The basic principle in crossing Salvias is the same as for
any plant, the designated female needs to be prevented from being prematurely
fertilised and then pollen from the desired male plant needs to be placed on
the receptive part of the stigma.
Preventing premature fertilisation requires that any
potential pollinators (birds, bees, hawk moths etc) are excluded and the anthers
are removed from the flower (i.e. emasculation) before the pollen is ready to
be released. For large flowers like S.
mexicana this emasculation can be done the afternoon before the flowers are
expected to open. However, I find that
for many species (even S. splendens) it is easier to do the emasculation later
in the night when the flower has developed a bit more. When anthers are removed check that they have
not dehisced and discard the flower if they have. If the intention is only to generate seed
from a species that does not normally set seed because of lack of a suitable
pollinator, then it is not necessary to emasculate.
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| The flower (S. fulgens) in the center is at a reasonable stage of development to emasculate. |
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| Best technique will vary with each species, For S. fulgens the upper petal can be grasped and peeled back. In some species the lower lip also needs to be pulled down. |
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| to reveal the immature stigma and anthers |
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| The anthers are tucked back in the flower |
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| Check that both are removed and have not started to dehisce. |
Salvia flowers are reported in the scientific literature to
be "protandrous", that is the anthers release pollen before the
stigma is receptive. I have not tested
this but just in case I delay pollinating flowers until the afternoon or even the
next day. When the anthers dehisce the
pollen mass is held together by pollenkitt so that it does not blow away. The pollen mass can remain sitting on the
anther for days in mild weather provided there are no bees around. At the peak of summer bees can be extremely
active and sometimes I have been forced to collect pollen, from varieties where
the anthers are exposed, before sunrise to beat the bees.
The receptive part of the Salvia stigma is the tip of the
lower fork. While the style of many
Salvia flowers has luxuriant hairs, applying pollen to the style or stylar hairs
does not result in any seed. I have also
not obtained seed from only pollinating the upper fork of the stigma. In species with coloured styles the receptive
part of the stigma is often clear.
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| S. fulgens flower ready to be pollinated, Stigma forks open. |
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| Apply pollen to tip of lower fork, this is the only receptive part. |
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| Check that there is a small amount of pollen adhering. |
Unlike many garden plants, Salvias have a dry stigma. On plants like lilies and roses the stigma is
covered by a nutrient rich fluid that provides an ideal environment for pollen
to germinate. In Salvias the pollen must
be recognised and accepted before the female makes moisture available for the
pollen grain to then imbibe and germinate.
This is a very selective process and many interspecific crosses fail at this
point.
After the pollen grain germinates it develops a pollen tube
that grows down the style. Interactions
can take place between the developing pollen tube and style that restrict
development of incompatible pollen. At
the base of the style the pollen tube must change direction and find the
entrance to the ovary so fertilisation can occur. The pollen tube is directed by specific
chemical signals from the ovary. The
result of all these interactions is that a species can restrict the potential
for foreign pollen to produce a seed.
Species can only exist if their genetic integrity is
maintained otherwise there is just one big mixing pot. Natural barriers to crossing exist where
species grown in different ecosystems or countries, have different pollinators
or flower at different times of the year.
These barriers are easily overcome in a breeding program or, if species are
grown together in a garden hybrid plants may occur naturally. Many interesting Salvia hybrids have resulted
in this way. However, if species have
been isolated for a very long time the changes that have accumulated in their
genetics and metabolism may prevent fertilisation or the development of viable
seeds. This situation is referred to as
incongruent. Such species would probably
be identified as very different by classical taxonomists and they are also
likely to be classified as very different based on DNA patterns.
Where species share the same ecosystem (sympatric) they
require specific mechanisms to prevent cross pollination so as to maintain their
genetic integrity. Such species may be
taxonomically closely related and have very similar DNA patterns. They are referred to as incompatible. Traditional plant breeders have a variety of
tricks that can sometimes overcome these specific mechanisms such as using
mixtures of pollen (mentor pollen) or bud pollination or grafting the stigma
from a male plant onto the female's style.
And there is always the sledgehammer, because these specific mechanisms
are rarely 100% effective, just making thousands of pollinations may get a few
seeds past the barrier.
For a plant breeder trying to decide what crosses are worth
making, taxonomy is a good starting point, but the only proof is to actually
make crosses. Mother Nature always has a
few surprises up her sleeve and there will always be exceptions to our
scientific expectations. New taxonomic
methods based on analysis of DNA etc have the potential to enhance our
understanding of relationships between Salvia species. However, these methods rely on probability
analyses to group species and those groupings may not reflect the relationships
in the natural world. When reading such
papers, I first look to see how they have grouped S. greggii and
S.microphylla. Where the distributions of
these two species overlap they intercross and their progeny are fully fertile,
so from a plant breeder's perspective they are the same species. New taxonomic analyses that do not reach this
conclusion are of no value to a plant breeder.
Sometimes if two species cannot be crossed directly, they
may both be compatible with a third species that can then act as a bridge to
enable characteristics from all three species to be combined.
Varieties within a species differ in their fertility. There is no point trying to make crosses with
varieties that are sterile. If suitable
pollinators are present, actual seed set can be used as an indicator of
fertility or plants can first be self-pollinated by hand to determine if they
are worth using as a parent. If an
anther does not produce sufficient pollen to enable it to dehisce, then the
plant is sterile. I also use a
microscope to examine pollen and a stain to test pollen viability.
Species and varieties also differ in other characteristics
that may make them difficult parents. S.
buchananii when used as a female has a propensity for the flower spike to
suddenly die before the seed is viable, however, as a male it produces a lot of
pollen that adheres well to stigmas. The
S. guaranitica varieties I have tried tend to drop florets before seed is
mature.
Achieving fertilisation and the development of a seed is not
the end of the task. The objective is to
have a viable plant and there are a range of additional potential obstacles.
There are three different classes of tissue in a salvia
seed. Although generally referred to as
a seed, the fruit of a Salvia is more like a very small nut. The outer-most layer is maternal and softer,
it corresponds say to the flesh of a prune.
Then there is then a thin hard layer, also maternal, that encapsulates
the cotyledons and embryo. These outer
layers have the same characteristics, including their contribution to dormancy,
as the female parent. Within this outer
shell there is the endosperm and the embryo.
The endosperm has two sets of genes from the female and one set from the
male parent while the embryo has half its genetic material from each
parent.
Sometimes fertilisation triggers development of the outer
maternal layers but the embryo aborts. This results in what looks like a normal
seed but it is empty. If I have plenty
of seed from a cross I dissect some before trying to germinate them. A more frustrating outcome is when
pollination triggers development of a seed but the pollen does not contribute
to the embryo. This may sound strange
but it is not rare (for jargonistas; this is termed "pseudogamous apomixis"). I have found this situation to be common in
crosses with S. coccinea although it took me a long while to convince myself this
was a natural effect and not a consequence of my emasculation technique.
Finally, when a seed has been germinated, the resulting
plant may lack vigour due to a lack of congruency in the metabolisms of the two
parents. Sterility is very common in
interspecific hybrids since the process to develop the reproductive cells requires
paring between chromosomes from the two original parents. Any differences between the parents are
magnified at this stage.
While sterility is a major problem for a plant breeder, it
can be a beneficial attribute in a plant variety. Flowers tend to drop soon after they are
fertilised so sterile plants often retain their flowers for much longer and
therefore are horticulturally attractive.
Sterile plants are also 'garden friendly' since their progeny cannot pop
up where they are not wanted.
