Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 337

4.2. Seed germination and ion homeostasis under abiotic stress
The initial events in stem propagule germination may differ in some respects those of seeds
but bud activation, elongation, and establishment events are similar. Germination of sugar
cane sets (stem cuttings) exhibited significant reduction in the rate and percentage of germi‐
nation due to NaCl damage [150]. These plants had an enhanced content of Na + and Cl - , a
concomitantly reduced content of potassium, calcium, nitrogen, and phosphorus and reduced
elongation and dry matter of seedlings.
Citrus rootstocks used to raise plantlets had a negative correlation of Cl+ with certain nutrients
[146]. Resting buds of salt-stressed poplar plant, grown in vitro, did not accumulate glycine‐
betaine and proline and thus had reduced growth of seedlings [2]. Similarly, tubers of hydrilla
showed signs of salt damage and reduced germination [147-148]. There remains a shortage of
information particularly about the salt tolerance of propagules during germination.
Exposure of seeds or seedlings to salinity results in the influx of ions with the imbibition of
water, which exerts an adverse effect on the growth of embryo [141; 143]. This may lead to a
marked decrease in the internal potassium concentration [143], a vital nutrient for protein
synthesis and plant growth [149]. Seedlings exposed to salinity are highly prone to excessive
ions, sometimes leading to their death shortly after emergence [142; 150]. The ability of plants
to cope with ion toxicity is principally related to the greater transport of ions to shoot [143-144].
Grasses show a strategy of salt tolerance by storing toxic ions in the mesocotyl up to a certain
limit [151-152]. This has significance in that the epicotyls and hypocotyl avoid ion toxicity, thus
ensuring better growth [141].
4.3. Regulation of Na + homeostasis in roots and shoots in tolerant walnut varieties
The fine-tuned control of net ion accumulation in the shoot involves precise in planta coordi‐
nation between mechanisms that are intrinsically cellular with those that are operational at the
intercellular, tissue or organ level [125, 157]. Several processes are involved, including the
regulation of Na + transport into the shoot, preferential Na + accumulation into the shoot cells
that are metabolically not very active and the reduction of Na + content in the shoot by
recirculation through the phloem back to the root [125-126].
Ions loaded into the root xylem are transported to the shoot largely by mass flow, driven by
the size of the transpirational sink [124-127]. A control response is to lower transpiration by a
reduction in stomata aperture; however, this is only effective as a short-term response because
plants need to maintain water status, carbon fixation and solute transport [157]. Controlling
ion load into the root xylem restricts accumulation in the shoot to a level where cells in this
organ can be effective ion repositories by vacuolar compartmentalization [125, 157]. In our
studies, tolerant walnut varieties showed such trends under both salt and drought stress [213].
Endodermal cells constitute a major control point in radial ion transport from the soil solution
to the root xylem since the Casparian strip is an impermeable barrier to apoplastic solute
movement [128]. However, bypass systems that function through 'leaks' in the Casparian strip
barrier or movement through areas of the root where the specialized endodermal cells are not
fully developed may be additional major entry points [129-130]. Regardless, vacuolar com‐