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adaptation aimed at scavenging photo respiratory H
2
O
2
produced during drought stress [203].
The reduced PPO activity in stressed walnut seedlings could be a response to increase the
abundance of antioxidative phenols. PPO could also be involved, through proteolytic action,
in removing proteins damaged by oxidative stress effects [211].
The increased POD, APX and CAT activities observed in the more drought and salt-tolerant
'Panegine20' and 'Chandler' seedlings, relative to 'Lara' and 'Serr' seedlings, underline the
effectiveness of 'Panegine20' and 'Chandler' antioxidative enzyme systems in protecting the
cellular apparatus under water deficit conditions. Furthermore, the higher proline accumula‐
tion observed in 'Panegine20' and 'Chandler' seedlings under WI was accompanied by higher
activities of SOD, APX, POD and CAT. These results suggest that proline accumulation could
activate the antioxidative defense mechanism in walnut trees as has been suggested by Yang
et al. [193] in salt-stressed soybean plants.
In conclusion, genotypic differences were observed among walnut seedlings in leaf water
status, photosynthetic performance, pigment content, proline accumulation and antioxidative
enzyme activity. The close relationship observed between photosynthetic rate (Pn) and proline
content points to an important role of this osmolyte in the maintenance of photosynthetic
activity and therefore in drought tolerance. These literature reviews show that differences in
SOD, APX, POD, PPO, LOX, PAO and CAT activities among walnut genotypes could be
attributed to differences in the mechanisms underlying oxidative stress injury and subsequent
tolerance to abiotic stress. Varietals differences in pigment content could be related to differ‐
ences in antioxidative enzyme activity. Notably, the 'Panegine20' and 'Chandler' seedlings,
which exhibit higher drought tolerance, also showed higher antioxidative enzyme activity than
other walnut seedlings. Seed of the later cultivars should be considered high-risk for planting
in dry areas. In addition, these results show that seedling genotypes with the higher photo‐
synthetic activity ('Panegine20' and 'Chandler') also had higher proline content and antioxi‐
dative enzyme activity. This supports an interaction between proline and the antioxidative
defense system as suggested by Yang et al [193]. To verify this hypothesis, we suggest further
studies focusing on the effects of exogenous application of proline and paraquat on the
activities of protective enzymes in walnut trees would be of interest.
4.12. Biotechnology and abiotic stress engineering in walnut
Breeding for drought and salinity tolerance in crop plants should be given high priority in
plant biotechnology programs. Molecular control mechanisms for abiotic stress tolerances are
based on the activation and regulation of specific stress-related genes. These genes are involved
in the whole sequence of stress responses such as signaling, transcriptional control, protection
of membranes and proteins, and free-radical and toxic-compound scavenging. A major
objective of walnut rootstock breeding is vigour, in order to promote rapid growth of the scion
under a variety of soil and environmental conditions and to quickly establish a full-sized
bearing canopy. Other objectives include resistance to diseases and pests, most notably
Phytophthora, nematodes and crown gall, and tolerance of soil-related problems including
waterlogging, salt accumulation and cold. There is interest in controlling tree size but not at
the cost of vigour. In walnut, breeding for abiotic stress tolerance or resistance has been limited
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