Agriculture Reference
In-Depth Information
9.2
Carbon Enrichment (CO
2
)
levels do not generate yield increases. This,
in turn, may be caused by an accumulation
of starch, which would limit photosynthe-
sis, or by an increase in the thickness of the
leaf (Hanan, 1998). When conditions occur
which induce high photosynthesis rates
(high radiation together with high CO
2
lev-
els, for instance; see Chapter 6), sometimes
an induced photosynthetic inhibition may
occur (feedback effect), due to an accumula-
tion of photosynthates (complex sugars
such as starch), as they are produced at a
higher rate than that with which they are
transported or exported to other organs of
the plant. The phenomenon is complex and
induces the Calvin cycle to stop (Lambers
et al
., 1998).
In a greenhouse, under normal stagnant
conditions it is very difficult to have a dif-
ference of CO
2
content between the external
air and the greenhouse air greater than 600
ppm (Seeman, 1974). In practice, CO
2
lev-
els that reach close to 1000 ppm is only fea-
sible (at an affordable cost) with closed
greenhouses.
Care must be taken with problems
derived from the pollution caused by the
supply of toxic gases, depending on the
source of CO
2
. From the point of view of
workers security the maximum limit is
5000 ppm (i.e. 0.5% of CO
2
in the air)
(Hicklenton, 1988).
Ventilation is the most economic
method to limit CO
2
depletion in the green-
house air, but it only allows the maximum
to reach levels close to those of the external
air (350 ppm). Besides, in many cases venti-
lating is not desirable, for other reasons.
Therefore, artificial enrichment is a usual
practice.
Maintaining high CO
2
levels involves
closing the vents to avoid leakage, which
may induce excess temperatures, in some
cases. Thus, under Mediterranean condi-
tions, a usual strategy is to maintain levels
of 350 ppm by injection and stopping the
injection when the vents must be kept
open to limit the thermal excesses; when
vents are closed, the CO
2
level is increased
to 600-700 ppm (Lorenzo
et al
., 1997c;
Sánchez-Guerrero
et al
., 1998; Segura
et al
., 2001).
9.2.1
Introduction
In a greenhouse, the limited air movement
hinders the supply of CO
2
to the stomata of
the leaves for photosynthesis. This gas
exchange is dominated by the boundary
layer resistance, as the air velocity, even
with mechanical ventilation, is hardly
above 0.3 m s
−1
(Hanan, 1998). Therefore, it
is necessary to achieve a minimum hori-
zontal air movement for CO
2
supply to the
leaf stomata. Increases in CO
2
levels gener-
ate an increase in photosynthesis and a sub-
sequent increase in yield. Besides, CO
2
enrichment induces an improvement in the
water use efficiency (Sánchez-Guerrero
et al
., 1998).
The nomenclature used for the presence
of CO
2
may take different forms: ppm (parts
per million), vpm (volumes per million) or
partial pressure. In this text the most com-
mon form (i.e. ppm) will be used. For more
details see Appendix 1 section A.7.
9.2.2
Recommended CO
2
concentrations
The recommended CO
2
concentration
depends on the species and the variety,
the climate conditions (especially PAR
and leaf temperature), as well as economic
reasons such as the price of CO
2
and ben-
efits of its use. For vegetables, it has been
recommended not to exceed 1500 ppm for
cucumber, or 1000 ppm for tomato and
pepper (Urban, 1997a). Nowadays, 1000
ppm is considered a suitable maximum
limit for all the species, except for cucum-
ber, aubergine and gerbera (Hanan, 1998).
In aubergine, 700 ppm must not be
exceeded (Nederhoff, 1984). The excess
of CO
2
in tomato plants may cause abnor-
mally short leaves or the rolling of the
leaves, whereas in other crops it may
cause
leaf
chlorosis
(Langhams
and
Tibbitts, 1997).
An adaptation of the plants to high CO
2
is possible, so that the increases in CO
2
