Question

Which one of the following statements about a plant carotenoid is FALSE?

• A. It is an accessory pigment which absorbs light at 600 - 700 nm.
• B. It protects chlorophyll a from photo-oxidation.
• C. It provides precursor for the synthesis of stress hormone in plants.
• D. It accumulates in chromoplasts during fruit ripening.

Hint:

Remember that pigments appear as the color they reflect.
Since carotenoids are yellow, orange, and red, they reflect wavelengths in the $600\text{--}700$ nm range.
Consequently, they must absorb in the opposite complementary region, which is the blue-green light range ($400\text{--}500$ nm).

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The question requires identifying the incorrect (false) statement regarding carotenoids, which are a class of accessory pigments found in plants.

Step 2 : Key Formulas and Approach:
The approach involves evaluating the physiological functions, biosynthetic pathways, and light absorption characteristics of plant carotenoids.

Step 3 : Detailed Explanation:

• Absorption Spectrum: Carotenoids (such as carotenes and xanthophylls) are accessory pigments that absorb light primarily in the blue-violet and blue-green regions of the spectrum (approximately $400\text{--}500$ nm).
The red-light region ($600\text{--}700$ nm) is absorbed primarily by chlorophyll pigments (chlorophyll a and b). Thus, stating that carotenoids absorb light at $600\text{--}700$ nm is incorrect.

• Photo-protection: Carotenoids act as antioxidants and quench singlet oxygen and excess excitation energy, protecting chlorophyll a molecules from damage caused by photo-oxidation. This statement is correct.

• Hormone Precursor: Carotenoids (specifically violaxanthin) serve as biosynthetic precursors for abscisic acid (ABA), which is the primary plant stress hormone. This statement is correct.

• Accumulation in Chromoplasts: During the ripening of fruits and senescence of leaves, chloroplasts transition into chromoplasts, which synthesize and accumulate high levels of carotenoids, giving them yellow, orange, or red colors. This statement is correct.

Step 4 : Final Answer:
The false statement is Option (A), as carotenoids do not absorb light in the $600\text{--}700$ nm range.