There are times when we need to store plant parts other than seeds to ensure that we
preserve diverse varieties of crops. Seeds don't actually capture the specific varieties of many
of our fruit and nut crops. For example, if you plant the seeds of a Honeycrisp apple
the resulting tree will not produce fruit that are the Honeycrisp variety.
This is because the fruit itself grows from maternal tissues on the Honeycrisp tree
and the seeds are a result of fertilization events using pollen from another apple variety.
Apples and most tree fruits and vines as well as nuts and tropical fruit crops
are propagated by methods such as cuttings or grafting, rather than seeds. Because of this,
we need to conserve non-seed tissues of the specific varieties. Our national genebank system
has about 30,000 varieties of 89 fruit and nut crops as well as some vegetables,
such as garlic and sweet potatoes, that may need to be stored as something other
than seeds at the National Laboratory for Genetic Resources Preservation.
Most of these plants are grown in the field locations all over the United States and these
field collections are susceptible to diseases, extreme weather, and natural disasters.
Therefore, it's particularly important that these collections be secured here at NLGRP.
With our collaborators, NLGRP research programs have developed methods to
cryopreserve buds from dormant twigs as well as one millimeter shoot tips from garlic,
citrus twigs, and many kinds of plants grown in tissue culture. Here's an
example of grapes and kiwifruit that we will be cryopreserving from these tissue culture plants.
We need to remove the free water for successful cryopreservation of plant tissues, otherwise
ice crystals form when we freeze the material, and that kills the plant cells upon rewarming.
We'll use the apple collection as an example to explain the cryopreservation of a fruit crop
collection in the form of dormant buds. There are 2,569 field trees in the USDA ARS National Plant
Germplasm System collection in Geneva, New York. 1,273 of these are different varieties of dessert
and cider apples. The rest are ornamentals, such as crab apples, and wild species.
First, twigs are collected from trees in our National Apple Collection in January. We need them
to be as dormant as possible so that the dormant buds can survive the drying and cooling steps.
After the twigs are harvested, they are shipped to Fort Collins. They are then cut into short
segments, each containing a dormant bud which will grow into a shoot after cryopreservation.
The segments are then dried to about 30 percent moisture content,
packaged into tubes with 10 bud segments per tube, and slow cooled to minus 30 degrees Celsius,
at which time they are plunged into liquid nitrogen.
The tubes are then transferred to the cryotanks in our vaults for long-term storage.
We have about two thousand different kinds of apple trees backed up in liquid nitrogen at NLGRP
and we cryopreserve them as dormant winter buds. The cryopreservation methods were
developed by USDA scientists and their colleagues at Colorado State University over 30 years ago.
We have five cryotanks that store the dormant buds in the vapor phase of liquid nitrogen,
which is about minus 185 degrees Celsius. It's particularly important that we back up the apple
collection at NLGRP because the field collection is vulnerable to threats that include fire blight
as well as waterlogged soils. We can use the same method for other temperate woody
crop collections that are in cold hardy locations in our national genebank system.
When the curation team in Geneva needs a new variety to replenish a tree in the field,
they can request a tube of dormant buds of that variety from NLGRP. A tube of
dormant buds is removed from liquid nitrogen, warmed, and rehydrated in moist peat moss.
After a week or two the buds can be grafted onto apple rootstocks and allowed to recover in the
greenhouse. When the trees are established, they are planted in the field collection.
The USDA ARS National Apple Collection is an important global resource for apple genetic
diversity. Researchers and plant breeders use materials in the collection for their programs.
The heritage trees in the collection have also been used to help identify unknown apple
varieties throughout the United States, providing a more complete picture of the diversity we need
to capture. It's important that we provide a security backup of our apple collection as
well as other vegetatively propagated collections in our national genebank. We do this to preserve
varieties of crops that are not usually propagated by seed as in the case of apples.