What is photoperiodism? Write at least one example of shortday plant, longday plant and day neutral plant.

What is Photoperiodism?

Photoperiodism is the ability of plants to sense and respond to the length of daylight hours. This response is mediated by phytochrome, a photoreceptor pigment that exists in two interconvertible forms: Pr (inactive) and Pfr (active). The ratio of red (R) to far-red (FR) light determines the equilibrium between these forms. During daylight, red light converts Pr to Pfr, while during night, Pfr reverts to Pr. The duration of the dark period is critical in determining the plant's response.

Types of Plants Based on Photoperiodic Response

Plants are broadly classified into three categories based on their photoperiodic responses:

Short-Day Plants (SDPs)

SDPs flower when the day length is shorter than a critical threshold, or when the night length exceeds a specific duration. They are often referred to as "long-night plants" because flowering is triggered by extended darkness.

• Mechanism: In SDPs, the phytochrome system senses the length of the dark period. A long night period leads to a high concentration of Pr, which inhibits flowering.
• Example: Chrysanthemums (Chrysanthemum morifolium) are classic examples of SDPs. They require nights of 12-14 hours to flower. Commercial chrysanthemum production often involves artificially shortening day length to induce flowering out of season.
• Other Examples: Poinsettias, Strawberries, Rice

Long-Day Plants (LDPs)

LDPs flower when the day length is longer than a critical threshold, or when the night length is shorter than a specific duration. They are often called "short-night plants" because flowering is triggered by a brief period of darkness.

• Mechanism: In LDPs, a short night period allows for the accumulation of Pfr, which promotes flowering. A long night period reverts Pfr to Pr, inhibiting flowering.
• Example: Spinach (Spinacia oleracea) is a well-known LDP. It requires days of 14-16 hours to flower. Farmers in regions with shorter daylight hours often use artificial lighting to extend the day length and induce flowering.
• Other Examples: Lettuce, Radish, Barley

Day-Neutral Plants (DNPs)

DNPs flower irrespective of day length. Their flowering is primarily influenced by other factors like maturity, temperature, and moisture.

• Mechanism: DNPs do not have a photoperiodic response; their flowering is not directly controlled by day length.
• Example: Tomatoes (Solanum lycopersicum) are a common example of DNPs. Their flowering is influenced more by the age of the plant and environmental conditions than by the length of the day.
• Other Examples: Corn, Cucumbers, Sunflowers

Plant Type	Critical Day Length	Night Length Requirement	Example
Short-Day Plant	Shorter than critical threshold	Longer than critical duration	Chrysanthemum
Long-Day Plant	Longer than critical threshold	Shorter than critical duration	Spinach
Day-Neutral Plant	No effect	No effect	Tomato

Applications and Significance

Understanding photoperiodism has significant implications for agriculture:

• Crop Improvement: Plant breeders can manipulate photoperiodic genes to develop varieties that flower at specific times of the year, aligning with desired harvest schedules.
• Greenhouse Cultivation: Artificial lighting and shading can be used to control day length in greenhouses, allowing for year-round production of crops that are normally seasonal.
• Ornamental Horticulture: Photoperiodism is exploited to control flowering in ornamental plants, ensuring blooms at desired times for commercial sales.

The Indian Council of Agricultural Research (ICAR) actively researches photoperiodism in various crops to improve productivity and adapt to changing climatic conditions. Recent research focuses on understanding the genetic mechanisms underlying photoperiodic responses in rice and other staple crops.