Agriculture Reference
In-Depth Information
that encloses the plant cell (Fig. 3.5,
Table 3.1). Cellulose is primarily and struc-
turally the main element of these fibrous
walls. The non-cellulose parietal elements
include other polysaccharides (pectins and
hemicelluloses), lignins as well as proteins
and various oligomers of sugar and phenol
(Turner, 1980; Jurasek, 1998; Buchanan
et al ., 2000). The relative proportions of
these various constituents depend on the
histological origins of the fibres (primary
fibre, secondary fibre, woody fibre), their
stage of growth and plant maturity, as well as
the plant variety. Ecophysiological factors
affecting the growth and development of the
plant will also cause variation in the cons-
tituents of the fibrous cell walls (van der Werf
et al ., 1994; Cappelletto et al ., 2001;
Mediavilla et al ., 2001).
bearing on the macroscopic mechanical
properties of the fibres, as is the case for flax
and wood (Roland et al ., 1995; Girault et al .,
1997; Focher et al ., 2001; Fratzl, 2003;
Bonatti et al ., 2004). The properties of cel-
lulose will also be influenced by the mechani-
cal stresses that the plant is subjected to
during growth, as well as the methods of
extraction used to recover the fibres.
Dislocated zones, in which the crystalline cel-
lulose has a different orientation, can be seen
as a result.
PECTINS . The pectins are a group of polymers
rich in galacturonic acid, rhamnose, arabinose
and galactose. These polymers are made up of
a primary chain with secondary chains branch-
ing off, which may themselves be branching or
have substitution (Ridley et al ., 2001). Pectins
are typically found in the middle lamella and pri-
mary walls of dicotyledons (and to a lesser extent
in monocotyledons; Robert and Catesson,
2000). The pectin acids are homopolymers of
galacturonic acid (polygalacturonan, PGA), which
is eventually methylated. The chains are held
together by ionic bonds and the resulting struc-
tures are stabilized by calcium ions. A rhamnose
residue may be introduced into the chain, pro-
ducing a deviation or 'elbow' among type I
rhamnogalacturonans (RGI, Fig. 3.7). More
complex rhamnogalacturonans, such as RGII,
are recognized by their short branches of
xylose and arabinose. Other types of pectin
correspond with neutral polysaccharides (β1,4-
galactones, α1,3-arabinones, β1,4-galactones
with an arabinose substitution at C3) (Chabbert
et al ., 2006).
Little further information on the structure
of Cannabis cell walls can be added to the
preceding general overview. A study of iso-
lated fractions of fibre pectins in macerated
hemp demonstrated similar arrangements to
RGI pectins (Fig. 3.7; Vignon and Garcia-
Jaldon, 1996).
CELLULOSE . Cellulose is a homopolymer made
up of a chain of glucose molecules linked by
β
1-4 glycosidic linkages. The repeating unit is
formed by the dimer of cellobiose (
- D -glucosyl-
(1-4)- D -glucose). The linear arrangement of the
glucan chains is stabilized by bonds between
the hydroxy group at 3 and oxygen (O-5) of
the contiguous cycle (Fig. 3.6). The glucan
chains then assemble themselves into a crystal-
line structure by the formation of hydrogen
bonds between and within chains. The microfi-
brils formed are approximately 3-5 nm thick
in most plants. They represent an alternating
of the crystalline and amorphous zones within
each structure. Hemp fibres are particularly
rich in crystalline cellulose (60-80%). That
said, cellulose crystallinity is less important in
the primary wall than in the secondary wall and
is related to the arrangement of microfibrils
and the composition of the parietal matrix
(that is, the other non-cellulose components of
the walls).
The organization of the cellulose microfi-
brils in the various parietal layers has a direct
β
Table 3.1. Illustration of the chemical composition of the periphloem fibres in hurds (Vignon et al ., 1995).
Weight (%)
Cellulose
Hemicellulose
Pectin
Lignin
Wax
Protein
Periphloem fibres
55
16
18
4
1
2
Hurds
44
18
4
28
1
3
 
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