1. Lesson C4–1 Understanding the Importance of Water in Plant Growth

2. Terms. Evapotranspiration Hydrologic cycle Irrigation Solvent
Translocation
Transpiration
Turgor
Wilting

3. 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 cacti are about 90 percent water.

5. Water molecules consist of two atoms of hydrogen and one atom of oxygen. As a bipolar
molecule it has positive and negative charges. It is a good coolant for organisms. Water is a universal solvent, meaning nearly everything will dissolve in it.
The exchange of water between earth and its atmosphere as a result of evaporation and
condensation is the hydrologic cycle.

7. Water plays an important role in all plant functions.
Water is necessary for a plant to maintain turgor, which gives a plant rigidity and shape.
Water aids in cell division and growth.
Water 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.

8. Energy released in respiration is moved through the plant by water.
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.

9. Water is pulled up through the plant by transpiration or the evaporation of water from stomates and plant surfaces. Evapotranspiration includes the water that transpires from plants and evaporates from soil.
As water evaporates, it helps to stabilize the temperature of the plant.

10. Plants are dependent upon water for healthy growth.
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.

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

12. Prolonged water stress results in permanent wilting and damage to plant tissues.
Plants differ in their ability to draw water from the soil and to conserve water in their tissues.

13. 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.

14. 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.

15. 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.