Why Do Plants Need Water?

A plant's need for water is explored by almost all elementary school students in the United States, often while experimentally growing their own plants at home as they learn how soil, water, and sunlight combine to help these living things grow.  Folks who have ever forgotten to water their plants have learned first hand how important water is for their survival, for many a plant owner has checked on their plants after forgetting to water them, only to find them quite wilted.

All living things need water to stay alive.  However, plants need much more water than animals do, because they use much more water than do animals. Plants also contain more water than animals do; they are 90 percent water, while the percentage of water in animals goes as low as 75 percent.  

How much water a plant uses and therefore needs depends on its type, how much light it gets, and how old the plant is.

Importance of Water

The reason plants wilt when they are not watered properly has to do with something called turgor, which is the water pressure inside the cells that make up the plants’ skeletons.

Water enters a plant through its stem and travels up to its leaves. When a plant is properly hydrated, there is enough water pressure to make the leaves strong and sturdy; when they are deprived of water, the pressure drops and the plants wilt.

Water and Photosynthesis

Plants need water for photosynthesis.  Photosynthesis is the means by which plants create their food, and water is critical to this process.  

Water enters a plant’s stem and travels up to its leaves, which is where photosynthesis actually takes place.  Once in the leaves water evaporates, as the plant exchanges water for carbon dioxide. This is called transpiration, and it happens through tiny openings in the leaves, called stomata.

The water from the leaves evaporates through the stomata, and filling its place, entering the stomata from the air, is carbon dioxide.  Plants need carbon dioxide to make food.

Transpiration – this exchange of water for carbon dioxide – only occurs during the day.  This explains why dew is found on plants in the morning. The plants contain a lot of water at that time, because all night long water has been entering through the stem and being pulled up into the leaves, but not evaporating.  

Since it doesn’t evaporate at night, the water has no choice but to remain on the leaves as dew, because it can’t all stay inside the leaves of the plant. Then it evaporates during the day.

The water given off during transpiration cools a plant on a hot, sunny day, similar to the way human beings cool off when perspiring. A mature house plant can transpire its body weight daily. This means it gives off a lot of water! If people needed that much water, an adult would drink 20 gallons of water a day.

Someone has estimated that in one growing season, an acre of corn plants transpires 400,000 gallons of water, enough to cover that field with 15 inches of water.

Other Uses for Water in Plants

It makes sense that plants need water in the summer to help keep them cool. However, plants also need water during the winter. Water is absorbed through and stored in roots for winter. When the soil is frozen, that water can’t move very much. Some plants curl up their leaves in the winter, because this limits the amount of leaf surface exposed to drying winds, which limits evaporation and keeps the plant hydrated.

Water acts as a form of transportation, helping to move minerals around the plant to where they need to be. Water absorbs these dissolved nutrients from the soil, acting as a solvent. These nutrients would not be easily moved around the plant if they were in a solid form; being mixed with water helps them to move around. In summary, plants need water to survive!