A common question when a person buys a potted houseplant or a new plant for outdoor planting is "How much water does it need?" Most people understand that plants need water to grow and thrive, but many people do not understand the physiological mechanisms that drive a plant's need for water.
Movement of Nutrients
Nutrients inside of a plant are water soluble, meaning that they dissolve in water. Water inside the plant moves nutrients from one part of the plant, where the nutrients are produced, to other parts of the plant where the sugars, proteins and other nutrients are needed.
Source of Hydrogen
Water is one of the basic building blocks of the sugars that a plant produces to create energy to grow. Water is made up of two atoms of hydrogen and one atom of oxygen. A plant splits the oxygen off and discards it as a waste product. The hydrogen combines with carbon dioxide in the air to form the sugars the plant needs to survive.
Cell Growth and Division
Plants grow in a couple of different ways. One is through cell division. The other is through cell expansion. Cell division causes growth through the creation of new cells. These new cells need water to be formed. Cell expansion growth is caused by cells that fill with water, much like a balloon fills with air. Both cell division and cell expansion rely on the presence of water in the plant.
Different plants have different water requirements. Some plants store the water they need over long periods to help the plant deal with long periods of drought. Cactus, for example, store water inside a thick stems and leaves. Succulents also store water for later use. Other plants, however, need regular water to thrive. Fruit bushes and trees, for example, need water to allow the fruit to properly develop.
Problems With Lack of Water
As a plant begins to suffer from water stress due to the lack of water, it is less able to make the sugars it needs to survive. Cells may break down and die, resulting in visible wilting of the plant. In addition, a plant that is not getting enough water may not be able to transfer nutrients from the roots to the upper part of the plant.