Each index can be assigned to the corresponding mea-
surement technique:
•
The maxima in the fl uorescence
emission spectra
correspond to chlorophyll and
phenols (Fig.
11.4
, bottom). The ratio of the intensities of the maximum chloro-
phyll fl uorescence supplies a simple but powerful index. This relation of
F680/
F730
is a measure for the chlorophyll content (Gitelson et al.
1999
). As described
before in Sect.
6.4.1
,
this ratio is negatively correlated to the chlorophyll content
due to self-absorption of the radiation below 700 nm. The blue fl uorescence
around 450 nm related to the far red fl uorescence 680 or 730 nm -
e
.
g
.
F450/
F680
- is related to the content of phenols (Lichtenthaler and Schweiger
1998
).
•
The dual
excitation
of chlorophyll with light in the ultraviolet and visible region
leads to the
F
UV
/F
VIS
quotient. For this the fl uorescence intensity in the far red
excited with UV light is related to the one that is excited with VIS - normally
blue or red light. This is a measure for the UV absorbing pigments in the leaf
epidermis (Cartelat et al.
2005
), namely the phenols.
•
The
kinetics
of the Kautsky effect are very complex and can be infl uenced by the
light scheme during measurement. The main parameter is the variable fl uores-
cence,
i
.
e
. the difference between maximum (F
M
) and minimum fl uorescence
(F
0
) related to the maximum fl uorescence, hence (
F
M
− F
0
)/
F
M.
This variation
occurs in the time of some seconds. The initial rise in the millisecond range can
be described with a special method (Strasser et al.
2004
).
In general the changes in plant properties -
e
.
g
. chlorophyll and phenol content
or photosynthesis - are detectable with the corresponding fl uorescence measure-
ment techniques. This is summarised in Table
11.1
. In detail there is a lot of litera-
ture dealing with stressed plants and fl uorescence. In the following, only a few
selected publications that deal with fungal infections as the stressor are listed:
•
Lüdeker et al. (
1996
) analysed wheat and barley infected with rust (
Septoria
) and
mildew (
Blumeria
) with ultraviolet excitation and found the blue
fl uorescence
