The active form of phytochrome is Pfr (option 1).

Option Breakdown

Phytochrome is a photoreceptor pigment in plants with two main interconvertible forms that respond to red and far-red light.

• (1) Pfr: This is the far-red absorbing form (peak at ~730 nm), biologically active, and triggers responses like seed germination, flowering, and stem elongation when produced from Pr via red light.

• (2) Pr: The red absorbing form (peak at ~660 nm), inactive, stable in the dark, and reverts to Pfr under red light but shows no physiological activity itself.

• (3) Fpr: Not a standard phytochrome form; likely a distractor or misspelling with no biological role in phytochrome signaling.​

• (4) Rp: No recognized form in phytochrome literature; possibly confuses with Pr but lacks validity.​

The active form of phytochrome plays a crucial role in plant photobiology, sensing red and far-red light to regulate growth processes like germination and flowering. Known as Pfr, this form converts from the inactive Pr under red light (~660 nm), enabling key physiological responses essential for competitive exams like GATE Life Sciences.

Phytochrome Forms and Light Conversion

Phytochrome exists primarily in two reversible forms: Pr (inactive, red-absorbing) and Pfr (active, far-red absorbing). Red light shifts Pr to Pfr, the active form of phytochrome, while far-red light or darkness reverts it to Pr. This cycle, called the phytochrome photostationary state, helps plants detect light quality through the canopy.

• Pfr (Active Form): Olive-green, peaks at 730 nm, moves to nucleus or membranes to activate genes for de-etiolation and shade avoidance.

• Pr (Inactive): Blue-red, peaks at 660 nm, cytosolic and non-responsive without conversion.

• Fpr and Rp: Invalid options; no evidence in plant biology texts, often exam distractors.​

Biological Roles of Active Pfr

As the active form of phytochrome, Pfr initiates responses such as:

• Promoting seed germination (e.g., lettuce requires brief red light).

• Inhibiting hypocotyl elongation in light-grown seedlings.

• Triggering photoperiodic flowering in long-day plants.

In shade, elevated far-red reverses Pfr to Pr, promoting internode growth. This reversible system underscores phytochrome’s role in photomorphogenesis.​

Exam Tips for Phytochrome MCQs

For questions like “The active form of phytochrome is:”, memorize Pfr as the trigger for responses. Practice reversibility: Red → Pfr (active); Far-red → Pr (inactive). Relate to plant physiology chapters in NCERT or GATE syllabi.