Can Plants Get Too Much Light? - The Science of Scorching

 This article explains the science behind light damage in plants. 

Photosynthesis is a complex, delicately balanced, and highly adaptive process. The rate at which it occurs varies between plants, as well as on the time of day it is taking place. 

Plants adapt their photosynthetic rate according to changing environmental conditions, including excess sunlight. When there is too much light, the chlorophyll which processes photon energy into biochemical energy will redirect excess photon energy to carotenoids. The carotenoids release this photon energy as heat. This protective process is called non-photochemical quenching. 

However, there is a limit to which excess light can be dissipated as heat. Damaging levels of sunlight will affect the photosynthetic ability of the plant. This is called photoinhibition. Once the photosynthesis apparatus can no longer function efficiently, plant growth and productivity become limited.

How does photosynthesis work?

The photosynthetic process is made up of two parts: photosystem I and photosystem II. These processes rely on two different light-capturing structures. Both sit in the thylakoids, flattened sacs inside a chloroplast. 

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Photosystem II

Photosystem II produces NADPH. Photosystem II revolves around chlorophyll molecules called P680. The process begins when photon energy agitates the P680 chlorophyll molecules. The electrons that are energized and transported through photosystem II are used to create NADPH. NADPH is required to carry electrons and protons toward creating sugar molecules. 

Once this has occurred, photosystem II replenishes the dislodged electrons by taking up new ones from water. It does so by splitting oxygen molecules from water - this is called oxidization. Photosystem II is responsible for most of the oxygen in our planet’s atmosphere. Hydrogen from the water is also released as a byproduct.

Photosystem I

Photosystem I is the second component of photosynthesis. It produces sugars called ATP and involves chlorophyll molecules called P700. When photos hit the P700 molecules, electrons are displaced and jump toward neighboring molecules. In the process, the electrons move from a high to a lower energy state. The energy released pumps hydrogen protons created by photosystem II out from the stroma (the fluid inside the chloroplast) and into the lumen (a structure within the chloroplast), lowering the density of hydrogen protons in the stroma. This difference in hydrogen proton density between the stroma and lumen is called the proton gradient. Once the lumen is emptied of hydrogen protons, hydrogen protons are once again drawn back into the stroma to equalize the density. The movement of protons between the lumen and stroma induced by the proton gradient provides the energy required to generate NADPH.  

How does excess light affect photosynthesis?

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Prolonged exposure to excessive light will decrease photosynthesis, damage the photosynthetic apparatus, and damage components responsible for the repair of the photosynthesis system.

While chlorophyll is efficient at absorbing light, the other processes involved in turning light into biochemical energy are slower. With excess photon energy, there can be a build-up of unprocessed electrons. This is called photooxidation and will cause bleached leaf tissue on the plant. 

The apparatus of photosystem II is more susceptible to light damage than photosystem I. When the plant receives more photon energy than it can process, ‘reactive oxygen species (ROS) are created. These are chemically unstable molecules that react with other molecules. They are what damages photosystem II and its repair systems. 

The part of its photosynthetic apparatus most likely to be damaged by excess light is called the ‘D1’ subunit. Usually, there is a rapid turnover of D1 proteins, but excess light prevents new D1 proteins from being synthesized. Excess light also damages MPH2, a protein responsible for repairing photosystem II.