As you all know that the plant growth regulators are simple organic molecules that play a vital role in the growth and development of the plants. Apart from these plant hormones or phytohormones, the plant growth is also influenced by certain external factors such as the temperature, the length of a day, light and dark exposures, etc.
From the observations of W. W. Garner and H. A. Allard, it was noticed that every plant does not require the same intensity of light and darkness for their flowering. Plants require only a specific phase of the light and the darkness for their flowering and this term is known to be the critical period. Based on these observations, they classified plants into three different categories which are:
Long Day Plants: The plants that require longer exposure to the light than their critical period. For eg: oats, and wheat
Short Day Plants: The plants that require a shorter period than their critical period. For eg: Chrysanthemum, and tobacco
Day-Neutral Plants: The plants in which the flowering is unaffected by the photoperiod. For eg. maize, and sunflower.
In this article, we will learn about photoperiodism, dormancy and vernalization in plants in detail. Let us first take a look at what is photoperiodism in plants.
Photoperiodism refers to the phenomenon that is seen in most of the plants. It is the developmental responses of the plants wherein they are exposed either more towards the day or night for a specific length of day for inducing the flowering.
The continuation of the dark period is equally important. Hence, it can be said that in several plants, the flowering depends on the duration of the light and the darkness exposures. This specific response to the light and dark is known as photoperiodism.
The plants that require more exposure to the light are known as the long-day plants while those that require less exposure to the light are known as short-day plants. The length of day or night depends on plants to plants.
Examples of the long-day plants are irises, lettuce, radishes, spinach, etc. Christmas cactus and poinsettias chrysanthemums are a few examples of the short-day plants.
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The term vernalization is used to describe the dependence of specific plants on exposure to low temperature for the flowering qualitatively or quantitatively. It inhibits the development of the reproductive organs during the growing season and permits the plant to have adequate time for reaching its maturity stage.
Vernalization is the phenomenon that is practised for the production of the earlier crops. They are usually cultivated in the places wherein they naturally do not grow. Vernalization helps in the acceleration of the plant breeding.
For example: the wheat plants that have two varieties, winter and spring. Beet, cabbage, turnips and onions are examples of vernalization.
The winter variety is planted in the autumn season. They get time for germination and come out as small seedlings in the winter, then continue to grow in the spring and are harvested in the mid-summer.
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Seed dormancy is defined as the state or a condition in which the seeds are prevented from germination even under the favourable environmental conditions for germination which includes temperature, water, gas, light, seed coats, and several other mechanical restrictions.
The main reason behind the occurrence of these conditions is that they require the period of rest before being capable of the germination. This condition varies from several days to months and even years. These conditions are the combination of light, water, gases, heat, seed coats and the hormone structures.
The seed dormancy is of the following different types:
It is the condition of the seeds that are incapable of germinating even if the conditions suitable for the seedling growth are supplied. This inability to germinate can be due in some certain species to the embryo being immature during the time of the dispersal.
It is the condition of the seeds that are incapable of germinating because of an environmental restraint that includes an adequate amount of moisture, light, oxygen, and a suitable temperature.
This type of seed dormancy occurs when the seed has imbibed water but it has been placed under extremely unfavourable conditions for germinating. Finally, the seed fails to germinate even when it is kept under more favourable conditions.
There are certain major causes of seed dormancy. Here are the few reasons for the seed dormancy that are mentioned below:
Hard seed coat
Period after ripening
Immaturity of seed embryo
Impermeable seed coat to the oxygen
Impermeable seed coat to the water
Mechanical resistance of the seed coat
Presence of the higher concentrate solutes
1. What Is The Importance Of Seed Dormancy?
Seed dormancy is important in the following ways:
It follows the storage of the seeds for their later use by animals and humans.
It helps with the dispersal of the seeds via the unfavourable environment.
Dormancy that is induced by the inhibitors present in the seed coats is extremely useful to the desert plants.
It allows the seeds to continue to be in the suspended animation without any harm during the cold or the high summer temperature and even under the drought conditions.
Dormancy helps the seeds to remain alive in the soil for many years and provides a continuous source of the new plants when all the mature plants of that area have died down because of the natural disasters.
2. How Is Vernalization Different From Photoperiodism?
Vernalization refers to the process of inducing the flowering in plants when they are exposed to the low temperatures. On the contrary, photoperiodism refers to the process by which the flowering in the plants is dependant by the relative lengths of the dark and the light periods. Photoperiodism is needed to regulate the flowering in plants. On the contrary, vernalization allows the plant for reaching the vegetative maturity even before reproduction occurs.