Agriculture Reference
In-Depth Information
light energy
light energy
photosynthesis
photosynthesis
heat
heat
F
F
normal leaf
fluorescence
damaged leaf
fluorescence
illumination
illumination
time
time
Fig. 11.5
Schematic of variable fl uorescence (F) in a normal leaf and in a damaged leaf where
photosynthesis is totally blocked. In the former, the fl uorescence shows typical kinetics during
illumination. In contrast to this, the damaged leaf has a higher but constant fl uorescence
when a dark adapted plant is illuminated with intense light. The effect in
normal plants is that the fl uorescence rises quickly up to fi ve times higher than
an initial low level and after a few seconds it decreases to an intermediate
stationary level. This is due to the processing of the light in photosynthesis,
and it is obvious that a damaged leaf which is “dead” or “nearly dead” shows
no or limited variable fl uorescence (Fig.
11.5
).
So in summary: there exist not only the spectral emission and excitation spectra
of fl uorescence, but also the temporal kinetic behaviour of the plant's fl uorescence.
11.4.2
Fungi-Plant-Interaction and Physiology
of Infected Plants
There exist many fungi (about 1.5 Mio) and plant species (250,000) and these have
had much time during their evolution to develop strategies against each other. Fungi
cause a
defence mechanism
in the infected plant. Elicitors (
e
.
g
. reactive oxygen spe-
cies - ROS - that are produced by fungi) trigger a hypersensitive response of the
plant. This could be the production of special substances - callose as a barrier for the
fungi or phenols with antifungal properties - or the dying of hypersensitive cells.
With common fungi in cereals, the defense reactions are either
necrotrophic
or
biotrophic
. For the fi rst case: septoria leaf spot and fusarium head blight are typical
examples in wheat. These fungi are destroying the plant cell walls with toxic sub-
stances and lead to necrosis of the plant leaf. Hereby compounds in the plant cells
transform into phenols (Osbourn
1996
).
The obligate biotrophic fungi need living plant material as they assimilate the
plant's nutrients by haustoria (extraction by roots). Typical examples are powdery
mildew (
Blumeria
) and rust (
Puccinia
) fungi. The effects on the plant parameters
are normally reduced photosynthesis but increased respiration. The biotrophic fungi
often form “green islands” on the leaf with a nutrient sink and higher nonphoto-
chemical quenching (energy dissipation of the light excitation) than the surrounding
area (Scholes and Rolfe
1996
).
