Agriculture Reference
In-Depth Information
Within dioecious varieties, a small propor-
tion of naturally occurring plants will be found
bearing flowers of both sexes. The incidence will
vary between one plant in a million and one in a
thousand. These plants are termed monoecious.
plants when days are short, and nitrogenous
fertilization has been shown to have a masculi-
nizing effect.
Different strategies have been introduced
by breeders to obtain and maintain this
characteristic:
MONOECITY
.
The first monoecious plants to
be recognized within a dioecious line were
identified by the German researcher
Bredeman, between World War I and World
War II, and subsequently by Von Sengbusch
and his collaborators. These plants do not
show heterogeneity when the sexes occur
separately and yield a greater quantity of
seeds, as all the plants are fruit bearing.
The expression of monoecity varies
between individuals in relation to the number
of male and female flowers borne by each
plant. A classification system was proposed by
Neuer and Von Sengbusch in 1943 to describe
these types:
1. Varieties have been created in which
100% of the population is monoecious. All
the monoecious types are represented, includ-
ing the masculinized lines. The sexual expres-
sion of these plants varies according to
pedoclimatic factors. Developed by all
researchers working on the monoecity of
hemp, these varieties are still used today in
Poland and Ukraine.
2. Varieties have been created in which the
population is mixed, consisting of a mixture of
female and monoecious plants. The produc-
tion of seed and seed stock is essentially assured
by the female plants and, to some degree, by
the feminized monoecious plants, while polli-
nation is assured by the monoecious plants.
This variety allows the impact of pedoclimatic
factors on sexual expression to be managed
and the proportion of masculinized monoe-
cious plants to be kept small. This practice was
developed in France in the 1960s, in parallel
with the original practice, namely that of the
dioecious-monoecious hybrids.
1. From 1+: one cross, monoecious plant
with a majority of male flowers, and therefore
heavily masculinized.
2. To 5+: five cross, monoecious plant with
a majority of female flowers, and therefore
heavily feminized.
The complexity of the genetics behind these
characteristics has not, however, allowed
a homogenous sexual phenotype to be con-
served across the generations.
Today all varieties developed and cultivated in
France are mixed, consisting of a mixture of
female plants and monoecious plants.
DIFFICULTIES ARISING FROM THE CREATION AND
PRESERVATION OF MONOECIOUS TYPES
.
The first
problem to be solved was that of eliminating
the male plants at the time of seed increase in
order to avoid a rapid return to a dioecious
state, or one with a high proportion of male
plants. To this day, the problem persists,
although these male plants can be managed
and are being kept in check.
The second problem to be addressed was
that of creating and maintaining monoecious
varieties with a high expression of female flow-
ers. This is important because masculinized
varieties produce less fruit and are more prone
to fungal parasitism with
Botrytis
.
That said, studies have shown that the
monoecious state is highly variable and not
very stable. It is very sensitive to pedoclimatic
factors. Photoperiod has a feminizing effect on
THE DIOECIOUS
-
MONOECIOUS HYBRID OPTION
.
Faced
with the challenge of producing and maintain-
ing monoecious varieties, research focused on
an original solution. This work was conducted
in collaboration with Professor Bócsa, of
Hungary, and sought to hybridize dioecious
female plants with monoecious plants. This
hybridization produced a novel F1 population
made up of a very high proportion (>95%) of
female plants.
The difficulties encountered in trying to
produce seed stock of these varieties, and, in
particular, the need to eliminate the entire pop-
ulation of dioecious male plants manually, meant
that this variety could not be distributed widely.
A number of techniques were found that
allowed the peculiarities of these variants to be
used inexpensively.
