Agriculture Reference
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growth suppression. A similar reduction of twig growth over several years after drought
was also seen in black walnut [214-215].
2.10. Cambial growth
Cambial growth slows or accelerates with rainfall. Cambial growth is constrained by water
supply of both the current and previous year. Last year's annual growth ring of wood affects
growth material supply on this year's growth [220]. This year's drought also will affect next
year's cambial growth. Such a delayed effect is the result of drought impacts upon crown
development, food production, and tree health. Drought will produce both rapid and delayed
responses along the cambium [220]. Shoot thickness of seedlings of sensitive and semi-tolerant
walnut genotypes decreased significantly in response to increased osmotic stress [212-213].
The stem of a woody plant comprises several different cell/tissue layers [222], from the
periphery and inwards: the protective outer bark; the inner bark with the phloem responsible
for sugar transport from leaves to roots; the vascular cambium responsible for growth of new
phloem outwards and new xylem inwards; and the mature xylem responsible for water
transport [222]. Transport occurs in conduits, comprising separate cell elements, inter-
connected by pores in their walls and/or series of cell elements forming vessels; all water
conducting cell elements die after completion of secondary cell wall growth and are then filled
with water. Zwieniecki et al. [37; 88] suggested that the interconnecting pores have a variable
diameter, since pectin is present in the pores and acts as a hydrogel in response to variable ion
concentration in the transported water.
2.11. Root growth
When roots are exposed to drought, the allocation of food to root growth may increase [220].
This provides more root absorptive area per unit area of foliage and increases the volume of
soil colonized. Extended drought leads to root suberization to prevent water loss to the soil.
Good water absorbing ability, coupled with a low transpiration rate for the amount of food
produced (high water-use efficiency), allows trees a better chance to survive drought condi‐
tions [220]. The annual root system (absorbing roots) takes up a majority of the water in a tree.
Annual roots are not the woody roots seen when a tree is dug. Large woody roots have bark.
Any bark crack or damage is quickly sealed-off so little water flows through these areas. It is
the young roots, the roots easily damaged by drought, which are the major absorbers of water
and essential elements in a tree [220].
In a study on walnut, under drought and salt stress, root length and dry weights for the
seedlings of many genotypes decreased significantly in response to increased osmotic stress
levels. Albeit under high osmotic pressure due to drought or salt stress root length was
greatest in the most tolerant varieties, 'Chandler' and 'Panegine20' [212-213]. Root dry
weight of most genotypes decreased significantly in tolerant genotypes vs. non-tolerant
ones. Tolerant genotypes ('Chandler' and 'Panegine20' and relatively 'Hartley'), had more
or less similar trend in term of root length and dry weight and did not show significant
differences at high Ψs.
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