Agriculture Reference
In-Depth Information
4.10. Effects of salt and drought on LOX activity
Both enzymatic and non-enzymatic lipid peroxidation have been previously implicated in ROS
perception. Oxylipins resulting from enzymatic oxidation via lipoxygenases (LOX) might
function in leaf senescence [159]. LOX activity in leaves of sensitive genotypes increased
markedly by the 5
th
d of WI and then continued to rise slightly. Also under salt stress conditions
the same trends were observed for walnut seedlings. Leaves were the most affected by water
deficit, showing a four-fold increase in LOX activity over control seedlings [213]. LOX activities
in root tissues were 1.7 and 1.6 times the control values at the maximum drought stress [213].
LOX activity of controls did not change significantly during the full 20 d of WI. There were no
significant increases in LOX activity in seedlings of 'Panegine20' and 'Chandler' [213]. LOXs
are a family of enzymes that catalyze the oxygenation of polyunsaturated fatty acids (PUFAs)
into lipid hydroperoxides (LOOHs) which are involved in responses to stresses [190]. Plant
LOXs may be involved in growth and developmental control processes through the biosyn‐
thesis of regulatory molecules and volatile compounds [198]. The high degree of lipid perox‐
idation observed could produce lipid derivatives acting as secondary messengers capable of
activating some drought stress associated genes by means of specific transcription factors,
triggering plant responses to desiccation [212]. Increase in LOX activity can be due to an
increased amount of enzymatic protein [204]. However, in this study a lower amount of the
enzymatic protein was found in drought-stressed seedlings of 'Panegine20' and 'Chandler'
than in controls [213].
4.11. Effects of salt and drought on antioxidant defense systems
A recent comprehensive study revealed that both salt and drought stresses led to down-
regulation of some photosynthetic genes, although most of the changes were small, possibly
reflecting the mild stress imposed. Compared to drought, salt stress affected more genes and
more intensely, possibly reflecting the combined effects of dehydration and osmotic stress
under salt-imposed conditions [194]. Desingh and Kanagaraj [226] pointed out that photosyn‐
thetic rate and RuBP carboxylase activity decreased with increasing salinity but some antiox‐
idative enzymes significantly increased. An important consequence of salt stress is the
excessive generation of reactive oxygen species (ROS) such as superoxide anion (O
2
-
), hydro‐
gen peroxide (H
2
O
2
) and the hydroxyl radicals (OH
-
), particularly in the chloroplast and
mitochondria [195].
In plant cells, ROS are generated in high amounts by both constitutive and inducible routes,
but under normal situations, the redox balance of the cell is maintained via the constitutive
action of a wide range of antioxidant mechanisms that have evolved to remove ROS [194]. ROS
are produced during photosynthesis and respiration, as by-products of metabolism, or via
dedicated enzymes. Cells are equipped with a range of efficient antioxidant mechanisms to
remove ROS. Changes in the cellular redox balance result from exposure to various abiotic and
biotic stresses, with induction of both ROS generation and removal mechanisms. Enzymatic
ROS scavenging mechanisms in plants include SOD (superoxide dismutase), present in many
cellular compartments; catalase, located in peroxisomes; and the ubiquitous ascorbate-
glutathione cycle. SOD catalyses the dismutation of superoxide to H
2
O
2
, and is thus one of the
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