Agriculture Reference
In-Depth Information
species. The latter would be divided into two
subspecies,
C. sativa spontanea
, the wild
form, and
C. sativa culta
, which collectively
describes the cultivated varieties used for the
industrial production of grain and fibre. This
last subspecies demonstrates considerable
physiological and morphological diversity
according to the environment in which it is
found (Bocsa and Karus, 1997). Consequently,
geographical types can be recognized accord-
ing to the size of the stem and whether they
flower early. In addition to environmentally
dependent characteristics, the industrial culti-
vars also demonstrate differences in fibre yield,
grain production and THC concentration. The
sexual dimorphism of this annual dioecious
flowering herb further complicates the mor-
phological and developmental diversity of this
plant. The genetic improvement of hemp has
led to the creation of a monoecious variety in
which the female phenotype predominates
(Arnoux
et al
., 1969). Hemp's diversity has
allowed the development and selection of culti-
vars according to the desired industrial end
product (paper fibre, fibre for textiles or seeds).
Recent studies have shown that this geographi-
cal and sexual polymorphism can be deter-
mined by molecular and protein analysis
(Lucchese
et al
., 2001; Mandolino and
Carboni, 2004). Current programmes can
identify molecular markers associated with the
desired morphological and chemical character-
istics of industrial hemp.
when compared to other economically impor-
tant annuals. The volume of the root system
varies according to the cultivation methods
used and the soil quality. It is also dependent
on sexual phenotype. On average, the root
volume comprises 8-9% of the total biomass.
Male plants have a shorter vegetative phase
than female plants, and consequently have a
less developed root system. The main root can
grow to a depth of 2 m, whereas the secondary
roots that form the bulk of the root system
reach lengths of between 10 and 60 cm.
The leaves
The leaves are composed of a petiole from
which arises 7-10 lanceolate folioles of unequal
size. The leaves arise following an opposite leaf
pattern every 10-30 cm, although this phyllo-
taxy evolves towards an alternating leaf pattern
as the plant flowers. This change can be identi-
fied towards the flower head. During the vege-
tative phase, hemp therefore demonstrates a
phyllotaxy with two synchronous helices. This
arrangement becomes asynchronous at the
start of the flowering period, producing an
alternating leaf pattern. The number of folioles
is always odd and increases progressively while
varying with cultivar. During its development,
the plant remains green. When mature,
the lower leaves on the stem may be shed,
especially where the sowing density is high.
(Lacombe, 1978; Bocsa and Karus, 1997).
3.3.2 Plant morphology
The inflorescence
Hemp is a dioecious species made up of male
and female plants. The male inflorescences
form a loose cluster of flowers on the end of a
central stem (rachis), with very few leaves. The
female reproductive apparatus forms at the
apex of the stem and the flowers are borne on
racemes. Genetic selection has led to the cre-
ation of dioecious strains in which the female
factor predominates. The male plants are
generally weaker and start to deteriorate at
the time of flowering, reducing the yield in grain
and fibre. Monoecious strains of hemp exist
that are exclusively female. The sexual
character of monoecious strains is very much
dependent on edaphic factors (photoperiod,
nitrogenation), with certain conditions favouring
C. sativa
is an annual plant with a stem that
can grow to a height of some 2-4 m depending
on the variety. The stem rarely branches and
has a diameter that averages between 1 and
3 cm. The plant's morphological characteristics
can vary according to the growing conditions
(sowing density, photoperiod, etc.). At low sow-
ing densities, branching is likely to increase,
whereas a high sowing density will favour the
development of a tall plant with long, straight
unbranched stems (Bocsa and Karus, 1997).
The root system
In comparison to the biomass above ground,
the root system of hemp is relatively reduced
