In this article we will discuss about:- 1. Definition of Vernalization 2. Site of Vernalization 3. Nature of Vernalization Process 4. Application.
Definition of Vernalization:
The precise definition of vernalization is not universally accepted. The term is best defined as the specific promotion of flowering by a cold treatment given to the imbibed seed or young plant. In certain seeds, and buds low temperature is required to break their dormancy but these responses are not included in our definition since they do not relate to flowering.
Temperature also affects floral initiation in some species but these are easily distinguishable from vernalization, which is an inductive process. After vernalization, floral buds are initiated after a specific photoperiod or when the plant is brought back to a specific temperature.
However, in some species of Brassica the biochemical changes that occur in response to cold and specific photoperiod are comparable. In general long day plants require vernalization and these may be annuals, biennials, or even perennials. If these plants are not exposed to cold, their flowering is delayed or may even fail.
In fact the effect of low temperature enhances with the exposure of exposure until there is saturation of response at a duration which is variable with species. The effective temperature varies widely e.g. from freezing to 10°C. At sub- optimal low temperatures, once the duration of exposure is enhanced, vernalization is complete.
Vernalization implies the conversion of winter varieties to the summer varieties by cold treatment. The low temperature treatment shortens the vegetative period and promotes flowering. Russian geneticist T.D. Lysenko observed that artificial cold treatment of seeds of winter wheat permitted them to behave like spring wheat plants in spring. This process is called vernalization.
In the annual plant, growth is started in the spring, flowers are developed in the summer and fruits and seeds are produced in the fall of the same year. The flowering is primarily under the control of photoperiod and the influence of temperature is secondary. Biennials on the other hand need a cold winter before flowering in its second growing season.
Without subjecting to a cold treatment, the majority of these plants maintain their vegetative stage. Hence they have a qualitative or an absolute requirement for cold e.g. Hyoscyamusniger. However, in Secalecereale the requirement is quantitative or facultative. For instance, under continuous light unvernalized winter rye will flower in 15 weeks, and if vernalized flowering is observed in 7 ⅟2 weeks. Obviously in this variety, vernalization shortens the time to flower.
Table 22-2 includes species which require vernalization:
For most biennials an “artificial” cold treatment followed by the correct photoperiod and temperature treatment will cause flowering during the first growing season. Thus in a biennial species, the flowering may be induced in the same time period as required for flowering of annuals.
The stage of vernalization also varies. For example, Secalecereale is vernalized in the seed stage while Hyoscyamus has to be vernalized when the plant is 10 days old and in the rosette stage. This is done by soaking and sprouting the seed in winter and keeping them in a vegetative stage by a low temperature or freezing until it can be shown in spring.
Flowering of many biennials is promoted by LD treatment following vernalization as in Hyocyamusniger. Other biennials are day neutral following low temperature treatment. In perennials grasses flowering is promoted by SD treatment following the low temperatures.
Thus, many different kinds of plants are stimulated to flower by cold periods which may have qualitative or quantitative effect. The flowering of some species also is promoted by a suitable day length.
Site of Vernalization:
Whenever vernalization occurs in the mature plants, the receptor of stimulus is the stem apex. In other words, the meristematic cells in the bud respond to the cold treatment. Thus, if a hormone-like substance is produced following vernalization, this occurs in the same cells in which it acts.
This fact is borne by the following experiment. If a plant is vernalized and its meristem is grafted into an unvernalized plant, the latter will flower, indicating as if it had been vernalized.
Conversely, if a meristem of an unvernalized plant is grafted onto vernalized plant where the vernalized meristem was removed, the transplanted, (unvernalized) meristem remains vegetative. Obviously vernalization is restricted to the meristematic tissues themselves.
Some workers have reported that excised leaves or isolated roots of Lunarisbiennis could also be vernalized. On close examination it was observed that new buds were formed on the leaf petioles.
Further these meristematic tissues had to appear before the vernalization was effective. It was proposed that vernalization requires not only the presence of a meristem but also the actively dividing cells. Indeed, replicating DNA was necessary for the perception of vernalization, no matter what their location in the plant was.
It is also suggested that the vernalization process requires energy. Thus, low temperature is effective only in the presence of oxygen, energy substrate (sucrose) and water contents at 40%. The percentage of flowering depends upon the duration of cold treatment, temperature used and the age of plant which varies with species.
New proteins appear following cold treatment. There is also synthesis of new mRNA. Gregory and Purvis have shown that embryo by itself can also be vernalized. In short receptor or perceptive mechanism of vernalization seems to be present in different parts of different plants.
Nature of Vernalization Process:
According to Chailakhyan, possibly two substances are involved in flower formation, one is GA or GA-like material and the other is a different compound. According to him, low temperature and LD requiring plants lack sufficient GA but have enough of the flower-inducing hormone, while SD plants contain sufficient GA but lack the flower-inducing hormone.
This point of view is supported by Melchers studies where a non-cold requiring (SD) tobacco plant (Maryland Mammoth) was grafted to a non-induced cold requiring long day Hyoscyamus plant and this induced the latter to flower.
Seemingly both the species contained one of the essential substances for the flowering process but was dependent on the other plant with which it was grafted for the second. This succeeded for the Hyoscyamus but not for the tobacco.
O.N. Purvis, the British physiologist, has proposed the following scheme of flowering in cereal plants (Fig. 22-14):
In this scheme, B is a compound that is part of a reaction system leading to flowering. D is a flowering hormone and C is an intermediate capable of initiating early stages in flowering initiation. The reaction system from B to D is controlled by photoperiod and leads to the production of floral hormone D.
In spring rye (an annual) B is either present in the embryo or is produced from A at normal temperature. In winter rye (biennial) B is insufficient. Exposure to low temperature accelerates its production. In the spring rye or vernalized winter rye, B accumulates in abundance.
Under long day conditions, B is slowly converted to C and C is rapidly converted to D, the flower hormone. Ultimately D reaches a critical stage and flowering ensues, while under short day condition the reaction C to D is inhibited thus forcing the back reaction, C to B to A to occur keeping the plant vegetative.
Application of Vernalization:
The shortening of vegetative period by vernalization in winter cereals enables to have the yield in the first year and helps them to escape drought in the regions having late summer drought.
Late varieties of wheat and oat can be cultivated in northern latitudes with short summer as vernalization helps fruiting in the short summer. Vernalization also increases yield of crop plants, removes wrinkleness of the grains and also enables to obtain flowering out of season.
In India several crops have been vernalized with varied success. For instance in rice, late Professor S. M. Sircar induced some delayed flowering following vernalization. Earlier Kar and Adhikary observed that presowing and high temperature induced early flowering in some varieties.
Presowing cold treatment of Corchoruscapsularis seeds caused delayed seed germination but produced high chlorophyll contents in the leaves and more healthy plants. When the seeds were subjected to prolonged low temperature it caused early flowering and also fruiting.
Some interesting results have been obtained on vernalization of gram, pea, mustard and linseed at different centres. The treatments were either given to the seeds, just emerged seedlings or even sprouted seeds.
The role of vernalization in reducing kernel shrivelling in triticale, a new synthetic genus and a polyploidy hybrid produced by cross breeding wheat and rye, has been shown. The basic mechanisms which regulate the kernel development were correlated with the vernalization response.
The lowering of α-amylase activity with high accumulation of endosperm starch leading to the reduction in kernel shrivelling could be obliterated through vernalization.