20. Which of the following mechanisms is NOT involved in providing photo protection to plants?
(1) Degradation of 01 protein.
(2) Zeaxanthin formation.
(3) Photolysis of water.
(4) Thermal dissipation

The mechanism that is NOT involved in providing photo protection to plants is (1) Degradation of D1 protein. D1 protein degradation is a marker of photodamage and photoinhibition, not a protective mechanism; instead, it signals the breakdown and renewal required due to stress or excessive light.​

Photoprotection Mechanisms in Plants: What Works and What Doesn’t

Introduction

Key phrase: photoprotection mechanisms in plants D1 protein degradation zeaxanthin thermal dissipation

Plants must defend themselves from light-induced stress with sophisticated photoprotective strategies. These include dissipating excess energy as heat (thermal dissipation), conversion of xanthophyll carotenoids (zeaxanthin formation), and rapid repair cycles. Understanding these mechanisms is key for improving crop resilience and photosynthetic efficiency.​

Explanation of Each Option

• (1) Degradation of D1 protein

  • Correct (as NOT protective). The D1 reaction center protein of PSII is highly susceptible to photodamage from excess light and must be regularly degraded and replaced during photoinhibition. This is a repair response, not a true photoprotection mechanism, as the protein’s initial breakdown signals failure to effectively dissipate damaging energy.​

• (2) Zeaxanthin formation

  • Protective. Zeaxanthin is created from violaxanthin during high light stress and enables non-photochemical quenching (NPQ), directly aiding plants to harmlessly dissipate excess excitation energy as heat in PSII.​

• (3) Photolysis of water

  • Protective. The splitting of water in PSII is a core process of photosynthesis; while not a classic photoprotective mechanism, its continuous operation ensures smooth electron flow and helps maintain energy balance.​

• (4) Thermal dissipation

  • Protective. Also known as non-photochemical quenching, thermal dissipation allows plants to convert dangerous excess energy into heat, preventing potential photo-oxidative damage and enzyme breakdown.​

Key Facts About Plant Photo Protection

• Zeaxanthin and thermal energy dissipation (NPQ) neutralize excitation stress.

• D1 protein turnover is a consequence of damage, not a photoprotective process.

• Efficient repair and protection together sustain healthy photosynthetic function.​