Front Range Seed Analysts
1997 Seed Forum Volume 11 Number 3
THOUGHTS ON SEED GERMINATION: Understanding the use of alternating temperatures. by Jim Bruce
In the last issue, I began to discuss issues of seed germination which I had not covered in detail when writing the Seed Germination and Viability portion of our correspondence course. The usage of KN03 was covered. As we all know, KN03 is used to enhance the breaking of certain types of dormancy, mainly light dormancy. In this issue I would like to pursue another aspect of breaking dormancy, alternating temperatures. An understanding of alternating temperatures is crucial to the germination most seed species. If one looks at the AOSA Rules for Testing Seed, it is quickly evident that most species need alternating temperatures to successfully complete the germination process.
I am not a seed physiologist and do not profess a complete understanding of the metabolic processes involved in alternating temperatures. My observations come from the germination laboratory and my garden.
Let's start by observing that some species, such as the small seeded legumes and cereals, germinate optimumly at constant temperatures. Many of these are our traditional agricultural crops. When I was learning laboratory germination in the'70's, I was told that constant temperatures could be explained by one of two lines of thought. First, with some species, the response to constant temperatures was the result of domestication by man and the selection over time for those cultivars which did not display dormancy. Makes sense, doesn't it? With other species I was told that it was a response to their native habitat and if one was to take note, most of these species had their origin in the Mediterranean region where the diurnal day to night fluctuation was minimal. (This was related to me by a seed physiologist who shall remain unnamed to prevent his embarassment.)
Let's take a look at these two reasons in more detail. The first reason concerning domestication may be quite valid. Domestication brought about larger seeds which were planted instead of failing on the top of the ground. Planting large seeds at greater depths allows the seed to come into contact with water more optimumly and for longer periods of time. At these depths, the temperature alternations are not as pronounced as near the surfaceof the soil. So, over time, selection by humans would most likely result in seeds which did not need to respond to daily fluctuations in temperature. But what about small seeded species?
We do find that most of our major crop species of small seeded legumes come from either the Mediterranean region or southern Asia where the temperatures in the spring and fall at planting time has little diurnal fluctuation. So, for many years I bought into the explanation which I had been presented. That is until I was looking at an alfalfa field one day. Imagine an alfalfa field being allowed to go to seed and reseed itself naturally. The seed drops off the plant and makes its way to the soil. The soil is shaded by the plants in the field and thus has a moderate temperature with less alternation. Here lies the explaination for constant temperature in nature, shading.
Now let's look at alternating temperatures. First the seeds themselves. Have you ever noticed that species which display dormancy tend to be the smaller seeded species? Having mentioned this, I have to qualify that question. I know that there are exceptions. Barley is a good example. Some cultivars display dormancy given the right conditions in the growing season. But barley does not show dormancy every year. What I'm talking aboutare those species which contain a certain percentage of seed dormancy yearly as part of their makeup. Most of these small seeded species need alternating temperatures to germinate. In addition, these small seeded species tend to have an interaction with light and alternating temperatures to stimulate the germination process. Small seeded species tend to be easily buried in the soil as a result of natural environmental processes, such asfrost heaving, rain, etc. These are the predominate species which survive buried in the "soil bank". Large seeded species tend to have a short survival period in the soil.
An interaction between light and alternating temperatures benefits the individual and the species when found under natural conditions. For annual species, light to the seed accompanied by wide fluctuations in temperatures tells the seed that the soil has been disturbed and that the seed is near or at the surface of the soil. For perennial species it signals that the seed has landed in a gap in the field or overlying canopy where sunlight comes through and there is room for growth of the plant.
The temperatures themselves tell us much about when the seed germinates under natural conditions. A quick glance through theAOSA Germination Tables makes it apparent that the most commonly found alternating temperatures are 15-25C and 20-30C. From viewing these two temperature regimes it should become obvious that seeds, in general, germinate at three periods throughout the growing season. First in the springtime at 15-25C, secondly in the early summer at 20-30C, and thirdly in the fall again at 15-25C.
Research has shown that by studying dormant the species present in the "soil seed bank" we find that species align themselves into four prominant types. The first could be typified by orchardgrass and ryegrass. These species are generally moderate- to large-seeded, lack pronounced dormancy mechanisms (except in abnormal years), have the ability to germinate over abroad range of temperatures, often can germinate in either light or dark conditions (although higher germination percentages are usually found under light conditions) and tend not to be persistent in the soil bank for long periods of time. These characteristics facilitate germination soon after the seed has fallen from the plant. Most of the species of this type set seed in late spring to early summer. If the seed set is early only lack of moisture will prevent germination. If the seed set is late, these species canrespond to a wide range of temperatures including cool falls. This type of germination pattern when studied shows that the majority of viable seed germinates soon after shattering while a small proportion remains dormant through the growing season in the soil. Usually none of the seed remains viable in the soil through the winter.
The second type of germination pattern shows species which have a winter dormancy followed by early germination in the spring. Most of the species within this grouping come from northern to midtemperate zonal areas. Species within this category usually tak emost of the growing season to flower and mature seeds. The seeds generally fall off the plant in the autumn and overwinter in the soil, germinating quickly in the spring. The longevity in the soil is only through the winter and the early summer. Seeds within this group have their germination inhibited by hot temperatures. Because of this and the lack of persistence in the soil, germination is limited only to spring and early summer when moisture conditions are conducive to survival. Most of the species are annuals.
A third type consists of species which essentially function like the first type, germinating quickly upon dissemination from the plant, with the exception that a proportion of the seeds are very dormant and fail to germinate directly after shattering. The dormant seeds are incorporated into the persistent soil bank and may survive there for several years. Unlike the first group, most of these species are small-seeded. Many of these species are winter annuals or small-seeded grasses such as the Poas. As a general rule, most of these species are autumn germinators.
The fourth type consists of many annual and perennial herbs which maintain large percentages of seed in the persistent soil bank. Few seeds germinate immediately following dispersal from the plant. They produce large numbers of small seeds. Germination of seeds within this group occurs over very restricted ranges of germination temperatures. Generally, germination is often suppressed by continuous darkness and enhanced only in the presence of light. Cooler temperatures often inhibit germination while warm temperatures enhance germination. Juncus spp., Potentilla, and the Amaranths can be placed in this category.
Observing your garden will give you much insight into these groups and how temperature affects them. In my garden for instance, Chenopods germinate readily in the early spring. They tend to be predominant in the sunny locations. Clovers germinate as the weather warms up, usually in the shady areas. Towards late spring as the weather gets warmer, germination of weeds subsides. As thesummer solstice passes, Amaranths and warm season grasses germinate profusely as the days are warm and the nights cool. Soon they subside in their germination. As fall is ushered in, cool season grasses and the Brassicas are found in profusion. Then all germination stops as winter snows cover the ground.
So much for now. We'll continue this discussion in the next issue. (If you are interested in the different germination patterns which we have discussed above I suggest that you read Seed Ecology by Jack Fenner.)