Appendix 1 - Greenhouse Technology and Management - page 288

k j = Thermal conductivity of the element j of
the greenhouse covering material (expressed
in W m −1 °C −1 )

must only be used if the soil or substrate tem-
perature is suitable, such as in heated green-
houses, because it may involve a yield
decrease in the case of undesirable cooling of
the soil or of the substrate in unheated green-
houses (Lorenzo et al ., 2001, 2005).

A.5 Chapter 6

A.5.1 Interception of radiation by the
canopy: extinction coefficient

A.5.2 Radiation absorbed by the crop

The PAR absorbed by the crop (Fig. 6.12) is
(Baille, 1999):

The radiation intensity intercepted by a cer-
tain layer of the canopy is proportionally
inverse to the LAI accumulated above such
a layer (Montsi and Saeki, 1953). The atten-
uation of the radiation follows the Lambert-
Beer law (Giménez, 1992):

PAR abs = ( PAR i - PAR t ) + ( PAR t,r - PAR r )

(6.4)

where:
PAR abs = PAR absorbed by the canopy
PAR i = PAR incident over the canopy
PAR t = PAR transmitted to the soil level
PAR t,r = PAR reflected from the soil
PAR r = PAR reflected by the canopy or albedo
Some authors neglect PAR t,r , considering
that it is not relevant, except in the case of
very low density crops. The absorption effi-
ciency of the radiation by the plant is very
influenced by the epidermal characteristics
of the leaf and its chlorophyll content.
The use of reflecting materials over the
greenhouse soil, such as white plastic
mulch, aims to increase PAR t,r in order to
raise PAR abs (Fig. 6.12).

I e = I × e − K (LAI) (6.3)

where:
I e = Radiation intensity that passes through
the considered layer
I = Radiation incident on the top of the
canopy
e = Mathematical constant (approximately
equal to 2.71828)
LAI = Leaf area index accumulated in the
considered layer
K = Radiation extinction coefficient
The extinction coefficient ( K ) is not a
constant value, because it depends on the
age of the crop, the average inclination of the
crop's leaves, the type of solar radiation
(direct or diffuse) and the solar elevation.
Fully developed crops with horizontal leaves
have higher K values, close to 1.0 (Fig. 6.11)
compared with crops with erect leaves,
which are more vertical, where K may
decrease to values around 0.3 (e.g. grasses,
gladiolus, garlic). The extinction coefficient
for diffuse light for cucumber and tomato is
of the order of 0.85 (Gijzen, 1995a).
When cultivating in rows, especially in
pair or double rows, the effect of the passage
on radiation interception is important when
the height of the plant is smaller than the
width of the passage, notably affecting pho-
tosynthesis (Gijzen and Goudriaan, 1989).
Under these conditions, it is beneficial to use
white mulch that reflects the light that
reaches the soil, directing it towards the crop
again (Gijzen, 1995a), but this technique

A.5.3 Growth parameters

Table 6.1. Main parameters used in the analysis
of growth (adapted from Berninger, 1989).

Accepted
abbreviation Units Name

Relative growth
rate

RGR g g −1

Leaf weight ratio LWR g (leaf) g −1 (total)

Leaf area ratio LAR m 2 (leaf) g −1

(total)

Specific leaf area SLA m 2 (leaf) g −1 (leaf)

Specific leaf
weight

SLW g (leaf) m −2 (leaf)

Leaf area index LAI m 2 (leaf) m −2

(soil)

Net assimilation
rate

NAR g m −2 h −1