Agriculture Reference
In-Depth Information
uniformity (Hanan, 1998). Specialized
computer software is available to assist in
the optimization of the positioning of the
electric lamps across the greenhouse area
and above the plants.
The illumination system is usually
switched on when the natural PAR radi-
ation level is below 10 W m
−2
(Baille,
1999) or 15 W m
−2
(Kamp and Timmerman,
1996), extending the day length up to a
total of 12-16 h, but not exceeding 18 h as
this would be harmful for some crops,
although some species such as lettuce
will benefit from constant illumination
(Nelson, 1985). However, constant illumi-
nation may cause leaf abscission in some
species, and is counterproductive in
tomato (Hanan, 1998). The recommended
illumination levels in vegetables range
between 12 and 24 W m
−2
PAR in cucum-
ber, pepper and tomato, and between 12
and 48 W m
−2
PAR for aubergine and let-
tuce (Hanan, 1998). When the PAR is very
low, the artificial light may double the
photosynthesis rate; as for example when
increasing PAR from 15 W m
−2
(without
complementary light) to 30 W m
−2
(Kamp
and Timmerman, 1996).
Pioneering work on the effects/bene-
fits of artificial light on greenhouse crops
(first on flowers and then on vegetable
crops) that started in Canada and the
Scandinavian countries in the last couple
of decades of the 20th century (Tsujita,
1977; Blain
et al
., 1987; Gislerød
et al
.,
1989) has now spread to many countries.
Many new findings have been reported in
proceedings of frequent international sci-
entific symposia in the first decade of the
21st century (Dorais, 2002, 2011; Moe,
2006; De Pascale
et al
., 2008).
The supply of power for illumination is
usually expensive, so it is frequent to resort
to cogeneration (simultaneous production
of heat, used for heating, and electricity),
which is more efficient and cheaper,
although it requires a higher initial invest-
ment. Using inter-lighting, instead of lights
only on top of the crop, and LEDs can sub-
stantially increase light and energy effi-
ciency (Heuvelink and González-Real, 2008;
Montero
et al
., 2010).
The use of artificial illumination
has not spread in the south of Europe.
Artificial light is more used in the north
of Europe, where, in the proximity of the
cities, its use is forbidden in the early
morning and at night for environmental
reasons (including a potential alteration
of the biological rhythms of humans). If
its use is restricted it becomes difficult to
make it profitable.
In order to lengthen the useful life of
the lamps, they must not be frequently
switched on and off. It is recommended to
keep them on for at least 20 min, in the case
of photosynthesis lamps, and switching
them off for 10-15 min, before switching
them on again (Van Meurs, 1995).
The lamps return a great deal of the
energy consumed as heat (in the order of
75%), which decreases the heating require-
ment, an aspect to consider in the manage-
ment of heating systems.
The lamps must be protected from the
fog systems. The profitability of the supple-
mentary illumination depends highly on
the ability of the grower to optimize the
growing conditions, with the aim of avoid-
ing other factors limiting productivity.
9.3.4
Partial light reduction
There are many methods to decrease the
solar radiation in greenhouses. The most
simple is to whitewash (i.e. spray the out-
side with white paint) the greenhouse, but
this does not allow for easy reversal of the
shading effect (Photo 9.5).
The usual objective is to reduce the
radiation to limit high temperatures in
the high radiation season. But if this radi-
ation reduction involves a PAR reduction,
as normally happens with conventional
shading screens or with whitewash, it
generates a decrease in growth and yield.
Therefore, whenever possible, it is prefer-
able not to shade but to increase the
capacity of the cooling system, which in
most
cases
means
more
and
better
ventilation.
It may be that, due to fruit quality prob-
lems (for instance, sun scald in tomato or
