Question

The photoreceptor pigment responsible for sexual reproduction in plants (A) Phytochrome (B) Phototropism (C) Photoperiodism (D) Thigmotropism (E) Gravitropism

Short answer

The correct answer is (C) Photoperiodism.

Step-by-step solution

Identification of the Correct Answer

To find the right answer, understanding the definitions and roles of the given options is essential. (A) Phytochrome: A type of photoreceptor in plants used to detect light for regulating plant growth.(B) Phototropism: This is a process but not a pigment. It refers to the orientation of a plant in response to light.(C) Photoperiodism: This isn't a pigment but a physiological reaction plants have to the length of day or night. Importantly, photoperiodism is responsible for the seasonal timing of flowering and sexual reproduction in many plants. Therefore, this is the correct choice.(D) Thigmotropism: This refers to the directional growth of a plant in response to touch. (E) Gravitropism: The orientation or growth of a plant in response to gravity.

Key concepts

Phytochrome

Phytochromes are specialized photoreceptor pigments in plants that play a vital role in detecting light changes. These pigments help plants sense environmental light conditions and adjust their growth and development accordingly. They exist in two forms, known as Pr and Pfr, which can interconvert based on the light they absorb. Phytochromes are essential for:

• Regulating germination
• Promoting seedling development
• Initiating flowering

By detecting the red and far-red spectrum of light, phytochromes balance various life cycle stages. For example, a plant may remain dormant or begin to sprout depending on the levels of light detected by phytochromes, ensuring growth occurs at the optimal time.

Phototropism

Phototropism is the growth movement of plants towards or away from a light source. This process enables plants to optimize their energy absorption by orienting their leaves and stems toward the sun. Phototropism is crucial for maximizing photosynthesis, the process of converting light energy into chemical energy. Key points about phototropism include:

• Positive phototropism: Growth towards light, as seen in stems and leaves
• Negative phototropism: Growth away from light, often occurring in roots

The hormone auxin plays a significant role in this process by distributing unevenly in the plant, causing it to grow faster on the side with less light, hence bending towards the light source.

Photoperiodism

Photoperiodism refers to a plant's response to the length of day and night. This biological process allows plants to synchronize their growth and reproduction with seasonal changes, ensuring that they flower and produce fruits at the most advantageous time. Importantly, photoperiodism influences:

• Timing of flowering
• Leaf shedding
• Dormancy periods

Most plants are classified into three categories based on their photoperiodism response: short-day, long-day, and day-neutral plants. For instance, short-day plants require lengthy periods of darkness to flower, while long-day plants need short nights. This adaptation means plants can respond dynamically to their environment, promoting survival and reproduction efficiency.

Thigmotropism

Thigmotropism is the directional growth movement plants undergo in response to physical contact or touch stimuli. This type of growth response is especially noticeable in climbing plants, vines, and tendrils that wrap around objects for support. Thigmotropism is critical for:

• Structural support in climbing plants
• Protection from environmental stressors
• Maximizing space by utilizing territories occupied by other structures

The touch stimulus causes specialized cells to promote uneven growth, allowing plants to move and adjust themselves effectively. This unique ability enables plants to thrive in crowded or competitive habitats where gaining height or support can be essential for survival.

Gravitropism

Gravitropism, or geotropism, is the growth orientation of plants in response to gravity. Plants exhibit two distinct types of gravitropic responses:

• Positive gravitropism: Growth towards gravity, primarily seen in roots
• Negative gravitropism: Growth against gravity, often in stems and shoots

This growth pattern is a critical factor for anchoring the plant into the soil and guiding shoots or stems upwards to optimize sunlight absorption. Gravitropism ensures that roots effectively reach nutrients and water in the soil, while stems reach aboveground light. Gravity-sensing cells in the plant, especially in the root caps and stem tips, help guide these processes, facilitating balanced and optimal growth.