AIR IS DIFFICULT to describe. We

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1 The Significance of Air in Plant Growth AIR IS DIFFICULT to describe. We cannot see it or smell it. We can feel the wind blow, but we cannot feel still air. Cold air can have a bite, and hot air can be stifling. Air is something we breathe and is something plants and humans need. Without air, life on Earth would not exist. Objective: Discuss the significance of air in plant growth. Key Terms: acid rain air air pollution atmosphere global warming humidity particulates pollutants smog water vapor Air The Earth s atmosphere is defined as a layer of gases surrounding the planet, which are held close to the Earth due to gravity. The Earth s atmosphere contains roughly 78 percent Page 1

2 nitrogen, percent oxygen, 0.93 percent argon, percent carbon dioxide, and approximately percent of other gases (including neon, helium, methane, krypton, and hydrogen). There is also water vapor, in varying amounts, but usually it is minimal (around 1 percent). The mixture of gases is commonly known as air, which is colorless and odorless. The atmosphere provides life on Earth with essential elements. For example, it protects organisms by absorbing harmful ultraviolet solar radiation. In addition, the atmosphere is important in reducing temperature extremes between day and night. CARBON DIOXIDE, OXYGEN, AND PLANT GROWTH The carbon dioxide and oxygen in air are critical for photosynthesis and respiration. The exchange of oxygen and carbon dioxide through stomata in the leaves keeps photosynthesis operating at peak efficiency. Carbon atoms from carbon dioxide serve as the building blocks for sugars produced by plants. Meanwhile, oxygen is used by plants during cellular respiration a process in which the energy stored in sugars is released. The ability of air to move in and out of the soil is important because it provides oxygen for healthy root growth. Root cells must have oxygen to undergo the vital life process of cellular respiration. A lack of oxygen in the root cells interrupts normal functions. Oxygen is unavailable for cellular respiration when soils are saturated, and the stress this creates often causes cells to sicken and die. Oversaturated roots also become susceptible to soil borne fungal and bacterial diseases. HUMIDITY Water vapor is the gaseous form of water, and humidity is the term used to discuss water vapor in the air. Humidity levels affect plant growth. The growth rate of plants increases under conditions of high humidity because the high humidity allows plants to conduct photosynthesis with efficiency. As a result, great quantities of sugars are produced. Lush, tropical forests exist, in part, because of frequent rain and high humidity. Tropical plants commonly have immense leaves because there is little water stress on plants growing in tropical conditions. Many plant diseases become problematic when the air has high humidity. Smuts, rusts, mildew, anthracnose, and other foliar diseases thrive in humid situations following wet weather. The causal agents include fungi and bacteria. If the humidity is low, the dryness of the air can put stress on the plant, which is especially true if soil moisture is inadequate and wilting occurs. Under extreme wilting conditions, photosynthesis ceases. Some plants have adapted to dry climates; such plants typically have small leaves and leaves covered with thick cuticles to reduce transpiration. Page 2

3 AIR POLLUTION Air pollution is defined as particles, chemicals, or biological agents that modify the natural characteristics of the atmosphere. The items that cause pollution are called pollutants. A mixture of pollutants and water vapor is called smog, which is often seen hovering over cities. Sources of Pollution Sources of air pollution include human activities and natural activities. Some human activities that result in air pollution include burning fuels (e.g., coal, wood, and gasoline); using aerosols; painting and using solvents; and spraying pesticides. The pollution might consist of sulfur oxides, nitrogen oxides, carbon monoxide, ozone, and unburned fuel fragments. Examples of natural pollutants are radon, materials from volcanic activity, methane from cattle, and smoke from wild fires. Volcanoes, dust storms, fires, sea spray, and living vegetation produce particulates, which are small solid or liquid particles in the air. Damage to Plants Air pollution can cause physical damage to plant tissues and weaken plant health. Dust in the air and dust from the air that settles on leaf surfaces can reduce light intensity and slow photosynthesis. Chemical pollutants, such as sulfur dioxide, can actually kill plant cells or the entire plant. Acid Rain FIGURE 1. Industrial plant exhausting smoke into the atmosphere. Acid rain is an atmospheric pollutant that damages leaf tissues and contributes to water pollution. Sulfur dioxide and nitrogen oxide released during the burning of fossil fuels combine with oxygen and water in the atmosphere. Coal and oil are major sources of sulfur dioxide. Automobile exhaust is a major source of nitrogen oxide. These compounds are converted to sulfurous and nitrous acids in the atmosphere. Winds carry these acids over a wide area through the atmosphere. Normal rain has a ph of 5.6 to 7.0, making it slightly acid. The addition of acid compounds to the atmosphere lowers the ph of rain to 4.0 or below. Some fog has been measured to have a ph as low as 1.7. When acid rain falls, it lowers the ph of lakes. In severe cases, it can cause the die off of fish and other aquatic organisms. Page 3

4 Forests and agricultural crops are adversely affected by acid rain. The acids cause nutrients to leach out of leaf tissues. Leaf tissues appear damaged and often die. Acid rain can lower the ph of soils and, in effect, restrict the uptake of some nutrients. In some agricultural areas where acid rain occurs, yields are significantly reduced. Global Warming Carbon dioxide, methane, and fluorocarbons contribute to global warming, which is an increase in the temperature of near-surface air and oceans. Burning fossil fuels contributes to the addition of these pollutants to the atmosphere. Global warming can alter the climate to which plants are adapted and lead to changes in agricultural production/yields. Summary: FIGURE 2. Shore beach after acid rain. The atmosphere is a layer of gases surrounding the planet, and air is a mixture of gases. Carbon atoms from carbon dioxide serve as the building blocks for sugars produced through photosynthesis. Oxygen is used by plants during cellular respiration. The ability of air to move in and out of the soil is important for healthy root growth. Humidity is a term used to discuss water vapor in the air. The growth rate of plants increases under conditions of high humidity, but many plant diseases become problematic when the air has high humidity. Air pollution is defined as particles, chemicals, or biological agents that modify the natural characteristics of the atmosphere. Sources of air pollution include human activities and natural activities. Air pollution can be damaging to plants. Acid rain is an atmospheric pollutant that damages leaf tissues and also contributes to water pollution. Carbon dioxide, methane, and fluorocarbons contribute to global warming. Page 4

5 Checking Your Knowledge: 1. What composes the atmosphere? 2. What roles do oxygen and carbon dioxide play in plant growth? 3. How does humidity affect plant growth? 4. What are some sources of air pollution? 5. How does acid rain injure crops? Expanding Your Knowledge: Devise an experiment in which the concentrations of oxygen and carbon dioxide are altered in tightly sealed jars. In the jars, grow the same species of plant; they should be the same age and size. Some ideas to alter the atmosphere are to add a piece of dry ice, a pad of wet steel wool, and oxygen from an oxygen tank. Observe the growth of the plants over several weeks. Web Links: Cycles of Life Effects of Air Pollution on Agricultural Crops Gas Exchange in Plants Particulate Agricultural Career Profiles Page 5