Agriculture Reference
In-Depth Information
The formation of reserve organs is influ-
enced by growth regulators. This is of no
interest to the usual greenhouse vegetables,
although it is important in ornamental
horticulture.
The shape and size of the product (fruit,
leaf) are very important from a qualitative
point of view. Although the effects of radia-
tion on tomato fruit size are due to the avail-
ability of assimilates (Cockshull, 1992),
other aspects of radiation, such as its qual-
ity, influence morphogenesis which affects
the size and shape of leaves, flowers and
fruits (Challa
et al
., 1995).
known as the 'harvest index' (HI) (Coombs
et al
., 1985).
Improvement in the productivity of the
crops can be achieved by minimizing the
respiration losses and maximizing the PAR,
the efficiency in light interception (
e
i
) and
the efficiency in biomass conversion (
e
b
).
In practice, the improvement in the
productivity of many crops has been
achieved by a better light interception,
derived from suitable fertilization and
proper cultural practices (Coombs
et al
.,
1985). The improvement in the efficiency of
the conversion of light into biomass in
greenhouses is feasible, mainly by means of
CO
2
enrichment, which also decreases pho-
torespiration (Coombs
et al
., 1985) and by
avoiding suboptimal climate conditions.
In plastic greenhouses, the use of light-
diffusing covering materials improves the
efficiency of radiation conversion (
e
b
), as
the proportion of diffuse radiation increases
(Baille, 1995), and its use is positive if it
does not significantly reduce the PAR
transmissivity.
The improvement in the HI has been
possible by breeding, as well as improving
the cultural practices (fertilization, protec-
tion against pests).
In vegetables, the HI (referred to as dry
weight) varies depending on the cultural
practices and the cultivar used. In
Mediterranean greenhouses, HIs expressed
as a decimal per unit or a percentage, have
been estimated at 0.3 for tomato grown in a
winter cycle (Castilla and Fereres, 1990),
very influenced by the suboptimal climate
conditions and the low dry matter content
in the fruit. In pepper, values of HI from
0.36 to 0.46 are usual in unheated green-
houses (Martínez-Raya and Castilla, 1993),
whereas in cucumber grown in an autumn
cycle it reaches a value of 0.59 (Castilla
et al
., 1991). The climate conditions notably
affect the HI values. For instance, in melon
cultivated in an unheated greenhouse in an
early cycle (colder), the HI is higher than in
a later cycle due to the lower vegetative
growth with suboptimal thermal conditions
(Castilla
et al
., 1996). Also, a notable influ-
ence on the HI values has been the length of
the cycle as well as the pruning and
6.9
Bioproductivity
harvest index (HI)
In any crop there are four factors which
determine its net productivity (
P
n
): (i) the
amount of incident PAR; (ii) the efficiency
of interception of this radiation by the green
organs of the plant (
e
i
); (iii) the efficiency in
photosynthetic conversion of the PAR into
biomass (
e
b
); and (iv) the biomass losses due
to respiration (
R
). Such factors are related
(Coombs
et al
., 1985):
P
n
= PAR × (
e
i
×
e
b
) −
R
(6.2)
P
n
= Net productivity or net gain of biomass
(g m
−2
) or net photosynthesis, result of
deducting the respiration losses from the
gross photosynthesis
e
i
= Efficiency of light interception (PAR) by
the crop, expressed per unit
e
b
= Efficiency of light (PAR) conversion
into biomass (g MJ
−1
)
PAR = accumulated PAR (MJ m
−2
)
R
= Biomass losses due to respiration
The product (
e
i
×
e
b
) is called the effi-
ciency in the use of radiation (light) by the
crop (Baille, 1995).
The economic performance of a crop
is the amount of this productivity (
P
n
)
which is destined to the harvestable organs
(the fruit in the case of tomato or pepper,
or the leaves in the case of lettuce). The
proportion of the total biomass represented
