Modified April 24, 2006.
Karl King (2004)
Iarovization is commonly translated as "vernalization", but this misses an important point. In general, iarovization protocols are designed to prevent germination, rather than to promote it. In fact, iarovized seeds are sometimes dried so they can be stored until needed, then planted with ordinary machinery.
Germination of a seed involves numerous processes. The most obvious is the emergence of the shoot and root. But there is also a great deal of enzymatic activity. Storage substances (e.g., starches, phytates, proteins) are digested to make water soluble nutrients available to the emerging seedling. Other enzymes are at work producing vitamins B1 and C, and converting non-essential amino acids to essential amino acids. In some cases, enzymes also degrade substances that keep the embryo dormant.
In the last case, obviously, enzymes must be active before the embryo awakens. According to Whyte's report1, "Saponikova (1935) analysed vernalized seed of Lupinus angustifolius. The content of reducing sugars in seeds treated at 6 to 7°C. increases with the progress of vernalization, a fact regarded as suggesting an increasing activity of the enzymes acting on carbohydrates; however the content of reducing sugars gradually falls in seeds vernalized at 4 to 5°C. The amount of active enzymes rises with vernalization at 6 to 7°C. to a maximum on the last day of treatment; enzymes increase less rapidly with vernalization at 4 to 5°C. during the first 12 days and then fall."
To iarovize a seed properly, we must find the combination of conditions — moisture, temperature and sometimes light — that will allow the enzymatic activities to proceed while keeping the embryo dormant. In this way, water soluble nutrients will be readily available when the embryo begins to grow. In untreated seeds the growth of the seedling is limited by the rate of enzymatic activity. In short, the sprout from a iarovized seed should grow faster and larger, which gives it a head start in life. Experiments have shown that plants grown from iarovized seeds give higher yields — herbage and seed — than plants from untreated seeds. In many cases maturity can be hastened by several days, which may be of particular concern in some areas.
Iarovization is not an "unnatural" process. In fact, many seeds are naturally iarovized. On the other hand, storing seeds indoors during winter and planting them only after the last frost is highly unnatural. We encourage seeds to germinate quickly, rather than undergo the natural iarovization: a slow action of enzymes preceding the emergence of the seedling.
Many gardeners have encountered seeds with hard seed coats. Experts assure us that the best way to handle such seeds is to nick, file, crack, scald or otherwise damage the seed coat to insure prompt germination. Such techniques usually work, but may not give the best seedlings. A hard seed coat slows the entry of water into the seed, which allows iarovization to proceed at a leisurely pace. But when we nick the coat, water is absorbed more quickly and may stimulate the embryo to start growing before an adequate supply of water soluble nutrients has been made available. The seedlings can survive, but may not be as vigorous as those from unmolested seeds.
It may be that nicking and artificial iarovization will give the best results.
Seeds with hard coats may vary in the thickness of the coat. This insures that some seeds will germinate in the first year, while others must wait for the second, third or later year. For a wild plant this is an obvious advantage. But a busy gardener may find it annoying. By nicking the coat, we remove this variable. And artificial iarovization will allow the enzymes to do their usual work before the embryo is awakened. All the seeds can germinate the first year, but the embryos will not be rushed into growth before the enzymes have prepared their soluble nutrients.
The effects of iarovization have been observed in the pollen of Viola diversifolia. According to Oakeley et al.2, citing Dajoz et al.3, "In Viola there are also two subtypes of pollen with either three or four potential pollen apertures. Both forms were able to germinate, also true for tobacco, but the four-aperture pollen germinated faster than the three- in vitro. Once germinated, the pollen tube from a three-aperture pollen grew faster."
That is, pollen grains with only 3 apertures absorb water more slowly, allowing the enzymes more time to digest the storage substances.
Iarovization also can be applied to storage organs such as tubers, corms, bulbs. Lee, et al.4 found that for Asiatic hybrid lilies, "The time required for shoot emergence and flowering decreased as bulb vernalization duration increased. More than four weeks of cold treatment were required for early shoot emergence and flowering, 100% flowering, and an increased number of flowers." The precise response varied with the clones. Griffiths and Wright5 found similar changes in the bloom time of Paperwhite Narcissus associated with differences in storage temperature.
It is useful to know that responses to iarovization are hereditary. One may, for example, select for late sprouting seeds. In the case of bulbous plants, we may select clones that give maximum response to shorter periods of treatment. Many of the next generation offspring should do as well with as little, or even less, pretreatment.
Note 3/29/2005: Iarovization also could be employed to reduce dependency on herbicides. Following proper iarovization, seedlings should grow substantially faster, giving them an advantage over weeds.