Agriculture Reference
In-Depth Information
(a)
(b)
Figure 10.6
(a) Hairy leaves of
Plectranthus argentatus
which appear silver; (b) water storage leaves of the jade
plant (
Crassula ovata
)
giganteum,
the giant redwoods, pull water up 100
metres from ground level. This apparent ability to flout
the laws of nature is probably due to the small size of
the xylem vessels, which greatly reduce the possibility
of the water columns collapsing.
A number of environmental factors affect the rate of
transpiration:
X
Humidity
.
If the air surrounding the leaf becomes
very
humid, then the rate of diffusion of water
vapour will be much reduced and the rate of
transpiration will decrease. In contrast,
moving
air
around the leaf will reduce the surrounding
humidity so will increase the rate of transpiration.
Windbreaks can help reduce windspeed and the
risk of plants drying out.
X
Temperature
affects the rate at which water
in the leaf evaporates and diffuses and thus
determines the transpiration rate. As temperature
rises, diffusion is speeded up, so the rate of
transpiration increases. At high temperatures
though,
the stomata will close and transpiration will
cease.
X
Light
affects the rate of transpiration due to the
response of stomata to light levels. At night, for
example, stomata close to conserve water loss
and transpiration is reduced. In the daytime,
stomata open to allow uptake of carbon dioxide
for photosynthesis and transpiration increases.
Shading may be used in glasshouses to reduce
temperature and light levels in the height of
summer, thus reducing the transpiration rate and
preventing water loss. Similarly cuttings are kept
out of direct sunlight to reduce transpiration until
they develop roots and are able to take up water
more effi ciently.
Structural adaptations to the leaf occur in many
species to enable them to reduce transpiration and
withstand low water supplies. For example, conifer
needles such as in
Pinus sylvestris
(see Figure 4.9)
have a reduced surface area and stomata sunk into the
leaf. Many evergreens such as holly (
Ilex aquifolium
)
have a very thick, waxy cuticle, while other plants
have many leaf hairs which trap humid air close to the
leaf as in the felted leaves of sage (
Salvia officinalis
)
and
Plectranthus argentatus
(Figure 10.6a). Others
have leaves which store water, such as the succulent
jade plant (
Crassula ovata
) shown in Figure 10.6b.
In extreme cases (e.g. cacti), the leaf is reduced to
a spine (see Figure 7.20f) and the stem takes over
the function of photosynthesis and is also capable of
water storage.
Mineral nutrient uptake and
movement in the plant
Essential mineral nutrients are inorganic substances
necessary for the plant to grow and develop (see
Chapter 14). They are dissolved in the soil water
and are taken up when this is absorbed by the root.
At the endodermis, nutrients have to cross the cell
membrane (see above) and since the concentration
of nutrients inside cells is almost always greater than
the concentration in the soil water, uptake is
against
a concentration gradient
.
Nutrients therefore cannot
enter the cell by simple diffusion and have to be taken
in by a process called
active transport
which requires
energy. This uptake is also
selective
, the plant only
absorbs the mineral nutrients it requires and rejects
others.
Essential mineral nutrients
are inorganic
substances necessary for the plant to grow and
develop.
