Have you ever wondered how a great redwood can bring water from its roots to its leaves? It can seem like a simple process for a small shrub or food crop, but the uptake and transport of water that defies gravity is a feat of natural engineering.

In this article, HerbSpeak will guide you through the process of how plants absorb water, why plants need so much water, and a quick science experiment that you can do at home to see how plants absorb water in real-time.  

Why Do Plants Need So Much Water?

While growing herbaceous sprouts growing on your windowsill might not need that much water each day, grand redwoods can absorb over 500 gallons of water each day. (1) Even a single, irrigated crop plant can absorb several hundred liters of water each day during the summer.

Why do plants need so much water?

Water is one of the essential elements of life in the plant kingdom. Without it, plants could not function, grow, or thrive. Water is one of the key ingredients that allow a plant to move nutrients from the soil into its system, as well as transfer energy captured through photosynthesis into glucose.

Plants absorb so much water because they only retain about 5% of the water they absorb through the soil; the rest evaporates into the atmosphere.

Why Do Plants Transpire?

This process of water evaporation through the plant’s system is called transpiration. So much water is lost in this process because plants must photosynthesize, and to do so, small pores in the leaves called stomata must open to allow absorption of carbon dioxide. However, when these pores open, the plant loses water.

“A single redwood can transpire as much as 500 gallons a day. That’s as much water as you might use in an hour-long shower!”

- California Department of Parks and Recreation

This is beneficial for the plant because it allows the plants to continue absorbing water through the roots. The evaporation of water vapor through the stomata creates a vacuum on the plant’s interior water pathway, the xylem, pulling the water up towards the leaves of the plant so that it may continue the process of photosynthesis and nutrient uptake through the soil.

Factors Affecting Rates of Transpiration

Many factors affect the rate of water uptake in plants, and you might not be surprised to learn that it is, to some degree, dependent on the health of the plant. (2) Other factors include wind speed, the intensity of light, humidity, and the temperature around the plant.

“Water flow and loss are controlled through stomata and regulation of hydraulic conductance via aquaporins. When water availability declines, water loss is limited through stomatal closure and by adjusting hydraulic conductance to maintain cell turgor. Plants also adapt to changes in water supply by growing their roots towards the water and through refinements to their root system architecture.”

- Johannes Daniel Scharwies

When the temperature is increased around the plant, the rate of evaporation and diffusion occurs at higher temperatures, meaning the rate of transpiration is increased.

Similarly, wind speed and light intensity increase the rate of transpiration because, in the case of wind speed, the air is being moved around quickly enough that humidity or water vapor is pulled away from the leaves of the plant, allowing more water to evaporate through the stomata.

In the case of light intensity, the rate of transpiration is increased because the stomata are open wider, allowing in much more carbon dioxide as the plant takes advantage of a stronger ability to photosynthesize. The wider opening allows more water to evaporate into the air.

Higher humidity will decrease the rate of transpiration because the air around the plant is already moist and it reaches a sort of stasis where the moisture inside the plant and the moisture outside the plant are similar.

What Affects Water Uptake in Plants?

Factors that affect water uptake in plants are different from factors that affect transpiration since it is on the opposite end of the process.

The rate of water uptake is affected by transpiration because only so much water can be in the plant’s tissues at once. (3) When more water escapes through the stomata, the more water can be absorbed through the roots.

Environmental factors also play a large role in how plants absorb water. The available moisture in the soil is an obvious but important factor. Soil temperature and the aeration level of the soil are also critical factors to consider.

In the case of soil moisture, the roots will passively absorb water that is present in the soil. This water is known as capillary water, which is the moisture that remains after gravitational water has been drained. The more moisture within the soil, the more the roots will attempt to absorb.

If the soil becomes water-logged with capillary water and left, the level of soil aeration will decrease and the roots will begin to degenerate, or rot, in the anaerobic environment. Different roots will have different tolerance for how much water each cell can hold.

How much oxygen is in the soil – soil aeration – is a critical factor in the transport of water between the soil and roots. Well-aerated soil allows roots to absorb water faster, while compacted soil will prevent roots from absorbing as much water, and there will, in most cases, be less moisture present in compacted soils.

Soil temperature is important not just for water uptake but also in activating the enzymes that control root hair growth. Lower or colder temperatures decrease the permeability of root cells and increase the viscosity of capillary water in the soil. This means that not only is it more difficult for the water to move, but the root will have a more difficult time absorbing the water.

This factor in how plants absorb water is one of the main reasons why plants tend to go into a cold dormancy in areas with cold winters. Normal rates of water absorption occur between 68°F and 95°F.

How Does Water Move Through a Plant?

Water moves through a plant from the soil, into the roots, through the plant cells, and finally ends in the leaves where it is then transpired out through the stomata.

Plants utilize two methods of water migration through cells to absorb water called osmosis and diffusion. Osmosis is the action of water molecules passing through permeable barriers, such as the cells of roots, while diffusion is the act of water equalizing itself. Higher concentrations of water will always want to equalize itself on the opposite side of that barrier, so more water is absorbed into the root cells.

In other words, the nature of water is that it must always equalize itself on both sides of the cell, allowing the plant to absorb more as it pulls the water up the stem.

Roots, and root hairs known as cilia, are semi-permeable, which means they allow water to pass through.

Most plants can absorb water on all parts of the plant to some degree, from the leaves to the stems and flower buds. The roots, however, are the primary source of water uptake for plants as their primary role is to uptake nutrients and water.  

See the xylem as circular, blue dots in this microscopic cross-section.

From the Soil into the Roots

At the very tip of the root, called the root cap, the root is still growing and searching for water. It is the most sensitive and permeable part of the root, allowing water to absorb easier. Water is also absorbed through the fine roots, also called cilia or root hairs. These root hairs are non-woody protrusions that both increase the surface area of the root and increase absorption.

 

From the Roots into the Stem and Leaves

Once water has been absorbed, it is the plant’s job to ensure it travels from the ground, through the stem, and into the leaves where it can then transpire. This occurs through capillary action; a phenomenon of water movement that occurs because water adheres to itself, as well as to other surfaces. This results in the water “pulling” itself up through a tube or, in this case, a root.

This is the same action that is occurring when you dip the tip of a paper towel into water and it continues to travel upwards through the paper fibers until gravity is stronger than the surface tension.

The roots contain a tube-like structure called the xylem, which, in vascular plants, is a series of tissue pathways that conduct water and dissolved nutrients from the root. This pathway extends from the roots, through the stem, to the leaves.

As transpiration in the leaves occurs, it will create a vacuum in the xylem as the leaf’s water pressure dissipates. This helps bring the water up through the narrow pathway of the xylem.

Water travels from the roots and into the stem through this pressure difference with the help of capillary action.

Once the water is pulled up the xylem through the vacuum of transpiration in the leaves, it must travel up to the leaves to finish the cycle again.

See Plant Water Uptake in Real Time: A Home Experiment

In this experiment, you will be able to see the capillary action of water in motion throughout the plant as it is transported through the xylem tissue into the leaves.

Of course, the roots are not present in the celery sticks, but this just means the water has a more direct route into the xylem. The same experiment would work with watering rooted plants with dyed water.

This process can work with any vascular plant, and this is how most artificially-colored bouquets are made, to preserve the lifelike feel of the petals and leaves.

Experiment - Colored Water with Celery

This experiment showcases the capillary action and transport of water through the xylem of vascular plants. After coloring the water and allowing the plant plenty of time to absorb the coloration, you will not only see the colored leaves, but also the colored xylem tubes in a cross-section.
Prep Time 1 d 1 hr
Total Time 1 d 1 hr
Course Science
Cuisine Science
Servings 3 celery

Equipment

  • 3 mason jars

Ingredients
  

  • 3 sticks celery leaves on
  • 3 colors food coloring

Instructions
 

  • Place 3 mason jars (or drinking glasses) on a countertop. Fill each jar halfway with room-temperature water.
  • Add a different color to each jar until the water is vibrant in each one.
  • Place 1 stick of celery, leaves on, into the water and leave overnight. If the leaves are not colored, leave an additional night.

Notes

Image - York.ac.uk

Once the leaves have become colored, dissect the celery: try cutting cross-sections and splitting the leaves to see where the xylem tubes are!
Thumbnail Image Credit: GardeningKnowHow
References
  1. Johannes Daniel Scharwies, Water Transport, Perception, and Response in Plants, https://pubmed.ncbi.nlm.nih.gov/30747327/
  2. California Department of Parks and Recreation, Survivors Through Time, https://www.parks.ca.gov/?page_id=24728
  3. Paul J. Kramer, Absorption of water by plants, https://link.springer.com/article/10.1007/BF02861198