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Many different vegetables are primed before they are planted. Priming is a water-based process that is performed on seeds to increase uniformity of germination and emergence from the soil, and thus enhance vegetable stand establishment. Priming decreases the time span between the emergence of the first and the last seedlings. Priming also increases the rate of emergence so the stand establishes itself faster. A uniform plant stand helps to ensure maximum cartons per acre at harvest. Wide ranges in seedling emergence decrease the amount of harvestable plants per acre, an undesirable situation. Lettuce is especially vulnerable to this particular field problem because what is harvested is what initially emerges, multiplied by the effects of photosynthesis. To attain the same size, seedlings must spend an equal amount of time in the sun. Seedlings that emerge 2 or 3 days later than the main crop never catch up in size because of the competitive effects of their bigger neighbors. This problem is analogous to the "runt of the litter" problem with hogs.

These attributes of priming have been well documented for many vegetables besides lettuce. Lettuce, however, has a unique problem in the world of vegetables that priming also solves. Lettuce seeds often need light and/or cool temperatures in order to germinate uniformly. When at least some seed within a lot fail to germinate in the dark (even when seeds are planted at a cool temperature as in figure 1), the seed lot is said to be "photo-sensitive" or "photodormant". The reason for the light requirement will be discussed in detail later. Please note for now that some seeds planted in the dark in Figure 1 did not germinate. Germination, or the lack thereof, is usually not an "all or nothing" response in lettuce. Some seeds within this lot could germinate in the dark while most could not. The light requirement problem is rare within the domesticated crop world, but similar germination requirements have been well documented in less domesticated species. Seeds evolved this type of dormancy mechanism so that they would not germinate if dispersed to a soil depth unfavorable for successful emergence.

The light requirement for germination in lettuce is further complicated by the interaction with germination temperature. Figure 2 illustrates the effects of increases temperature of lettuce seed germination, even when light is not an issue. In this case, lettuce seed was planted in constant light but at two different germination temperatures. Seed planted at the higher temperature (82°F) did not germinate as well as seed planted at the lower temperature (72°F). High temperatures can thus cause the same type of inhibition that darkness can cause. Similar to the light response, high temperature responses vary from seed to seed within a lot. 

The worst case scenario for lettuce seed germination is when a lot is placed in a light-deprived environment that is also at a high temperature (see Figure 3). The two environmental factors interact together, producing a greater inhibitory response than either of the factors acting separately. The complete absence of germination on the right-hand blotter in Figure 3 illustrates the severe negative effect that this environmental "double whammy" has on lettuce seed germination. If these negative environmental effects are not alleviated within a particular length of time, the lettuce seed will not germinate even if environmental conditions later become optimal. The seed has entered a state of dormancy and will not germinate under optimal conditions for a long time. Hence the terms "photodormancy" or "thermodormancy" are often heard when lettuce seeds emerge erratically or not at all after planting. Needless to say, all thoughts of a uniform plant stand have long ago vanished.

The origin of these germination requirements lies within the structure of the lettuce seed itself (Figure 4, reprinted by permission of Drew and Gray). There are several tissue layers surrounding the embryo of the lettuce seed. The pericarp is the outer seed layer, derived from the ovary wall of the flower. The integument layer is derived from the ovule and, in many seeds, develops into a "true" seed coat. The endosperm layer lies next to the embryo and is the remnant of nutritive tissue from the embryo sac. In monocots (e.g. corn), the endosperm is a bulky storage tissue. In lettuce, however, the endosperm becomes the "seed coat" because this tissue layer regulates germination. The endosperm layer causes the light and cool temperature requirements. If the endosperm layer is removed from the embryo, the seed germinates fine. Thus, in lettuce seed, the endosperm layer acts as a seed coat instead of supplying energy reserves to the germinating embryo, and the tissues that usually regulate germination, i.e. pericarp or integuments, play little of no role in regulation. To say the least, lettuce is an unusual seed.

The degree of regulation caused by the endosperm-embryo interaction varies from seed to seed within a lot, from variety to variety, and from lettuce type to lettuce type. For example, Tango (a spring-mix lettuce type) is extremely photosensitive. Light is required for the seed to germinate even at a moderate temperature of 73°F. To obtain a uniform plant stand, Tango always needs to be primed, because mechanical planting always causes some seed to be completely buried. Parris Island Cos is a romaine type of lettuce and is much less photosensitive. This type of seed will germinate if planted in the dark, provided the germination temperature is not high (<78-80°F). Thus, these seeds only need to be primed if there is a chance of encountering high temperatures during germination. Lastly, crisp head type lettuce varieties are very tolerant of high temperatures and darkness during germination. They seldom need to be primed if planted as raw seed, and then only under extreme conditions.

Light and temperature are thought to regulate germination by affecting the strength of the endosperm to be punctured by the growing radicle. Studies have shown that less force is required to puncture the endosperm in thermo-tolerant genotypes and in the presence of light. Light is thought to weaken the endosperm by causing increased activity of the enzyme endo-B-mannanase. This enzyme degrades the cell walls of the endosperm, which are mainly composed of mannans. Light has also been shown to affect the ratio of phytochrome configuration in the embryo. If the level of one form of phytochrome (Pfr) is higher than the other form (Pr), seed tends to germinate. If these levels are reversed, seeds tend not to germinate. Genetics and how the seed was produced may also affect these phytochrome forms (Pfr/Pr ratio). The exact interaction between light and temperature and their effects on enzyme activity and/or phytochrome ratios have not been fully elucidated. We do not know whether the environment promotes germination only by weakening the endosperm layer or by a combination of endosperm weakening and radicle strengthening. Priming lettuce seeds has also been shown to decrease the strength needed to puncture the endosperm. How priming affects the endosperm and/or embryo is not fully understood yet. We do know that primed seeds germinate well in high temperatures with or without light (see Figure 5). Please also note that root length of the seedlings are all about the same (no seed appears to have "just" germinated) and there appears to be quite a proliferation of root hairs on each radicle. A secondary benefit of priming any seed species appears to be proliferation of root hairs.

Pelleting the seed exacerbates the vulnerability of lettuce seeds to photo and/or high temperature inhibition. No matter how small a mechanical constraint the pellet places on the germinating seed, the pellet will always prevent at least some light from reaching the seed. Planting pelleted seeds beneath the soil surface (a recommended practice to keep the pellets wet and as cool as possible during germination) does add a stress that raw seeds do not encounter when planted on the surface. Choosing a pellet type that allows germination under stress environments is critical. Not all pelleted seed products are equal.

The preceding paragraphs have been an attempt to explain why lettuce seeds often need to be primed, especially if they are pelleted. Uniformity of emergence is not as important to home gardeners as it is to the commercial producer. However, everyone benefits in understanding some of the complexities of lettuce germination and why seeds do or do not germinate.