The low temperature treatment given to the seed or plant for stimulating flowering in them is called vernalization.
Some seeds required low temperature treatment to break dormancy. Similarly shoot bud dormancy in some plants like Acer and Betula is overcome by low temperature. But the low temperature requirement for processes other than flowering is not taken as vernalization.
Seed is given low temperature treatment during vernalization. Its effect persists and flowering takes place much later. Thus the effect of vernalization is iuductive in nature. Therefore, the plant need not be exposed to low temperature at the time of flowering.
Discovery of vernalization
J. Gustav Gassner (1928) was a German plant physiologist. He was working with rye (Secak cereale) found that temperature is critical during early stages of seed germination. He showed flowering and fruiting depends on temperatures during this critical stage. There are two varieties of rye:
- Winter Petkus rye: Winter Petkus rye is sown in the field in the fall of the year. It germinates and develops a few leaves. Then winter comes. So further growth is checked for several months due to low winter temperatures. Some plants are even covered by snow. The temperature becomes moderate in the spring. Now the plants resume growth and produce a grain crop.
- Spring Petkus rye: The spring Petkus rye is sown in the spring. It germinates immediately and its vegetative and reproductive growth occurred. It is harvested in winter.Experiment of Gassner.He planted winter rye in spring. The seeds germinate and produce vegetative plants. But they did not flower until late in the growing season. The winter variety requires low temperature. Gassrier found that winter Petkus rye could be given low temperature requirement artificially. He placed imbibed seeds in chilling temperatures between 2 to 5 °C for 5 to 6 weeks. He planted the chilled seeds in the spring. They grew and flowered like ordinary Petkus rye. Thus he discovered the phenomenon of vernalization.
Vernalization in Annual and Biennial Forms(Qualitative and Quantitative Vernalization)
- There are two types of plants:
- Annual forms (Quantitative vernalization)
The plants which survive one growing season are called annual plants. Its examples are rye, pea and gram. They are quantitative in nature. These plants can flower without vernalization. But vernalization develops the flowering earlier in these plants. For example Petkus rye flowers in 15 weeks without vernalization. But it takes only 7 weeks when it is given low temperature treatment.
- Biennial forms (Qualitative vernalization)
The plants which survive two growing seasons are called biennial plants. Its example is Hyoscyamus.. It has qualitative vernalization requirement. Their first growing season is before winter. They are sown in summer. They develop vegetative body in first growing season. They store food in their roots. Then winter comes. Vegetative plant dies. After winter the under ground root give rise to new plant body. It is their second growing season. The plant develops flowers and seeds. It is harvested in spring. These plants fail to flower without vernalization. They remain vegetative indefinitely without vernalization.
SITE OF VERNALIZATION
There are two sites of vernalization:
I. Stem tip: The localized cooling treatment was given to stem tip. It receives the vernalization effect. The stem tip is chilled. The rest of the plant may be kept at high temperature. It vernalizes all the plants.
2. Seed: Seeds are also vernalized before sowing. The embryo in seed receives the vernalization stimulus. Then seed is sown. It produces flowers–..
- Root tips and leaves: The root tips and leaves of Lunaira biennia contain dividing cells. Therefore, its root tip and leaves can be vemalized. Thus, divididg cells are necessary for vernalization.