Marijuana Botany by Robert Connell Clarke

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Layering

Layering is a process in which roots develop on a
stem while it remains attached to, and nutritionally sup-
ported by the parent plant. The stem is then detached and
the meristematic tip becomes a new individual, growing
on its own roots, termed a layer. Layering differs from
cutting because rooting occurs while the shoot is still
attached to the parent. Rooting is initiated in layering by
various stem treatments which interrupt the downward
flow of photosynthates (products of photosynthesis) from
the shoot tip. This causes the accumulation of auxins,
carbohydrates and other growth factors. Rooting occurs in
this treated area even though the layer remains attached
to the parent. Water and mineral nutrients are supplied by
the parent plant because only the phloem has been inter-
rupted; the xylem tissues connecting the shoot to the
parental roots remain intact (see illus. 1, page 29). In this
manner, the propagator can overcome the problem of keep-
ing a severed cutting alive while it roots, thus greatly in-
creasing the chances of success. Old woody reproductive
stems that, as cuttings, would dry up and die, may be
rooted by layering. Layering can be very time-consuming
and is less practical for mass cloning of parental stock
than removing and rooting dozens of cuttings. Layering,
however, does give the small-scale propagator a high-success
alternative which also requires less equipment than cuttings.
Techniques of Layering

Almost all layering techniques rely on the principle of
etiolation. Both soil layering and air layering involve de-
priving the rooting portion of the stem of light, promoting
rooting. Root-promoting substances and fungicides prove
beneficial, and they are usually applied as a spray or pow-
der. Root formation on layers depends on constant mois-
ture, good air circulation and moderate temperatures at
the site of rooting.
Soil Layering
Soil layering may be performed in several ways. The
most common is known as tip layering. A long, supple
vegetative lower limb is selected for layering, carefully bent
so it touches the ground, and stripped of leaves and small
shoots where the rooting is to take place. A narrow trench,
6 inches to a foot long and 2 to 4 inches deep, is dug paral-
lel to the limb, which is placed along the bottom of the
trench, secured with wire or wooden stakes, and buried
with a small mound of soil. The buried section of stem
may be girdled by cutting, crushed with a loop of wire, or
twisted to disrupt the phloem tissue and cause the accumu-
lation of substances which promote rooting. It may also be
treated with growth regulators at this time.
Serpentine layering may be used to create multiple
layers along one long limb. Several stripped sections of the
limb are buried in separate trenches, making sure that at
least one node remains above ground between each set of
roots to allow shoots to develop. The soil surrounding the
stem is kept moist at all times and may require wetting
several times a day. A small stone or stick is inserted under
each exposed section of stem to prevent the lateral shoot
buds rotting from constant contact with the moist soil sur-
face. Tip layers and serpentine layers may be started in
small containers placed near the parental plant. Rooting
usually begins within two weeks, and layers may be re-
moved with a sharp razor or clippers after four to six
weeks. If the roots have become well established, trans-
planting may be difficult without damaging the tender root
system. Shoots on layers continue to grow under the same
conditions as the parent, and less time is needed for the
clone to acclimatize or harden-off and begin to grow on its
own than with cuttings.
In air layering, roots form on the aerial portions of
stems that have been girdled, treated with growth regula-
tors, and wrapped with moist rooting media. Air layering
is an ancient form of propagation, possibly invented by the
Chinese. The ancient technique of goo tee uses a ball of clay
or soil plastered around a girdled stem and held with a
wrap of fibers. Above this is suspended a small container
of water (such as a bamboo section) with a wick to the
wrapped gootee; this way the gootee remains moist.
The single most difficult problem with air layers is the
tendency for them to dry out quickly. Relatively small
amounts of rooting media are used, and the position on
aerial parts of the plant exposes them to drying winds and
sun. Many wraps have been tried, but the best seems to be
clear polyethylene plastic sheeting which allows oxygen to
enter and retains moisture well. Air layers are easiest to
make in greenhouses where humidity is high, but they may
also be used outside as long as they are kept moist and
don't freeze. Air layers are most useful to the amateur
propagator and breeder because they take up little space
and allow the efficient cloning of many individuals.

 

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Making an Air Layer

A recently sexed young limb 3-10 mm (1/8 to 3/8
inch) in diameter is selected. The site of the layer is usually
a spot 30 centimeters (12 inches) or more from the limb
tip. Unless the stem is particularly strong and woody, it is
splinted by positioning a 30 centimeter (12 inch) stick of
approximately the same diameter as the stem to be layered
along the bottom edge of the stem. This splint is tied in
place at both ends with a piece of elastic plant-tie tape.
This enables the propagator to handle the stem more con-
fidently. An old, dry Cannabis stem works well as a splint.
Next, the stem is girdled between the two ties with a twist
of wire or a diagonal cut. After girdling, the stem is sprayed
or dusted with a fungicide and growth regulator, sur-
rounded with one or two handfuls of unmilled sphagnum
moss, and wrapped tightly with a small sheet of clear poly-
ethylene film (4-6 mil). The film is tied securely at each
end, tightly enough to make a waterproof seal but not so
tight that the phloem tissues are crushed. If the phloem is
crushed, compounds necessary for rooting will accumulate
outside of the medium and rooting will be slowed. Plastic
florist's tape or electrician's tape works well for sealing air
layers. Although polyethylene film retains moisture well,
the moss will dry out eventually and must be remoistened
periodically. Unwrapping each layer is impractical and
would disturb the roots, so a hypodermic syringe is used to
inject water, nutrients, fungicides, and growth regulators.
If the layers become too wet the limb rots. Layers are
checked regularly by injecting water until it squirts out
and then very lightly squeezing the medium to remove any
extra water. Heavy layers on thin limbs are supported by
tying them to a large adjacent limb or a small stick an-
chored in the ground. Rooting begins within two weeks
and roots will be visible through the clear plastic within
four weeks. When the roots appear adequately developed,
the layer is removed, carefully unwrapped, and trans-
planted with the moss and the splint intact. The layer is
watered well and placed in a shady spot for a few days to
allow the plant to harden-off and adjust to living on its
own root system. It is then placed in the open. In hot
weather, large leaves are removed from the shoot before
removing the layer to prevent excessive transpiration and
wilting.
Layers develop fastest just after sexual differentiation.
Many layers may be made of staminate plants in order to
save small samples of them for pollen collection and to
conserve space. By the time the pollen parents begin to
flower profusely, the layers will be rooted and may be cut
and removed to an isolated area. Layers taken from pistil-
late plants are used for breeding, or saved and cloned for
the following season.
Layers often seem rejuvenated when they are re-
moved from the parent plant and begin to be supported by
their own root systems. This could mean that a clone will
continue to grow longer and mature later than its parent
under the same conditions. Layers removed from old or
seeded parents will continue to produce new calyxes and
pistils instead of completing the life cycle along with the
parents. Rejuvenated layers are useful for off-season seed
production.

 

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Grafting

Intergeneric grafts between Cannabis and Humulus
(hops) have fascinated researchers and cultivators for
decades. Warmke and Davidson (1943) claimed that Humu-
lus tops grafted upon Cannabis roots produced ". . . as
much drug as leaves from intact hemp plants, even though
leaves from intact hop plants are completely nontoxic."
According to this research, the active ingredient of Canna-
bis was being produced in the roots and transported across
the graft to the Hum ulus tops. Later research by Crombie
and Crombie (1975) entirely disproves this theory. Grafts
were made between high and low THC strains of Cannabis
as well as intergeneric grafts between Cannabis and Humu-
lus, Detailed chromatographic analysis was performed on
both donors for each graft and their control populations.
The results showed ". . . no evidence of transport of inter-
mediates or factors critical to cannabinoid formation
across the grafts."
Grafting of Cannabis is very simple. Several seedlings
can be grafted together into one to produce very interesting
specimen plants. One procedure starts by planting one seed-
ling each of several separate strains close together in the
same container, placing the stock (root plant) for the cross
in the center of the rest. When the seedlings are four weeks
old they are ready to be grafted. A diagonal cut is made
approximately half-way through the stock stem and one of
the scion (shoot) seedlings at the same level. The cut por-
tions are slipped together such that the inner cut surfaces
are touching. The joints are held with a fold of cellophane
tape. A second scion from an adjacent seedling may be
grafted to the stock higher up the stem. After two weeks,
the unwanted portions of the grafts are cut away. Eight to
twelve weeks are needed to complete the graft, and the
plants are maintained in a mild environment at all times.
As the graft takes, and the plant begins to grow, the tape
falls off.

 

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Pruning

Pruning techniques are commonly used by Cannabis
cultivators to limit the size of their plants and promote
branching. Several techniques are available, and each has
its advantages and drawbacks. The most common method
is meristem pruning or stem tip removal. In this case the
growing tip of the main stalk or a limb is removed at
approximately the final length desired for the stalk or limb.
Below the point of removal, the next pair of axial growing
tips begins to elongate and form two new limbs. The
growth energy of one stem is now divided into two, and
the diffusion of growth energy results in a shorter plant
which spreads horizontally.
Auxin produced in the tip meristem travels down the
stem and inhibits branching. When the meristem is re-
moved, the auxin is no longer produced and branching may
proceed uninhibited. Plants that are normally very tall and
stringy can be kept short and bushy by meristem pruning.
Removing meristems also removes the newly formed tissues
near the meristem that react to changing environmental
stimuli and induce flowering. Pruning during the early part
of the growth cycle will have little effect on flowering, but
plants that are pruned late in life, supposedly to promote
branching and floral growth, will often flower late or fail
to flower at all. This happens because the meristemic
tissue responsible for sensing change has been removed and
the plant does not measure that it is the time of the year
to flower. Plants will usually mature fastest if they are
allowed to grow and develop without interference from
pruning. If late maturation of Cannabis is desired, then
extensive pruning may work to delay flowering. This is
particularly applicable if a staminate plant from an early-
maturing strain is needed to pollinate a late-maturing pistil-
late plant. The staminate plant is kept immature until the
pistillate plant is mature and ready to be pollinated. When
the pistillate plant is receptive, the staminate plant is
allowed to develop flowers and release pollen.
Other techniques are available for limiting the size and
shape of a developing Cannabis plant without removing
meristematic tissues. Trellising is a common form of modi-
fication and is achieved in several ways. In many cases
space is available only along a fence or garden row. Posts 1
to 2 meters (3 to 6 feet) long may be driven into the
ground 1 to 3 meters (3 to 10 feet) apart and wires
stretched between them at 30 to 45 centimeters (12 to 18
inches) intervals, much like a wire fence or grape trellis.
Trellises are ideally oriented on an east-west axis for maxi-
mum sun exposure. Seedlings or pistillate clones are placed
between the posts, and as they grow they are gradually
bent and attached to the wire. The plant continues to grow
upward at the stem tips, but the limbs are trained to grow
horizontally. They are spaced evenly along the wires by
hooking the upturned tips under the wire when they are 15
to 30 centimeters (6 to 12 inches) long. The plant grows
and spreads for some distance, but it is never allowed to
grow higher than the top row of wire. When the plant be-
gins to flower, the floral clusters are allowed to grow up-
ward in a row from the wire where they receive maximum
sun exposure. The floral clusters are supported by the wire
above them, and they are resistant to weather damage.
Many cultivators feel that trellised plants, with increased
sun exposure and meristems intact, produce a higher
yield than freestanding unpruned or pruned plants. Other
growers feel that any interference with natural growth
patterns limits the ultimate size and yield of the plant.
Another method of trellising is used when light expo-
sure is especially crucial, as with artificial lighting systems.
Plants are placed under a horizontal or slightly slanted flat
sheet of 2 to 5 centimeters (1 to 2 inches) poultry netting
which is suspended on a frame 30 to 60 centimeters (12 to
24 inches) from the soil surface perpendicular to the direc-
tion of incoming light or to the lowest path of the sun. The
seedlings or clones begin to grow through the netting al-'
most immediately, and the meristems are pushed back
down under the netting, forcing them to grow horizon-
tally outward. Limbs are trained so that the mature plant
will cover the entire frame evenly. Once again, when the
plant begins to flower, the floral clusters are allowed to
grow upward through the wire as they reach for the light.
This might prove to be a feasible commercial cultivation
technique, since the flat beds of floral clusters could be
mechanically harvested. Since no meristem tissues are re-
moved, growth and maturation should proceed on schedule.
This system also provides maximum light exposure for all
the floral clusters, since they are growing from a plane
perpendicular to the direction of light.
Sometimes limbs are also tied down, or crimped and
bent to limit height and promote axial growth without
meristem removal. This is a particularly useful technique
for greenhouse cultivation, where plants often reach the
roof or walls and burn or rot from the intense heat and
condensation of water on the inside of the greenhouse.
To prevent rotting and burning while leaving enough room
for floral clusters to form, the limbs are bent at least 60
centimeters (24 inches) beneath the roof of the green-
house. Tying plants over allows more light to strike the
plant, promoting axial growth. Crimping stems and bending
them over results in more light exposure as well as inhibit-
ing the flow of auxin down the stem from the tip. Once
again, as with meristem removal, this promotes axial
growth.
Limbing is another common method of pruning Can-
nabis plants. Many small limbs will usually grow from the
bottom portions of the plant, and due to shading they re-
main small and fail to develop large floral clusters. If these
atrophied lower limbs are removed, the plant can devote
more of its floral energies to the top parts of the plant
with the most sun exposure and the greatest chance of
pollination. The question arises of whether removing entire
limbs constitutes a shock to the growing plant, possibly
limiting its ultimate size. It seems in this case that shock
is minimized by removing entire limbs, including propor-
tional amounts of stems, leaves, meristems, and flowers;
this probably results in less metabolic imbalance than if
only flowers, leaves, or meristems were removed. Also, the
lower limbs are usually very small and seem of little signifi-
cance in the metabolism of the total plant. In large plants,
many limbs near the central stalk also become shaded and
atrophied and these are also sometimes removed in an
effort to increase the yield of large floral clusters on the
sunny exterior margins.
Leafing is one of the most misunderstood techniques
of drug Cannabis cultivation. In the mind of the cultivator,
several reasons exist for removing leaves. Many feel that
large shade leaves draw energy from the flowering plant,
and therefore the flowering clusters will be smaller. It is
felt that by removing the leaves, surplus energy will be
available, and large floral clusters will be formed. Also,
some feel that inhibitors of flowering, synthesized in the
leaves during the long noninductive days of summer, may
be stored in the older leaves that were formed during the
noninductive photoperiod. Possibly, if these inhibitor-laden
leaves are removed, the plant will proceed to flower, and
maturation will be accelerated. Large leaves shade the inner
portions of the plant, and small atrophied floral clusters
may begin to develop if they receive more light.

In actuality, few if any of the theories behind leafing
give any indication of validity. Indeed, leafing possibly
serves to defeat its original purpose. Large leaves have a
definite function in the growth and development of Can-
nabis. Large leaves serve as photosynthetic factories for the
production of sugars and other necessary growth sub-
stances. They also create shade, but at the same time they
are collecting valuable solar energy and producing foods
that will be used during the floral development of the
plant. Premature removal of leaves may cause stunting,
because the potential for photosynthesis is reduced. As
these leaves age and lose their ability to carry on photo-
synthesis they turn chloro tie (yellow) and fall to the
ground. In humid areas care is taken to remove the yellow
or brown leaves, because they might invite attack by fun-
gus. During chlorosis the plant breaks down substances,
such as chlorophylls, and translocates the molecular com-
ponents to a new growing part of the plant, such as the
flowers. Most Cannabis plants begin to lose their larger
leaves when they enter the flowering stage, and this trend
continues until senescence. It is more efficient for the plant
to reuse the energy and various molecular components of
existing chlorophyll than to synthesize new chlorophyll at
the time of flowering. During flowering this energy is
needed to form floral clusters and ripen seeds.

 

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Removing large amounts of leaves may interfere with
the metabolic balance of the plant. If this metabolic change
occurs too late in the season it could interfere with floral
development and delay maturation. If any floral inhibitors
are removed, the intended effect of accelerating flowering
will probably be counteracted by metabolic upset in the
plant. Removal of shade leaves does facilitate more light
reaching the center of the plant, but if there is not enough
food energy produced in the leaves, the small internal
floral clusters will probably not grow any larger. Leaf re-
moval may also cause sex reversal resulting from a meta-
bolic change.

If leaves must be removed, the petiole is cut so that
at least an inch remains attached to the stalk. Weaknesses
in the limb axis at the node result if the leaves are pulled
off at the abscission layer while they are still green. Care is
taken to see that the shriveling petiole does not invite
fungus attack.

It should be remembered that, regardless of strain or
environmental conditions, the plant strives to reproduce,
and reproduction is favored by early maturation. This pro-
duces a situation where plants are trying to mature and
reproduce as fast as possible. Although the purpose of
leafing is to speed maturation, disturbing the natural pro-
gressive growth of a plant probably interferes with its rapid
development.
Cannabis grows largest when provided with plentiful
nutrients, sunlight, and water and left alone to grow and
mature naturally. It must be remembered that any altera-
tion of the natural life cycle of Cannabis will affect pro-
ductivity. Imaginative combinations and adaptations of
propagation techniques exist, based on specific situations
of cultivation. Logical choices are made to direct the
natural growth cycle of Cannabis to favor the timely
maturation of those products sought by the cultivator,
without sacrificing seed or clone production.

 

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Chapter 3 - Genetics and Breeding of Cannabis

The greatest service which can be rendered to
any country is to add a useful plant to its culture.
-Thomas Jefferson


Genetics

Although it is possible to breed Cannabis with limited
success without any knowledge of the laws of inheritance,
the full potential of diligent breeding, and the line of action
most likely to lead to success, is realized by breeders who
have mastered a working knowledge of genetics.
As we know already, all information transmitted from
generation to generation must be contained in the pollen
of the staminate parent and the ovule of the pistillate
parent. Fertilization unites these two sets of genetic infor-
mation, a seed forms, and a new generation is begun. Both
pollen and ovules are known as gametes, and the trans-
mitted units determining the expression of a character are
known as genes. Individual plants have two identical sets of
genes (2n) in every cell except the gametes, which through
reduction division have only one set of genes (in). Upon
fertilization one set from each parent combines to form
a seed (2n).
In Cannabis, the haploid (in) number of chromo-
somes is 10 and the diploid (2n) number of chromosomes
is 20. Each chromosome contains hundreds of genes, influ-
encing every phase of the growth and development of
the plant.
If cross-pollination of two plants with a shared genetic
trait (or self-pollination of a hermaphrodite) results in off-
spring that all exhibit the same trait, and if all subsequent
(inbred) generations also exhibit it, then we say that the
strain (i.e., the line of offspring derived from common an-
cestors) is true-breeding, or breeds true, for that trait. A
strain may breed true for one or more traits while varying
in other characteristics. For example, the traits of sweet
aroma and early maturation may breed true, while off-
spring vary in size and shape. For a strain to breed true for
some trait, both of the gametes forming the offspring must
have an identical complement of the genes that influence
the expression of that trait. For example, in a strain that
breeds true for webbed leaves, any gamete from any parent
in that population will contain the gene for webbed leaves,
which we will signify with the letter w. Since each gamete
carries one-half (in) of the genetic complement of the
offspring, it follows that upon fertilization both "leaf-
shape" genes of the (2n) offspring will be w. That is, the
offspring, like both parents, are ww. In turn, the offspring
also breed true for webbed leaves because they have only
w genes to pass on in their gametes.

On the other hand, when a cross produces offspring
that do not breed true (i.e., the offspring do not all re-
semble their parents) we say the parents have genes that
segregate or are hybrid. Just as a strain can breed true for
one or more traits, it can also segregate for one or more
traits; this is often seen. For example, consider a cross
where some of the offspring have webbed leaves and some
have normal compound-pinnate leaves. (To continue our
system of notation we will refer to the gametes of plants
with compound-pinnate leaves as W for that trait. Since
these two genes both influence leaf shape, we assume that
they are related genes, hence the lower-case w and upper-
case W notation instead of w for webbed and possibly P for
pinnate.) Since the gametes of a true-breeding strain must
each have the same genes for the given trait, it seems logi-
cal that gametes which produce two types of offspring
must have genetically different parents.

Observation of many populations in which offspring
differed in appearance from their parents led Mendel to his
theory of genetics. If like only sometimes produces like,
then what are the rules which govern the outcome of these
crosses? Can we use these rules to predict the outcome of
future crosses?

 

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Assume that we separate two true-breeding popula-
tions of Cannabis, one with webbed and one with
compound-pinnate leaf shapes. We know that all the
gametes produced by the webbed-leaf parents will contain
genes for leaf-shape w and all gametes produced by the
compound-pinnate individuals will have W genes for leaf
shape. (The offspring may differ in other characteristics,
of course.)
If we make a cross with one parent from each of the
true-breeding strains, we will find that 100% of the off-
apring are of the compound-pinnate leaf phenotype. (The
expression of a trait in a plant or strain is known as the
phenotype.) What happened to the genes for webbed leaves
contained in the webbed leaf parent? Since we know that
there were just as many w genes as W genes combined in
the offspring, the W gene must mask the expression of the
w gene. We term the W gene the dominant gene and say
that the trait of compound-pinnate leaves is dominant over
the recessive trait of webbed leaves. This seems logical
since the normal phenotype in Cannabis has compound-
pinnate leaves. It must be remembered, however, that many
useful traits that breed true are recessive. The true-breeding
dominant or recessive condition, WW or ww, is termed the
homozygous condition; the segregating hybrid condition
wW or Ww is called heterozygous. When we cross two of
the F1 (first filial generation) offspring resulting from the
initial cross of the ~1 (parental generation) we observe two
types of offspring. The F2 generation shows a ratio of
approximately 3:1, three compound pinnate type-to-one
webbed type. It should be remembered that phenotype
ratios are theoretical. The real results may vary from the
expected ratios, especially in small samples.
In this case, compound-pinnate leaf is dominant over
webbed leaf, so whenever the genes w and W are combined,
the dominant trait W will be expressed in the phenotype.
In the F2 generation only 25% of the offspring are homo-
zygous for W so only 25% are fixed for W. The w trait is
only expressed in the F2 generation and only when two w
genes are combined to form a double-recessive, fixing the
recessive trait in 25% of the offspring. If compound-pinnate
showed incomplete dominance over webbed, the geno-
types in this example would remain the same, but the
phenotypes in the F1 generation would all be intermediate
types resembling both parents and the F2 phenotype ratio
would be 1 compound-pinnate :2 intermediate :1 webbed.
The explanation for the predictable ratios of offspring
is simple and brings us to Mendel's first law, the first of the
basic rules of heredity:
I. Each of the genes in a related pair segregate from
each other during gamete formation.
A common technique used to deduce the genotype of
the parents is the back-cross. This is done by crossing one
of the F1 progeny back to one of the true-breeding P1
parents. If the resulting ratio of phenotypes is 1:1 (one
heterozygous to one homozygous) it proves that the
parents were indeed homozygous dominant WW and
homozygous-recessive ww.

 

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The 1:1 ratio observed when back-crossing F1 to P1
and the 1:2:1 ratio observed in F1 to F1 crosses are the two
basic Mendelian ratios for the inheritance of one character
controlled by one pair of genes. The astute breeder uses
these ratios to determine the genotype of the parental
plants and the relevance of genotype to further breeding.
This simple example may be extended to include the
inheritance of two or more unrelated pairs of genes at a
time. For instance we might consider the simultaneous
inheritance of the gene pairs T (tall)/t (short) and M (early
maturation)/m (late maturation). This is termed a poly-
hybrid instead of monohybrid cross. Mendel's second law
allows us to predict the outcome of polyhybrid crosses
also:
II. Unrelated pairs of genes are inherited indepen-
dently of each other.
If complete dominance is assumed for both pairs of
genes, then the 16 possible F2 genotype combinations will
form 4 F2 phenotypes in a 9:3:3:1 ratio, the most frequent
of which is the double-dominant tall/early condition. In-
complete dominance for both gene pairs would result in 9
F2 phenotypes in a 1:2:1:2:4:2:1:2:1 ratio, directly re-
flecting the genotype ratio. A mixed dominance condition
would result in 6 F2 phenotypes in a 6:3:3:2:1:1 ratio.
Thus, we see that a cross involving two independently
assorting pairs of genes results in a 9:3:3:1 Mendelian
phenotype ratio only if dominance is complete. This ratio
may differ, depending on the dominance conditions present
in the original gene pairs. Also, two new phenotypes,
tall/late and short/early, have been created in the F2 genera-
tion; these phenotypes differ from both parents and grand-
parents. This phenomenon is termed recombination and
explains the frequent observation that like begets like, but
not exactly like.
A polyhybrid back-cross with two unrelated gene
pairs exhibits a 1:1 ratio of phenotypes as in the mono-
hybrid back-cross. It should be noted that despite domi-
nance influence, an F1 back-cross with the P1 homozygous-
recessive yields the homozygous-recessive phenotype
short/late 25% of the time, and by the same logic, a back-
cross with the homozygous-dominant parent will yield the
homozygous dominant phenotype tall/early 25% of the
time. Again, the back-cross proves invaluable in determin-
ing the F1 and P1 genotypes. Since all four phenotypes of
the back-cross progeny contain at least one each of both
recessive genes or one each of both dominant genes, the
back-cross phenotype is a direct representation of the four
possible gametes produced by the F1 hybrid.
So far we have discussed inheritance of traits con-
trolled by discrete pairs of unrelated genes. Gene inter-
action is the control of a trait by two or more gene pairs.
In this case genotype ratios will remain the same but
phenotype ratios may be altered. Consider a hypothetical
example where 2 dominant gene pairs Pp and Cc control
late-season anthocyanin pigmentation (purple color) in
Cannabis. If P is present alone, only the leaves of the plant
(under the proper environmental stimulus) will exhibit
accumulated anthocyanin pigment and turn a purple color.
If C is present alone, the plant will remain green through-
out its life cycle despite environmental conditions. If both
are present, however, the calyxes of the plant will also ex-
hibit accumulated anthocyanin and turn purple as the
leaves do. Let us assume for now that this may be a desir-
able trait in Cannabis flowers. What breeding techniques
can be used to produce this trait?

 

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First, two homozygous true-breeding ~1 types are
crossed and the phenotype ratio of the F1 offspring is
observed.
The phenotypes of the F2 progeny show a slightly
altered phenotype ratio of 9:3:4 instead of the expected
9:3:3:1 for independently assorting traits. If P and C must
both be present for any anthocyanin pigmentation in leaves
or calyxes, then an even more distorted phenotype ratio of
9:7 will appear.
Two gene pairs may interact in varying ways to pro-
duce varying phenotype ratios. Suddenly, the simple laws
of inheritance have become more complex, but the data
may still be interpreted.

Summary of Essential Points of Breeding

1 - The genotypes of plants are controlled by genes
which are passed on unchanged from generation to
generation.
2 - Genes occur in pairs, one from the gamete of the
staminate parent and one from the gamete of the pistillate
parent.
3 - When the members of a gene pair differ in their
effect upon phenotype, the plant is termed hybrid or
heterozygous.
4 - When the members of a pair of genes are equal in
their effect upon phenotype, then they are termed true-
breeding or homozygous.
5 - Pairs of genes controlling different phenotypic
traits are (usually) inherited independently.
6 - Dominance relations and gene interaction can
alter the phenotypic ratios of the F1, F2, and subsequent
generations.

 

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Polyploidy

Polyploidy is the condition of multiple sets of chro-
mosomes within one cell. Cannabis has 20 chromosomes in
the vegetative diploid (2n) condition. Triploid (3n) and
tetraploid (4n) individuals have three or four sets of chro-
mosomes and are termed polyploids. It is believed that the
haploid condition of 10 chromosomes was likely derived
by reduction from a higher (polyploid) ancestral number
(Lewis, W. H. 1980). Polyploidy has not been shown to
occur naturally in Cannabis; however, it may be induced
artificially with colchicine treatments. Colchicine is a poi-
sonous compound extracted from the roots of certain
Colchicum species; it inhibits chromosome segregation to
daughter cells and cell wall formation, resulting in larger
than average daughter cells with multiple chromosome
sets. The studies of H. E. Warmke et al. (1942-1944) seem
to indicate that colchicine raised drug levels in Cannabis. It
is unfortunate that Warmke was unaware of the actual
psychoactive ingredients of Cannabis and was therefore
unable to extract THC. His crude acetone extract and
archaic techniques of bioassay using killifish and small
freshwater crustaceans are far from conclusive. He was,
however, able to produce both triploid and tetraploid
strains of Cannabis with up to twice the potency of dip-
bid strains (in their ability to kill small aquatic organisms).
The aim of his research was to "produce a strain of hemp
with materially reduced marijuana content" and his results
indicated that polyploidy raised the potency of Cannabis
without any apparent increase in fiber quality or yield.
Warmke's work with polyploids shed light on the
nature of sexual determination in Cannabis. He also illus-
trated that potency is genetically determined by creating a
lower potency strain of hemp through selective breeding
with low potency parents.
More recent research by A. I. Zhatov (1979) with
fiber Cannabis showed that some economically valuable
traits such as fiber quantity may be improved through
polyploidy. Polyploids require more water and are usually
more sensitive to changes in environment. Vegetative
growth cycles are extended by up to 30-40% in polyploids.
An extended vegetative period could delay the flowering of
polyploid drug strains and interfere with the formation of
floral clusters. It would be difficult to determine if canna-
binoid levels had been raised by polyploidy if polyploid
plants were not able to mature fully in the favorable part
of the season when cannabinoid production is promoted
by plentiful light and warm temperatures. Greenhouses
and artificial lighting can be used to extend the season and
test polyploid strains.

 

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The height of tetraploid (4n) Cannabis in these exper-
iments often exceeded the height of the original diploid
plants by 25-30%. Tetraploids were intensely colored,
with dark green leaves and stems and a well developed
gross phenotype. Increased height and vigorous growth, as
a rule, vanish in subsequent generations. Tetraploid plants
often revert back to the diploid condition, making it diffi-
cult to support tetraploid populations. Frequent tests are
performed to determine if ploidy is changing.
Triploid (3n) strains were formed with great difficulty
by crossing artificially created tetraploids (4n) with dip-
bids (2n). Triploids proved to be inferior to both diploids
and tetraploids in many cases.
De Pasquale et al. (1979) conducted experiments with
Cannabis which was treated with 0.25% and 0.50% solu-
tions of colchicine at the primary meristem seven days
after generation. Treated plants were slightly taller and
possessed slightly larger leaves than the controls, Anoma-
lies in leaf growth occurred in 20% and 39%, respectively,
of the surviving treated plants. In the first group (0.25%)
cannabinoid levels were highest in the plants without
anomalies, and in the second group (0.50%) cannabinoid
levels were highest in plants with anomalies, Overall,
treated plants showed a 166-250% increase in THC with
respect to controls and a decrease of CBD (30-33%) and
CBN (39-65%). CBD (cannabidiol) and CBN (cannabinol)
are cannabinoids involved in the biosynthesis and degrada-
tion of THC. THC levels in the control plants were very
low (less than 1%). Possibly colchicine or the resulting
polyploidy interferes with cannabinoid biogenesis to favor
THC. In treated plants with deformed leaf lamina, 90% of
the cells are tetraploid (4n 40) and 10% diploid (2n 20).
In treated plants without deformed lamina a few cells are
tetraploid and the remainder are triploid or diploid.
The transformation of diploid plants to the tetraploid
level inevitably results in the formation of a few plants
with an unbalanced set of chromosomes (2n + 1, 2n - 1,
etc.). These plants are called aneuploids. Aneuploids are
inferior to polyploids in every economic respect. Aneu-
ploid Cannabis is characterized by extremely small seeds.
The weight of 1,000 seeds ranges from 7 to 9 grams (1/4
to 1/3 ounce). Under natural conditions diploid plants do
not have such small seeds and average 14-19 grams (1/2-
2/3 ounce) per 1,000 (Zhatov 1979).
Once again, little emphasis has been placed on the
relationship between flower or resin production and poly-
ploidy. Further research to determine the effect of poly-
ploidy on these and other economically valuable traits of

 

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Cannabis is needed.

Colchicine is sold by laboratory supply houses, and
breeders have used it to induce polyploldy in Cannabis.
However, colchicine is poisonous, so special care is exer-
cised by the breeder in any use of it. Many clandestine
cultivators have started polyploid strains with colchicine.
Except for changes in leaf shape and phyllotaxy, no out-
standing characteristics have developed in these strains and
potency seems unaffected. However, none of the strains
have been examined to determine if they are actually poly
ploid or if they were merely treated with colchicine to no
effect. Seed treatment is the most effective and safest way
to apply colchicine. * In this way, the entire plant growing
from a colchicine-treated seed could be polyploid and if
any colchicine exists at the end of the growing season the
amount would be infinitesimal. Colchicine is nearly always
lethal to Cannabis seeds, and in the treatment there is a
very fine line between polyploidy and death. In other
words, if 100 viable seeds are treated with colchicine and
40 of them germinate it is unlikely that the treatment in-
duced polyploidy in any of the survivors. On the other
hand, if 1,000 viable treated seeds give rise to 3 seedlings,
the chances are better that they are polyploid since the
treatment killed all of the seeds but those three. It is still
necessary to determine if the offspring are actually poly-
ploid by microscopic examination.
The work of Menzel (1964) presents us with a crude
map of the chromosomes of Cannabis, Chromosomes 2-6
and 9 are distinguished by the length of each arm. Chromo-
some 1 is distinguished by a large knob on one end and a
dark chromomere 1 micron from the knob. Chromosome 7
is extremely short and dense, and chromosome 8 is assumed
to be the sex chromosome. In the future, chromosome
*The word "safest" is used here as a relative term. Coichicine has
received recent media attention as a dangerous poison and while
these accounts are probably a bit too lurid, the real dangers of expo-
iure to coichicine have not been fully researched. The possibility of
bodily harm exists and this is multiplied when breeders inexperi-
enced in handling toxins use colchicine. Seed treatment might be
safer than spraying a grown plant but the safest method of all is to
not use colchicine.
mapping will enable us to picture the location of the genes
influencing the phenotype of Cannabis. This will enable
geneticists to determine and manipulate the important
characteristics contained in the gene pool. For each trait
the number of genes in control will be known, which
chromosomes carry them, and where they are located
along those chromosomes.

Breeding
All of the Cannabis grown in North America today
originated in foreign lands. The diligence of our ancestors
in their collection and sowing of seeds from superior
plants, together with the forces of natural selection, have
worked to create native strains with localized characteris-
tics of resistance to pests, diseases, and weather conditions.
In other words, they are adapted to particular niches in the
ecosystem. This genetic diversity is nature's way of pro-
tecting a species. There is hardly a plant more flexible than
Cannabis. As climate, diseases, and pests change, the strain
evolves and selects new defenses, programmed into the ge-
netic orders contained in each generation of seeds. Through
the importation in recent times of fiber and drug Cannabis,
a vast pool of genetic material has appeared in North Amer-
ica. Original fiber strains have escaped and become acclima-
tized (adapted to the environment), while domestic drug
strains (from imported seeds) have, unfortunately, hybrid-
ized and acclimatized randomly, until many of the fine
gene combinations of imported Cannabis have been lost

 

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Changes in agricultural techniques brought on by
technological pressure, greed, and full-scale eradication
programs have altered the selective pressures influencing
Cannabis genetics. Large shipments of inferior Cannabis
containing poorly selected seeds are appearing in North
America and elsewhere, the result of attempts by growers
and smugglers to supply an ever increasing market for mari-
juana. Older varieties of Cannabis, associated with long-
standing cultural patterns, may contain genes not found in
the newer commercial varieties. As these older varieties and
their corresponding cultures become extinct, this genetic
information could be lost forever. The increasing popular-
ity of Cannabis and the requirements of agricultural tech-
nology will call for uniform hybrid races that are likely to
displace primitive populations worldwide.
Limitation of genetic diversity is certain to result
from concerted inbreeding for uniformity. Should inbred
Cannabis be attacked by some previously unknown pest or
disease, this genetic uniformity could prove disastrous due
to potentially resistant diverse genotypes having been
dropped from the population. If this genetic complement
of resistance cannot be reclaimed from primitive parental
material, resistance cannot be introduced into the ravaged
population. There may also be currently unrecognized
favorable traits which could be irretrievably dropped from
the Cannabis gene pool. Human intervention can create
new phenotypes by selecting and recombining existing
genetic variety, but only nature can create variety in the
gene pool itself, through the slow process of random
mutation.
This does not mean that importation of seed and
selective hybridization are always detrimental. Indeed
these principles are often the key to crop improvement,
but only when applied knowledgeably and cautiously. The
rapid search for improvements must not jeopardize the
pool of original genetic information on which adaptation
relies. At this time, the future of Cannabis lies in govern-
ment and clandestine collections. These collections are
often inadequate, poorly selected and badly maintained.
Indeed, the United Nations Cannabis collection used as the
primary seed stock for worldwide governmental research
is depleted and spoiled.
Several steps must be taken to preserve our vanishing
genetic resources, and action must be immediate:

• Seeds and pollen should be collected directly from
reliable and knowledgeable sources. Government seizures
and smuggled shipments are seldom reliable seed sources.
The characteristics of both parents must be known; conse-
quently, mixed bales of randomly pollinated marijuana are
not suitable seed sources, even if the exact origin of the
sample is certain. Direct contact should be made with the
farmer-breeder responsible for carrying on the breeding
traditions that have produced the sample. Accurate records
of every possible parameter of growth must be kept with
carefully stored triplicate sets of seeds.

• Since Cannabis seeds do not remain viable forever,
even under the best storage conditions, seed samples should
he replenished every third year. Collections should be
planted in conditions as similar as possible to their original
niche and allowed to reproduce freely to minimize natural
and artificial selection of genes and ensure the preservation
of the entire gene pool. Half of the original seed collection
should be retained until the viability of further generations
is confirmed, and to provide parental material for compari-
son and back-crossing. Phenotypic data about these subse-
quent generations should be carefully recorded to aid in
understanding the genotypes contained in the collection.
Favorable traits of each strain should be characterized and
catalogued.

• It is possible that in the future, Cannabis cultiva-
tion for resale, or even personal use, may be legal but only
for approved, patented strains. Special caution would be
needed to preserve variety in the gene pool should the
patenting of Cannabis strains become a reality.
• Favorable traits must be carefully integrated into
existing strains.
The task outlined above is not an easy one, given the
current legal restrictions on the collection of Cannabis
seed. In spite of this, the conscientious cultivator is making
a contribution toward preserving and improving the genet-
ics of this interesting plant.
Even if a grower has no desire to attempt crop im-
provement, successful strains have to be protected so they
do not degenerate and can be reproduced if lost. Left to
the selective pressures of an introduced environment, most
drug strains will degenerate and lose potency as they accli-
matize to the new conditions. Let me cite an example of a
typical grower with good intentions.

 

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A grower in northern latitudes selected an ideal spot
to grow a crop and prepared the soil well. Seeds were
selected from the best floral clusters of several strains avail-
able over the past few years, both imported and domestic.
Nearly all of the staminate plants were removed as they
matured and a nearly seedless crop of beautiful plants re-
sulted. After careful consideration, the few seeds from
accidental pollination of the best flowers were kept for the
following season, These seeds produced even bigger and
better plants than the year before and seed collection was
performed as before. The third season, most of the plants
were not as large or desirable as the second season, but
there were many good individuals. Seed collection and cul-
tivation the fourth season resulted in plants inferior even to
the first crop, and this trend continued year after year.
What went wrong? The grower collected seed from the best
plants each year and grew them under the same conditions.
The crop improved the first year. Why did the strain
degenerate?
This example illustrates the unconscious selection for
undesirable traits. The hypothetical cultivator began well
by selecting the best seeds available and growing them
properly. The seeds selected for the second season resulted
from random hybrid pollinations by early-flowering or
overlooked staminate plants and by hermaphrodite pistil-
late plants. Many of these random pollen-parents may be
undesirable for breeding since they may pass on tendencies
toward premature maturation, retarded maturation, or
hermaphrodism. However, the collected hybrid seeds pro-
duce, on the average, larger and more desirable offspring
than the first season. This condition is called hybrid vigor
and results from the hybrid crossing of two diverse gene
pools. The tendency is for many of the dominant charac-
teristics from both parents to be transmitted to the F1 off-
spring, resulting in particularly large and vigorous plants.
This increased vigor due to recombination of dominant
genes often raises the cannabinoid level of the F1 offspring,
but hybridization also opens up the possibility that unde-
sirable (usually recessive) genes may form pairs and express
their characteristics in the F2 offspring. Hybrid vigor may
also mask inferior qualities due to abnormally rapid growth.
During the second season, random pollinations again
accounted for a few seeds and these were collected. This
selection draws on a huge gene pool and the possible F2
combinations are tremendous. By the third season the gene
pool is tending toward early-maturing plants that are accli-
matized to their new conditions instead of the drug-
producing conditions of their native environment. These
acclimatized members of the third crop have a higher
chance of maturing viable seeds than the parental types,
and random pollinations will again increase the numbers of
acclimatized individuals, and thereby increase the chance
that undesirable characteristics associated with acclimati-
zation will be transmitted to the next F2 generation. This
effect is compounded from generation to generation and
finally results in a fully acclimatized weed strain of little
drug value.
With some care the breeder can avoid these hidden
dangers of unconscious selection. Definite goals are vital to
progress in breeding Cannabis. What qualities are desired in
a strain that it does not already exhibit? What character-
istics does a strain exhibit that are unfavorable and should
be bred out? Answers to these questions suggest goals for
breeding. In addition to a basic knowledge of Cannabis
botany, propagation, and genetics, the successful breeder
also becomes aware of the most minute differences and
similarities in phenotype. A sensitive rapport is established
between breeder and plants and at the same time strict
guidelines are followed. A simplified explanation of the
time-tested principles of plant breeding shows how this
works in practice.
Selection is the first and most important step in the
breeding of any plant. The work of the great breeder and
plant wizard Luther Burbank stands as a beacon to breeders
of exotic strains. His success in improving hundreds of
flower, fruit, and vegetable crops was the result of his
meticulous selection of parents from hundreds of thou-
sands of seedlings and adults from the world over.

 

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Bear in mind that in the production of any new
plant, selection plays the all-important part.
First, one must get clearly in mind the kind of
plant he wants, then breed and select to that end,
always choosing through a series of years the
plants which are approaching nearest the ideal,
and rejecting all others.
-Luther Burbank (in James, 1964)

Proper selection of prospective parents is only possible
if the breeder is familiar with the variable characteristics of
Cannabis that may be genetically controlled, has a way to
accurately measure these variations, and has established
goals for improving these characteristics by selective breed-
ing. A detailed list of variable traits of Cannabis, including
parameters of variation for each trait and comments per-
taining to selective breeding for or against it, are found at
the end of this chapter. By selecting against unfavorable
traits while selecting for favorable ones, the unconscious
breeding of poor strains is avoided.

The most important part of Burbank's message on
selection tells breeders to choose the plants "which are ap-
proaching nearest the ideal," and REJECT ALL OTHERS!
Random pollinations do not allow the control needed to
reject the undesirable parents. Any staminate plant that
survives detection and roguing (removal from the popula-
tion), or any stray staminate branch on a pistillate her-
maphrodite may become a pollen parent for the next gen-
eration. Pollination must be controlled so that only the
pollen- and seed-parents that have been carefully selected
for favorable traits will give rise to the next generation.
Selection is greatly improved if one has a large sample
to choose from! The best plant picked from a group of 10
has far less chance of being significantly different from its
fellow seedlings than the best plant selected from a sample
of 100,000. Burbank often made his initial selections of
parents from samples of up to 500,000 seedlings. Difficul-
ties arise for many breeders because they lack the space to
keep enough examples of each strain to allow a significant
selection. A Cannabis breeder's goals are restricted by the
amount of space available. Formulating a well defined goal
lowers the number of individuals needed to perform effec-
tive crosses. Another technique used by breeders since the
time of Burbank is to make early selections. Seedling
plants take up much less space than adults. Thousands of
seeds can be germinated in a flat. A flat takes up the same
space as a hundred 10-centimeter (4-inch) sprouts or six-
teen 30-centimeter (12-inch) seedlings or one 60-centimeter
(24-inch) juvenile. An adult plant can easily take up as
much space as a hundred flats. Simple arithmetic shows
that as many as 10,000 sprouts can be screened in the
space required by each mature plant, provided enough seeds
are available. Seeds of rare strains are quite valuable and
exotic; however, careful selection applied to thousands of
individuals, even of such common strains as those from
Colombia or Mexico, may produce better offspring than
plants from a rare strain where there is little or no oppor-
tunity for selection after germination. This does not mean
that rare strains are not valuable, but careful selection is
even more important to successful breeding. The random
pollinations that produce the seeds in most imported mari-
juana assure a hybrid condition which results in great seed-
ling diversity. Distinctive plants are not hard to discover if
the seedling sample is large enough.
Traits considered desirable when breeding Cannabis
often involve the yield and quality of the final product, but
these characteristics can only be accurately measured after
the plant has been harvested and long after it is possible to
select or breed it. Early seedling selection, therefore, only
works for the most basic traits. These are selected first, and
later selections focus on the most desirable characteristics
exhibited by juvenile or adult plants. Early traits often give
clues to mature phenotypic expression, and criteria for
effective early seedling selection are easy to establish. As an
example, particularly tall and thin seedlings might prove to
be good parents for pulp or fiber production, while seed-
lings of short internode length and compound branching
may be more suitable for flower production. However,
many important traits to be selected for in Cannabis floral
clusters cannot be judged until long after the parents are
gone, so many crosses are made early and selection of seeds
made at a later date.

 

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Hybridization is the process of mixing differing gene
pools to produce offspring of great genetic variation from
which distinctive individuals can be selected. The wind
performs random hybridization in nature. Under cultiva-
tion, breeders take over to produce specific, controlled
hybrids. This process is also known as cross-pollination,
cross-fertilization, or simply crossing. If seeds result, they
will produce hybrid offspring exhibiting some characteris-
tics from each parent.
Large amounts of hybrid seed are most easily pro-
duced by planting two strains side by side, removing the
staininate plants of the seed strain, and allowing nature to
take its course. Pollen- or seed-sterile strains could be devel
oped for the production of large amounts of hybrid seed
without the labor of thinning; however, genes for sterility
are rare. It is important to remember that parental weak-
nesses are transmitted to offspring as well as strengths.
Because of this, the most vigorous, healthy plants are al-
ways used for hybrid crosses.

Also, sports (plants or parts of plants carrying and
expressing spontaneous mutations) most easily transmit
mutant genes to the offspring if they are used as pollen
parents. If the parents represent diverse gene pools, hybrid
vigor results, because dominant genes tend to carry valu-
able traits and the differing dominant genes inherited from
each parent mask recessive traits inherited from the other.
This gives rise to particularly large, healthy individuals. To
increase hybrid vigor in offspring, parents of different geo-
graphic origins are selected since they will probably repre-
sent more diverse gene pools.

Occasionally hybrid offspring will prove inferior to
both parents, but the first generation may still contain
recessive genes for a favorable characteristic seen in a par-
ent if the parent was homozygous for that trait. First gen-
eration (F1) hybrids are therefore inbred to allow recessive
genes to recombine and express the desired parental trait.
Many breeders stop with the first cross and never realize
the genetic potential of their strain. They fail to produce
an F2 generation by crossing or self-pollinating F1 offspring.
Since most domestic Cannabis strains are F1 hybrids for
many characteristics, great diversity and recessive recombi-
nation can result from inbreeding domestic hybrid strains.
In this way the breeding of the F1 hybrids has afready been
accomplished, and a year is saved by going directly to F2
hybrids. These F2 hybrids are more likely to express reces-
sive parental traits. From the F2 hybrid generation selec-
tions can be made for parents which are used to start new
true-breeding strains. Indeed, F2 hybrids might appear with
more extreme characteristics than either of the P~ parents.
(For example, P1 high-THC X P1 low-THC yields F1 hybrids
of intermediate THC content. Selfing the F1 yields F2 hy-
brids, of both P1 [high and low THC] phenotypes, inter-
mediate F1 phenotypes, and extra-high THC as well as
extra-low THC phenotypes.)

 

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Also, as a result of gene recombination, F1 hybrids
are not true-breeding and must be reproduced from the
original parental strains. When breeders create hybrids they
try to produce enough seeds to last for several successive
years of cultivation, After initial field tests, undesirable
hybrid seeds are destroyed and desirable hybrid seeds
stored for later use. If hybrids are to be reproduced, a clone
is saved from each parental plant to preserve original paren-
tal genes.
Back-crossing is another technique used to produce
offspring with reinforced parental characteristics. In this
case, a cross is made between one of the F~ or subsequent
offspring and either of the parents expressing the desired
trait. Once again this provides a chance for recombination
and possible expression of the selected parental trait. Back-
crossing is a valuable way of producing new strains, but it is
often difficult because Cannabis is an annual, so special
care is taken to save parental stock for back-crossing the
following year. Indoor lighting or greenhouses can be used
to protect breeding stock from winter weather. In tropical
areas plants may live outside all year. In addition to saving
particular parents, a successful breeder always saves many
seeds from the original P1 group that produced the valuable
characteristic so that other P1 plants also exhibiting the
characteristic can be grown and selected for back-crossing
at a later time.
Several types of breeding are summarized as follows:
1 - Crossing two varieties having outstanding qualities
(hybridization).
2 - Crossing individuals from the F1 generation or
selfing F1 individuals to realize the possibilities of the ori-
ginal cross (differentiation).
3 - Back crossing to establish original parental types.

4 - Crossing two similar true-breeding (homozygous)
varieties to preserve a mutual trait and restore vigor.

It should be noted that a hybrid plant is not usually
hybrid for all characteristics nor does a true-breeding strain
breed true for all characteristics. When discussing crosses,
we are talking about the inheritance of one or a few traits
only. The strain may be true-breeding for only a few traits,
hybrid for the rest. Monohybrid crosses involve one trait,
dihybrid crosses involve two traits, and so forth. Plants
have certain limits of growth, and breeding can only pro-
duce a plant that is an expression of some gene already
present in the total gene pool. Nothing is actually created
by breeding; it is merely the recombination of existing
genes into new genotypes. But the possibilities of recombi-
nation are nearly limitless.
The most common use of hybridization is to cross two
outstanding varieties. Hybrids can be produced by crossing
selected individuals from different high-potency strains of
different origins, such as Thailand and Mexico. These two
parents may share only the characteristic of high psycho-
activity and differ in nearly every other respect. From this
great exchange of genes many phenotypes may appear in
the F2 generation. From these offspring the breeder selects
individuals that express the best characteristics of the par-
ents. As an example, consider some of the offspring from
the P1 (parental) cross: Mexico X Thailand. In this case,
genes for high drug content are selected from both parents
while other desirable characteristics can be selected from
either one. Genes for large stature and early maturation
are selected from the Mexican seed-parent, and genes for
large calyx size and sweet floral aroma are selected from
the Thai pollen parent. Many of the F1 offspring exhibit
several of the desired characteristics. To further promote
gene segregation, the plants most nearly approaching the
ideal are crossed among themselves. The F2 generation is a
great source of variation and recessive expression. In the F2
generation there are several individuals out of many that
exhibit all five of the selected characteristics. Now the
process of inbreeding begins, using the desirable F2 parents.
If possible, two or more separate lines are started,
never allowing them to interbreed. In this case one accept-
able staminate plant is selected along with two pistillate
plants (or vice versa). Crosses between the pollen parent
and the two seed parents result in two lines of inheritance
with slightly differing genetics, but each expressing the
desired characteristics. Each generation will produce new,
more acceptable combinations.
If two inbred strains are crossed, F1 hybrids will be
less variable than if two hybrid strains are crossed. This
comes from limiting the diversity of the gene pools in the
two strains to be hybridized through previous inbreeding.
Further independent selection and inbreeding of the best
plants for several generations will establish two strains
which are true-breeding for all the originally selected traits.
This means that all the offspring from any parents in the
strain will give rise to seedlings which all exhibit the
selected traits. Successive inbreeding may by this time have
resulted in steady decline in the vigor of the strain.

 

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When lack of vigor interferes with selecting pheno-
types for size and hardiness, the two separately selected
strains can then be interbred to recombine nonselected
genes and restore vigor. This will probably not interfere
with breeding for the selected traits unless two different
gene systems control the same trait in the two separate
lines, and this is highly unlikely. Now the breeder has pro-
duced a hybrid strain that breeds true for large size, early
maturation, large sweet-smelling calyxes, and high THC
level. The goal has been reached!
Wind pollination and dioecious sexuality favor a heter-
ozygous gene pool in Cannabis. Through Anbreeding, hy-
brids are adapted from a heterozygous gene pool to a
homozygous gene pool, providing the genetic stability
needed to create true-breeding strains. Establishing pure
strains enables the breeder to make hybrid crosses with a
better chance of predicting the outcome. Hybrids can be
created that are not reproducible in the F2 generation.
Commercial strains of seeds could be developed that
would have to be purchased each year, because the F1
hybrids of two pure-bred lines do not breed true. Thus, a
seed breeder can protect the investment in the results of
breeding, since it would be nearly impossible to reproduce
the parents from F2 seeds.
At this time it seems unlikely that a plant patent
would be awarded for a pure-breeding strain of drug Can-
nabis. In the future, however, with the legalization of cul-
tivation, it is a certainty that corporations with the time,
space, and money to produce pure and hybrid strains of
Cannabis will apply for patents. It may be legal to grow
only certain patented strains produced by large seed com-
panies. Will this be how government and industry combine
to control the quality and quantity of "drug" Cannabis?

 

[Milonix420]

Acclimatization

Much of the breeding effort of North American culti-
vators is concerned with acclimatizing high-THC strains of
equatorial origin to the climate of their growing area while
preserving potency. Late-maturing, slow, and irregularly
flowering strains like those of Thailand have difficulty
maturing in many parts of North America. Even ~:n a green-
house, it may not be possible to mature plants to their full
native potential.
To develop an early-maturing and rapidly flowering
8train, a breeder may hybridize as in the previous example.
However, if it is important to preserve unique imported
genetics, hybridizing may be inadvisable. Alternatively, a
pure cross is made between two or more Thai plants that
most closely approach the ideal in blooming early. At this
point the breeder may ignore many other traits and aim at
breeding an earlier-maturing variety of a pure Thai strain.
This strain may still mature considerably later than is ideal
for the particular location unless selective pressure is ex-
erted. If further crosses are made with several individuals
that satisfy other criteria such as high THC content, these
may be used to develop another pure Thai strain of high
THC content. After these true-breeding lines have been
established, a dihybrid pure cross can be made in an
attempt to produce an F1 generation containing early-
maturing, high-THC strains of pure Thai genetics, in other
words, an acclimatized drug strain.
Crosses made without a clear goal in mind lead to
strains that acclimatize while losing many favorable charac-
teristics. A successful breeder is careful not to overlook a
characteristic that may prove useful. It is imperative that
original imported Cannabis genetics be preserved intact to
protect the species from loss of genetic variety through ex-
cessive hybridization. A currently unrecognized gene may
be responsible for controlling resistance to a pest or disease,
and it may only be possible to breed for this gene by back-
crossing existing strains to original parental gene pools.
Once pure breeding lines have been established, plant
breeders classify and statistically analyze the offspring to
determine the patterns of inheritance for that trait. This is
the system used by Gregor Mendel to formulate the basic
laws of inheritance and aid the modern breeder in predict-
ing the outcome of crosses,

1 - Two pure lines of Cannabis that differ in a particu-
lar trait are located.
2 - These two pure-breeding lines are crossed to pro-
duce an F1 generation.
3 - The F1 generation is inbred.
4 - The offspring of the F1 and F2 generations are
classified with regard to the trait being studied.
5 - The results are analyzed statistically.
6 - The results are compared to known patterns of
inheritance so the nature of the genes being selected for
can be characterized.

 

[Milonix420]

Fixing Traits
Fixing traits (producing homozygous offspring) in
Cannabis strains is more difficult than it is in many other
flowering plants. With monoecious strains or hermaphro-
dites it is possible to fix traits by self-pollinating an individ-
ual exhibiting favorable traits. In this case one plant acts as
both mother and father. However, most strains of Cannabis
are dioecious, and unless hermaphroditic reactions can be
induced, another parent exhibiting the trait is required to
fix the trait. If this is not possible, the unique individual
may be crossed with a plant not exhibiting the trait, inbred
in the F1 generation, and selections of parents exhibiting
the favorable trait made from the F2 generation, but this is
very difficult.
If a trait is needed for development of a dioecious
strain it might first be discovered in a monoecious strain
and then fixed through selfing and selecting homozygous
offspring. Dioecious individuals can then be selected from
the monoecious population and these individuals crossed
to breed out monoecism in subsequent generations.
Galoch (1978) indicated that gibberellic acid (GA3)
promoted stamen production while indoleacetic acid (IAA),
ethrel, and kinetin promoted pistil production in prefloral
dioecious Cannabis. Sex alteration has several useful appli-
cations. Most importantly, if only one parent expressing a
desirable trait can be found, it is difficult to perform a
cross unless it happens to be a hermaphrodite plant. Hor-
mones might be used to change the sex of a cutting from
the desirable plant, and this cutting used to mate with it.
This is most easily accomplished by changing a pistillate
cutting to a staminate (pollen) parent, using a spray of 100
ppm gibberellic acid in water each day for five consecutive
days. Within two weeks staminate flowers may appear.
Pollen can then be collected for selfing with the original
pistillate parent. Offspring from the cross should also be
mostly pistillate since the breeder is selfing for pistillate
sexuality. Staminate parents reversed to pistillate floral
production make inferior seed-parents since few pistillate
flowers and seeds are formed.
If entire crops could be manipulated early in life to
produce all pistillate or staminate plants, seed production
and seedless drug Cannabis production would be greatly
facilitated.
Sex reversal for breeding can also be accomplished by
mutilation and by photoperiod alteration. A well-rooted,
flourishing cutting from the parent plant is pruned back
to 25% of its original size and stripped of all its remaining
flowers. New growth will appear within a few days, and
several flowers of reversed sexual type often appear.
Flowers of the unwanted sex are removed until the cutting
is needed for fertilization. Extremely short light cycles
(6-8 hour photoperiod) can also cause sex reversal. How-
ever, this process takes longer and is much more difficult
to perform in the field.