Agriculture Reference
In-Depth Information
11
that water losses by transpiration are the
unavoidable cost for the plant in order to be
able to fix CO
2
from the air, essential for
photosynthesis and plant growth.
Transpiration requires energy (nor-
mally solar) for the water evaporation pro-
cess. If the energy decreases, transpiration
decreases. The energy supplies from the
greenhouse heating systems also contribute
to the evaporation process.
The majority of the water is absorbed
passively by the roots, as a result of transpi-
ration. When transpiration stops, there is no
passive absorption of water and nutrients,
which can have negative effects for the
plant. Therefore, in areas of low solar radi-
ation some energy is applied to the green-
house by maintaining a certain minimum
pipe temperature to promote transpiration.
The roots can obtain energy, burning
sugars, and absorb water actively, when there
is no transpiration or during the night. This
active absorption can be enhanced by increas-
ing the soil temperature, and thus, the roots.
Normally, on a sunny day, transpiration
increases quickly immediately after sunrise
and, as the roots cannot absorb the transpired
water at the same pace, the plant transiently
uses the water stored in its tissues. When
transpiration decreases, the root absorption
restores the water deficit of the tissues, com-
pleting rehydration during the night.
Water serves a number of basic functions
in a plant's life, constituting up to 95% of
fresh weight (Sutcliffe, 1977). Water dis-
solves several substances and is the trans-
port vehicle for the nutrients in plants. By
means of cell turgor it provides rigidity
and gives shape to several plant organs. It is
necessary for photosynthesis and partici-
pates in a large number of chemical reac-
tions of plant metabolism. In addition, it
allows plants to be cooled through its evap-
oration, by means of transpiration, absorb-
ing heat and cooling the leaf surfaces.
Transpiration may reach up to a maximum
of 98% of the total amount of water absorbed
by cultivated plants in their life cycle
(Sutcliffe, 1977), but normally it represents
95% (Kramer, 1983), the rest being used in
plant metabolism.
Transpiration takes place, mainly,
through the stomata of the leaves, which
must open to capture the required CO
2
for photosynthesis, and transfers water
vapour from the plant to the atmosphere
(see Chapter 6). One way of looking at it is
