Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 334

physiological disorders increase tree vulnerability to secondary stresses like insect damage,
frost or another drought [168].
4. Molecular responses to abiotic stresses:
4.1. Mineral composition and Ion homeostasis under abiotic stress
Perhaps the most significant change in plant electrophysiological studies, beginning about 25
years ago, was a shift in focus from more basic electrical and biophysical properties of plant
membranes to pursuing an understanding of the physiological and cell biological functions of
individual plant ion channel types [114]. In the 1990s, ion channels were characterized as
targets of upstream signal transduction mechanisms, and in the later 1990s powerful combined
molecular genetics, patch clamp, and plant physiological response analyses further manifested
the importance of ion channels for many biological and stress responses of plants [114].
Essential metals and ions in the intracellular and intraorganellar spaces of plant cells contribute
to the activities of regulatory proteins, signal transduction, and to the maintenance of turgor
pressure, osmoregulation, toxic metal chelation, and membrane potential control. A large
number of studies on mineral nutrition have sustained the profitable cultivation of plant
growth and development and provided important knowledge on mechanisms of mineral
absorption from soils [114]. Lotfi et al. [180] tested the mineral composition of promising
walnut varieties under both salt and drought stress. Their results showed that differences in
the range of sodium accumulation were minimal as compared with other minerals at different
salt and drought stress levels. In control plants, the average sodium content ranged from 0.34
to 1.82 mg g -1 dry weight (DW), whereas the shoots of the sensitive cultivars (Lara, Vina, and
Serr) had significantly higher sodium contents than other cultivars [212].
In salt-treated plants, the average sodium content was higher than in control plants (nearly
twice) and ranged from 0.52 to 7.92 mg g -1 DW [212], and the Chandler seedlings had signifi‐
cantly less sodium content than the others. Sodium levels in roots were higher than in the
shoots in almost all varieties, especially in the tolerant and semi-tolerant varieties [212]. In
contrast, the increase in sodium content was more evident in shoots of sensitive and semi-
sensitive varieties.
Results of mineral composition analysis showed that the calcium and potassium accumula‐
tions were increased by the increase in salt and drought stress levels, especially in shoots of
semi-sensitive and tolerant cultivars [212]. Also, variations of magnesium accumulation in root
and shoot samples were significant at all stress levels and were dependent on cultivar [212].
Several classes of Ca 2+ permeable channels have been characterized in the plasma membrane
of plant cells, including depolarization-activated Ca 2+ channels [139; 140] and hyper polariza‐
tion-activated Ca 2+ influx channels [114]. In general, plant Ca 2+ channels are not entirely Ca 2+
selective but also show permeability to other cations [166]. However, the genes encoding
plasma membrane Ca 2+ channels remain less well-clarified. Two gene families are likely to
provide possible candidates. One family includes 20 genes in the Arabidopsis genome and