Plant Magic: Creating Food out of Thin Air

by Laura Huenneke

This geranium produced leaves with red pigment while living outdoors on a sunny porch, but switched to light green leaves when moved indoors.

Back when I taught college botany, I began each semester promising that we’d learn how plants find mates, avoid enemies, and help their offspring get a good start in life. This description shocked many students who had never really thought of plants as being “alive.” But the most amazing thing that plants do is create their own “food” or source of material for growth – something magical to most of us.

You might think plants “eat” mineral nutrition in the soil and consume water to build their tissues, grow, and carry out activities like reproduction. But scientists in 1780-1800 devised elegant experiments demonstrating that plant tissues (leaves, roots, and flowers) are chiefly built out of thin air – specifically, the carbon atoms from carbon dioxide in the atmosphere, and secondarily atoms from water from the soil. Soil minerals make only very minor contributions to plant growth.

We think of air as being weightless -- the absence of material stuff. But in fact air is full of stuff – nitrogen, carbon dioxide, oxygen, and many other molecules that do have weight! But in gaseous air the molecules are far apart and the density is low relative to things we perceive as more solid. Nearly all the mass a plant accumulates, whether the delicate flower of a violet or the hundred-plus pounds of a giant pumpkin, comes from carbon in the surrounding air.

Those same 18^th century scientists found that in daylight, plant leaves release oxygen in amounts proportional to the amount of carbon dioxide taken up. At night, though, plants consume oxygen. Because living plants need energy to function, they use oxygen to burn stored carbohydrates just like animals do, to release energy and power their activities. So it’s not as simple as “plants breathe carbon dioxide and let off oxygen, while animals breathe oxygen and let off carbon dioxide.” Plants actually do it all.

Basic photosynthesis – building sugar and starch by gluing together carbon atoms from carbon dioxide – follows a standard pattern for most plants. Stomates, or pores, on the surface of leaves open in daytime, allowing carbon dioxide to move into the leaf’s interior and reach living cells filled with green chlorophyll. The cells absorb that carbon dioxide, use sunlight to split off the carbon and attach it to what will become sugar or starch, and release oxygen as part of the water-splitting reaction that powers the process. This chemistry happens during the day, powered by light from the sun, whose energy is captured and stored in those carbohydrates.

Why does this chemical magic matter to gardeners? To grow well, our plants need supplies for photosynthesis: carbon dioxide, water, the soil nutrients that build enzymes to do the chemical work of photosynthesis, and of course light. When stomates are open in the daytime, and sunlight is available, then the plant inevitably loses water by evaporation through the stomates. The plant must be well hydrated; if conditions are too dry, the stomates close to preserve water and the leaf quickly runs out of carbon dioxide to use. The plant may survive, but it won’t grow much because it can’t make more carbohydrates.

(Some plants, especially cacti and other succulents, cleverly avoid this tradeoff of water for carbon. They open their stomates in the cool night, taking in carbon dioxide and holding it temporarily. In the hot daytime, they close the stomates, let the carbon dioxide out of storage, and use sunlight then to make carbohydrates.)

Pigments or color substances in the leaves carry out the initial capture of light energy. Different pigments capture different wavelengths of light, so a plant that needs full sun, like corn or the top leaves of an oak tree, has different pigments than something that thrives in shade, like lettuce or wildflowers on the forest floor. Some plants even change the pigments they produce in different leaves, depending on what conditions are like when each leaf is formed! My geranium switched from red-patterned leaves to lime-green ones when I brought it indoors after summer on the sunny porch.

Some scientists today are working to create artificial leaves – structures that can capture sunlight and manufacture carbohydrates from air, minerals, and water. That seems a little too artificial for my old-fashioned taste. For now, I’m grateful for the many plants reliably pulling carbon out of our thin mountain air to make our beans, corn, and tomatoes.