Photoperiodism, Vernalilzation and Seed Dormancy | Agriculture Optional Notes for UPSC PDF Download

Photoperiodism in Plants

• Plant Growth Regulators:
  • Plant growth regulators are organic compounds that play a crucial role in governing the growth and development of plants.
  • In addition to these phytohormones, external factors like temperature, day length, and exposure to light and darkness also significantly influence plant growth.
• Observations of Garner and Allard:
  • W.W. Garner and H.A. Allard made observations that not all plants require the same duration of light and darkness to initiate flowering.
  • Plants have specific light and dark phases, known as the critical period, during which flowering occurs.
  • Based on these findings, they categorized plants into three distinct groups.

Types of Plants Based on Photoperiodism:

• Long Day Plants:
  • These plants necessitate longer exposure to light compared to their critical period for flowering.
  • Examples include oats and wheat.
• Short Day Plants:
  • Short day plants require a shorter duration of light than their critical period for flowering.
  • Notable examples are Chrysanthemum and tobacco.
• Day-Neutral Plants:
  • The photoperiod does not affect the flowering of day-neutral plants.
  • Examples of such plants include maize and sunflower.

Definition of Photoperiodism

Photoperiodism is a phenomenon observed in a majority of plants, involving their developmental responses to specific durations of exposure to either daylight or darkness in order to initiate the process of flowering.

The uninterrupted dark phase is of equal significance in this context. Consequently, it can be stated that for many plants, the flowering process relies on the duration of their exposure to both light and darkness. This distinct responsiveness to light and dark conditions is termed photoperiodism.

Significance of Photoperiodism in Plants

Plants exhibit varying requirements for light exposure, which play a crucial role in their growth and development. Long-day plants are those that necessitate more extensive exposure to light, while short-day plants thrive with less exposure to light. The specific length of the day or night that triggers flowering varies from one plant species to another.

• Examples of Long-Day Plants: Long-day plants, such as irises, lettuce, radishes, and spinach, require a longer duration of daylight to induce their flowering.
• Examples of Short-Day Plants: Short-day plants, including Christmas cacti, poinsettias, and chrysanthemums, are those that initiate flowering with shorter exposure to daylight.

Vernalization Process

Vernalization is a term used to characterize the reliance of certain plants on being exposed to low temperatures in order to trigger flowering, either in terms of the quality or quantity of flowering. It acts to impede the development of the plant's reproductive organs during the growing season, allowing the plant to allocate sufficient time to reach its mature stage.

Importance of Vernalization

• Vernalization is a crucial process employed to cultivate early crops in regions where they would not typically thrive. It plays a significant role in expediting plant breeding.
• For instance, consider wheat, which comes in two varieties: winter and spring. Vernalization is applied to crops like beets, cabbage, turnips, and onions.
• The winter variety is sown during the autumn season. These plants undergo germination and emerge as small seedlings during the winter, continuing to grow in the spring, and they are ultimately harvested in mid-summer. Vernalization enables the successful cultivation of these crops in areas where they would not naturally flourish.

Seed Dormancy Definition

Seed dormancy is a state in which seeds are unable to sprout, even when the environmental conditions necessary for germination, such as temperature, water, gas, and light, are favorable. Several mechanical barriers, including seed coats, contribute to this prevention.

The primary reason for these conditions is that seeds need a period of dormancy or rest before they can successfully germinate. This dormant state can persist for varying durations, ranging from days to months or even years. These conditions involve a combination of factors like light, water, gases, heat, seed coats, and hormonal structures.

Types of Seed Dormancy

There are several distinct types of seed dormancy:

• Inherent Dormancy:
  • In this type of dormancy, seeds are unable to sprout even when provided with optimal conditions for seedling growth.
  • This inability to germinate may occur in certain species due to the immaturity of the embryo at the time of dispersal.
• Imposed Dormancy:
  • Imposed dormancy refers to a situation where seeds cannot germinate due to external factors in their environment.
  • These factors encompass the availability of sufficient moisture, light, oxygen, and an appropriate temperature.
• Induced Dormancy:
  • Induced dormancy occurs when a seed has absorbed water but is subsequently subjected to extremely unfavorable conditions for germination.
  • Ultimately, the seed fails to sprout even when placed under more favorable conditions.

Causes of Seed Dormancy

There are certain major causes of seed dormancy. Here are the few reasons for the seed dormancy that are mentioned below:

• Temperature
• Light
• Hard seed coat
• Germinating inhibitors
• 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

Difference between Photoperiodism and Vernalisation

Tabulated below are the important differences between photoperiodism and vernalization: