Water is the life-sustaining liquid needed both for the physical movement of materials through the plant and for the chemical processes that make all the plant’s food.
Use of Water by Plants
Plants use water as the medium for transporting dissolved minerals from the soil to the leaves. This upward flow of water and minerals occurs in microscopic tubes, called the xylem (ZYE-lem). The xylem tubes extend from the roots through the stem and into the leaf. Water then passes through the leaf material, where most of it evaporates (a process called transpiration) through microscopic openings on the undersides of leaves, called stomata. In the process of transpiration, the evaporating water cools the plants, just as a wet towel cools by evaporation.
Three factors can reduce the flow of water up the xylem to the leaves. The soil may be too dry, the root system may not be functioning properly, or the xylem may be plugged or cut for some reason. If the xylem flow is slowly reduced over a period of time, the leaves begin to show nutrient deficiencies, usually by turning pale green and yellow. If the xylem flow is suddenly reduced, and the transpiration rate is high, perhaps because of hot weather or wind, the leaf wilts, then scorches or sunburns as it overheats.
Once in the leaf, water is used in the process of photosynthesis. Photosynthesis is the chemical reaction that locks up the sun’s energy in the form of simple compounds that form the basis for all plant and animal life. Photosynthesis can only occur when the stomata are open. If a plant is stressed due to lack of water, it will close its stomata, which stops photosynthesis and plant growth.
The plant does not transpire all of the water in its system. Some of it flows back from the leaves carrying sugars and other chemicals to the flowers, fruits, growing tips, and roots. This second transport system in a plant is called the phloem (FLO-em). The phloem nourishes the other plant parts. If you peel back the bark of a tree, the phloem is visible as the white part of the bark. The flow of material in the phloem is by diffusion and is much slower than water movement in the xylem.
Plant Requirements for Water
The plant’s size and its ability to conserve water affect its need for water. Plants native to desert or windy locations have developed ingenious mechanisms to conserve water. Such plants often have small, thick leaves with waxy coatings. Other plants are adapted to areas where it rains every day, such as tropical rain forests. These plants can’t tolerate even short periods of drought. They often have large, thin leaves. Most plants are between these extremes. They are adapted to occasional drought, but tolerate regular water, too.