15. Plants dissipate excess excitation light energy as heat so as to protect from photo-oxidative damage. The mechanism is known as
(1) Photo chemical quenching
(2) Non Photochemical quenching
(3) Photo-inhibition
(4) Red drop effect

The mechanism is (2) Non Photochemical Quenching (NPQ). Plants use NPQ to dissipate excess excitation energy as heat, protecting themselves from photo-oxidative damage when light energy exceeds their photosynthetic capacity.​

Non Photochemical Quenching: Plant Protection from Excess Light

Introduction

Key phrase: non photochemical quenching protection excess light plants

Non Photochemical Quenching (NPQ) is a vital photoprotective mechanism plants use to defend against photo-oxidative damage in high light conditions. By harmlessly converting excessive light energy into heat through molecular vibrations, NPQ ensures the efficiency and survival of photosynthetic organisms in fluctuating environments.​

Explanation of Each Option

• (1) Photochemical quenching

  • Incorrect. Photochemical quenching refers to the use of absorbed light energy for productive photosynthetic electron transport, leading to ATP and NADPH formation rather than protective heat dissipation.​

• (2) Non Photochemical quenching

  • Correct. NPQ enables plants to dissipate excessive excitation energy as heat, primarily within the PSII light-harvesting antenna, helping prevent damage to photosynthetic machinery by reducing excited-state chlorophyll lifetimes and limiting reactive oxygen species generation.​

• (3) Photo-inhibition

  • Incorrect. Photo-inhibition is the result of photodamage to the photosynthetic apparatus due to excess light, not a protective mechanism itself.​

• (4) Red drop effect

  • Incorrect. The “red drop effect” describes the sharp decrease in photosynthetic efficiency observed at wavelengths longer than 680 nm, unrelated to photoprotection or NPQ.​

Key NPQ Facts

• NPQ involves rapid and reversible changes in the PSII antenna complexes, Xanthophyll cycle pigments (zeaxanthin, violaxanthin), and proton gradient across thylakoid membranes.​

• NPQ is essential for plant fitness and high crop yields under fluctuating light.​

• The most effective NPQ component, qE, responds within seconds to excess light.​