Photoperiodism – Definition, Types, Importance

What is Photoperiodism?

• Photoperiodism is the biological phenomenon by which organisms detect and respond to variations in day length. This response is critical for synchronizing physiological processes with seasonal changes. The underlying principle is based on photoperiod, which refers to the duration of light and dark periods in a 24-hour cycle. This cycle is influenced by the Earth’s rotation and its axial tilt, which results in different lengths of daylight and darkness across seasons. For instance, summer days typically feature extended daylight hours, while winter days are characterized by longer periods of darkness.
• In plants, photoperiodism governs developmental processes, including flowering and growth. Plants are categorized into three types based on their photoperiodic responses: short-day plants, which require long nights to flower; long-day plants, which need short nights; and day-neutral plants, which do not respond to changes in photoperiod. This classification highlights how plants use the length of day and night to time critical life stages.
• Similarly, in animals, photoperiodism, often termed seasonality, involves physiological adjustments in response to changing day lengths. These adjustments help animals adapt to seasonal variations, such as reproductive cycles and hibernation patterns, aligning their activities with environmental changes.
• Historical research into photoperiodism began in the early 20th century. Notably, in 1920, plant physiologists W.W. Garner and H.A. Allard conducted pioneering studies that elucidated this phenomenon. They investigated two plants: the Biloxi variety of soybeans (Glycine max) and the Maryland Mammoth tobacco (Nicotiana tabacum). Garner and Allard observed that the flowering timing in these plants was consistent regardless of the sowing date, indicating an internal mechanism regulated by day length.
• The Biloxi soybeans demonstrated that a plant’s growth and flowering could be influenced by the duration of light exposure. Conversely, the Maryland Mammoth, a variant of Maryland tobacco, illustrated that even within a species, different varieties could have contrasting photoperiodic requirements. The Maryland Mammoth, a large-leaved variant, flowers in winter, unlike its narrow-leaved counterpart that flowers in summer.

Classification of plants based on Photoperiodism

Plants are classified into several categories based on their response to the length of light and dark periods. These categories reflect how different species time their flowering and growth in accordance with seasonal day-length variations. The primary classifications include:

1. Short-Day Plants (SDP):
   • These plants, also known as long-night plants, flower when the day length is shorter than a certain critical threshold.
   • They typically bloom in early spring or autumn when nights are longer.
   • Examples: Biloxi variety of soybeans (Glycine max), Maryland Mammoth tobacco (Nicotiana tabacum), Cocklebur (Xanthium strumarium).
2. Long-Day Plants (LDP):
   • Also referred to as short-night plants, these species flower when day length exceeds a specific critical limit.
   • Their flowering usually occurs in late spring or early summer when days are longer.
   • Examples: Henbane (Hyoscyamus niger), onion (Allium cepa).
3. Day-Neutral Plants (DNP):
   • These plants are indifferent to the length of day or night for flowering.
   • They can flower under varying day lengths, from as little as 5 hours to as much as 24 hours of light.
   • Examples: Tomato (Solanum lycopersicum), cotton (Gossypium), sunflower (Helianthus annuus).
4. Long-Short Day Plants:
   • These plants are primarily short-day plants but require long days during their initial growth stages to trigger flowering.
   • Example: Bryophyllum species.
5. Short-Long Day Plants:
   • These are mainly long-day plants, but they need exposure to short days during their early growth to flower later.
   • Examples: Certain varieties of wheat (Triticum) and rye (Secale).
6. Intermediate-Day Plants:
   • These plants require a photoperiod that falls between the requirements for long-day and short-day plants for flowering.
   • Examples: Sugarcane (Saccharum) and Coleus (Plectranthus scutellarioides).

Photoperiodic Induction

Photoperiodic induction refers to the process by which plants are triggered to flower based on the relative length of day and night, known as the photoperiod. In a 24-hour cycle, an appropriate combination of light and dark periods constitutes one inductive cycle, which stimulates flowering. The number of inductive cycles required for flowering varies across plant species, and these cycles can be continuous or discontinuous depending on the plant’s specific needs.

Key points about photoperiodic induction include:

1. Inductive Cycles:
   • Plants require one or more inductive cycles to flower. An inductive cycle is the right combination of light and dark periods in a single day.
   • The number of inductive cycles necessary for flowering differs among species. For example, some plants may need only a single cycle, while others require several cycles.
2. Discontinuous Inductive Cycles:
   • Flowering can still occur if a plant experiences unfavourable photoperiods between inductive cycles. In this case, the inductive cycles are spread out over time but still lead to flowering.
   • This phenomenon is known as discontinuous induction, where a plant doesn’t receive continuous light-dark cycles but still achieves flowering after several breaks in favorable photoperiods.
3. Impact of Inductive Cycle Quantity:
   • Increasing the number of inductive cycles can accelerate flowering. For instance, Xanthium, a short-day plant (SDP), normally requires one inductive cycle and 64 days to flower. However, with four to eight inductive cycles, it can flower in as little as 13 days.
   • Continuous inductive cycles, where the plant receives consistent photoperiods without interruption, generally result in earlier flowering than discontinuous cycles.
4. Species-Specific Requirements:
   • Different species have varying needs for the number of inductive cycles. For example:
     • The Biloxi variety of soybeans (SDP) requires two inductive cycles for flowering.
     • Salvia occidentalis (LDP) requires 17 inductive cycles.
     • Plantago lanceolata (LDP) needs 25 inductive cycles to initiate flowering.

Photoperiodic Stimulus or Floral Hormone

Photoperiodic stimulus refers to the signal plants use to initiate flowering in response to changes in day length. This stimulus is perceived by the leaves, where a floral hormone, referred to as florigen, is produced. Florigen then moves to the apical tip of the plant, where it triggers the development of floral primordia, leading to flowering.

Key points regarding the photoperiodic stimulus and its role in plant flowering are as follows:

1. Perception of Photoperiodic Stimulus:
   • The stimulus is primarily perceived by the leaves of a plant. When leaves detect the appropriate photoperiod (length of day and night), they produce a floral hormone.
   • This hormone is then transported to the apical tip, initiating the formation of flowers.
   • Experiments on cocklebur (Xanthium pennsylvanicum), a short-day plant, demonstrated that defoliating the plant prevents flowering. However, flowering occurs if even one leaf remains. This emphasizes the critical role of leaves in photoperiod perception.
   • In long-day conditions, defoliation prevents flowering, but if one leaf is exposed to short-day conditions while the rest are in long-day conditions, the plant still flowers.
2. Transmission of Photoperiodic Stimulus:
   • Experiments show that the photoperiodic stimulus can be transmitted between different parts of the plant. In a two-branched cocklebur plant, if one branch is exposed to short-day conditions and the other to long-day conditions, flowering occurs in both branches.
   • The stimulus is clearly transmitted from one branch to another, proving that the photoperiodic signal moves within the plant.
   • However, if all leaves are removed from the branch in short-day conditions, no flowering occurs, showing that the presence of leaves is essential for perceiving the stimulus.
3. Translocation of the Stimulus:
   • Grafting experiments have further demonstrated the mobility of the photoperiodic signal. When a cocklebur plant exposed to short-day conditions is grafted to one under long-day conditions, flowering occurs in both plants, proving that the stimulus can translocate from one plant to another.
   • If both plants are kept in long-day conditions, flowering does not occur, indicating that the signal needs to originate from a plant exposed to the correct photoperiod.
4. Florigen – The Hypothetical Floral Hormone:
   • The term florigen was proposed by Mikhail Chailakhyan to describe the floral hormone produced in response to photoperiod. Despite extensive research, florigen remains a hypothetical hormone, as it has not yet been isolated.
   • Scientists suggest that florigen might be highly unstable, making extraction difficult.
   • Florigen is believed to be composed of gibberellin and anthocyanin, two compounds involved in flowering. Experiments show that applying gibberellin to long-day plants under short-day conditions can induce flowering, but the same does not hold true for short-day plants under long-day conditions.
   • This indicates that the hormone responsible for flowering may be composed of different factors depending on the plant’s photoperiodic requirements.
5. Role of Gibberellin and Anthesine:
   • Gibberellin and anthesine, though hypothetical, are believed to be the key components of florigen. In short-day plants, anthesine may be necessary for flowering, while in long-day plants, gibberellin appears to play a similar role.
   • These compounds together may form the photoperiodic stimulus, allowing plants to flower under the appropriate conditions.

Importance of Photoperiodism

Photoperiodism plays a crucial role in regulating various growth and reproductive processes in plants, particularly by determining the timing of flowering and fruiting based on the length of day and night. This phenomenon is essential for both agricultural productivity and natural plant cycles. The significance of photoperiodism can be explained through several key points:

1. Determination of Flowering and Fruiting Seasons:
   • Photoperiodism ensures that plants flower and produce fruit during optimal seasons. The length of the day acts as a signal, helping plants synchronize their reproductive stages with favorable environmental conditions, such as the availability of pollinators and suitable weather.
2. Enhanced Vegetative Growth:
   • In certain crops like radish and carrot, photoperiodism helps maintain a prolonged vegetative growth phase, delaying the transition to reproductive stages. This delay results in larger and more productive plants, contributing to higher crop yields. In contrast, it supports plants like pea in entering the reproductive phase at the right time, enhancing their seed production.
3. Facilitates Cross-breeding:
   • Photoperiodism is instrumental in the cross-breeding of plant varieties. By controlling the flowering time through photoperiodic manipulation, breeders can mix the traits of different varieties, producing new cultivars with improved characteristics. These new varieties often exhibit higher yields, greater pest resistance, and enhanced appeal to pollinators, leading to overall better plant performance.
4. Induction of Early Flowering – Vernalization:
   • Photoperiodism aids in early flowering through a process known as vernalization, where plants are exposed to prolonged cold temperatures. This cold treatment, combined with the right photoperiod, can trigger early flowering, a technique often used in agricultural settings to accelerate the production cycle of certain crops.
5. Year-round Flowering in Greenhouses:
   • By controlling light exposure, photoperiodism enables plants to flower throughout the year in controlled environments such as greenhouses. This is especially beneficial for horticultural industries, allowing growers to produce flowers and fruits outside of their natural growing seasons.

Difference between short day plants and long day plants

The following are the key differences between these two types of plants:

1. Photoperiod Requirement:
   • Short Day Plants (SDPs): These plants flower when the photoperiod (day length) is shorter than a critical length. They typically bloom in autumn or early spring when the nights are longer.
   • Long Day Plants (LDPs): These plants flower when the photoperiod is longer than a critical length. They usually bloom in late spring or summer when the days are longer.
2. Effect of Light Interruption:
   • SDPs: Interruption of the light period with darkness does not inhibit flowering in short day plants.
   • LDPs: In long day plants, flowering is inhibited if the light period is interrupted by darkness.
3. Effect of Dark Period Interruption:
   • SDPs: Flowering in short day plants is inhibited if the long dark period is interrupted by a flash of light.
   • LDPs: Flowering in long day plants is stimulated if the long dark period is interrupted by a flash of light.
4. Importance of Continuous Dark Period:
   • SDPs: A long, continuous, and uninterrupted dark period is critical for flowering in short day plants.
   • LDPs: The dark period is not as crucial for flowering in long day plants.
5. Response to Alternating Light and Dark Cycles:
   • SDPs: These plants will not flower under alternating cycles of short day and short light periods.
   • LDPs: Long day plants can flower under alternating cycles of short day followed by even shorter dark periods.
6. Phytochrome Response (Pfr/Pr Ratio):
   • SDPs: Flowering is inhibited when the accumulation of phytochrome in its active form (Pfr) is high, resulting in a Pfr/Pr ratio of less than 1.
   • LDPs: Flowering is stimulated by the accumulation of Pfr, with a Pfr/Pr ratio greater than 1.
7. Common Names:
   • SDPs: Also called “long night plants” because they require long nights for flowering.
   • LDPs: Also known as “short night plants,” since shorter nights are sufficient for their flowering.
8. Flowering in Complete Darkness:
   • SDPs: These plants can flower in complete darkness if provided with sufficient nutrients.
   • LDPs: Long day plants cannot flower in complete darkness, even if nutrients are supplied.
9. Seasonal Flowering:
   • SDPs: Typically flower in the autumn or early spring when nights are longer and days are shorter.
   • LDPs: Usually bloom in late spring or summer when days are longer and nights are shorter.
10. Examples:
    • SDPs: Chrysanthemum, sugarcane, strawberry, soybean.
    • LDPs: Radish, spinach, wheat, lettuce.