Agriculture Reference
In-Depth Information
correctly and on time (suppressing newly
formed shoots, without allowing their
growth). In sophisticated greenhouses,
therefore, HI values are expected to be
higher than those obtained in unheated
greenhouses during the cold season (see
Table 6.2 in Appendix 1 section A.5.4).
4
3
2
1
6.9.2
Interception of radiation
by the crop
0
50
100
Days after sowing
Fig. 6.10.
evolution of the leaf area index (LAI)
of a cucumber crop, along its cropping cycle
(autumn-winter), in an unheated plastic
greenhouse (Mediterranean area).
Leaf area index (LAI) and crop growth rate
The interception of solar radiation by the
leaves is essential to convert the solar energy
into vegetable matter (biomass). At the
beginning of a crop growing cycle, when the
plants are small, a large part of the radiation
is not intercepted, impinging on the ground
and not being profited by the crop.
The basic parameter that relates the
radiation intercepted by a crop and the inci-
dent solar radiation is the LAI (Watson,
1958). This quantifies the surface of leaves
of a crop per unit soil ground area:
LAI = leaves' surface (m
2
)/soil surface (m
2
).
At the beginning of the crop growing
cycle, leaf development is slow and the LAI
increases slowly. At this stage, a large part
of the radiation is not intercepted by the
crop. Later, the LAI increases exponentially
if there are no limiting factors for growth
(lack of water, inappropriate temperatures)
until it reaches its maximum values
(Fig. 6.10). Later changes in LAI depend of
the type of growth of the crop. In plants of
determined growth, after reaching the maxi-
mum values of LAI, this parameter decreases
when senescence starts. In crops of undeter-
mined growth, high values of LAI will be
maintained during a great part of the cycle,
as the senescent leaf area is compensated
for by the production of new leaves.
Temperature has a great influence on
the growth and development of the leaves;
so, temperature needs to be managed accord-
ingly to achieve the maximum LAI in the
minimum possible time.
A LAI index between 3 and 4 is consid-
ered necessary, so that radiation interception
reaches 95%, with the usual planting
densities for herbaceous crops (Giménez,
1992). Crops with more vertical leaves (garlic,
onion, gladiolus, cereals) may reach higher
values with LAI of 5-10 for maximum inter-
ception. In greenhouses, as the crops are
grown in rows (in paired lines in many occa-
sions) the situation is more complex. Some
authors (Baille, 1995) estimate, as an approx-
imation, that interception is 100% (
e
i
= 1, in
Eqn 6.2) if the LAI is equal or higher than 3.
Until the plant does reach a LAI of 4, photo-
synthesis rates increase in parallel to the LAI
(Challa and Schapendok, 1984).
The 'critical LAI' (Broughman, 1956) is
the value above which there are no more
increases in the crop's growth rate, which
usually corresponds to an interception of
95% (Giménez, 1992), that is,
e
i
= 0.95.
Later increases in LAI, when the inter-
ception is virtually total, there may be a
decrease in the growth rate in some cases,
when the photosynthesis rate of the shad-
owed leaves does not compensate for the
respiration losses. In other cases, an accli-
mation of the shadowed lower leaves occurs,
adapting their respiration rates to the photo-
synthesis rates, without changing their
growth rate (Giménez, 1992).
The influence of cultural practices
(manual defoliation, plant density, training,
etc.) on the LAI is relevant. In Mediterranean
greenhouses, values of LAI of 3.5 for cucum-
ber in autumn-winter have been estab-
lished, whereas for undetermined growth
green-bean values of 6.2 have been recorded
