Agriculture Reference
In-Depth Information
•
Short-day plants (nyctoperiodic) - short
therefore influenced directly by the date of
sowing and the latitude, as these two factors
influence the photoperiod. The variety of hemp
also influences the sensitivity to light, and this
factor can therefore also influence the yield.
The start of flowering, corresponding with
the appearance of the first female flowers, is
also influenced by the photoperiod. This stage
is termed the beginning of flowering.
This phenomenon is still poorly under-
stood. It is more complex in nature and is
termed photothermoperiodism.
Experimental studies have demonstrated
that, as nights lengthen, temperature require-
ments are reduced:
day, long night produces flowering.
Day-neutral plants (aperiodic) - indifferent
•
to day length.
Intermediate plants (amphiperiodic) - requir-
•
ing a stricter set of conditions. This group
was added by Allard (1938).
Hemp, like the majority of tropical plants,
belongs to the short-day plant category.
Influence of photoperiod on hemp
The principal effect of photoperiod on hemp is
on the flowering of this plant. More specifi-
cally, the photoperiod influences the end of the
development of the inflorescences, and there-
fore the opening of the last female flowers.
At this point, it can be said that the hemp is
fully flowered.
It has been observed experimentally that
growth is arrested as soon as the last female
flowers are fertilized. This fertilization takes
place 1 week after these last female flowers
open, and terminates the flowering stage.
The yield in hurds is thus under the strict
control of the photoperiod (night length). It is
•
Summer nights are short and warm, result-
ing in flowering occurring more rapidly.
The nights at the end of spring are long
•
and cooler, resulting in slower flowering.
The nights at the start of spring are short and
•
cool, resulting in a more unpredictable flow-
ering pattern depending on the climate.
The interaction of these two effects can have
specific consequences on hemp cultivation
(Table 3.2). No other developmental stage
Table 3.2.
Agronomic consequences of photoperiodism.
Photoperiodic
requirements
to bring about
the end of
flowering
Active growth
during days
(length)**
Thermal
requirements
for flowering
Timing of
sowing
Start of
flowering
Stage of full
flowering
Normal*
(15 April-15
May)
Max.imal
(end of June)
Maximum
Standard -
start of July
Standard
(example/
norm)
Standard (start
of August)
Late (1 June)
Shortest
(start of July)
Weak
Slightly later,
although
more or less
the same
Identical
Identical (to
the example/
norm)
Very late
(15 June)
Shortest
(mid July)
Very weak
Later
Identical
(although
flowering is
poorly
developed)
Later
Early (1 April)
Shortest
(start of May)
Weak
Early
Standard
Identical (to
the example/
norm)
Very early
(20 March)
Very short
(end of April)
Very weak
Very early
Standard
Identical (to
the example/
norm)
* We consider the 'normal' sowing date to require a qualifi ed standard.
** Active growth starts about 4 weeks after sowing.
