Agriculture Reference
In-Depth Information
training; (iv) pruning; and (v) use of
mulching (Castilla, 1994). It is interesting
to experiment with novel growing tech-
niques, prior to their general adoption. In
this respect, it is important to highlight
the potentially negative influence in pro-
ductivity of the use of white mulching in
autumn-winter to increase the radiation
intercepted by the crop, in unheated
greenhouses under certain conditions,
because of concomitant significant reduc-
tions in root temperature, both in crops
grown in the soil or in artificial substrates
(Lorenzo
et al
., 1999, 2005; Hernández
et al
., 2001).
There are temperature differences inside
a greenhouse, the east, west and north bor-
ders usually being colder (in the northern
hemisphere), which can be minimized with
double cladding (Berninger, 1989).
The thermal inversions in unheated
greenhouses may occur on calm nights,
with a clear sky, when the radiation losses
towards the atmosphere are larger in the
interval known as 'atmospheric window'
(Rose, 1979; Fig. 2.14), if the greenhouse
covering material is permeable to radiation
in that interval (Day and Bailey, 1999).
3.4.2
Plant temperature
A thermometer located (without protec-
tion) inside a greenhouse during the night
may provide a reading different to the
actual air temperature. By approximation,
we call it the 'radiative temperature' or
'actinothermal index' (see Chapter 2). These
differences are larger with a normal PE
cover than with a glass cover. This 'radi-
ative temperature' better represents the
plant temperature than the air temperature,
during the night.
During the daytime, there are large dif-
ferences in plant temperature, with respect
to the air and also between parts of the plant,
depending on the radiation intercepted, the
water evaporation and the air movement,
among other factors. The temperature of the
flowers and the fruits depends greatly on
their colour, which influences the absorp-
tion of radiation. For instance, green fruit is
usually colder than red fruit.
Plant temperature has traditionally
been assessed on the basis of the air tem-
perature, corrected with the temperature of
the greenhouse walls and the ground sur-
face, and on the rate of evapotranspiration
(Berninger, 1989). However, now technol-
ogy is available for the direct measurement
of plant temperature and the theory/philos-
ophy of the 'speaking plant' in greenhouse
crop management is widely accepted
(Takakura, 1989; Challa and Bakker, 1995).
The recommended values for the green-
house air temperature, for the majority of the
In an unheated greenhouse the main source
of heat during the daytime is solar radiation,
part of which is stored in the soil. During the
night, the energy comes mainly from the
soil, in the form of far IR radiation.
3.4.1
Air temperature
The air temperature inside the greenhouse
is the result of the energy balance of the pro-
tection (Fig. 3.2). The greenhouse effect gen-
erally has two consequences:
1.
At night, due to the limitation of IR radi-
ation losses, the minimum temperatures are
similar or slightly higher (1-3°C higher,
depending on the covering material) than
the outside (Plate 8). Nevertheless, on clear
nights without wind, 'thermal inversions'
may occur.
2.
During the daytime, due to the 'heat trap'
effect and the reduction in the convective
exchanges (as the air is confined), the air
temperature is higher indoors than out-
doors, being possibly excessive when the
radiation is high and the greenhouse is not
efficiently ventilated (Plate 8).
The measurement of the air tempera-
ture must be performed in a representative
location of the greenhouse, protected from
direct sunlight and below a flow of air.
