Agriculture Reference
In-Depth Information
S2 layer of hemp. Unlike the plant fibres
found in wood, the secondary wall has little
hemicellulose and lignin-impregnated matrix.
Little information is available about the pre-
cise architecture of the secondary walls of
hemp. It is known, however, that the extra-
xylem fibres of hemp bear a strong resem-
blance to those of flax, in terms of their
composition and histology (Rahman, 1979),
the importance of the S2 layer and the pres-
ence of a much reduced S1 layer. The sec-
ondary wall fibres are characterized by an
unusual crystalline form of cellulose that
forms microfibrils which lie parallel to the cel-
lular axis (Bonatti
et al
., 2004).
The formation of the different parietal lay-
ers is a sequential dynamic process that accom-
panies cell growth. The lignification of the
walls leads to the parietal structures becoming
both impermeable and rigid. The particularity
of the non-xylem fibres of hemp lies in their
low concentration of lignin. As with flax, lignin
is found primarily in the intercellular cement of
the middle lamella and primary wall. The
deposition of lignin is greatest at the time of
flowering and mainly affects the outer layers of
the secondary wall.
membranes. Plant cells are further character-
ized by an external structure called the cell
wall.
The physical structures of the cell are
made up of complex assemblies of molecules
and/or macromolecules. Almost 100 mole-
cules play a role in the architecture and
compartmentalization of plant cells. The life
processes of a cell, tissue, organ or, indeed,
the whole plant rely on a collection of meta-
bolic pathways working in concert to assure
the various functions necessary for a plant's
reproduction and survival. These pathways are
controlled by the genes and the proteins and
enzymes the genes encode. In total, some
50,000 molecules are implicated in the various
plant anabolic and catabolic processes
(Pichersky and Gang, 2000).
While many of the principal metabolic
activities (including respiration, assimilation,
transport and differentiation) are common to
all cells, there are a number of proteins syn-
thesized by specific cells or tissues. The
molecular reservoirs (starch, lipids and pro-
teins) deposited and stored in seeds are of
particular importance. Other molecules,
known as secondary metabolites, are less
important to plant survival. Their presence
may, however, have direct or indirect conse-
quences for plants. These include molecules
that are attractive or repellent to insects,
toxins, allelopathic or antifungal molecules
and phytohormones.
Considering the complexity, diversity and
multiplicity of biochemical structures impli-
cated in the structure and metabolism of
plants, this treatment of
C. sativa
will focus
on the principal molecules of an industrial,
agro-edible or pharmacological interest.
Therefore, the various products will be pre-
sented, rather than an exhaustive presenta-
tion of the hundreds of molecules identified to
date (Turner, 1980).
It should be noted that:
1. With the exception of the cannabinoids,
the majority of molecules isolated from hemp
are not unique in and of themselves, as they
are found in other plants (annuals and
perennials).
2. The hemps used for industrial and pharma-
ceutical purposes differ in their cannabinoid
content.
3.4 Part II: Constituents and
Chemical Composition
3.4.1 General points
Plants are highly organized biological sys-
tems in which each tissue or organ fulfils a
precise function. Generally speaking, it is
possible to distinguish two biochemical con-
stituents: (i) those arising from the plant's
physical structures and (ii) those dependent
on the plant's metabolism.
As in all living organisms, the cell is the
basic unit of plants (Fig. 3.5). Plant cells
typically possess an envelope or plasma
membrane, as well as a fibrous reservoir of
microtubules that make up the filamentous
cytoskeleton. The cell encloses the cytoplasm
and the collection of organelles (mitochondria,
ribosomes, reticulum . . .), of which some are
plant specific (chloroplasts, vacuoles), that
make up the metabolic machinery of the cell.
The organelles are themselves delimited by
