Grafting Citrus Trees under a Microscope

Grafting Citrus Trees under a Microscope


In this video I will show techniques, including grafting citrus trees under a microscope that
California scientists use to avoid all known
graft-transmissible diseases when introducing a citrus
variety to the state. Any citrus variety can be introduced to the state of California through the Citrus Clonal
Protection Program or CCPP at the University of California Riverside. I have requested some
citrus variety introductions and at the end of the video
will provide more information about variety introduction
and how you can order a citrus variety that
is not in California. Because citrus trees are vulnerable to a number of
graft-transmissible diseases, all citrus varieties to be
introduced in California must by law go through the CCPP. The CCPP will remove any
disease-causing organisms and certify that the variety is healthy for release in California. If you submit a new variety to the CCPP, the cuttings must be washed in soapy water after removing the leaves. This is an important
step to remove insects that could be on the cuttings. After rinsing and air drying, the cuttings are sealed
inside of a plastic bag and shipped using an
express courier service. When the cuttings arrive at the CCPP lab, the lab technician first photographs them to document their condition. The cuttings are then surface sterilized in a solution of chlorine bleach. Next the cuttings are
cut into shorter pieces and inserted into test tubes containing a plant growth medium. The tubes are then placed
inside an incubator. Within a few days, buds on
the cuttings start to grow. Once there is sufficient growth, the cuttings are removed from
the incubator to be grafted. These rootstock seedlings, which were planted in test
tubes with a plant growth medium and grown inside of a dark incubator, are the right size to be grafted. A United States penny
will give you a sense of scale under the microscope. First the rootstock seedling is removed from the test tube and
placed on a petri dish. The top of the seedling is cut off. Next, new growth of a
shoot from the variety being introduced is
placed on the petri dish. This shoot may contain bacteria, viruses, and other organisms
that can cause diseases, but scientists have
discovered that the very tip of the shoot is free of
disease-causing organisms. These organisms have difficulty invading the very tip of the shoot
called the apical meristem. By cutting off this healthy
part of the shoot tip, it is possible to grow a tree free of disease-causing organisms. The tiny leaves are removed with a scalpel and the apical meristem
is cut off very carefully. Precision is critical. If too little of the shoot tip is removed, the graft will fail. If too much of the shoot tip is removed, it could contain diseased tissues. A triangular incision is cut
into the rootstock seedling and the shoot tip is carefully
moved onto the incision. Even a slight misstep could crush the delicate shoot tip and
cause the graft to fail. The grafted plant is then
inserted back into a test tube and placed into the incubator. The survival rate of citrus
shoot tip grafting is low, about 10%, so many
grafts must be performed to achieve a single
successful shoot tip graft. Once the graft of the
introduced variety has grown to a good size, it is propagated onto a bigger rootstock seedling. In order to produce fast growth, the rootstock typically
used is rough lemon. First the lemon seedling is bent over to overcome apical dominance, which would prevent further
growth of the graft. Next a T-shaped incision is
cut into the lemon seedling. Then the tiny tree of
the introduced variety is removed from the test tube and any rootstock suckers are removed. A cut is made on the rootstock
part of the tiny tree. The rootstock part of the tiny tree is then inserted into the incision on the lemon seedling
to make a double graft. The graft is wrapped very
carefully with grafting tape to avoid breaking the soft, tiny tree. An easy-to-read label and
a securely-fastened label are both added to the container. The double grafted tree is then placed in a growth chamber until
the new graft has healed. When the tree has put
on sufficient growth, tissue samples are taken and tested for all known citrus
graft-transmissible bacteria, viruses, and other organisms
that cause diseases. Although it has been shoot tip grafted, there is a chance that the
shoot tip could contain infected tissue, so it is
critical to test the tree. High-tech molecular diagnostic tests determine whether plant tissue samples contain disease-causing organisms by looking for their DNA fragments. If DNA fragments of these
disease-causing organisms are detected, the tree fails the test. Other tests are performed
where tissue samples of the new tree are grafted
onto special indicator plants that will show specific disease symptoms when disease is present. If certain disease symptoms are seen, the tree fails the test. If the tree fails any test, the shoot tip grafting is repeated. If the tree passes all tests, the tree will be released from quarantine and budwood of the
recently introduced variety will be made available to everyone. The CCPP has long worked
to help all Californians access the citrus varieties
that they would like to grow by maintaining and
introducing new varieties based on industry and consumer interest. Although the CCPP maintains hundreds of different citrus varieties, there are still varieties
that Californians would like to grow that are currently unavailable from the CCPP. There is no reason to smuggle budwood into California because anyone can request a citrus variety introduction
by contacting the CCPP. This action helps to
prevent the introduction and spread of deadly citrus
diseases in California that could threaten not
only our citrus industry, but also the trees in our back yards. If you are interested in learning more about new variety introduction, please visit the link in
the description below. If you enjoyed this video,
please be sure to share it and subscribe to this YouTube channel. Thank you.

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