Nature of Vernalization Stimulus – Vernalin or Gibberellin
Many attempts were made to isolate vernalin but all failed. However, Anton Lang in 1957 showed that application of gibberellins to certain vernalization requiring biennials like henbane induce flowering in them without cold temperature treatment. Purvis in 1961 induced winter annuals to flowers induced by treating their seeds with gibberellins. It was also found that natural gibberellins are formed in greater amounts in vernalization requiring species when these are exposed to low temperatures, e.g., in Chrysanthmum and rudbeckia species. These results indicate that properties of gibberellins may be similar to those expected for vernalin.
But when gibberellins are applied to, vernalization requiring rosette plants, elongation of stem take place first followed by initiating of floral buds on those shoot. However, when plants are provided with cold temperature in order to induce flowering, the floral buds appeared as soon as the shoot begin to elongate (bolt). The results indicate that flowering response to applied gibberellins is not equal to flowering response induced naturally, by cold-temperature treatment.
Mikhail Chailakhyan (1968) in Russia suggested that there are two substances involved in flower formation, one a gibberellin or gibberellin-like substances and anthesin, a hypothetical substance like vernalin. According to him the plants that require low temperatures or long days or both might lack sufficient gibberellins until they have been exposed to inducing environment, i.e., to low temperatures, long day lengths or both; whereas short day plants might contain sufficient gibberellins bit lack anthesin.
Chailakhyan suggested that vernalization requiring plants produce vernalin when subjected to low temperatures and the vernalin is then converted to gibberellins in response to long days, at least in those plants that require exposure to long days after low temperature treatment.
Melchers grafting experiment with photo-inductive Maryland mammoth tobacco plant to anon-inductive henbane plant suggest the presence of two substances as proposed by Chailakhyan. Apparently, each contained one of the substances necessary for inducing flowering. Thus, they obtained these substances from each other when theses are grafted. Henbane was successful in getting the requiring substances whereas the tobacco was not.
However, in many vernalization requiring plants gibberellins failed to replace cold requirement for flowering. Later, on experimental evidence was provided suggesting that gibberellins are less important in flowering.
Factors or Conditions Necessary for Vernalization
Certain conditions or factors are necessary for vernalization. Since, vernalization is dependent on a sequence of steps leading to production of an active substance, the presence of these factors are necessary for the vernalization process. These factors also influence the efficiency of the vernalization process. Some of these important conditions are as followings:
Temperature and its Duration
Lang’s experiment with henbane showed relationship between temperature and time of exposure and the influence of this relationship on the efficiency of vernalization. He exposed vernalization requiring henbane to different temperatures from 3°C to 17°C for varying periods of time. The efficiency of low temperature to induce flowering was determined by the number of days the plant took after the treatment. Lang found that temperature range (3 to 17°C) is effective if the period of vernalization is 105 days. The flower initiation occurred in 8 days. But when the vernalization period was shortened to 15 days, different temperatures were effective differently. A temperature of 10°C initiating flowers in 23 days was most important. When the cold treatment period was extended to 42 days, the most effective temperature range was from 3 to 6 °C, requiring 10 days for flower initiation.
Hansel studied the effect of a wide range of temperature on Petkus rye. He found that vernalization fails before -4°C, from 1 to 7°C there is a shortening of number of days to flowering, and there is rapid fall in the rate of vernalization when the temperatures are increased from 7 to 15°C. Exposure to high temperature range result in devernalization.
The age of plant plays a pivotal role in its response vernalization. The age at which a plant is sensitive to vernalization differ from species. For example, in cereals germinating seeds and even embryo as can be receive the stimulus of vernalization, whereas certain biennials require a certain period of growth before responding to the stimulus of vernalization. For instance, Hyoscymus niger must have completed at least 10 days growth in the rosette stage, to be vernalized. The most effective time is when the plant is 30 days old. Other plants such as Oenothera can be vernalized only when at least six to eight leaves are present. The time when a plant is sensitive to vernalization is something referred to as ripeness-to-flower, a term originally used for photoperiod.
The need for a certain amount of vegetative growth to take place in plant so it becomes sensitive to low temperature suggests that some factor accumulates that perhaps receives vernalization stimulus. The substance may be synthesized during photosynthesis and in cases where seeds are vernalized during seedling stage this substance may already be present, either donated by the mother plant or synthesized during the development of the embryo.
An indication that this substance is produced during photosynthesis is provided during experiments with Arabidopsis thaliana. The seeds of this plants are most sensitive to vernalization and sensitivity decreases with the development of seedling. It reaches minimum when the seedling is two weeks old. But as the new leaves develop the sensitivity to low temperature increases once again. The loss of sensitivity may due to depletion of stored food of the seed and increase in sensitivity may be due to synthesis of carbohydrates as a result of photosynthesis. Evidence for the involvement of carbohydrates in vernalization has also been provided by Petkus winter rye embryos. When isolated embryos grown in a medium containing sucrose and minerals these proved to be sensitive to vernalization, but as the sucrose supplies are decreased the vernalization retarded.
Vernalization of dry seeds is impossible unless the seeds have imbibed some moisture.
Purvis pointed out that enough moisture must be present to initiate a small but visible degree of germination. She found that winter strain of Petkus rye must imbibe water 60% or 80% of the absolute dry weight for effective vernalization.
Experiments with grains showed that absence of oxygen make plants unresponsive to low temperature treatment, even if they are provided with adequate supply of water. It was also found that oxygen is also necessary for the vernalization of whole plants, such as henbane.
Both sugars and oxygen are required for the effect of low temperature. This requirement indicates the activation of some metabolic reaction essential for flowering. But in winter wheat (Triticum aestivum) formation of new proteins was found after vernalization. The protein pattern after vernalization resembles that of spring wheat does not require vernalization to flower. This suggests that proteins synthesis is another condition necessary for vernalization.
Nature of Vernalization Stimulus – Vernalin or Gibberellin