Agriculture Reference
In-Depth Information
time for 'Chandler' and 'Panegine20' was 2.3 to 5.2 d and 2.6 to 5.4 shorter, respectively, than
those observed in the other cultivars [192; 212].
In the study of 18 walnut cultivars, the lengths of nuts were varied from 2.07 (± 0.47) cm
for 'Lara' to 4.08 (± 0.78) cm for 'K72'. Seed size could be a factor affecting germination in
stressed media [212]. Many studies have shown that various seed sizes and weights may
behave differently in terms of germination under stress conditions [180; 212]. It is general‐
ly believed that large seed sizes have a higher propensity for germination in saline and
dry media. However, some previous studies, found a negative relationship between seed
size and germination capacity in Trianthema triquetra L. Within the range of seed sizes
studied, we did not observe any significant differences in the germination response and
analysis failed to show any relationship between percent germination and seed weight
under both salt and drought stress.
2.6. Wilting
The amount of water lost before visible leaf wilting varies by species. Temporary wilting is the
visible drooping of leaves during the day followed by rehydration and recovery during the
night. During long periods of dry soil, temporary wilting grades into permanent wilting.
Prolonged permanent wilting kills trees [14; 212; 220]. The relation between water loss from
leaves and visible wilting is complicated by large differences among species in the amount of
supporting tissues leaves contain. Leaves of black cherry (Prunus), dogwood (Cornus), birch
(Betula), and basswood (Tilia) wilt readily. Leaf thickness and size do not prevent wilting.
Rhododendrons are also extremely sensitive to drought with leaves that curl, then yellow and
turn brown. By comparison, the leaves of holly and pine are supported with abundant
sclerenchyma tissue (i.e. tough, strong tissue) and do not droop readily even after they lose
considerable water.
2.7. Leaf shedding
In normal abscission, an organized leaf senescence process, which includes the loss of chlor‐
ophyll, precedes leaf shedding. With severe drought, leaves may be shed while still full of
valuable materials [220]. For example, sycamore ( Platanus ) sheds some leaves, and buckeye
( Aesculus ) may shed all of its leaves, as drought continues. On the other hand, leaves of
dogwood ( Cornus ) usually wilt and die rather than abscise. Many times these leaves are stunted
[220]. Walnut is also known to shed leaves in response to drought [61]. Sometimes drought-
caused leaf shedding may not occur until after rehydration. Abscission can be initiated by
water stress but cannot be completed without adequate water to shear-off connections between
cell walls. The oldest leaves are usually shed first [220].
Injury to foliage and defoliation are most apparent in portions of the crown that are in full sun.
These leaves show drought associated signs of leaf rolling, folding, curling, and shedding.
Over the past 20 years, our knowledge of the hydraulic architecture of trees has increased and
some hypotheses have been raised to explain how trees might be designed hydraulically to
help them cope with period of drought [220]. Hypotheses have generally invoked a mechanism
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