How Can Sunlight Hurt Plants?

Since my previous article looked at some ways plants detect light, I thought it might be fun to look at what happens when plants get too much sunlight. If you've ever wondered what happens to shade plants when put into full sun, you'll probably find this interesting.

The specific questions we'll look at are:

How are plants damaged by sunlight, and how do they protect themselves?

The general outline for answering these questions is:

1. Photosynthesis Refresher
2. How Does Excess Light Causes Damage?
3. How Do Plants Protect Themselves?

Photosynthesis Refresher

We need to start this journey in the chloroplast, so here is a reference figure to orient yourself:

First, we need to consider that photosynthesis converts light energy into chemical energy. This process happens on the thylakoid membrane in the chloroplast during the light-dependent reactions as shown below:

During the light reactions photosystems I and II absorb energy which is used to concentrate hydrogen ions inside the thylakoids(i.e. in the thylakoid lumen). The result is potential energy, because the concentration of hydrogen ions is higher inside the thylakoid than outside [1].

The chloroplast takes advantage of the potential energy as these ions flow out of the thylakoid. The only path for them to exit is through a protein called ATP synthase. As the ions pass through, ATP synthase harnesses the potential energy to create adenosine triphosphate(ATP) from ADP and phosphate. ATP is then used as a source of chemical energy to power remaining steps of photosynthesis [1].

ADP/ATP cycle

How Does Excess Light Causes Damage?

Since sunlight is a form of energy, it makes sense that too much of it would cause some damage to the plant. This is similar to what you would expect if you plugged your computer into a greater power source than it was designed for.

The first line of damage occurs when the energy absorbed by chlorophyll cannot be directed toward photosynthesis. When this happens, the chlorophyll molecule can react with oxygen to form singlet oxygen [2]. These singlet oxygen molecules are very reactive and can cause major damage to components in the cell [3].

Another type of damage can occur if protein complexes absorbing light energy for photosynthesis are damaged. This directly results in a reduction in photosynthetic capability. When this happens it is called photoinhibition, because the photosynthetic process is inhibited by the damage [4]. The most vulnerable photosynthetic component is photosystem II which is a complex composed of many proteins as shown below:

The most vulnerable protein is named D1 [1][2]. Once the D1 proteins are damaged, it reduces the ability of the plant to concentrate the hydrogen ions inside the thylakoid lumen. So, the photosynthetic rate will be reduced, since it cannot use the potential energy to create ATP.

The only way for the plant to recover is to remove the damaged D1 proteins from the PSII complex and create new ones from scratch [1]. This can take sometime for a plant to recover from photoinhibition and it could very well die before it fully recovers.

How Do Plants Protect Themselves?

Plants evolved to be solar powered, so it's no surprise that they have some clever ways to avoid damage from too much solar energy hitting them. I'll be breaking these down into short-term responses and long-term responses focusing on developmental responses.

Short-Term Strategies

Chloroplast Movement

Even though plants are sessile organisms, it doesn't mean that parts of them can't move. For example, chloroplasts within leaves can actually be moved around! So, one simple way plants avoid excess light is to simply reduce the surface area of exposed chloroplasts by moving them within the leaves [5].

Heat Dissipation

Once excess light is hitting the chloroplasts, it's possible to just transform that absorbed energy into heat to remove it from the system. All of the mechanisms involved have not been elucidated and confirmed yet, but we can look at one of the more prominent examples discovered so far. It involves pigment molecules called carotenoids. These are the same class of molecules that give carrots their color.

More specifically, a subclass of carotenoids called xanthophylls are involved in a process that converts excess energy into heat. The system consists of three xanthophyll molecules called violaxanthin, antheraxanthin, and zeaxanthin. Enzymes can interconvert these molecules forming a cycle[6]:

violaxanthin --> antheraxanthin --> zeaxanthin

The cycle goes from violaxanthin to zeaxanthin under high light and backwards under low light. There is evidence that zeaxanthin can release heat when it interacts with excited chlorophyll molecules [7].

Long-Term Strategies

There are also some strategies plants use to acclimate to living in environments with high sun. Acclimation refers to changes in the development of a plant to optimize to its environment.

One response is to change the shape of its leaves. Typically, leaves acclimated to high light conditions will be thicker than shade leaves [1]. I think this makes sense if you think back to the chloroplast movement example above. There are probably other reasons this is helpful as well though.

Another way plants acclimate to high light conditions is to adjust the concentrations of chlorophyll and xanthophyll in their leaves. Leaves acclimatd to high light often have lower concentrations of chlorophyll and higher concentrations of xanthophylls compared to shade leaves [1].

---

Thank you for reading and I hope everyone has a great evening.

---

Sources

Images

Chloroplast diagram: https://commons.wikimedia.org/wiki/File:Chloroplast_(borderless_version)-en.svg

Thylakoid membrane diagram: https://commons.wikimedia.org/wiki/File:Thylakoid_membrane_3.svg

ATP cycle: https://fr.m.wikipedia.org/wiki/Fichier:ADP_ATP_cycle.png

Photosystem II: https://commons.wikimedia.org/wiki/File:Photosystem-II_2AXT.PNG

Chloroplast movement: https://commons.wikimedia.org/wiki/File:Chloroplast_movement.svg

Carrot: https://commons.wikimedia.org/wiki/File:Carrot-fb.jpg

Xanthophyll cycle: https://commons.wikimedia.org/wiki/File:Violaxanthin_cycle.png

References

[1] Taiz, Lincoln, et al. Plant Physiology and Development. Sinauer Associates, Inc., Publishers, 2015.

[2] Krieger-Liszkay, A. “Singlet Oxygen Production in Photosynthesis.” Journal of Experimental Botany 56, no. 411 (November 29, 2004): 337–46. doi:10.1093/jxb/erh237.

[3] Triantaphylides, C., M. Krischke, F. A. Hoeberichts, B. Ksas, G. Gresser, M. Havaux, F. Van Breusegem, and M. J. Mueller. “Singlet Oxygen Is the Major Reactive Oxygen Species Involved in Photooxidative Damage to Plants.” PLANT PHYSIOLOGY 148, no. 2 (August 8, 2008): 960–68. doi:10.1104/pp.108.125690.

[4] Powles, Stephen B. "Photoinhibition of photosynthesis induced by visible light." Annual Review of Plant Physiology 35, no. 1 (1984): 15-44.

[5] Park, Youn-Il, Wah Soon Chow, and Jan M. Anderson. “Chloroplast Movement in the Shade Plant Tradescantia Albiflora Helps Protect Photosystem II against Light Stress.” Plant Physiology 111, no. 3 (1996): 867–875.

[6] Niyogi, Krishna K., Arthur R. Grossman, and Olle Björkman. “Arabidopsis Mutants Define a Central Role for the Xanthophyll Cycle in the Regulation of Photosynthetic Energy Conversion.” The Plant Cell 10, no. 7 (1998): 1121–1134.

[7] Horton, P. “Molecular Design of the Photosystem II Light-Harvesting Antenna: Photosynthesis and Photoprotection.” Journal of Experimental Botany 56, no. 411 (November 29, 2004): 365–73. doi:10.1093/jxb/eri023.