Agriculture Reference
In-Depth Information
(a)
(b)
Figure 10.4
Reversible wilting: (a) the
Primula
is wilting because the water in the pot is frozen (physiological
drought) - the roots are unable to replace the water lost by the leaves so turgor pressure is lost; (b) a few hours
later the water in the pot has thawed, turgor pressure is restored and the plant has recovered
will cease to enter the cell and will eventually start to
move out of the cells.
Turgor
pressure is lost and the
plant will
wilt
(Figure 10.4). This will not be a problem
if the water supply is only reduced temporarily,
the plant will be able to recover once water loss
is reduced or water supply is restored. However, if this
continues, the cells may then become
plasmolysed
, a
situation
where the cell contents shrink away from
the cell walls leading to irreversible damage and cell
death. Plasmolysis can also occur if there is a build-up
of salts in the soil or, for example, where too much
fertilizer is added causing
root scorch
. Water moves
out of the root cells because the solute concentration
is greater outside the cell than inside. Such situations
can be avoided by applying the correct dosage
of fertilizer to soils (and leaves where patches of
plasmolysed cells appear as leaf scorch if foliar feeds
are too concentrated (see p. 173)).
Most of the water uptake takes place in the
root hair
zone
where the root surface area is greatly enlarged
(see p. 81). As well as passing into the root hair cells,
water also flows through the cell walls surrounding
them.
water and have very little specialized support
tissue. In order to survive, any organism must
carry out complex chemical reactions, such as
photosynthesis and respiration, described in
Chapter 9. Raw materials for these chemical
reactions must be transported and brought into
contact with each other by a suitable medium;
water is an excellent
solvent
, that is, many
substances are able to dissolve in it. One of
the most important processes in the plant is
photosynthesis
, and a small amount of water is
used up as a raw material in this process. Water
may also be used for
seed
,
fruit
and
pollen
dispersal
in aquatic plants and in more primitive
plants such as mosses and liverworts, water is
needed for
reproduction
.
Movement of water in the roots
It is the function of the root system to take up water
and mineral nutrients from the growing medium, and
the system is constructed accordingly, as described
in Chapter 6. Detailed work by Dr E.G. Coker showed
the extent of the root system in a mature apple tree,
as revealed in the image in Figure 10.1.
Water enters the plant through the root primarily
though the root hairs (Figure 10.3). Initially it crosses
the epidermis either entering the root hair cells by
osmosis or passing between the epidermal cells
through the relatively porous cell walls. It then meets
the
cortex
layer, which is often quite extensive, and
moves across it to reach the transporting tissue that is
in the centre of the root (see p. 80). Water movement
is relatively unrestricted as it moves through the
intercellular spaces and the lattice work of cell walls,
Functions of water
The plant consists of about 95% water, which
is the main constituent of
protoplasm
or the
living matter of cells. When the plant cell is
full of water, or turgid, the pressure of water
enclosed within a membrane or vacuole acts as
a means of
support
for the cell and therefore
the whole plant, so that when a plant loses
more water than it is taking up, it may wilt.
Aquatic plants are supported largely by external
