Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 347

at best. One of the first attempts is transformation of somatic embryos of Persian walnut with
a gene isolated from a cyanobacter. This gene controls expression of flavodoxin. The role of
flavodoxin in response to salinity and osmotic conditions is known [229].
Ferredoxins are very ancient proteins widely used by anaerobic organisms for many metabolic
pathways. Ferredoxin (Fd) is up-regulated by light, indicating that under autotrophic growth,
Fd is the normal electron carrier [230]. As a replacement for Fd, Flavedoxin gene (fld) is induced
under various environmental sources of stress including oxidative stress in enterobacteria and
salinity stress in cyanobacteria [231]. Results showed that transgenic plantlets of walnut
harboring the fld gene clearly grow better at 200 mM NaCl than the non-transgenic controls.
The control plants did not produce any callus and turned brown and died after 10 days, while
transgenic lines showed no brown symptoms, produced callus, and continued their growth
for up to 45 days on 200 mM NaCl [229]. Compared to salt stress, the decrease in evaluated
parameters of transgenic and non-transgenic SEs caused by PEG-induced stress was relatively
lower. At the 1.5% PEG, the number of cotyledonary embryos was significantly increased in
both transgenic and non-transgenic somatic embryos (SEs) [229]. With increasing concentra‐
tions of PEG in culture medium to 5% and 10%, significant differences between transgenic and
non-transgenic SEs for most of the evaluated parameters were observed. The results showed
that transformants reduced stress in both salt and osmotic stress conditions and the degree of
response was greater to salt than to PEG. Over-expression of the fld gene in transgenic lines
of Persian walnut partially decreases some of the hostile effects of salinity stress. Production
of callus and new shoots by transgenic plants expressing this gene and grown on stress-
inducing media is in agreement with previous reports in tobacco [232]. All findings reported
show clearly that expression of cyanobacterial proteins can be a powerful tool to enhance the
stress tolerance of some plants.
5. Conclusions and perspectives
• Cavitation avoidance is a likely physiological function associated with stomatal regulation
during abiotic stress in walnut. This suggests that stomata are responding to leaf water status
as determined by transpiration rate and plant hydraulics and that P rachis might be the
physiological parameter regulated by stomatal closure during water stress, which would
have the effect of preventing extensive developments of cavitation during water stress.
• Hydraulic segmentation for walnut trees ( Juglans regia ) by petioles displaying a large
vulnerability to abiotic stresses in sensitive genotypes. This phenomenon disconnects leaves
through massive cavitation during stress and avoids irreversible damage to perennial parts
of the tree.
• Photosynthesis is limited by stomatal closing during drought and by direct effects of heat
on leaf biochemistry. This suggests that hot and dry weather reduces photosynthesis and
potential productivity in walnut even in the absence of soil water deficit. But, some
promising varieties show the sufficient net assimilation rate and photosynthesis under
abiotic stress conditions.