Agriculture Reference
In-Depth Information
The turgor changes come as a response
to different external stimuli, such as light,
CO
2
content, presence of potassium ions
(K
+
) and water supply. Under normal condi-
tions of water supply, most of the higher
plants open their stomata during the day, as
a response to light, and close them at night.
If the water conditions are less favourable,
which affects their turgor, the stomata may
close partially or totally. The CO
2
content in
the intercellular spaces also affects the
stoma opening. The temperature also affects
the movement of the stomata, influencing
the speed of response, which is slower at
lower temperatures.
Regulating the opening, the stomata
maintain a balance between photosynthesis
and water transpiration, to achieve the
higher levels of photosynthetic assimilation
while avoiding desiccation.
In plants with CAM metabolism, the
stomata open at night and close during
the day, to preserve precious water supplies
in extremely dry climates.
affect water vapour, CO
2
and O
2
fluxes,
influencing photosynthesis as well as tran-
spiration and respiration.
All the factors which induce stomatal
closure decrease photosynthesis. Lack of air
movement also reduces photosynthesis as
the thickness of the boundary layer
increases, and the resistance to the diffu-
sion of CO
2
molecules increases (Nobel,
1974a, b) (Fig. 6.3). The boundary layer is
the layer of motionless air that surrounds
the leaves where gaseous exchange takes
place by molecular diffusion. In the green-
house, the absence of wind compared with
open field cultivation generates thick
boundary layers. The resistance to gaseous
diffusion of the boundary layer of the leaves
may be notably higher than that of stomata
when air movements are very weak, such is
the case in closed greenhouses (Urban,
1997a). In practice, the grower must main-
tain a certain air movement in the green-
house for efficient photosynthesis and
proper production.
The accumulation of photosyntheti-
cally assimilated products in the leaves may
have a depressant effect on their own photo-
synthesis. These assimilates must be trans-
ported to other organs of the plant. This
transport process is regulated by several fac-
tors. For instance, high temperature, as well
as the presence of nitrates, favours this
translocation of assimilates. The lack of
nitrogen involves accumulation of starch in
the
6.3.3
Internal factors affecting
photosynthesis
To access the chloroplasts of the leaf tissues
(where photosynthesis takes place) the CO
2
must diffuse from the external air to the sto-
matal cavity (Fig. 6.2). Access to the stomata
by CO
2
is hindered by the stability of the air
layers, which surround the leaf (boundary
layer) and the stomatal cavity. The CO
2
must
overcome these two barriers, which are
quantified by their resistance: (i) the bound-
ary layer resistance; and (ii) the stomatal
resistance (Gijzen, 1995a). Both barriers
leaves,
decreasing
photosynthesis
(Acock
et al
., 1990).
The availability of proteins is funda-
mental in photosynthesis. A reduction of
nitrogen involves a decrease in the rate of
photosynthesis (Urban, 1997a).
Wind
Boundary layer
Fig. 6.3.
The leaf and its boundary layer (layer of motionless air which surrounds the leaf).
