Our eyes and plant leaves have much in common

Discovery initially led to accusations of flouting the law of thermodynamics, newest college Professor of Distinction recalls

By Clint Talbott

One might ask what human eyes and photosynthesis have in common. The answer, a pioneering scientist at CU-Boulder says, is, “More than you might think.”

Barbara Demmig-Adams has been named the 2013 College Professor of Distinction by the College of Arts and Sciences. She discussed her work during a public presentation celebrating the honor this fall.

“Intense sunlight can actually damage the human eye, as well as leaves.” The eye absorbs light for the purpose of interpreting information, and leaves absorb light for energy to make food.

“Therefore, it is maybe not entirely surprising that eyes and leaves share things. They employ the same powerful protection against any damage from this intense light, except that plants manufacture this protection just as is needed, but we can neither manufacture this protection nor do we seem to know what we need to eat.”

Humans need to consume certain compounds that protect our eyes and other parts of the bodies against damage.

I stood up there and proposed that plants do dissipate any excess light just fine, and this ‘damage’ thing had to be looked at again. … Nothing could have prepared me for the firestorm that broke loose.”

Demmig-Adams described her interest in photosynthesis this way: It is the basis of life on Earth and human civilization.

For those seeking evidence for that statement, she said: “If you need oxygen—and I know I do—every last molecule of oxygen on this planet has been made by photosynthesis” sometime over the last 3.5 billion years.

Further, photosynthesis is the basis of virtually all food chains, fiber, building material and fuels. She noted that popular distinction between “fossil fuels” and “biofuels.”

“I would say the fossil fuels are really biofuels, too, because every calorie that’s in there has been laid down by a plant or other photosynthetic organism, now dead … So fossil fuels are really biofuels, too, only made by organisms that are no longer alive.”

All that energy is provided by the sun, which can sometimes be too intense. “So if there’s an excess of absorbed sunlight, there’s a potential for certain chemicals to be formed that can damage biological organisms”—unless organisms can rid themselves of the excess.

William and Barbara Demmig-Adams take in the Colorado sun in 2006 in front of two of their evergreen study plants Ponderosa pine (Pinus ponderosa; left), Douglas fir (Pseudotsuga menziesii; center) and the ground cover kinnikinnick (bearberry; Arctostaphylos uva-ursi). Photo by Melanie Adams.

“That’s what my work is about,” Demmig-Adams said.

Three decades ago, she recalled, she was studying responses of plants, and signs she was observing were thought to be a sign of actual sunlight-induced damage to plants. However, she was working with plants that were genetically adapted to grow in high-light conditions.

“I was growing them under full sunlight, and I was thinking, ‘You know, they don’t look that droopy to me.’” Demmig-Adams began to think that perhaps the “damage” she was observing was “actually a sign of what they’re doing to defend themselves against this excess light and a sign of them getting rid of this excess light.”

She read a paper that noted a leaf that formed a carotenoid pigment under high-light conditions but quickly lost that pigment in the absence of light.

“I was thinking that sounds like a great opportunity to do what the system needs to do-get rid of nothing but the excess light, because every photon a plant can get a hold and actually utilize in photosynthesis should be used in photosynthesis.”

“And so the plant needs to carefully count its photons” and determine which to use and which to dispose of. She surmised this fast-forming and quickly disposed-off pigment might be involved: a molecule called zeaxanthin, which (in addition to being present in sunlight-flooded leaves) gives the yellow color to an ear of corn or an egg yolk and “which is a really good thing to eat for your eye health.”

When attending scientific conferences after her post-doctoral research appointment had ended, “I stood up there and proposed that plants do dissipate any excess light just fine, and this ‘damage’ thing had to be looked at again.” Further, she proposed that zeaxanthin was the dissipater.

“Nothing could have prepared me for the firestorm that broke loose,” she recalled. “I was called many things that I cannot repeat here, but the most professional accusation I received was that I was breaking the laws of thermodynamics. I took that pretty hard.”

Plants put themselves out there where there is some good energy to be absorbed, and it turns out that every plant pretty much every day has to contend with more light than it can actually handle through photosynthesis.”

The law Demmig-Adams was allegedly flouting could be framed this way: Just as Boulder Creek does not flow up the canyon, “energy could not flow uphill, which was thought to be the direction from chlorophyll to carotenoids.”

However, Demmig-Adams did suggest that energy was flowing from chlorophyll to zeaxanthin and was then disposed of as heat. “It took 25 years before the jury came back and said, ‘Barbara’s OK; she doesn’t have to go to jail.’”

The delay was due to the fact that carotenoids such as zeaxanthin have difficult-to-observe states that allow excess energy to be accepted from chlorophyll and then dissipated.

Demmig-Adams and her husband, William Adams, went into the field with instruments that measure the responses of leaves to light. They wanted to determine how often plants had to manage an excess of light. The answer? “Every day.”

“Plants put themselves out there where there is some good energy to be absorbed, and it turns out that every plant pretty much every day has to contend with more light than it can actually handle through photosynthesis.”

And were there any plants that grow so quickly that they can use all the available energy? No.

“There is no plant on this planet that can use all of the energy of full sunlight,” she said. Even the fastest-growing plants will use at most half of the light they absorb when the sun is highest. Plants that grow more slowly might use 5 percent or 10 percent of the energy at high noon, she said.

So the conclusion is that photosynthesis would not be possible without having a mechanism that dissipates excess energy every day. Those mechanisms are biochemical and biophysical, she said.

She and her research team found that this mechanism of dissipation has a huge amount of variation and “specific adaptations to every possible environment on Earth.”

“These plants have been there for many millions of years, and they know what they’re doing.”

In high, subalpine mountain regions like those in Colorado, evergreen trees actually shut down photosynthesis during the winter while they do not grow because the soil water is frozen. In winter, they dissipate 100 percent of the light they absorb every day, Demmig-Adams said.

In Australia, by contrast, she and her team found that vines exposed to intermittent periods of bright light and shade when growing under the canopy of a eucalypt forest with fluttering leaves, oscillate between efficient light collection and effective dissipation of the sun’s energy just as quickly.

Demmig-Adams and her students determined together that there are “many parallels between plant function and animal / human function” with respect to dissipation of sunlight’s energy.

The human eye and a green leaf have “so much in common,” she said. “They handle light, and they need to know how to do this, and it turns out that our zeaxanthin … is an important required ingredient for the human eye.”

Cataracts and age-related blindness have been linked to a dearth of zeaxanthin in the human eye. A dearth of zeaxanthin also has negative effects in younger people, in whom such a deficit is associated with a decline in visual acuity.

“We are amazed at how many parallels there are,” Demmig-Adams said. The focus of her current work is on how the need for dissipation (and the fraction of full sunlight turned into food or fuel) can be manipulated by the whisking-away of energy-rich molecules formed in photosynthesis through the export pipelines from a leaf to the rest of the plant—or by draining energy-rich molecules from photosynthetic algae.

The College Professor of Distinction title is reserved for scholars and artists of national and international acclaim whom their college peers also recognize as exceptionally talented teachers and colleagues. Honorees of this award hold this title for the remainder of their careers in the College of Arts and Sciences at CU-Boulder.

Dean Steven Leigh had high praise for Demmig-Adams. “Our students are learning from a global academic leader,” he said.


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