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that in 'Panegine20', contrary to 'Chandler', “ion osmosis” is another important osmotic
regulator under drought and salt stress [212].
Proline content increases significantly in relation to the severity of drought stress, in particular
in roots of tolerant walnut genotypes [180]. In two and three years old walnut seedlings proline
content of both roots and shoots was elevated initially and increased significantly with length
of drought stress in tolerant genotypes [180]. During 16 days of water stress, root proline
content increased 1.48 fold in 'Panegine20' and 1.38 fold in 'Chandler' seedlings [180].
Similarly, leaf proline content increased 2.07 times in 'Panegine20' and 1.50 times in 'Chandler'
seedlings compared to the control plants [180]. The increase in proline content was greater in
'Panegine20' than in 'Chandler' and greater in roots than in shoots [180].
4.5. Total soluble sugars and starch variation under abiotic stress
Salinity and drought cause the accumulation of soluble sugars, free proline, and soluble
proteins [141; 154]. Parida and Das [177] reported that lower osmotic potential allows leaves
to withstand a greater evaporative demand without loss of turgor. This requires an increase
in osmotica, either by the uptake of soil solutes or by the synthesis of metabolically compatible
solutes [138]. These findings appear to apply to olives since Tattini et al. [179] showed a
correlation between leaf glucose and increasing levels of salinity in the root zone.
Drought and salt stress significantly increased the total soluble sugar content of roots and
leaves only in 'Panegine20' and 'Chandler' varieties [180]. Leaf soluble sugar content increased
1.39 times in 'Panegine20' and 1.59 times in 'Chandler' compared to the controls. The increase
in sugar concentration may result from the degradation of starch [202]. Soluble sugar content
was elevated initially and increased progressively in drought stressed tissues of the tolerant
genotypes. Sugars may act directly as osmotica or may protect specific macromolecules and
thereby contribute to the stabilization of membrane structures [197]. In general, soluble sugar
content tends to be maintained in the leaves of drought-stressed plants even though rates of
carbon assimilation are partially reduced. In this study, observed increases in soluble sugar
concentration coincided with decreases in starch content as the water potential dropped.
Metabolites may prove to be beneficial to germination, first by reducing osmotic inhibition
and second by providing substrates for the growth of embryonic tissues [150; 153]. Imposition
of different polyethylene glycol treatments on promising genotypes of walnut seedlings
significantly increased total soluble sugar content [180]. Compared to the control, a drastic
increase was observed in shoots and roots. Root content soluble sugar increased 1.65 times in
'Panegine20' progeny and 1.70 times in 'Chandler', and shoot soluble sugar content increased
1.73 times in 'Panegine20' and 1.60 times in 'Chandler' relative to control plants. Starch content
significantly decreased in roots and shoots of both genotypes. Total starch content of roots
decreased 49.46% in 'Panegine20' and 38.18% in 'Chandler'. This decrease was 52.79% in
'Panegine20' and 47.42% in 'Chandler' relative to the control plants [180].
Ability of LEA proteins to act synergistically with non-reducing sugars to form a glassy matrix,
and thus confer drought protection, is an attractive hypothesis [170]. This hypothesis is
supported by the abundance of LEA proteins and reducing sugars in desiccation-tolerant plant
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