Agriculture Reference
In-Depth Information
that permits plants to shed expendable distal components of its shoots while preserving other
parts that represent years of carbon investment. Leaf shedding is a potentially cost-effective
way for plants to deal with drought stress by a plant segmentation mechanism.
2.8. Growth inhibition
Growth of vegetative and reproductive tissues of walnut is constrained by cell initiation
shortages, cell enlargement problems, and inefficient food supplies. Cell enlargement depends
upon hydraulic pressure for expansion and is especially sensitive to water stress. Cell division
in generating new cells is also decreased by drought.
2.9. Shoot growth
Internal water deficits in trees constrain the growth of shoots by influencing development of
new shoot units (nodes and internodes). A period of drought has a carry-over effect in many
species from the year of bud formation to the year of expansion of that bud into a shoot.
Drought also has a short-term effect by inhibiting extension of shoots within any one year. The
timing of leaf expansion is obviously later than that of shoot extension. If shoot extension
finishes early, a summer drought may affect leaf expansion but not shoot extension [220].
Shoots of some trees elongate for only a few weeks in late spring. This growth form is called
fixed or determinant growth. Other species elongate shoots over a period of several months
which is called multiple flushing or continuous growths. A late July drought may not affect
current-year shoot elongation in species with fixed growth, like oaks. Oak shoots expand only
during the early part of the growing season [220]. A late July drought can inhibit expansion of
shoots from multiple flushing species, like sycamore, which elongate shoots during much of
the summer. Spring and summer droughts damage both types of trees. In the southern pines,
late summer droughts will influence expansion of shoots in the upper crown to a greater extent
than those in the lower crown [212; 220]. This is because the number of seasonal growth flushes
varies with shoot location in the crown. Shoots in the upper crown normally exhibit more
seasonal growth flushes than those in the lower crown. Buds of some lower branches may not
open at all [220].
In walnut, the maximum decrease in shoot fresh weight was observed after 4 days of osmotic
stress treatment [192; 212-213]. Response of half-sib families differed as the severity of water
and salt stress increased. Under severe osmotic stress (-1.50 MPa), offspring of 'Panegine20'
and 'Chandler' produced the greatest shoot fresh weight [212].
Available water, more than any other resource, determines the annual growth potential of
individual trees. Variations in water availability account for up to 80% of the inter-
annual variability in size increment in temperate stands. Tree water deficits dramatically
reduce height and radial growth as well as bud production [168]. Abiotic stress experi‐
ments on one and two year old trees of promising walnut varieties showed the same trends
[192; 212-213]. Twig growth patterns are affected during several years, as demonstrated by
Fulton and Buchner [14] for Persian walnut in California. Recovery from the previous
drought was still not complete when the next drought began, and induced even further
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