Agriculture Reference
In-Depth Information
internodes, lower chlorophyll concentra-
tion and lower dry weight. High radiation
levels induce the stimulation of branching,
the proliferation of growing points, possible
photodestruction of chlorophyll (known as
bleaching), and, in extreme cases, gener-
ation of symptoms of stress attributable to
radiation excesses in some ranges of the
spectrum, as the increase in the production
of anthocyanins (Langhams and Tibbitts,
1997). High radiation levels may also
induce, due to their higher energy supply,
heating of the leaves increasing water use,
and causing desiccation in extreme cases.
In nature, under low light conditions,
in an adaptation process of the plants that
involves a long-term mechanism of natural
selection, the leaves tend to place them-
selves horizontally, to intercept the maxi-
mum radiation. When the light intensity is
high, the leaves tend to adopt a more verti-
cal position.
The efficiency in the use of radiation by
photosynthesis (CO
2
fixed in relation to
absorbed PAR) varies little among C3 plant
species (Ehleringer and Pearcy, 1983), but
varies more if we refer to incident radiation
rather than to absorbed radiation.
At low latitudes the predictable maxi-
mum values of global radiation are slightly
higher than 1000 W m
−2
, in open field, ele-
vated locations, with a semi-arid climate
and low air turbidity (Hanan, 1998), whereas
at sea level the maximum global radiation is
of the order of 900 W m
−2
(Salisbury, 1985).
On the Spanish Mediterranean coast, the
maximum values of global radiation inten-
sity are close to 1000 W m
−2
at the summer
solstice. The average daily transmissivity,
in low-cost type commercial greenhouses
with a shallow roof slope, oscillates during
this season around 61.5% as an average
value (Morales
et al
., 1998), which is very
influenced by the dirtiness of the plastic
cover, since washing the plastic results in
an increase of transmissivity of about 14%
(Montero
et al
., 1985; Morales
et al
., 1998).
At the winter solstice, in low-roof-slope
commercial greenhouses, the average
transmissivity ranges from 57% for north-
south orientation, to 63% for east-west ori-
entation (Morales
et al
., 2000), with less
months, because in the winter the rain usu-
ally washes the plastic cover. In low-cost
parral-type greenhouses, increasing the
slope of the south side of each span, with
east-west orientation, allows increases in
transmissivity of up to 73% at the winter
solstice (Castilla
et al
., 2001).
Temperature
Photosynthetic activity has a clear response
to temperature; it is at a minimum at about
5°C, reaching an optimum at temperatures
from 25 to 35°C in the majority of horticul-
tural species and it decreases at higher tem-
peratures (Urban, 1997a).
The optimum temperature increases
with the radiation and CO
2
levels (Acock
et al.
, 1990). In practice, it is of no interest
to maintain high temperatures with low
radiation (not much heating on days with
little light). Under high temperatures, crops
grow better with high radiation; therefore,
shadows must be avoided in usual horticul-
tural crops (which are radiation demanding).
CO
2
At relatively low CO
2
levels in the air, if
radiation and temperature are high enough
not to become limiting factors, the photo-
synthetic rate is almost proportional to the
air CO
2
content (Urban, 1997a) (Fig. 6.7).
The critical CO
2
threshold below which the
carbon balance is negative (respiration is
higher than photosynthesis) is, normally,
lower than 200 ppm (Gijzen, 1995a).
Higher CO
2
contents induce a higher
value of the CO
2
:O
2
ratio, increasing the
activity of the enzymes which favour photo-
synthesis (ribulose-1,5-bisphosphate car-
boxylase/oxygenase, commonly known by
the shorter name RuBisCO) and limiting
photorespiration, improving the carbon bal-
ance (Urban, 1997a).
The CO
2
levels in the air have increased
during the last century, from values of
280 ppm (Nederhoff, 1995) to levels of 360-
370 ppm. It is forecasted that this increase
will continue during the next years, due to
human activity.
The atmospheric CO
2
values that max-
imize
