Agriculture has benefitted from low-temperature science in numerous ways. The most dramatic effect has come from the simple introduction of the refrigerator and freezer. Agricultural products can now be stored for an extended time without resorting to canning fruit or drying meat. Not only can people keep vegetables from the garden around longer, but also foods can easily be shipped around the world so shoppers now find strawberries -- which grow only in warm temperatures -- in the supermarket in the winter. (For a price!)
While basic refrigeration has had a profound effect on farmers' lives worldwide, far lower temperatures are now used in agriculture as well. The field of cryopreservation -- storage at temperatures of -196 C or lower -- is put to use for artificial insemination of livestock and preserving plant seeds. Breeding livestock for special traits has long been a tradition on farms, so being able to store a male animal's sperm for artificial insemination gives farmers much more flexibility.
Breeding for specific traits can eventually have repercussions too. When it comes to plants it's estimated that since 1900 about 75percent of crop diversity has been lost as growers have focused on hardier or pest-resistant plants. But what's good for growing in one area of the world now might not turn out to be good for another part of the world later on. Or if a new disease attacks a crop, one might wish to incorporate genes from a different plant to increase resistance. To preserve plants for future agriculture, some 1,470 gene banks around the world are trying to maintain more than 5.4 million samples of plants, the vast majority of which are crop plants.
Most of these samples are stored as seeds. The majority of seeds won't deform when frozen at sub-zero temperatures (see the activity on animalsfor more information on why freezing a cell can destroy it). These seeds are dried and kept in deep freezers, where they can survive for up to 200 years with minimal degradation. But a lot of seeds need more care -- they need to be flash frozen so quickly that they don't change shape. Such seeds are usually stored in liquid nitrogen at -320 F (-196 C). This temperature is so cold that everything in the seed ceases to function. Scientists think a seed could remain intact like this for hundreds or even thousands of years, ensuring the diversity of crops for centuries to come.
Many zoos use cryopreservation to help with in vitro fertilization of endangered animals. A record number of captive pandas -- 25 worldwide -- were born in 2005. All of them were artificially inseminated. The zookeeper made use of a technique discovered in 1980 where panda sperm can be stored in liquid hydrogen until needed. Can you research other endangered animals that have benefitted from low-temperature storage? Discuss the pros and cons of trying to increase the numbers of endangered animals.
Hands on Activity
Freezing Seeds This activity will take a couple weeks. A demonstrator might cut off a few days by freezing the seeds ahead of time.
Seeds of different varieties -- at least thirty of each. Some good ones to try:
Supersweet corn
Onion
Lima bean
Cabbage, raddish, or broccoli
Lettuce
Liquid nitrogen
Petri dishes
Cotton rolls
Water
a standard freezer
A toaster or laboratory oven, or an incubator
1. Set five of each type of seed aside. Put five of each type of seed in the freezer -- without covering them. Suspend the remaining seeds in liquid nitrogen -- keeping the various kinds separated so you know which set is which. Leave over night.
2. After one day, remove five of each type of seed from the nitrogen. Remove another set of five seeds on each succeeding day, so that when you're finished you will have sets of seeds that have been frozen for one, two, three, and four days.
3. After four days remove the seeds from the freezer.
3. You are going to get the seeds to sprout in a petri dish. Set the petri dishes up by cutting circles of cotton that will fit inside the dish. Soak the cotton in water and then drain them for five minutes. (Too much water will drown the seeds!) Place the seeds in the petri dish and put them in the oven at about 70 to 80 F. (In a pinch, just keep the petri dishes someplace warm -- on top of the refrigerator for example -- they might not sprout as quickly without a perfectly regulated temperature, but they should sprout eventually.) Make sure to label which seeds were kept at room temperature, which were frozen, and which were kept at cryogenic temperatures for however many days.
4. Observe and record the rate of germination for each of the seeds every 24 hours. Compare and contrast the rates at which the seeds germinate. Can you explain the difference?
Thank you to our Underwriters: National Science Foundation, Alfred P. Sloan Foundation.
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