Agriculture Reference
In-Depth Information
6
6.1
Introduction
compounds are moved and consumed.
The roots are a source of water and mineral
elements, whereas the leaves are a source of
carbohydrates. The main 'sinks' are the
growing parts (young leaves, flower buds,
fruits) and the reserve storage organs (tubers,
bulbs). To operate appropriately the plant
requires a balance between 'sources' and
'sinks'.
Physiologists distinguish between two
aspects of the vegetative activity of the
plant: (i) growth; and (ii) development. Both
are conditioned, directly or indirectly, by
environmental factors. Growth is a quantita-
tive notion corresponding to the variation
in size and weight of the different plant
organs (Berninger, 1989). Development is a
qualitative notion related to the changes of
stage: (i) germination; (ii) leaves and inter-
nodes succession; (iii) bud differentiation;
and (iv) flowering.
The yield, in a broad sense, includes
not only the quantitative aspects of produc-
tion but also the qualitative ones. These are
more complex to measure, due to the diffi-
culties in evaluating quality, whose impact
on the economic return of high added value
vegetables for fresh consumption is large.
Good product quality is determined,
normally, by proper crop growth and devel-
opment conditions. The amount produced,
quantified in vegetables by the weight of
The relationship between the different
organs of a plant are represented schemati-
cally in Fig. 6.1. The leaves receive solar
energy for its conversion into biomass
(vegetable matter) by means of photosyn-
thesis. The required gas exchange for pho-
tosynthesis takes place through the stomata
of the leaves, as does transpiration (trans-
fer of water from the leaves to the sur-
rounding air). The roots, besides anchoring
the plant to the soil or the substrate, absorb
water and mineral elements and may also
serve as reserve storage organs. The stem
and the branches, besides accomplishing a
support function, contain the vessels
through which the ascending and descend-
ing sap flows between the different organs
of the plant. The apical meristems are
responsible, as their cells multiply, for the
formation of new organs. Other meristems
are responsible for branching and for other
growth functions. The meristems are cen-
tres of intense biochemical activity, espe-
cially in the synthesis of hormones which
regulate the harmonic growth of the plant
(Berninger, 1989).
The plant can be conceived as a set of
'sources' providing compounds necessary
for the activities of its parts and a set
of 'sinks' or destinations where these
