Agriculture Reference
In-Depth Information
Those in the phloem or secondary phloem are
termed secondary fibres. They differ from each
other and from the fibre of wood by their dia-
meter, length and the degree of lignifications
(Fig. 4.11).
The proportion of primary and secondary
fibres within the bark of the stem of the hemp
plant is dependent on the agronomic condi-
tions of the crop and does not appear to be of
importance with regard to the use of these
fibres in the paper industry.
An increase in the concentration of total
cortical fibres of hemp varieties remains an
essential selection criterion.
This trait is highly heritable and this herit-
ability has simplified selection. From a practical
point of view and taking into account how dif-
ficult fibre is to extract from the rest of the cor-
tical part of the stem, measuring techniques
were simplified as early as the 1960s by evalu-
ating the proportion of the stem made up of
bark, a factor that correlates closely with fibre
concentration.
Today, the varieties developed and culti-
vated in France show proportions of bark in
the stem between 30 and 35% (depending on
agronomic conditions) compared with values
of 20-25% before these selection programmes
were initiated. Two new varieties of hemp,
called Santhica 27 and Santhica 70, show a
proportion of bark as high as 35-40%.
Selection of varieties with low
concentrations of delta-9-THC
Both EU and French legislation impose a
maximum level for the constituent psycho-
trope, delta-9-tetrahydrocannabinol (Δ9 THC).
Consequently, as early as the 1970s, a selec-
tion programme was instituted in order to
make available to producers varieties that con-
formed to the legislation.
The maximum levels were:
•
0.30% in an average mixed sample made
up of the upper third of 200 plants (female
and monoecious) collected in the 10 days
following the end of flowering (up until
2000).
0.20% in an average mixed sample made
•
up of the top 30 cm of 50 plants (female
and monoecious) collected during the
period extending from 20 days after flow-
ering up until 10 days after the end of
flowering (from 2001).
Fig. 4.11.
Detailed view of the fibres.
