Abiotic Stress Tolerance in Plants with Emphasizing on Drought and Salinity Stresses in Walnut - Abiotic Stress: Plant Responses and Applications in Agriculture - page 338

partmentalization in cells that form the interconnected network between the soil solution and
the root xylem progressively lowers the content of ions that are entering the transpirational
stream. It is presumed that NHX-like cation/H + transporters have a major function in this
process [131; 157].
4.4. Osmotic homeostasis: Compatible osmolytes
Osmotic balance in the cytoplasm is achieved by the accumulation of organic solutes that do
not inhibit metabolic processes, called compatible osmolytes. These are sugars (mainly sucrose
and fructose), sugars alcohols (glycerol, methylated inositols), complex sugars (trehalose,
raffinose and fructans), ions (K + ), charged metabolites (glycine betaine) and amino acids such
as proline [156; 157]. The function of the compatible solutes is not limited to osmotic balance.
Compatible solutes are typically hydrophilic, and may be able to replace water at the surface
of proteins or membranes, thus acting as low molecular weight chaperones [157]. These solutes
also function to protect cellular structures through scavenging ROS [6; 10; 157]. Salt tolerance
requires that compatible solutes accumulate in the cytosol and organelles where these function
in osmotic adjustment and osmoprotection [187]. With exceptions like K + , most compatible
osmolytes are organic solutes. Genes that encode enzymes that catalyze the biosynthesis of
compatible solutes enhance salt and/or drought tolerance in gain-of-function strategies [155].
Proline occurs widely in higher plants, and normally accumulates to large quantities in
response to environmental stresses [205]. In addition to osmotic adjustment, it is involved in
prevention of protein denaturation and preservation of enzyme structure and activity [187].
Most of research on proline as an osmoregulatory compound has been carried out on the
vegetative parts of the plants. Little attention has been paid to the reproductive organs,
especially seeds. Recently, information has been published on osmotic adjustment of seeds
under stress conditions. Salt stress increased proline accumulation in the cotyledons and roots
of germinating ground-nut seeds [162]. Proline accumulated in the endosperm and radicles of
germinating barley seeds with increasing NaCl concentrations in the growing media [163-164].
This proline probably originated from the degradation of stored protein in the endosperm.
Walnut seeds average 15-25 g protein per 100 g of kernel and the proline content of seeds varies
with genotype, ranging from 1100 to 1500 mg/100g kernel. A high amount of proline was
detected in embryonic axis and leaves [181]. Our previous study revealed that the amount of
proline in seeds of different genotypes of walnut, especially in semi-tolerant and tolerant
genotypes, is high [212]. Even at the beginning of a drought period, the machinery for proline
accumulation was most activated in the tolerant genotypes 'Chandler' and 'Panegine20' of
walnut [180]. These initial differences in proline content, observed among the genotypes at
day zero, prior to application of WI, and notably high in 'Panegine20' and 'Chandler', could
be due to the natural adaptation to abiotic stress of the germplasm from which these genotypes
were derived. Proline content of both 'Chandler and 'Panegine20' were elevated and similar
to each other early in the drought period, but at the end the proline content of 'Panegine20'
was higher than that of 'Chandler' [180]. Proline appears to be a major osmotic regulator in
'Panegine20' and 'Chandler' under drought stress. Also, our previous study demonstrated