White Plant Photosynthesis: How Plants That Aren’t Green Photosynthesize
Did you ever wonder how plants that aren’t greenphotosynthesize? Plantphotosynthesis occurs when sunlight creates a chemical reaction in theleaves and stems of plants. This reaction turns carbon dioxide and water into aform of energy that can be used by living things. Chlorophyll is the greenpigment in leaves that captures the sun’s energy. Chlorophyll appears green toour eyes because it absorbs other colors of the visible spectrum and reflectsthe color green.
How Plants That Aren’t Green Photosynthesize
If plants require chlorophyll to produce energy fromsunlight, it’s logical to wonder if photosynthesis without chlorophyll canoccur. Other photopigments can also utilize photosynthesisto convert the sun’s energy.
Plants that have purplish-red leaves, like Japanesemaples, use the photopigments that are available in their leaves for theprocess of plant photosynthesis. In fact, even plants that are green have theseother pigments. Think about deciduoustrees that lose their leaves in the winter.
When autumn arrives, the leaves of deciduous trees stop theprocess of plant photosynthesis and the chlorophyll breaks down. The leaves nolonger appear green. The color from these other pigments become visible and wesee beautiful shades of yellows, oranges and reds in the fall leaves.
There is a slight difference, however, in the way greenleaves capture the sun’s energy and how plants without green leaves undergophotosynthesis without chlorophyll. Green leaves absorb sunlight from both endsof the visible light spectrum. These are the violet-blue and reddish-orangelight waves. The pigments in non-green leaves, like the Japanese maple, absorbdifferent light waves. At low light levels, non-green leaves are less efficientat capturing the sun’s energy, but at midday when the sun is the brightest,there is no difference.
Can Plants Without Leaves Photosynthesize?
The answer is yes. Plants, like cacti,don’t have leaves in the traditional sense. (Their spines are actually modifiedleaves.) But the cells in the body or “stem” of the cactus plant still containchlorophyll. Thus, plants like cacti can absorb and convert energy from the sunthrough the process of photosynthesis.
Likewise, plants like mossesand liverworts also photosynthesize. Mosses and liverworts are bryophytes, orplants that have no vascular system. These plants don’t have true stems, leavesor roots, but the cells that compose the modified versions of these structuresstill contain chlorophyll.
Can White Plants Photosynthesize?
Plants, like some types of hosta,have variegatedleaves with large areas of white and green. Others, like caladium,have mostly white leaves that contain very little green color. Do the whiteareas on the leaves of these plants conduct photosynthesis?
It depends. In some species, the white areas of these leaveshave insignificant amounts of chlorophyll. These plants have adaptationstrategies, such as large leaves, that allow the green areas of the leaves toproduce sufficient amounts of energy to support the plant.
In other species, the white area of the leaves actuallycontains chlorophyll. These plants have changed the cell structure in theirleaves so they appear to be white. In reality, the leaves of these plantscontain chlorophyll and use the process of photosynthesis to produce energy.
Not all white plants do this. The ghostplant (Monotropa uniflora), for example, is an herbaceous perennialthat contains no chlorophyll. Instead of producing its own energy from the sun,it steals energy from other plants much like a parasitic worm robs nutrientsand energy from our pets.
In retrospect, plant photosynthesis is necessary for plantgrowth as well as the production of the food we eat. Without this essentialchemical process, our life on earth wouldn’t exist.
How Does a Plant With Red Leaves Support Itself Without Green Chlorophyll?
Q. My tree has red leaves all year. How does a plant support itself without green chlorophyll?
A. Some parasitic plants lack chlorophyll entirely and steal the products of photosynthesis from their green hosts, said Susan K. Pell, director of science at the Brooklyn Botanic Garden. Other plants, like a red-leafed tree, have plenty of chlorophyll, but the molecule is masked by another pigment.
Chlorophyll absorbs red and blue light, “reflecting, and thus appearing, green,” Dr. Pell said. Chlorophyll uses this electromagnetic energy, along with carbon dioxide and water, to make glucose and oxygen.
Most plants also have other pigments: carotenoids, which usually appear yellow to orange, and anthocyanins, which are red to purple. One pigment usually dominates. So a plant with red leaves probably has higher than usual amounts of anthocyanins, Dr. Pell said. But chlorophyll is still present and at work.
“We used to think that all fall foliage color change resulted from the revealing of already-present carotenoids and anthocyanins when chlorophyll was broken down in preparation for dormancy,” she said. We now know that leaves actually produce additional anthocyanins into old age, she said.
The evolutionary advantages are not fully understood, Dr. Pell said. One theory is that extra anthocyanins provide shade under which chloroplasts (structures within cells) can break down their chlorophyll, helping the plant reabsorb its building blocks, especially valuable nitrogen. Another theory is that anthocyanins, which are powerful antioxidants, protect the plants in preparation for winter.
Photosynthesis in Leaves That Aren’t Green
Q: How does photosynthesis occur in plants that are not obviously green, such as ornamental plum trees with deep purple-colored leaves? [Paul, Santa Cruz]
A: Photosynthesis (which literally means “light put together”) is that very elegant chemical process that jump-started life as we know it some 4 billion years ago. So to answer your question, we’ll need a short chemistry lesson. Basically six molecules of water (H2O) plus six molecules of carbon dioxide (CO2) in the presence of light energy produce one molecule of glucose sugar (C6H12O6) and emit six molecules of oxygen (O2) as a by-product. That sugar molecule drives the living world. Animals eat plants, then breathe in oxygen, which is used to metabolize the sugar, releasing the solar energy stored in glucose and giving off carbon dioxide as a by-product. That’s life, in a nutshell.
All photosynthesizing plants have a pigment molecule called chlorophyll. This molecule absorbs most of the energy from the violet-blue and reddish-orange part of the light spectrum. It does not absorb green, so that’s reflected back to our eyes and we see the leaf as green. There are also accessory pigments, called carotenoids, that capture energy not absorbed by chlorophyll. There are at least 600 known carotenoids, divided into yellow xanthophylls and red and orange carotenes. They absorb blue light and appear yellow, red, or orange to our eyes. Anthocyanin is another important pigment that’s not directly involved in photosynthesis, but it gives red stems, leaves, flowers, or even fruits their color.
Many plants are selected as ornamentals because of their red leaves— purple smoke bush and Japanese plums and some Japanese maples, to name just a few. Obviously they manage to survive quite well without green leaves. At low light levels, green leaves are most efficient at photosynthesis. On a sunny day, however, there is essentially no difference between red and green leaves’ ability to trap the sun’s energy. I have noticed the presence of red in the new leaves of many Bay Area plants as well as in numerous tropical species. The red anthocyanins apparently prevent damage to leaves from intense light energy by absorbing ultraviolet light. There is also evidence that unpalatable compounds are often produced along with anthocyanins, which may be the plant’s way of advertising its toxicity to potential herbivores. So red-leaved plants get a little protection from ultraviolet light and send a warning to leaf-eating pests, but they lose a bit of photosynthetic efficiency in dimmer light.
Botanists have been wondering about red versus green leaves for the past 200 years and there is still much research to be done in this arena. So you are in good company, Paul.
What if photosynthesis stopped happening?
It's a concept most children learn in science class: Photosynthesis converts light energy to chemical energy. Essentially, photosynthesis is the fueling process that allows plants and even algae to survive and grow. So what would happen if photosynthesis suddenly stopped happening?
If photosynthesis came to an abrupt end, most plants would die within short order. Although they could hold out for a few days -- or in some cases, a few weeks -- how long they lived would largely be a factor of how much sugar they had stored within their cells. Large trees, for example, may be able to soldier on for several years — perhaps even a few decades — because of their energy stores and the slow rate of use. However, the majority of plants would meet a withering end, and so would the animals that rely on them for nourishment. With all the herbivores dead, the omnivores and carnivores would soon follow. Although these meat-eaters could feed on all the carcasses strewn about, that supply wouldn't last more than a few days. Then the animals that temporarily relied on them for sustenance would die.
That's because for photosynthesis to cease to exist, Earth would have to plunge into darkness. To do this, the sun would have to disappear and plunge Earth's surface temperatures into a never-ending winter of bitter cold temperatures. Within a year, it would bottom out at minus 100 degrees Fahrenheit (minus 73 degrees Celsius), resulting in a planet of purely frozen tundra [source: Otterbein].
Ironically, if the sun burned too bright, it could cause photosynthesis to stop occurring. Too much light energy would damage plants' biological structure and prevent photosynthesis from happening. This is why the photosynthetic process, in general, shuts down during the hottest hours of the day.
Whether the culprit were too much sunlight or not enough, if photosynthesis stopped, plants would stop converting carbon dioxide -- an air pollutant -- to organic material. Right now, we rely on photosynthetic plants, algae and even bacteria to recycle our air. Without them, there would be less oxygen production [source: Hubbard].
Even if all the plants on Earth were to die, people would remain resourceful -- especially if their lives depended on it. An artificial photosynthesis process being developed by scientists could just become the world's biggest problem-solver. Using an artificial "leaf," scientists have successfully harnessed sunlight and recreated photosynthesis. The leaf is actually a silicon solar cell that, when put in water and exposed to light, then generates oxygen bubbles from one side and hydrogen bubbles from the other -- essentially splitting oxygen and hydrogen. Although the idea was designed as a way to potentially produce clean energy, there are implications for recreating a photosynthetic atmosphere as well [source: Chandler].
Phototropism Obstacle Course
Without light, a plant can’t make its food by photosynthesis.
Sunlight is so important to a plant that it will change the way it grows so that it points toward the light.
This is called phototropism, from the Greek words for ‘light’ and ‘turn.’ You can see this with a houseplant: the leaves grow to point toward the window. If you turn it around it will eventually move to orient itself toward the window again.
How strong is this attraction to sunlight? Can a plant grow around obstacles to find the light? Time to find out!
What You Need:
- Shoe box with a lid
- A couple pieces of cardboard
- Matte black paint (spray paint is easiest)
- Small flowerpot or styrofoam cup
- Potting soil
- Bean seed
What You Do:
- Cut two pieces of cardboard that are as deep as the box and about 2/3 as wide.
- Paint the inside of the box, the inside of the lid, and the two pieces of cardboard with black paint. This will help cut down on light reflection.
- When the paint is dry, tape one of the cardboard pieces to the inside of the box so that it extends out into the middle of the box. (During the experiment the box will stand on its end – leave enough room for your flower pot or cup to stand below the piece of cardboard.) Tape the other piece of cardboard to the opposite side of the box a few inches above the first one.
- Stand the box on its end and cut a small hole (about the size of a dime) in the top end.
- Plant one or two bean seeds 3/4-inch deep in some damp potting soil in the flower pot or styrofoam cup. Place it in the bottom of the the box and put the lid on. (Make sure the lid fits tightly enough that no light can get in except through the hole in the top of the box.)
- Keep your bean plant watered and check on it once a day to see how it is growing. Draw or take pictures of how it grows.
The bean plant grows towards the only source of light, the hole in the top of the box, even if it means growing around the cardboard obstacles you placed inside the box!
The energy it needs to sprout and start growing was stored in the seed, but eventually that food will be all used up and the plant will need to make more through photosynthesis. The plant spends the energy from the seed trying to find light so it can survive.
(This project is adapted from The Amateur Naturalist by Nick Baker.)