Agriculture Reference
In-Depth Information
the use of radiation
these indices may be increased by up to
20% (Baille, 1995) or 30% (Challa
et al.
,
1995) if a concentration of 1000 ppm of CO
2
is maintained. It has been reported that a
reduction of 1% of radiation caused a
decrease of 1% in the yield of cucumber
and tomato (De Visser and Vesseur, 1982;
Cockshull
et al.
, 1992).
On a fresh weight basis, assuming a HI
of 0.7 in tomato, a dry matter content in the
fruit of 5%, and, an efficiency in the use of
radiation of 1 g of dry matter MJ
−1
of global
radiation, the tomato productivity would be
14 g of fresh fruit MJ
−1
for a developed crop
without other limiting factors under normal
conditions (Baille, 1995).
The potential greenhouse production
may be roughly estimated for average mete-
orological conditions and depending on the
greenhouse characteristics (Challa and
Bakker, 1999). In practice, it is estimated
that the proportion of absorbed PAR, which
is used for dry matter production, ranges
from 4% to a maximum of 10%, in the best
conditions (Baille, 1999).
The use of radiation in a greenhouse is
one of the most efficient of all agricultural
ecosystems, although radiation is reduced
compared with that of an open field.
However, in terms of total energy use, mod-
ern heated greenhouses are the most inten-
sive of all the agricultural ecosystems
(Baille, 1999) (Fig. 6.12; see Appendix 1
section A.5.2).
Crop management must pursue optimiza-
tion of the photosynthetic process, to maxi-
mize the yield and the quality. Early sowings,
when the climate conditions are good for
plant growth or planting with sufficiently
developed transplants, allow for a good and
early interception of radiation. Achieving a
fast leaf development by the use of cultivars
adapted to the local climate conditions allow
for an improvement of radiation intercep-
tion and photosynthesis. Cultural practices,
such as optimum management of fertiliza-
tion and irrigation, also affect the final yield,
as photosynthesis is improved.
The management of plant density is
one of the ways used to achieve efficient
interception of radiation (Giménez, 1992;
Papadopoulos and Pararajasinghma, 1997).
However, in fruit vegetables, besides adapt-
ing the plant density to the climate condi-
tions (radiation, mainly), it must be taken
into account that a high density may affect
the fruit size (Castilla, 1995). For a certain
density, pruning and training of the plants
must pursue the optimization of photosyn-
thesis and promote the distribution of
assimilate towards the plant organs which
are required (Papadopoulos and Ormrod,
1988, 1991).
Diffuse radiation represents an impor-
tant fraction of solar radiation entering
PAR
i
PAR
r
PAR
t,r
PAR
t
Fig. 6.12.
The different components of the PAR for the calculation of the radiation absorbed by a crop.
PA R
i
, PAR incident on the crop;
PA R
r
, PAR reflected by the crop or albedo;
PA R
t
, PAR transmitted at ground
level;
PA R
t,r
, PAR reflected from the ground (see Appendix 1 section A.5.2, adapted from Baille, 1999).
