# DBT JRF 2022 — Question 83

Exam: DBT JRF 2022 · Section: B · Topic: Plant Biology › Action Spectrum & Photobiology · Difficulty: Medium

## Question

A spectrum that depicts the magnitude of response of a biological system to light as a function of wavelength is called:

## Options

- **A.** Action spectrum
- **B.** Absorption spectrum
- **C.** Visible spectrum
- **D.** Hypothetical spectrum

## Answer

Option A

## Explanation

Action spectrum in plant physiology: An action spectrum is a graph that shows the relative effectiveness of different wavelengths of light in causing a specific biological response (photobiological process): • X-axis: wavelength (nm) • Y-axis: relative rate or magnitude of the biological response per unit photon energy Applications: • Photosynthesis action spectrum: peaks in red (~660–680 nm) and blue (~430–450 nm) — identifies chlorophyll a, b as primary pigments. • Photomorphogenesis action spectrum: peaks in red/far-red — identifies phytochromes. • Phototropism action spectrum: peaks in blue/UV-A — identifies phototropins. Distinction: • Absorption spectrum: shows which wavelengths a pigment absorbs (in vitro). • Action spectrum: shows which wavelengths actually DRIVE the biological process (in vivo). If action spectrum matches absorption spectrum → that pigment is responsible for the process. Answer: An action spectrum plots the effectiveness of different light wavelengths in inducing a biological response.

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