Ever think about how the water you — or Mother Nature — adds actually moves inside your plants? All gardeners know that plants need water, but how does water in the soil get to the top of the plant?
Water moves through the entire plant through an intricate plumbing system, or series of cells, that starts at the roots, where the water is absorbed from the soil. The water moves upward through the cells to all parts of the plant where it is diffused from the leaves into the surrounding air. This sounds simple, but there is considerably more to it.
First of all, the water enters the roots by a process called osmosis through the surface of the root cell. Osmosis is the movement of water from an area of higher concentration (the soil) to an area of lower concentration (the root).
The initial process is rather slow, because incoming water is passed from cell to cell inside the root’s surface. Osmosis is not the final answer in water movement, because pressures generated through this process would allow water to move less than an inch. This doesn’t explain the presence of water at the top of a 300-foot tree or even a three-foot tomato plant.
Osmosis is the start of water movement in the plant. The power needed to suck water up from the soil into the plant is furnished by the high absorptive potential of dry air. As water moves from the wet interior leaf to the dry air through a process called transpiration, more water is pulled into the leaf tissue from the cell structure within the plant.
Another phenomenon that aides in water movement is the cohesive nature of water. Water molecules stick together strongly. That’s why it’s possible to fill a cup slightly above the brim with water. Cohesive means that the water is pulled into the leaves by a long chain of plant cells running from the root hairs to the leaf area. As water is lost through the leaf, a vacuum is created in the system and new water moves in to fill the void.
It has been calculated that extreme pressures can be generated in the plant system to allow for the movement of water in something as small as a rose bush or a giant redwood tree. If a plant is going to function properly, water must flow continuously through its system.
When the total plant system is filled with water, the plant has “turgor” and is seen as an upright, fully expanded plant. On the other hand, when a plant loses more water through transpiration than it can take in through the root system, the plant will lose its internal water pressure and wilt. If water loss continues, the total plant system may collapse.
It may sound like a plant is similar to a straw, where water enters the bottom and is lost out the top. But Mother Nature is smarter than that. She has built is a set of checks to prevent rampant water loss. Small pores, called stomata, allow water to escape into the atmosphere. They contain a mechanism that controls the rate of water loss. At the edge of the stomata are pairs of guard cells that regulate the size of the opening like small pressure valves. They close in times of reduced water supply and open when water is available.
The value and function of the root and its root hairs should not be overlooked in this water movement process. The root on a healthy plant is actively growing and searching for water.
A balance exists between the top shoot growth and root system on the plant. When soil conditions are suitable, a root system will continue to grow and provide needed water to replace that, which is lost from the leaves.
The microscopic root hairs are the real workhorse of any root system. Scientists have checked the roots on a 4-month-old winter rye plant and found a growth rate of more than three miles a day. New root hairs were produced at the rate of more than 100 million a day. These great numbers explain the plant’s ability to maintain the movement of water and nutrients into the total system.
Water accounts for 80 to 95 percent of the weight of most soft tissue plants. The living plant cells usually contain 95 to 98 percent water. Water can be found inside the plant cell, the cell walls and inter-cellular spaces to form a continuous liquid phase throughout the total plant.
Why is water so important? Water keeps the plant erect. It serves as a solvent and transport agent for nutrients, gases and plant products to move into and through the plant.
Water also cools the plant, the same way sweating cools people. When water transpires from the leaf surface, it takes heat energy with it and maintains optimum internal temperatures. Finally, water is an essential element in the process of photosynthesis, the system in which sugars and related plant foods are manufactured to feed the root system.
To sum up the entire process, all life depends of water. Water is the most essential plant nutrient because without it, all plant functions stop operating.
Bill Lamont is professor emeritus of vegetable crops in the Department of Plant Science at Penn State and can be reached by email at firstname.lastname@example.org.