The Word: Transpiration
Bruce Palmer
Cuten, California

        It's high summer in the Northern Hemisphere. Many of us are having to give our rhododendrons a lot of water to keep them healthy. Why do plants require so much more water than animals? Part of the answer lies with the word for this issue, transpiration. The word comes from the Latin transpirare, to breathe. It means to give off moisture from a surface. Technically, it includes perspiration (Latin perspirare, to breathe everywhere), but in practice we tend to separate the two and apply transpiration primarily to plants. Transpiration in plants is the release of water from leaves and other surfaces. The nutrients required to make cells are mostly soluble in water and so water is required to get them around a multicelled body. Vertebrate animals have a closed circulatory system and so are able to conserve water quite effectively. Plants do not have such a system. The primary way plants get nutrients from one end to the other is to depend on processes that occur in nature outside living organisms. For plants that dependence is quite wasteful of water, and a typical plant may require up to twenty times as much water as a vertebrate animal of comparable size. When I lived in Hawaii, the mantra was it took one thousand pounds of water for sugar cane to make a pound of sugar. That's the high end of the scale, but all plants have tremendously high throughputs of water compared to the solid substances they make as a result.
        The task of getting water to a leaf begins at the roots with the process called diffusion (Latin: diffusio, to disperse). In diffusion, substances move from where they are more concentrated toward where they are less concentrated until they are equally distributed. You can see that process in action when you put a teabag into a cup of hot water. In a very short time the coloring and caffeine are evenly distributed and you can take the teabag out of the cup. The diffusion of water works the same way, but in living organisms it typically occurs across cell membranes and is called osmosis (Greek: osmos, impulse). Water and the nutrients dissolved in it are carried across cell membranes from one end of the plant to the other as long as each cell in turn has a lesser concentration of water. That's where transpiration comes in. The movement of water out of leaves (transpiration) occurs mainly through cell groupings called stomata (Greek: stomen, mouth). Stomata control the diffusion of water and oxygen out of a leaf, and carbon dioxide into the leaf, an entire topic for another time. What we need to know here is that the cells inside the leaves carrying out photosynthesis need to have wet surfaces for the carbon dioxide to diffuse into them. This means they must be kept wet, and to keep them wet, the water must be there at all times. Since plants don't have closed circulation, they give off water from the leaf surfaces, creating a lower water concentration on the leaf surfaces, which in turn allows osmosis to move the water upward constantly. Plants are thus caught in a double bind. The internal cells need to have wet surfaces, and to keep them wet the plant must give off water (transpire) all day long which creates a need for water also all day long. Over 90 percent of the water a plant takes up is given off through the physical process of evaporation into the air by the biological plant process of transpiration.

        There are a number of simple lab experiments that demonstrate how much water is transpired by leaves. Most of them can't be done at home conveniently, but there is a phenomenon in your yard that shows what is happening. It's called guttation (Latin: gutta, a drop). Sometimes when you go out in your garden in the morning, the grass looks as if it has dew on it. Look closely and you will see that it is not dew, but droplets of water near the tips of the grass blades. The droplets are there because the roots have been taking up water all night but photosynthesis and transpiration haven't been operating. The water had to go somewhere, so the grass blades excrete it, mostly though special cells in small areas, and thus the drops. As soon as the light gets bright enough for photosynthesis, the drops evaporate away and disappear and transpiration takes over. So, the next time you're out watering in the morning, look for guttation. Plants may not have an efficient water use system, but it works and they have adapted in many ways to various wet and dry environments.

Bruce Palmer is a member of the Eureka ARS Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii system for 25 years.