Agriculture Reference
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primary mediators of H
2
O
2
production from intracellular sources of superoxide. Unlike most
organisms, plants have multiple forms of the different types of SODs encoded by multiple
genes [216]. According to our previous study, SOD activity in water-stressed 'Panegine20'
walnuts increased 58% and 29% relative to controls in leaves and roots respectively. In
'Chandler' seedlings this increase was 51% and 33%, respectively [213]. In 'Serr' seedlings, the
decline was 54% and 42% in the different tissues, respectively and in 'Lara' walnuts; the decline
was 67% and 53% in leaves and roots. For all cultivars, the increase in SOD activity under WI
conditions in roots was less than those recorded in leaves [213].
Increasing SOD activity induces a higher tolerance to oxidative stress under salt or drought
stress [205]. Metallo enzyme SOD, which is ubiquitous in all aerobic organisms and in all
subcellular compartments prone to ROS mediated oxidative stress, is the most effective
intracellular enzymatic antioxidant [184]. This enzyme provides the first line of defense against
the toxic effects of elevated levels of ROS. Yang et al. [193] found that under drought conditions
and high light SOD activity increased significantly relative to low light.
The patterns of SOD, CAT, POD and APX activities were roughly parallel in all the tissues
examined, showing a significant increase under salt and drought-treatment conditions in
tolerant walnut varieties [213] but at different levels among genotypes and plant tissues.
'Panegine20' and 'Chandler' seedlings of walnut showed the highest levels of antioxidative
enzyme activity. The increases in APX and CAT activities in 'Panegine20' seedlings were
significant in leaves and roots tissue and activity was greater in leaves than roots. In 'Chandler'
seedlings, the increase in APX activity under WI was significant only in leaves [213]. In
'Panegine20' and 'Chandler' seedlings APX activity increased more than SOD or CAT activity
under WI conditions [213]. Activities of SOD, CAT and APX peaked at the 7
th
d of WI, but POD
activity climaxed on the 5
th
d and was higher in 'Panegine20' than 'Chandler' [213].
Abiotic stresses, such as drought stress, cause molecular damage to plant cells, either directly
or indirectly, through the formation of AOS. In this study, the plants exposed to abiotic stress
showed a significant increase in CAT, APX, SOD and POD activity. MDA is regarded as a
biomarker of lipid peroxidation and stress-induced damage to the plasmalemma and organelle
membranes [189]. In this study, the amount of MDA in tolerant varieties decreased with
increasing drought stress. CATs are tetrameric heme-containing enzymes with the ability to
directly convert H
2
O
2
into H
2
O and O
2
and are indispensable for ROS detoxification under
stress conditions [204]. APX is thought to play an essential role in scavenging ROS and
protecting cells by scavenging H
2
O
2
in water-water and ASH-GSH cycles and utilizing ASH
as the electron donor.
APX has a higher affinity for H
2
O
2
(μM range) than CAT and POD (mM range) and may have
a more crucial role in the management of ROS during stress. As expected, the activities of all
these enzymes changed significantly in walnut seedlings under water stress. The observed
greater increase in APX activity in leaves of water-stressed plants than in roots could be due
to localization of APX in chloroplasts. The significant increase in APX activity seen in leaves
could be a mechanism developed by walnut trees for protection of chloroplasts, which under
stress conditions develop sustained electron flows and are the main producers and targets of
ROS action [195]. The increase in CAT activity in leaves of water-stressed plants may be an
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