A red pigment called anthocyanin has been considered a sunscreen that protects plants from becoming sunburned, much like the sunscreens we use to protect us, from ultraviolet radiation (UV).
Look at newly emerging leaves from buds and notice the red color of the delicate tissues that have not yet “hardened.” When leaves expand from the bud, they are somewhat like a water balloon. They fill with water but the plant cannot build the necessary support tissues that rapidly. Feel newly expanded leaves to notice how delicate they are. The cellular tissues remain thin for days.
The leaves of trees and shrubs expand rapidly but it takes much longer to reinforce cells with cellulose and other strengthening tissues. The first line of defense to protect delicate tissues from UV radiation would reasonably be found in the protective outer cell layer called the epidermis. This layer lacks the green chlorophylls that make leaves green and it also has a low concentration of anthocyanin. Anthocyanin is more abundant deeper in leaf tissues called palisade cells, where vertical rows of cells stand next to each other and circulate green chloroplasts to capture sun energy. It also is more abundant in photosynthetic cells beneath the palisade cells know as spongy mesophyll cells. Studies are trying to understand the mystery UV protection.
Think of the palisade cells like a series of farm silos packed closely together to fill a checkerboard. They are tall and slim. Imagine each silo filled with water and beach balls. The balls represent the chloroplasts that form a moving loop inside silo like an internal Ferris wheel. The chloroplasts are like seats on the Ferris wheel following others as they rise to the top and circulate back down to bottom. The spongy mesophyll cells below the palisade cells are more globular in water filled spaces between cells.
UV can cause damage to DNA in the cells of the two layers, just like damage can cause cancer in our skin tissues. Anthocyanin filters radiation to varying degrees and helps protect plants. Melanin in our skin serves that function and is built when our skin is exposed to UV and makes us tan.
Shade tolerant plants in the understory of forests are protected from intense sun radiation by the forest canopy. When trees are clear cut, the ground plants are suddenly exposed to UV and respond. They produce large quantities of anthocyanin and become intensely red. Unfortunately, it is not adequate to save them and most succumb to sunburn. Plants adapted to tolerate open sunny nature niches colonize the new sunny habitat. When you see a clear-cut forest, stop to notice how red the ground plants become when exposed.
Explore with family members to notice new growth on dogwood shrubs, maples, sassafras, oaks, and cherries. Choose any tree or shrub and feel how soft and delicate new tissues are and that they are pigmented red until they harden and feel sturdy. It is universal that the new tissues concentrate anthocyanin. The water-soluble pigment has other functions also but it plays a role as protective sunscreen. Phenolic acids in corn and other crops are UV-absorbing compounds so anthocyanin is not the only sunscreen. More mysteries are waiting discovery.
Declining levels of ozone in the upper atmosphere have generated concern because more UV radiation is entering the lower atmosphere where we live. In our latitudes, UV has risen by 3 to 5 percent in recent decades. Closer to the poles it has risen 6 to 8 percent. Increased skin cancer in people is occurring. People are not the only species impacted by UV radiation but we tend to think we are isolated from nature niches. That is not now nature works. What happens to plants happens to people. We do not live alone and sustainable care for other life is essential for our own health. Food and forest productivity depend on how we care for ozone layers.
Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at firstname.lastname@example.org Ody Brook Nature Sanctuary, 13010 Northland Dr, Cedar Springs, MI 49319-8433. 616-696-1753.