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production in both plants and bacteria when shikimic acid (SA) or 3-DHS were used as
substrates and NADP
+
as a cofactor. Finally, they showed that purified
E. coli
and
J. regia
SDH produced GA in-vitro. Also, they proposed that the C-terminal, AroE/SDH domain of
the plant enzyme is the region of the protein responsible for GA production [235]. Be‐
cause of the importance of GA as an antioxidant in plants, controlling its production and
accumulation in plants could significantly increase the nutritional value and tolerance of
walnut for abiotic stresses. Further expression studies using fragments of the walnut gene(s)
will be performed to verify the activity of each individual domain in GA production [235].
Anderson [158] examined antioxidative tannins and related polyphenols in foods and nuts,
isolating 16 polyphenolic constituents including three new hydrolysable tannins, along with
adenosine and adenine, from commercial walnuts. Under abiotic stress, the profile of total
phenols and PPO activity was similar in both roots and leaves of all genotype seedlings
subjected to salt and drought stress, but amounts of phenolics and levels of PPO activity
were higher in leaves than in roots [213]. A significant increase (25.3% and 38.4%) in total
phenolic concentration was observed within 20 d of water deficit treatment in leaves of both
'Chandler' and 'Panegine20' [213] in contrast to a small and not significant increase in total
phenolic concentration in seedlings of some varieties, especially in root tissues [213]. The
reverse pattern was observed for PPO activity, with 'Chandler' and 'Panegine20'showing a
slight decline in root and leaves while PPO activity in 'Lara' and 'Serr' increased sharply
during drought in both tissues. Among the antioxidative enzymes analyzed, PPO was the
only one clearly down-regulated under WI conditions. 'Chandler' and 'Panegine20' leaves
showed a marked decline in PPO activity during water deficit stress but in roots PPO activity
decrease were less sharp [213]. A significant increase in PPO activity in water stressed leaves
of 'Lara' and 'Serr' (112% and 76%) was apparent after 7 d of drought [213]. In these varieties,
PPO activity linearly increased until the Ψw was -1.84 MPa or more (during the 7th d of
drought period) and then remained at a similar level [213]. The antioxidant properties of
plant phenolic compounds are well-documented [206]. These are synthesized de novo [207]
and can influence auxin metabolism, membrane permeability, respiration, oxidative
phosphorylation, and protein synthesis [199] and their activity also has been related to the
occurrence of physiological injury [208]. The changes in phenolic production and PPO
activity observed in drought-stressed walnut seedlings show that some varieties, namely
'Lara' and 'Serr', are more sensitive to drought than the tolerant varieties 'Panegine20' and
'Chandler' [213].
4.8. Effects of salt and drought on Malondialdehyde (MDA) content
Earlier, Jouve et al. [225] found that the endogenous level of MDA did not vary in control
and in the salt stressed aspen (
Populus tremula
L.) plants. This indicates that the level of lipid
peroxidation was similar in stressed and non-stressed plants. Likewise, MDA concentra‐
tion changed with increasing salt concentration in the shoots of tolerant walnut varieties,
decreased slightly at 100mM, while increased at 200 and 250mM salt stress which sug‐
gests that walnut shoots are better protected from oxidative damage under salt stress [213].
Changes in the MDA content of leaf tissues subjected to drought and salt were well
documented by Lotfi [213]. Application of drought for 20 d caused a linear increase in the
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