Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
Unfavorable soil temperature, excessive moisture, and attack by pests can all prevent a seed from growing into a seedling. Sometimes it amazes me that any seeds manage to come up. But, horticultural researchers are seeking ways to surmount these problems, and they are also finding out just what goes on in a germinating seed.
One of the most promising methods of improving germination under adverse conditions, as well as good conditions, is a process known as osmotic priming. This technique has produced faster and more uniform germination in crops including lettuce, sweet corn, carrots, tomatoes, and others.
Osmotic priming consists of soaking seeds in an aerated solution for a period of time, usually a week or so in length, then either immediately planting the seeds or drying them and planting them later. Osmosis is the movement of water from a region of low solute concentration into a region of high solute concentration. The priming solution concentration is adjusted to control the rate of osmosis into the seed, and afterwards, the treated seeds are primed for action. Hence the name, osmotic priming.
Horticulturalists are pleased with the results. For example, lettuce, primed for 14 days in a polyethylene glycol solution, showed accelerated germination at low temperature, and priming for as little as 1 day allowed germination at high temperatures where untreated seed failed to germinate. Botanists are pleased, too, for by figuring out why osmotic priming improves germination, they are figuring out germination itself.
One theory is that there are two stages to germination imbition and radical or root tip emergence. During imbibition, water swells the seed and its metabolism fires up. The seed begins to accumulate various solutes as the end result of that initial metabolism, and through osmosis, the solutes cause more water to enter the seed. When enough water has entered the seed, radicle emergence begins.
Osmotic priming may result in faster germination by taking the seed through the first step of germination while keeping the second step on hold. Since the priming solution has a relatively high concentration of solutes, sufficient water from the priming solution is not drawn into the seed, and radicle emergence is not triggered. But when the seed is dried, the solutes generated in early germination remain in the seed, and when it is planted, these solutes quickly draw water from the soil into the seed and start the root tip growing.
Osmotic priming probably affects germination at adverse temperatures because the initial germination processes are more temperature-sensitive than radicle emergence. Again, priming takes the seed past the early stages and holds it ready to start growth in the next phase of germination.
Osmotically primed seeds maintain their enhanced germination even after storage for several months. Whether they will germinate after longer periods, allowing seeds to be primed for sale to gardeners and farmers, remains to be seen. I wouldn't be surprised, though, to see specially prepared seeds on the market someday. In the meantime, I'm even more amazed by the growing process than I was before.
(Originally published as "Research Roundup: Germination," by Ellen M. Silva, Extension Technician, Consumer Horticulture, in The Virginia Gardener Newsletter, Volume 10, Number 3.)
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