Plant Water Use From The Ground Up

Plants are more than eighty percent water by weight.  However, only a very small fraction of the water that moves through a plant is kept.  Plants are conduits that allow water to move from the soil, into its vascular tubes and exit through special gates called stomata which are located on the lower surfaces of leaves.  The water transport system, called the xylem, is a “straw” made of cells stacked directly on top of one another.  Depending on temperature, sunlight, wind and other factors, stomata will either close to slow the movement of water through the xylem, or they’ll open and allow the leaves to “sweat” water vapor into the air.

Water is the engine of all plant life.  It’s everywhere.  Even the air we breathe, work, and play in holds enormous volumes of the stuff (humidity).   It drives storms and buffers temperatures as it changes from a solid state, to liquid, to gas, and back again.  Only three percent of the Earth’s water is fresh.  The United States Geological Survey estimates more than two-thirds of fresh water is frozen and as much as thirty percent is underground.  A tiny three-tenths of a percent is readily available as surface water.

Imagine for a moment that you’re a molecule of water in the soil.  To a plant, you’re the most important yet limited natural resource it needs.  You’re made of a single oxygen atom flanked by two hydrogen atoms.  Your unique physical and chemical properties cause you to stick like glue to nearby molecules.  Thin cells on the outer surface of roots, called hairs, reach into the surrounding soil and draw you in.  Each molecule of water is taken up by a root hair and added one at a time to a slow moving microscopic river of dissolved minerals.  The stream flows against gravity as it moves toward the leaves.  Plants don’t directly spend any energy to move water in this way.

If a root hair pulled you in during the cool of the morning, you might feel the inward pressure of neighboring molecules crowding against you.  Plant cells are full of water in the morning.  Overnight, while the sun and wind are less intense, the stomata are closed and less water vapor is lost at the leaf surface.  A bottleneck of sorts develops.  You may even hear popping and cracking sounds of nearby cells as they bulge from water entering faster than it is leaving.

On the other hand, if you moved into a root hair during the hottest part of the day, you may feel a strong tug from the water molecule that entered just before you.  In turn, you’d be pulling hard on the one following you.  During hot summer days, evaporation through stomata often happens much faster than water can be taken up by roots.  If you were a water molecule under these conditions, you may occasionally hear the retort of entire columns of water in the xylem as they snapped like overstretched rubber bands.  Plant cells near the break would soon dry out and die if the water stream isn’t restored.

Left to its own devices, the water status of a plant is partially dependent upon its ability to react to injury, drought, heat, cold, poor quality water and any number of other environmental conditions.  Some of those conditions in West Texas include a soil pH above of 7.0 (alkaline), fourteen or fewer inches of rain annually, and temperatures that occasionally reach over 110 degrees Fahrenheit or dip below 25 degrees Fahrenheit.  While all of these must be considered, it is the quantity and quality of water which imposes the strongest limits on our selection of perennial landscape plants in the long run.

There is little that a homeowner can do to reduce the loss of plants from water stress.  Those things that we do have some ability to influence belong in one of three broad categories; we can select plants that perform well in our region, we can “train” landscape plants to use water more efficiently, and we can influence the availability of soil water.  We’ll take a look at these things in a second installment.

To learn more about the fascinating world of plants or how to become a Texas Master Gardener, contact your local Texas A&M AgriLife Extension office



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