Agriculture Reference
In-Depth Information
can happen only from leaks through the greenhouse envelope. If the internal air
circulation is very low, the remaining CO
2
deficit is not recoverable.
Whereas the traditional straw bale cultural technique for cucumbers is one of the
oldest and simplest methods of CO
2
enrichment in greenhouses, its importance has
increased with the trend towards producing crops in nutrient film, rockwool, and
other substrates, where natural CO
2
concentration is small compared to CO
2
coming
from the soil profile. Indeed, in greenhouses, the soil is frequently covered with plas-
tic sheets when alternative soilless media are used (Hicklenton
1988
; Slack
1986a
).
On the other hand, CO
2
enrichment methods have continued to develop sources of
nonpolluting CO
2
(Mortensen
1987
), so that the negative effects associated with the
burning of hydrocarbons have been reduced (Gruda
2005
).
Maintaining high levels of CO
2
is sometimes difficult, when solar radiation and/
or inside air temperatures are high inside the greenhouse, because roof and/or side
windows need to be opened, to ventilate the greenhouse in order to reduce the air
temperature and/or regulate the VPD at optimal values. New perspectives recently
developed closed and semi-closed greenhouses which reduce the energy consump-
tion. In such greenhouses window ventilation is usually reduced or replaced by an
active cooling system. In addition, energy saving measures have been implemented
where excess solar energy is collected and stored, in order to be reused at night or in
periods in which the solar radiation is limited, e.g. in cloudy days or in the winter.
Under these conditions it is possible and preferable to keep high CO
2
concentrations
even at high light levels.
Generally, concentrations of 800-1,000 µmol mol
−1
in greenhouse atmosphere
are used for different plants in the daytime, in order to promote photosynthesis and
inhibit light respiration. According to Drake et al. (
1997
) elevated CO
2
reduces
stomatal conductance as well as transpiration rate and improves WUE, while at
the same time stimulates higher rates of photosynthesis and increases light-use ef-
ficiency. Many studies reported these positive effects on physiology, growth, and
productivity of plants. Besford et al. (
1990
), for instance, found more than double
photosynthetic rates in mature leaves of tomatoes, an increase of the fresh weight
per unit area of leaf, and in general increases of crop yields, due to an increase of
CO
2
concentrations to 1,000 µmol mol
−1
. Mortensen (
1987
) reported that horti-
cultural greenhouse plants exhibited positive effects due to CO
2
enrichment by in-
creasing dry weight, plant height, number of leaves, and lateral branching, whereas
Mortensen and Moe (
1995
) found that the development rate of miniature roses could
be accelerated by 4-5 days at elevated CO
2
. Plant quality of ornamentals, expressed
by growth habit and number of flowers, is often enhanced by CO
2
enrichment.
Peet and Willits (
1987
) reported that CO
2
enrichment significantly increased the
yield of cucumbers. Mortensen (
1994
) showed that increasing CO
2
concentration
in plastic “field chambers” from ambient to 800-900 μmol mol
−1
could increase
the dry weight of lettuce, carrot, and parsley by 18, 19, and 17 %, respectively.
Enrichment with CO
2
(900 μmol mol
−1
, 8 h day
−1
) and supplementary lighting for
approximately 3 weeks before transplanting, increased accumulation of dry matter
in shoots by 50 % for tomato and pepper seedlings, as compared with the control
