Agriculture Reference
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composition of the tissues and some alterations in the relative rates of photosynthesis and
respiration.
Under salt and drought stress, different seedlings of walnut cultivars show significant
differences in RWC content. Semi-tolerant ('Hartley') and tolerant ('Chandler' and 'Pane‐
gine20') cultivars of walnut have moderate and high levels of TWC and RWC at osmotic stress
level [212]. RWC below 80 % usually implies a water potential on the order of -1.5 MPa or less,
and this would produce changes in the metabolism, reduced photosynthesis, increased
respiration and increased proline and abscisic acid accumulation.
2.13. Root biomass
Walnut root growth differs significantly between the dry season and wet season. Mean root
length in both the upper (0-30 cm) and deep (30-80 cm) soil layers was shortest in early July
when the soil water content and air relative humidity were lower [17]. After rewatering events,
the total root length in late August and early October increased by 128% and 179%, respec‐
tively, compared with that in early July [17]. The abundance of new roots significantly
increased in both the upper and deep soil layers in response to the recovery events. The growth
of new roots was greater in the upper soil profile than in the deep soil profile. Dead root length
in the upper soil layer was significantly higher in the wet season than in the dry season, while
no difference in dead roots was detected in the deep soil layer between the seasons. In walnut,
the diameter of the roots did not significantly change by season [17]. The increase in osmotic
drought level was accompanied by a substantial decrease in root relative water content and
differences between genotypes at different osmotic levels were highly significant [213].
There was a significant variation in the vertical distribution of roots under stress condition
(Table 1). Roots were the most abundant at 10-30 cm depth, followed by 0-10 cm depth. Root
biomass decreased with depth below 30 cm. Generally, most of the root surface area, root
length density and root biomass were confined to the upper soil layers (0-30 cm), and ac‐
counting for 60.9, 62.2 and 78.9% of the total root measurements from the 0-80 cm soil layers,
respectively.
2.14. Leaf architecture and position
Annual heavy nut production will require selection of seedlings of walnut cultivars with
multiple leaf layers to maximize photosynthetic production, tendencies toward lateral bearing,
good resistance to anthracnose, and efficient use of photosynthates for tree growth and nut
production [212].
Leaves are extraordinarily variable in form, longevity, venation architecture, and capacity for
photosynthetic gas exchange. Much of this diversity is linked to water transport capacity [17].
The pathways through the leaf constitute a substantial (≥30%) part of the resistance to water
flow through plants, and thus influence rates of transpiration and photosynthesis. Leaf
hydraulic conductance (Kleaf) varies more than 65-fold across species, reflecting differences
in the anatomy of the petiole and the venation architecture, as well as pathways beyond the
xylem through living tissues to sites of evaporation.
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