Agriculture Reference
In-Depth Information
Net
photosynthesis
p
High radiation
Medium radiation
Low radiation
CO
2
content
of the air
Net
respiration
Fig. 6.7.
Net photosynthesis as a function of the content of CO
2
in the air, for different radiation
intensities. The graph portrays the 'limiting factors principle' applied to radiation (adapted from Urban,
1997a). P, Line of potential net photosynthesis.
1000 ppm (Kimball, 1986; Hicklenton,
1988). But this response at the leaf level
does not guarantee a corresponding increase
at a whole plant level in biomass produc-
tion (Urban, 1997a). Besides, when plants
are under high CO
2
levels for long periods,
they become adapted to these levels and
limit their response (Woodward, 1987).
On a practical level, values above
750 ppm have been recommended for
tomato and cucumber crops, in Northern
European countries (Urban, 1997a). The dif-
ficulty arises when ventilation is necessary
due to excess temperature, which involves
losing CO
2
to the atmosphere and higher
costs. Therefore under Mediterranean con-
ditions, where higher temperatures require
frequent ventilation, the CO
2
fertilization
strategy is different and it is recommended
to maintain levels of 360 ppm of CO
2
in the
internal air, while the vents are open, and
600-700 ppm when the vents are closed
(Lorenzo
et al
., 1997c, 2005).
water supply (non-limiting irrigation) and
in the absence of salinity problems, photo-
synthesis is not affected by a low environ-
mental humidity (Urban, 1997a). However,
if plants are under a very high evaporative
demand caused by low humidity or if there
are difficulties in the water supply from the
roots, photosynthesis may be limited,
because of stomatal closure due to the low
water status of the leaves (Gijzen, 1995a, b).
In fact, the positive effect of high humidities
on photosynthesis is only observed when
water absorption by the roots is limiting
(Grange and Hand, 1987).
Therefore, under non-limiting irrigation
conditions and in the absence of salinity
problems, humidification of the atmosphere
is not justified to improve photosynthesis.
Inhibition of photosynthesis
When carbohydrate demand is lower than
the supply, for instance after the harvesting
of fruit (sinks), there is an increase in the
carbohydrate content (starch) in the leaves
which may induce a photosynthesis reduc-
tion as a 'feedback' effect (Stitt, 1991).
The presence of highly polluting gases
(SO
2
, CO, NO
x
) may also reduce photosyn-
thesis if high levels are reached.
Ambient humidity
The ambient humidity does not directly
interfere with photosynthesis. Its role is
indirect through its influence on stomatal
