Agriculture Reference
In-Depth Information
duration of the astronomic day increased by a
period of 40-60 min (Berninger, 1989).
The seeds of some species require radi-
ation to germinate, which they get if their
seeds are on or near to the soil surface (less
than 5 mm deep) and receive red radiation
(Langhams and Tibbits, 1997).
The seasonal control of plant develop-
ment by means of photoperiodism allows
them to be in synchrony with the climate
conditions and other organisms (Hart, 1988).
production sites (sources) to the places in
which they are used (sinks).
Normally the harvestable product of a
crop is only a part of the total produced bio-
mass. A good agronomic management must
ensure that the distribution of assimilates is
mainly destined for the harvestable organs
of the plant.
The mineral elements are mainly trans-
ported through the xylem. The organic ele-
ments are transported through the phloem.
In many horticultural species the most
important differences in harvest between
cultivars are a result of the differences
in the distribution of assimilates (Challa
et al
., 1995).
This distribution of assimilates is regu-
lated, mainly, by the 'sink strength' of indi-
vidual organs, which is the capacity of a
sink to accumulate assimilates and is
related to its growing potential (Marcelis
and De Koning, 1995). Climate factors influ-
ence the distribution of assimilates in the
short term, affecting the sink strength of
different organs and, in the long term, alter-
ing their number (Marcelis, 1989; Marcelis
and De Koning, 1995).
6.5
Respiration
Respiration is the essential process of energy
release, which is necessary for the processes
of life. It involves absorption of O
2
and the
release of CO
2
, with consumption of carbo-
hydrates in a reaction that we may consider
as the reverse of photosynthesis.
Respiration consumes carbohydrates
produced by photosynthesis. It has two basic
components: (i) maintenance respiration,
which is proportional to the dry weight of
the plant or of its active organs (excluding
reserves); and (ii) growth respiration, which
is proportional to the products of photosyn-
thesis. There is a third form of respiration,
photorespiration, relevant in C3 plants,
which only exists in the presence of light,
which fulfils a defence function against the
toxic effect of some ions (Berninger, 1989).
Growth respiration is less sensitive to
temperature than maintenance respiration,
which doubles for every 7-10°C increase in
temperature. Therefore it is desirable to
limit high temperatures, especially at night,
to improve the overall carbon balance.
Growth respiration consumes, approxi-
mately 20-30% of the photosynthesized
carbohydrates (Berninger, 1989).
6.6.2
Distribution of assimilates
between organs
Different organs compete for assimilates but
this is regulated by hormones (Russel, 1977).
A short supply of water and nutrients
increases the distribution of assimilates to
the root, to favour its growth, and in this way,
reduce these deficiencies (Brouwer, 1981).
Generally assimilates produced in a
certain point are transported to the closest
sink (Wardlaw, 1968). To maximize photo-
synthesis, it is necessary for the sinks to
have enough capacity to consume the avail-
able assimilates, otherwise they would
induce a reduction in photosynthesis,
which would re-adjust to the actual assimi-
late demand (Giménez, 1992); or, the assim-
ilate distribution to other organs would be
prioritized, such as in crops of undeter-
mined growth, when the scarcity of fruits
(due to pruning or harvest) induces a higher
vegetative growth (stem and leaves).
6.6
Distribution of Assimilates and
Sink-Source Relations
6.6.1
Introduction
The translocation or distribution of assimi-
lates is the transport of these from the
