Agriculture Reference
In-Depth Information
9.3
Light
and the associated costs of the inputs in
relation to the corresponding values of the
produced yields.
Nowadays, in sophisticated facilities,
it is possible to carry out dynamic control
of the CO
2
supply. The CO
2
enrichment
threshold varies depending on the heat
demand conditions, radiation, wind speed
and opening of the vents and this is repre-
sented in Plate 20 (Nederhoff, 1995). A
high CO
2
set point (line A in Plate 20) is
used if the heating is independent of venti-
lation and radiation. Line B is taken as the
set point when heating is not required and
radiation exceeds a certain threshold. If the
radiation is lower than a pre-fixed value
and heating is not used the CO
2
set point
follows line C. A last option for a CO
2
set
point is chosen when the greenhouse is
highly ventilated (from a certain percent-
age, the 'Min.' line). How far the vents are
open (the percentage), which is determined
by the wind speed among other parameters,
influences the CO
2
thresholds (Plate 20);
an increase in the wind speed moves lines
B and C towards the left.
The grower can maintain a higher CO
2
level when using carbon enrichment by
avoiding ventilation but it does not seem
advisable.
9.3.1
Introduction
Light regulation is practised in a greenhouse
for the following reasons: (i) to alter the
length of daylight hours (increasing or
reducing them); (ii) to interrupt the dark-
ness at night (briefly, for regulation of pho-
toperiod); (iii) to extend or reduce the dark
period of the night using artificial light or
darkening screens; (iv) to increase photo-
synthesis (complementing the naturally
available light and/or extending the length
of the day with artificial light, Photo 9.3);
and (v) to decrease the light intensity (e.g.
with shading screens when the air tempera-
ture gets too high).
The objective is to maximize photosyn-
thesis by maximizing the light interception
(PAR) by the greenhouse, which involves
optimizing its design and orientation. In
order to make the radiation useful for pho-
tosynthesis it must be intercepted by the
crop, which will require the crop rows to be
appropriately orientated (north-south) and
a proper arrangement and density of the
plants (lower in winter than in high radia-
tion seasons), depending on the species,
cultivar and crop conditions.
Photo 9.3.
Artificial light is used in areas where there is a deficit of solar radiation.