Agriculture Reference
In-Depth Information
Under normal conditions, the LAI
(leaf area index) is an indicator of the
light interception (see Chapter 6). During
the first stages of the crop a high plant
density allows for better light intercep-
tion, so early production will increase (in
relation to a normal density). Once the
crop covers all the available space the
plant density is less relevant. A high
planting density involves a decrease in
the quality of the product in most species,
and beyond a certain threshold a decrease
in yield, when expressed on a per unit
area basis.
When the solar radiation is insufficient,
it may be complemented with artificial
light, to increase the PAR level above the
radiation compensation point and maintain
an active growth. The positive effect of an
increase of the PAR on the growth is more
relevant at low PAR levels (Hanan, 1998).
Artificial light may also be used to extend
the period of photosynthetic activity in the
winter season. In vegetable cultivation in
the Mediterranean area, artificial light is not
economically feasible in most cases. The
vegetables usually grown in greenhouses
are insensible to photoperiod, under normal
conditions, but tomato, as an example, will
become chlorotic when the day length
exceeds 18 h.
Whitening the greenhouse cover is effi-
cient at decreasing excess temperatures in
the high radiation season but it notably lim-
its radiation, which involves a decrease in
the potential yield (Morales
et al
., 2000). It
is preferable to improve the ventilation sys-
tem to limit thermal excesses. The use of
permanent shading screens has similar
effects to whitening (Pérez Parra
et al
.,
2003c). The use of mobile screens, which
are deployed only during the hours around
midday when the radiation is too high (and
so is the temperature), is another option of
possible interest.
of light to the crop, such as: (i) painting the
greenhouse structural components white;
(ii) applying a white plastic film as soil
mulch (Hernandez
et al
., 2001); and (iii) in
general making extensive use of other light-
reflecting materials.
A usual practice is to use reflecting
walls, such as in the lean-to greenhouses in
China (Fig. 7.5). Several reflection devices
have been proposed to increase radiation,
but they are usually uneconomic. However,
the reflectors perform well with direct light
and not with diffuse light, and unfortunately
the highest interest for increasing the light
availability is in the winter months when
diffuse radiation prevails (Hanan, 1998).
Artificial light is the most reliable and
effective method to increase the light
availability.
9.3.3
Artificial light to increase
the illumination
The use of supplementary artificial light
is common in sophisticated greenhouses
for high added value crops, in latitudes
above 40°N in America and 50°N in
Europe (Nelson, 1985). The main goals of
supplementary light are to increase photo-
synthesis (daytime illumination) and to
extend the length of the day (photoperiod)
which allows for an increase of the accu-
mulated daytime radiation (Huijs, 1995).
Traditionally, its most popular use has been
in cut flower crops (rose and chrysanthe-
mum) and during the first growth stage of
young plants, but in the last decade the use
of artificial light has also spread to other
flower species (Moe
et al
., 1992) and, to a
limited extent, to high-tech greenhouse
vegetable production areas of northern
countries.
The types of greenhouse lamps are:
(i) incandescent; (ii) fluorescent; and (iii) high-
intensity discharge lamps. The recent inno-
vation on the use of LEDs (light emitting
diodes) for lighting appears very promising,
but needs further research on its use. The
incandescent lamps have a very low energy
efficiency in converting electricity into PAR
9.3.2
Light increase
Inside the greenhouse, various techniques
have been used to improve the availability
