Agriculture Reference
In-Depth Information
During the initial stage of vegetative
growth, roots, stems and leaves compete
for the assimilates produced by the leaves.
The young leaves initially need to import
assimilates until they are self-sufficient,
normally before reaching their final size
(Giménez, 1992).
From the time of flowering, the fruits
are the main sinks, attracting the available
assimilates and limiting the translocation
to the vegetative organs. Then, if the crop
is of determined growth, the growth of
stems and leaves slows down until it stops.
But if the crop is of undetermined growth,
there is a coexistence of growth of fruits
and vegetative organs, whose balance must
be controlled by cultural practices (removal
of stems, leaves or fruit; but, also, appro-
priate
In plants of undetermined growth, the
pattern of distribution of assimilates
between fruits and vegetative organs is not
constant through the cycle (Marcelis and De
Koning, 1995). Low temperatures, in gen-
eral, limit translocation. At the end of a day
in which the rate of photosynthesis has
been high, the assimilates accumulate in
the leaves and they can, if they are not
transported, limit the photosynthesis of the
following day. For this reason, it is recom-
mended to maintain a greenhouse night
temperature that is high enough to transport
these assimilates from the leaves to other
organs. By contrast, at the end of a day in
which the rate of photosynthesis has been
low (for instance, a day with low radiation)
it is not necessary to transport so many
assimilates, so the night temperature does
not have to be as high (Calvert and Slack,
1974). Other environmental factors, such as
light and CO
2
, have only an indirect influ-
ence in the distribution of assimilates by
affecting the rate of photosynthesis and, as a
consequence, the availability of assimilates
(Marcelis and De Koning, 1995).
Temperature can alter the distribution
of assimilates by creating new sinks. In gen-
eral, high temperature enlarges the inter-
nodes and decreases branching (Challa
et al
., 1995).
Humidity affects the size of the leaves.
For instance, high humidities induce larger
leaves in cucumber, which does not occur
in tomato (Bakker, 1991).
High CO
2
levels increase the aerial
part:root ratio, inducing thicker leaves and
favouring lateral branching, decreasing the
apical dominance (Enoch, 1990).
The priority in the demand for translo-
cation of assimilates towards a particular
sink organ depends on: (i) the nature of the
sink; (ii) its stage of development; (iii) its
age; and (iv) its position within the plant.
Normally, sink organs located in lower posi-
tions promote the translocation of assimi-
lates towards them, with respect to ones in
higher locations (Urban, 1997a), at least
when the distance is not excessive.
Competition between organs which act
as sinks, obviously, influences the growth
and development of the plant and, therefore,
nutritional
and
environmental
control).
During the senescence of the leaves,
and when the demand for assimilates by the
sinks is not satisfied by production in the
active sources, the remobilization of carbo-
hydrates, nitrogen compounds and other
mobile compounds from the senescent
leaves to other active sinks of the plant
offers another option (Giménez, 1992). In a
similar way, remobilization occurs if the
plant has reserve organs (e.g. tubers).
6.6.3
Management of the assimilate
distribution
In fruit vegetables (horticultural crops that
are grown for their fruit), it is essential to
achieve rapid leaf development, to ensure
optimal development of the future sources
of assimilates (leaves) that will meet the
future high demand for assimilates by the
fruits. So it is wise to maintain high tem-
peratures at the beginning of the cropping
cycle, as well as pruning the first fruits to
avoid competition with leaf development.
Later, with a developed crop, the quantity
of fruit is regulated by means of pruning
(and the harvest itself if they are staggered),
adapting it according to plant density. The
vegetative growth adapts to the fruit load
by means of pruning and elimination of
stems.
