Agriculture Reference
In-Depth Information
Figure 5.
Chloroplast high light avoidance movements. Dark-acclimated
Arabidopsisthaliana
leaf strips have been ex‐
posed to a laser beam and the autofluorescence of chloroplasts has been recorded directly after switching on the laser
(A), 7 min (B) and 30 min (C) after laser exposure.
Plants have also developed several mechanisms for removing the excess of energy. Dissipa‐
tion of EEE can be achieved by the combination of photochemical (qP) and non-photochemi‐
cal quenching (NPQ) processes. Photochemical quenching increases the utilization of
photosynthetic electrons by metabolic pathways such as the water-water cycle or photores‐
piration. The consumption of electrons through water-water cycle is achieved by combined
action of the O
2
reduction at PSI to O
2
−
and the chloroplast antioxidant system. The reduc‐
tion of O
2
is much lower than the disproportionation of O
2
−
, catalysed by SOD and the fol‐
lowing H
2
O
2
processing to H
2
O, catalysed by APX. Therefore, the water-water cycle
shortens the lifetime of O
2
−
and H
2
O
2
, suppresses the production of
•
OH, and prevents pho‐
toinhibition [77]. Another energy sink preventing photoinhibition of photosynthetic appara‐
tus by EEE is photorespiration. During this process, photo-produced ATP and reducing
equivalents are consumed, preventing the overreduction of PET. However, the photorespir‐
atory cycle leads to the production of H
2
O
2
that has to be elimainated by antioxidant sys‐
tems [151].
Non-photochemical quenching processes relay on the transfer of excitation energy to carote‐
noids that are able to dissipate it as heat during the xanthophyll cycle (VAZ cycle). The xan‐
thophyll cycle involves the conversion of violaxanthin to de-epoxidised zeaxanthin, via the
intermediate antheraxanthin. This enzymatic cycle is performed by violaxanthin de-epoxi‐
dase and plays a key role in the stimulation of energy dissipation within light-harvesting an‐
tenna [152]. In
Arabidopsis
, the chlorophyll binding protein - PSII subunit S, encoded by
PsbS
gene has been proven to be required for NPQ [153].
Excess energy is sensed by the photosynthetic apparatus not only as a result of HL, but also
other environmental factors such as UV radiation, limitations in nutrient availability,
drought, salinity or high/low temperatures. All these abiotic stimuli are accompanied by ox‐
idative stress, manifested in the overproduction of reactive oxygen species. If the level of
ROS is too high for antioxidant system to eliminate them, cellular macromolecules and
structures can be damaged, which triggers PCD. Several studies clearly demonstrate that
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