Plant Physiology

Step 1: Understanding the diagram.
The diagram shows a pigment (X) converting into a signal which leads to flowering. This represents the concept of photoperiodism, where light-sensitive pigments control flowering in plants.
Step 2: Identification of pigment ‘X’.
The pigment labelled ‘X’ is Phytochrome. It is a light-sensitive pigment that detects changes in light duration.
Step 3: Site of production.
Phytochrome is produced in the leaves of the plant, which perceive light signals.
Step 4: Mechanism of inducing flowering.
Phytochrome exists in two forms (active and inactive) and responds to light and dark periods. Based on day length, it triggers the production of a flowering hormone (florigen), which acts as a signal.
Step 5: Final effect.
This signal (florigen) is transported to the shoot apex, where it induces flowering in the plant.

Step 1: Understand seed dormancy.
Seed dormancy is a condition in which seeds do not germinate even under favourable conditions. It is an important mechanism that helps plants survive adverse environmental conditions.
Step 2: Identify the hormone responsible.
Abscisic acid (ABA) is known as a growth-inhibiting hormone. It plays a major role in inducing and maintaining seed dormancy by inhibiting germination.
Step 3: Analyze other hormones.

• (A) Auxin: Promotes cell elongation and growth.
• (B) Gibberellin: Promotes seed germination and growth.
• (C) Ethylene: Involved in fruit ripening.
• (D) Abscisic acid: Correct. It inhibits growth and maintains dormancy.

Step 4: Conclusion.
Therefore, the hormone responsible for maintaining seed dormancy is abscisic acid.
Final Answer:} Abscisic acid.