Which of the following statements about phytochrome is not correct?
a. It is photoreversible.
b. Its function was understood long before the protein itself was isolated.
c. The Pfr form activates the responses to red light.
d. It is involved in guard-cell opening.

Verified step by step guidance

1. Understand the role of phytochrome: Phytochrome is a photoreceptor in plants that is sensitive to light in the red and far-red region of the electromagnetic spectrum. It plays a crucial role in regulating various plant developmental processes.
2. Review the concept of photoreversibility: Phytochrome exists in two interconvertible forms, Pr and Pfr. The Pr form absorbs red light and converts to Pfr, while the Pfr form absorbs far-red light and converts back to Pr. This photoreversibility is a key feature of phytochrome.
3. Examine the historical context: The function of phytochrome in plant development, such as seed germination and flowering, was studied and understood before the actual protein was isolated and characterized.
4. Analyze the role of Pfr: The Pfr form of phytochrome is the active form that triggers various physiological responses in plants, such as seed germination and flowering, in response to red light.
5. Consider the involvement in guard-cell opening: Phytochrome is primarily known for its role in processes like seed germination and flowering, but it is not typically associated with the opening of guard cells, which is more directly influenced by blue light receptors.

Key Concepts

Phytochrome

Phytochrome is a photoreceptor in plants that detects light and is crucial for regulating various developmental processes. It exists in two forms, Pr and Pfr, which are interconvertible depending on the light conditions. This photoreversibility allows plants to respond to changes in light quality, influencing processes like seed germination and flowering.

Photoreversibility

Photoreversibility refers to the ability of phytochrome to switch between two forms, Pr and Pfr, in response to red and far-red light. This reversible transformation enables plants to sense and adapt to their light environment, playing a critical role in processes such as photoperiodism and shade avoidance.

Guard-cell Function

Guard cells are specialized cells in the epidermis of plant leaves that regulate the opening and closing of stomata, controlling gas exchange and water loss. While phytochrome influences many light-dependent processes, its direct involvement in guard-cell opening is not well-established, making this statement potentially incorrect in the context of phytochrome's known functions.