1. Animal, Plant & Soil Science E4-8 The Importance of Water in Plant Growth

2. Interest Approach
Obtain an inflatable Earth balloon. It can be purchased through a school supply store or catalog, or through the National Geographic Society. Toss the Earth balloon among the students. Ask them to note where their right index finger is touching the balloon, and to call out “water” or “land,” depending on which one the index finger is touching. Ask one student to tally the responses on the board. Continue the toss until the “water” responses are approximately 65–75 percent of the total number of tosses. Discuss the results of the toss in terms of the similarity to the actual estimated ratio of water to land on Earth’s surface.

3. Objectives 1 Identify important characteristics of water.
2 Explain the hydrologic cycle.
3 Describe the functions of water in plant growth.
4 Explain plant responses to water shortage and to excess water.

4. Terms absolute humidity adhesion aquifers cloud cohesion condensation
dew
dew point
evaporation
evapotranspiration

5. Terms fog frost groundwater humidity hydrologic cycle irrigation
precipitation
relative humidity
solute
solution

6. Terms solvent specific humidity sublimation surface water
translocation
transpiration
turgor
universal solvent
wilting

7. What are the important characteristics of water?
I. Water covers three-fourths of the earth’s surface and it is available for plants in most areas of the world. It is also a major component of plants and animals. Humans are about 70 percent water and most plants are about 90 percent water.

8. What are the important characteristics of water?
A. Water molecules consist of two atoms of hydrogen and one atom of oxygen. As a bipolar molecule it has positive and negative charges. This attribute causes individual molecules to be attracted to one another. Water can form hydrogen bonds with up to four adjacent water molecules.

10. What are the important characteristics of water?
B. Water is a universal solvent, meaning nearly everything will dissolve in it. Plants and other organisms are composed of solutions consisting of atoms and molecules dissolved in water. A solution is defined as a uniform mixture of two or more substances. Solutions consist of solvents and solutes. The solvent is usually present in the greatest amount. The solute is present in a lesser amount. For example soil solutions have a solute (minerals/elements) dissolved in a solvent (water). Plant roots absorb the soil solution.

11. What are the important characteristics of water?
C. Water has both cohesive and adhesive forces. Because of the hydrogen bonds, water molecules stick together. The tendency of water molecules to stick together is called cohesion. Water molecules also stick to other substances, such as the surface of a leaf or soil particle. The ability of molecules of one substance to adhere to a different substance is known as adhesion. Cohesion and adhesion play an important role in the movement of water up through a plant. These forces are also involved in the capillary action of water rising through pore spaces of soil.

13. What are the important characteristics of water?
D. Water molecules have a slight tendency to split apart or dissociate. The result is that hydrogen ions (H+) and hydroxyl (OH-) ions are found in solution. These same ions readily reunite to form water molecules. There are always some hydrogen and hydroxyl ions found in water. In pure water the concentration of hydrogen ions (H+) and (OH-) ions is equal.

14. What is the hydrologic cycle?
II. The exchange of water between earth and its atmosphere is the hydrologic cycle or water cycle. The hydrologic cycle involves continuous movement of water and many processes. Water also changes states from gas to liquid to solid.

15. What is the hydrologic cycle?
A. The gaseous form of water is water vapor. Humidity is the amount of water vapor in the air as compared to the amount of water vapor the air can hold at a certain temperature. Three terms used to refer to humidity are absolute humidity, relative humidity, and specific humidity.

16. What is the hydrologic cycle?
1. Absolute humidity is the amount of water in a volume of air.
2. Relative humidity is the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage. For example, air might be measured as having 90% humidity.
3. Specific humidity is the ratio of water vapor to “dry air plus water vapor” in a particular volume.

17. What is the hydrologic cycle?
B. Water vapor in the atmosphere may condense to the liquid of solid states to form rain, snow and other forms of precipitation.
1. The term condensation means the change in matter of a substance to a denser phase. Water vapor changes to a liquid as air cools. Clouds form as water vapor condenses into tiny droplets or frozen crystals. Fog is similar to a cloud, but it is in contact with the ground. Another form of condensation is dew.

18. What is the hydrologic cycle?
2. Dew is water in the form of droplets that appears on objects such as grass in the morning or evening. For example, as grass blades cool water vapor condenses on the leaves forming water droplets. The temperature at which droplets form is called the dew point. When air temperatures are low enough, dew freezes and is called frost.

19. What is the hydrologic cycle?
3. Precipitation includes the forms of water that fall from the sky. Rain, snow, sleet, and hail are common forms of precipitation. Precipitation is a product of condensation.

20. What is the hydrologic cycle?
C. Precipitation falls to the oceans and land. Runoff from precipitation forms streams, rivers, and lakes. These waters are referred to as surface water. Some water infiltrates the ground and becomes groundwater that slowly returns to the oceans. Groundwater is water found in pores, cracks, and openings in soil and rock.

21. What is the hydrologic cycle?
1. Groundwater can be extracted through wells. Some wells tap into large underground reserves of water called aquifers. Aquifers contain vast amounts of water that have collected over thousands of years.
2. Some water is absorbed by plants.

22. What is the hydrologic cycle?
D. Evaporation is the process by which water leaves the surface of the earth and becomes water vapor.

23. What is the hydrologic cycle?
1. Water absorbed by plants is lost to the atmosphere in a process called transpiration. Much of the water loss is through stomata and from plant surfaces.
a. The rate of transpiration is affected by humidity, temperature, wind, and soil moisture. The rate increases when humidity is low, temperatures high, wind speeds high, and soil moisture adequate.
b. Leaf shape, leaf surface area, and the number of stomata are other factors that determine the rate of transpiration. Broad, thin leaves with an abundance of stomata transpire more than narrow, thick leaves with few stomata.

24. What is the hydrologic cycle?
2. Evapotranspiration includes the water that transpires from plants and evaporates from soil.
3. Sublimation is a form of evaporation in which water vaporizes from ice without becoming water first. This occurs when the air temperature is colder than the temperature of the snow or ice. The phenomena can be observed in winter as snow slowly disappears under cold conditions.

26. What are the functions of water in plant growth?
III. Plants get most of their water through the root system. In some situations, plants can soak up water through their stems or take in water through their leaves. Water plays an important role in all plant functions.

27. What are the functions of water in plant growth?
A. Water is necessary for a plant to maintain turgor, which gives a plant rigidity and shape.
B. Water aids in cell division and growth.

28. What are the functions of water in plant growth?
C. Water provides internal pressure that helps roots push through soil.
D. Water molecules are split in photosynthesis releasing oxygen into the atmosphere and contributing hydrogen atoms to the manufacture of carbohydrates.

29. What are the functions of water in plant growth?
E. Cellular respiration requires water and energy released through the process is moved through the plant by water.
F. Water dissolves minerals in the soil. Roots absorb the water with the dissolved minerals and transport them throughout the plant where they serve as raw materials for the making of numerous compounds. This movement of materials is called translocation.

30. What are the functions of water in plant growth?
G. The process of transpiration helps the plant to draw water and minerals up through the plant.
H. As water evaporates, it helps to stabilize the temperature of the plant.

31. How do plants respond to water shortage and to excess water?
IV. Plants are dependent upon water for healthy growth. Too little water and an excess of water can have an adverse effect on plant health.

32. How do plants respond to water shortage and to excess water?
A. Water stress occurs when the soil is too dry for root hairs to absorb water or if the evaporation and transpiration rates from the leaves exceed the rate at which water can be moved up the stem. Several things occur when plants lack water. The most common sign is wilting or the limpness to the plant tissue. Water stress might also appear as leaf rolling, leaves changing color from a deep green to a blue-green, yellowing or browning of leaves, and leaf drop. Prolonged water stress results in permanent wilting and damage to plant tissues.

33. How do plants respond to water shortage and to excess water?
1. Optimal plant growth and crop yields are achieved when a plant is never placed under water stress.
2. Plants differ in their ability to draw water from the soil and to conserve water in their tissues. Factors that improve a plant’s ability to tolerate drought include a deep, well-developed root system, waxy leaf surfaces, hairy leaves, shiny or light colored leaves that reflect light, and leaves that fold up or drop under water stress conditions.

34. How do plants respond to water shortage and to excess water?
B. Too much water in the root zone can be equally damaging to plants as too little water. Excessive water around the roots reduces the oxygen levels. Without adequate oxygen for respiration, root hairs die. The death of root hairs leads to an inability to absorb available water and therefore moisture stress within the plant. The symptoms of excessive water are very similar to those symptoms when the soil lacks moisture.

35. How do plants respond to water shortage and to excess water?
C. Irrigation practices, the artificial application of water, should be aimed at providing the right amount of water to avoid water stress, but not too much to limit oxygen to the roots.

36. REVIEW 1. What are the important characteristics of water?
2. What is the hydrologic cycle?
3. What are the functions of water in plant growth?
4. How do plants respond to water shortage and to excess water?