Geoscience Reference
In-Depth Information
Initial water
content
0.3
0.2
0.1
0.0
0
3
6 9
Distance from root surface (mm)
12
15
18
21
Figure 6.2 Water content distribution around a single root of radish after 10 hours of
water uptake (Hainsworth and Aylmore, 1986 ).
cortex always has to enter the cell protoplasm. Here at the endodermis low between
neighbouring cell wands or intercellular air spaces is obstructed by suberization of
the lateral cell walls, the barrier named Casparian strip ( Figure 6.3 ). At the endoder-
mis layer the plant has its last chance to modify the composition of solutes coming
in with water before they are transported from the root. Nutrients in abundance and
nonessential or even toxic ions can be excluded from passing onwards to the vascular
system (Ehlers and Goss, 2003 ; Kirkham, 2005 ).
The hydraulic resistance of the cortex depends largely on the permeability of the
cell membranes. And the permeability depends to a large extent on cell respiration.
Respiration again relies upon temperature and oxygen supply. These interrelations
indicate a phenomenon that plants may wilt in poorly drained or waterlogged soils.
The plants show signs of water deiciency, while large amounts of soil moisture occur
in the root zone!
For maintenance of root tissue permeability, most of crop plants need an oxygen
supply to the roots via the pore system of the soil and an adequate soil temperature.
In the soil, oxygen is transported from the soil surface to the oxygen respiring roots
by diffusion (Cook, 1995 ; Bartholomeus et al., 2008 ). As long as some soil pores
are drained, oxygen diffusion through the aerated pore system may satisfy oxygen
requirements. But in soil with excessive water, oxygen diffusion is greatly impaired,
as the diffusion coeficient of oxygen in water is 10 4 times smaller than in air. As a
result, the oxygen supply rate to the roots is very much reduced. Table 6.1 lists the
minimum volumetric air contents required in the top soil for potential root water
uptake of different crops in a humid climate. It shows that bulb and beet crops require
higher air contents than grain crops and grasses. In clay soil, a lower air content is
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