The Importance of Water in Plant Growth - Oconto Falls Agricultural ...

The Importance of Water in Plant Growth

WATER covers ? 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. For example, humans are approximately 70 percent water, and most plants are about 90 percent water.

Objective:

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Explain the importance of water to plant growth.

Key Terms:

? absolute humidity adhesion aquifers clouds cohesion condensation dew dew point evaporation evapotranspiration

fog frost groundwater humidity hydrologic cycle irrigation precipitation relative humidity solute solution

solvent specific humidity sublimation surface water translocation transpiration turgor universal solvent wilting

Water

Water is a chemical substance essential to all forms of life. Life on Earth could not exist without water.

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THE CHEMISTRY OF WATER

Water molecules consist of two atoms of hydrogen and one atom of oxygen. As a bipolar molecule, water 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.

Universal Solvent

Water is a universal solvent, so 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 a uniform mixture of two or more substances, which consist of solvents and solutes. The solvent is usually present in the greatest amount, while the solute is present in a lesser amount. For example, soil solutions have a solute (minerals/elements) dissolved in a solvent (water), and plant roots absorb the soil solution.

Cohesive and Adhesive Forces

Water has cohesive and adhesive forces. Because of the hydrogen bonds, water molecules stick together, which is called cohesion. Water molecules also stick to other substances, such as the surface of a leaf or a 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 through a plant, and these forces are involved in the capillary action of water rising through pore soil spaces.

Hydrogen and Hydroxyl Ions

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, but 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.

HYDROLOGIC CYCLE

The exchange of water between earth and its atmosphere is the hydrologic cycle (water cycle), which involves the continuous movement of water. The four main parts of the cycle are condensation, precipitation, collection, and evaporation. Water also changes states from gas to liquid to solid.

Condensation

The gaseous form of water is water vapor. Humidity is the amount of water vapor in the air compared to the amount of water vapor the air can hold at a certain temperature. Absolute humidity, relative humidity, and specific humidity are three terms used to refer to humidity.

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Moist Air Condensation

Evaporation Transpiration Evaporation

Precipitation Precipitation

Plants & Trees

Ocean

River Lake

Mountains Forest

Groundwater

FIGURE 1. The hydrologic cycle.

Absolute humidity is the amount of water in a volume of air. 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 percent humidity. Specific humidity is the ratio of water vapor to "dry air plus water vapor" in a particular volume.

Water vapor in the atmosphere may condense to the liquid or solid states to form rain, snow, and other forms of precipitation. Condensation refers to the change in matter of a substance to a denser phase. For example, 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, which is water in the form of droplets that appears on objects, such as grass in the morning or evening. As grass blades cool, water vapor condenses on the leaves forming water droplets. The temperature at which water droplets form is called the dew point. When air temperatures are low enough, dew freezes and is called frost.

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Precipitation

Precipitation includes the forms of water that fall from the sky. Rain, snow, sleet, and hail are common forms of precipitation, which is a product of condensation.

Collection

Precipitation falls to the oceans and land. Some precipitation is absorbed by plants. 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, which can be extracted through wells. Some wells tap into large underground reserves of water called aquifers, which contain vast amounts of water that have collected over thousands of years.

Evaporation

Evaporation is the process by which water leaves the surface of the earth and becomes water vapor. Three evaporative processes are transpiration, evapotranspiration, and sublimation.

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. The rate of transpiration is affected by humidity, temperature, wind, and soil moisture. The rate increases when humidity is low, temperatures are high, wind speeds are high, and soil moisture is adequate. Leaf shape, leaf surface area, and the number of stomata are other factors that determine the transpiration rate. Broad, thin leaves with an abundance of stomata transpire more than narrow, thick leaves with few stomata.

UNDER INVESTIGATION...

LAB CONNECTION: Photosynthesis

Green plants convert sunlight into chemical energy during photosynthesis. A number of environmental factors influence the rate of photosynthesis, including light, temperature, air, and water.

A laboratory exercise can be conducted to measure the rate of photosynthesis by testing for carbohydrates in plant tissues. The presence of carbohydrates in leaf tissue suggests that photosynthesis is active. Lugol's iodine turns a dark color in the presence of starch (carbohydrates).

The effect of water on photosynthesis can be tested. Withhold water from a plant until it is severely wilted. Another plant of the same species should be watered properly. Keep both plants under optimal light conditions. Take leaf samples from each plant. Extract the chlorophyll from the samples by immersing them first in boiling water and then in boiling alcohol. Remove the leaf when it has lost its green color and place it in a petri dish where it can be flooded with Lugal's iodine solution.

What do you expect the results to be?

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Evapotranspiration includes the water that transpires from plants and evaporates from soil.

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.

FUNCTIONS OF WATER IN PLANT GROWTH

Plants receive 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. For example, water is necessary for a plant to maintain turgor, which gives a plant rigidity and shape. Water aids in cell division and growth and provides internal pressure that helps roots push through soil. Water molecules are split in photosynthesis, releasing oxygen into the atmosphere and contributing hydrogen atoms to the manufacture of carbohydrates. Cellular respiration requires water, and energy released through the process is moved through the plant by water. Water dissolves minerals in the soil. Then 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. The process of transpiration helps the plant to draw water and minerals up through the plant. As water evaporates, it helps to stabilize the temperature of the plant.

WATER SHORTAGE AND EXCESS

Plants are dependent upon water for healthy growth. Too little water or an excess of water can have adverse affects on plant health.

Water Stress

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. Optimal plant growth and crop yields are achieved when a plant is never placed under water stress.

Several things occur when plants lack water. Wilting, or limpness to the plant tissue, is one sign of a lack of water. In addition, water stress might appear as

FIGURE 2. Plants growing in a drought area.

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