Agriculture Reference
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soil and death of roots needed to absorb adequate soil water during periods of high transpi‐
ration. On soils with restrictive layers in the walnut rooting zone, soil water accumulates above
the restrictive layer leading to a perched water table during the dormant season. Walnut roots
within the perched water table die from a lack of oxygen. If these roots are not replaced during
the growing season, it results in a reduced capacity to absorb soil moisture during the following
growing season, followed by stomatal closure from moisture stress and subsequent decreases
in the rate of photosynthesis [14].
2.4. Deficient water supply
Insufficient available soil moisture causes stresses that can lead to wilting and premature
defoliation under extreme conditions. Under less extreme conditions, the stomata close to
decrease the rate of transpiration. When this occurs, carbon dioxide can no longer enter
into the leaves through the stomata and photosynthesis decreases. If walnut orchards are
not going to be irrigated, then soil depth and water holding capacity become very
important during site selection for the walnut orchard. The water held within the root‐
ing zone determines if adequate soil moisture is available during dry spells. In the central
hardwood region, droughts usually occur in late summer when there is a high demand
for photosynthesis to fill the developing nuts. Lack of adequate soil moisture in late
summer can also affect the physiological condition of the tree and suppress the initiation
of female flowers necessary for the following year's crop [58].
2.5. Germination under abiotic stress conditions
The germination percentage of walnut seeds of eighteen cultivars decreases significantly in
response to decrease in (more negative) water potentials and increases of salinity level.
Decreasing the water potential to -1.0 MPa reduced the germination of all varieties to less than
50% and at -1.50 MPa, the germination decreased to less than 25% [192]. The drought and salt
stress treatments were unaffected by the size of seeds or seed weight and there was not a
significant correlation between percent germination and either seed or kernel weight [212].
Seedlings of walnut cultivars showed differential responses to salt stress under greenhouse
conditions. Increase in salinity levels decreased root and shoot length, diameter, and fresh and
dry mass, especially those of shoots. Seedlings of 'Lara' and 'Chandler' were most and least
affected by salt stress, respectively. The increase in salinity levels was accompanied by a
substantial decrease in root RWC (relative water content) [212]. Seed germination rates were
generally more rapid in control (no salt stress) than in salt containing media. The FGP (final
germination percentage) values were significantly lower at higher salinity levels and there
were also differences in FGP among species [212].
The mean germination time differed both among different treatments and cultivars and also
a significant interaction was found between these two factors under salt stress condition [212].
For all the seedlings of walnut cultivars studied, the mean germination time was shorter in the
control than in the other treatments [212]. There were also differences in the mean germination
time among cultivars. At high salinity levels (200 and 250 mM), the average mean germination
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