How Plants Use Water: Wetting Fronts, Transpiration, Root Zones and Photosynthesis

When rainwater falls onto soil, or when water is applied through irrigation, it doesn’t just pool on surfaces. If the surface is impermeable, like concrete, caliche or other hard surface, then the water simply takes the path of least resistance and flows off.

Water Acts Differently on Porous Surfaces

if the surface is permeable (porous) a different process takes place. It seeps down through layers of soil, filling that layer to capacity. Once that layer is filled, it moves down into the next layer.

In sandy soil, water moves quickly, while it moves much slower through clay or silty soil.

The movement of water through soil layers is called the “wetting front.”

The Wetting Front and The Root Zone

When enough water gets to the surface (either by rainfall or irrigation), it moves the wetting front into the root zone of plants. At that point, the roots absorb the moisture. This moisture then moves up the roots to the stem, leaves and fruit of the plant. Roots must have a constant supply of moisture, because of two factors; transpiration and evaporation.

Plant Transpiration

Leaves have thousands of microscopic openings, called stomates. Water (in the form of water vapor) is lost through these stomates. This process is called transpiration. Transpiration is a constant process in plants (just as respiration is in animals). Meanwhile, in the root zone, roots must absorb a constant supply of water to replace that lost through transpiration.

Evaporation

In addition to water vapor transpiring through stomates in the leaves, water also evaporates from the soil into the air. The combination of these two processes is called evapotranspiration.

Evapotranspiration

The rates of evapotranspiration vary, depending on the length of days, temperature, humitiy, cloud cover, wind, number of plants growing in a given area, size of those plants and mulch.

Necessity of Water

Water is an essential part of the physiological process of all plants. Through water, necessary minerals are moved from the roots to the parts of the plants that require them. Water moves chemicals from one part of the plant to another. It is also required for photosynthesis and for metabolism. It also helps cool plant surfaces (through transpiration).

Plants not receiving enough water will be lower in production of fruit, seed, roots and flowers. Without enough water, plants will close their stomates. This will result in leaf curling and rolling, reducing leaf and stem growth and fruit yield.

If plants are showing some of these symptoms, roots may not be taking in enough water. One of the reasons could be that the “wetting front” is not reaching all the roots of the plant.