Agriculture Reference
In-Depth Information
Photosynthesis per unit leaf area
reaches a maximum approximately when
the leaves reach their maximum size and
decreases as the leaves age, as they lose
functionality from a photosynthetic point of
view.
The absorption spectrum of the photo-
synthetic pigments shows that green light is
less efficient, the main absorption peaks
(intervals of higher efficiency) being in the
red and the blue areas, due to the light absorp-
tion by the carotenoids that accompany the
chlorophyll in the chloroplast membranes
(Plate 11) (Whatley and Whatley, 1984). The
red light is more efficient than the blue light
for photosynthesis (McCree, 1972). Within a
canopy, the lower layers receive radiation
with a higher proportion of green light than
the higher levels, which filter the light.
There is a curvilinear response of pho-
tosynthesis of individual leaves to the
absorbed PAR, if other factors such as CO
2
and temperature are not limiting (Fig. 6.4)
(Urban, 1997a). At low radiation, the photo-
synthesis may be lower than the respiration
losses. Gains and losses become equal at
the radiation compensation point (Fig. 6.4).
A plant cannot subsist for a long time below
the compensation point. With high radiation
the increase in photosynthesis reaches a
maximum and is no longer proportional to
the increase in radiation; this point is the
light saturation point (Fig. 6.4). When the
radiation is very low artificial light is more
6.3.4
External factors influencing
photosynthesis
Radiation
The quantity of photosynthesis carried out
by a plant is influenced by three properties
of the light: (i) the 'quality' of the light;
(ii) its intensity; and (iii) its duration.
Only a fraction of the global solar radia-
tion is used in photosynthesis. This fraction
is known as photosynthetically active radia-
tion (PAR). Within the PAR range (400-
700 nm) not all the photons of different
wavelengths have the same photosynthetic
efficiency. Within the range of 500-600 nm
(green colour) the radiation is not well
absorbed by the chlorophyll (reflecting part
of it), giving the plants their typical green
colour (Monteith and Unsworth, 1990).
Net
photosynthesis
Radiation
saturation
Photodestruction
A
0
Intensity of PA R
Net
respiration
Fig. 6.4.
Response of net photosynthesis to photosynthetically active radiation (PAR). A, Radiation
compensation point (adapted from Hall and Rao, 1977).
