The photon intensity captured by green plants is used in the following processes:
(i) Photosynthesis
(ii) Generation of heat
(iii) Production of toxic products such as superoxide, singlet oxygen etc.
(iv) Damage to D1 protein of PSII
Based on the above facts, photoinhibiton will happen when the:
(1) entire photon intensity is used for photosynthesis
(2) excess photon intensity is completely used for heat generation
(3) excess photon intensity is used for heat generation and formation of toxic products
(4) excess photon intensity leads to the damage of D1 protein

Photoinhibition will happen when (4) excess photon intensity leads to the damage of D1 protein. Photoinhibition occurs when the light absorbed by plants exceeds the capacity for safe energy usage, resulting in the production of reactive oxygen species and direct damage to the D1 protein in PSII, which impairs photosynthetic activity.​

Photoinhibition: When Excess Light Damages PSII

Introduction

Key phrase: photoinhibition excess photon intensity D1 protein damage

Photoinhibition is a process where the photosynthetic machinery is compromised due to excess photon energy, particularly under intense light. The central event in photosystem II (PSII) is the damage of the D1 protein, an integral reaction center subunit, which leads to reduced photosynthetic capacity and efficiency.​

Explanation of Each Option

• (1) Entire photon intensity is used for photosynthesis

  • Incorrect. Plants rarely utilize all absorbed light for photosynthesis; there are upper limits to electron flow and energy processing. Excess light always exists under saturating or stress conditions.​

• (2) Excess photon intensity is completely used for heat generation

  • Incorrect. While non-photochemical quenching safely dissipates some excess energy as heat, it has capacity limits. Not all excess is converted to heat, especially under extreme light or environmental stress.​

• (3) Excess photon intensity is used for heat generation and formation of toxic products

  • Partially correct. Some excess energy forms reactive oxygen species (ROS) such as singlet oxygen and superoxide, but photoinhibition specifically refers to light-induced inactivation of photosynthetic capacity linked to direct damage of core proteins (not just ROS presence).​

• (4) Excess photon intensity leads to the damage of D1 protein

  • Correct. The hallmark of photoinhibition is inactivation of the D1 protein in PSII reaction centers. This damage often results from ROS formation when excess energy cannot be dissipated or utilized, directly impairing electron transport and requiring continuous D1 repair and replacement for recovery.​

Key Photoinhibition Mechanisms

• Photoinhibition is marked by D1 protein degradation and inhibited PSII repair.

• Reactive oxygen species causing oxidative damage and PSII center shutdown.

• Plants have repair cycles for D1 protein, but chronic excess light overwhelms repair efficiency.​