Red light induces seed germination in imbibed seeds, whereas far-red light inhibits it, as shown in the graph for this CSIR NET Life Sciences question. The correct option is A based on the described experiment with lettuce seeds. This follows phytochrome-mediated photoreversibility, where the final light exposure determines the outcome after 72 hours in darkness at 25°C.​

Option Analysis

A: Red light induces seed germination whereas far-red light inhibits it.
This matches classic lettuce seed experiments where red light (~660 nm) converts phytochrome to active Pfr form, promoting germination via gibberellin increase and abscisic acid decrease. Far-red light (~730 nm) reverts it to inactive Pr, inhibiting germination; the graph shows high % for red, low for far-red.​

B: Red light and far-red light inhibit seed germination.
Incorrect, as red light promotes (high bar), not inhibits; only far-red inhibits strongly under low fluence or continuous exposure.​

C: Red light inhibits seed germination whereas far-red light induces it.
Opposite of evidence; red induces (high % germination), far-red inhibits (low %).​

D: Both red and far-red light induce seed germination.
Wrong, as far-red inhibits; both do not induce equally per graph and phytochrome studies.​

Introduction to Light-Regulated Seed Germination

Red light induces seed germination far-red inhibits it through phytochrome photoreceptors, a core concept in plant physiology for CSIR NET aspirants. In lettuce (Lactuca sativa) experiments, imbibed seeds exposed to red light show high germination percentages after 72 hours darkness at 25°C, while far-red sharply reduces it, as graph bars indicate.​

Phytochrome Role in Red and Far-Red Light Effects

Phytochrome exists as Pr (absorbs red, converts to active Pfr) and Pfr (absorbs far-red, reverts to Pr). Red light induces seed germination far-red inhibits by shifting to Pfr, boosting gibberellins for embryo growth and testa weakening. Far-red maintains Pr, elevating abscisic acid to enforce dormancy, preventing endosperm rupture.​

• Red (~660 nm): Promotes ~80-90% germination in graphs.​

• Far-red (~730 nm): Inhibits to <20%, reversible by subsequent red.​

• Final exposure decides: R→FR→R germinates; R→FR inhibits.

Graph Interpretation for Exam Success

The figure displays four bars: high for red light (induces), low for far-red (inhibits), confirming option A. This low-fluence response (LFR) or very low-fluence (VLFR) aids seedling escape from soil/shade avoidance.​

CSIR NET Exam Tips

Master reversibility: Last light quality controls outcome. Practice similar MCQs on phytochrome in molecular biology units.​