How Water Works with Plants

This article was inspired by a similar article from the South Africa Fern Society Newsletter on the subject of Soil Moisture Tension and answering the question “I water my plant, why is it dying? Some research led to that answer and a bunch of others about the relationship of plants and water. Interestingly, it all starts at the top.

Much of the information in this article came from web pages. The included John Kimball’s Biology Pages (www.ultranet.com/~jkimball/BiologyPages/) and the Iowa State Agronomy 154 pages (www.pals.iastate.edu/agron154/Agron_154/). This discussion includes some technical terms that will be explained as we go.

Let’s start with plants’ breathing. That’s called “transpiration.” Transpiration is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. Stomata are the pores on the leaves that allow gases to be exchanged.

Air that is not fully saturated with water vapor (100% relative humidity) will dry the surfaces of cells with which it comes in contact. So, the photosynthesizing leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots and stem. Xylem is the “tubing” in plants that conducts water and dissolved minerals from the roots to all the other parts of the plant. Phloem is the tubing that conducts the nutrients created during photosynthesis from the leaves to the roots, growing tips, and flowers.

All right, that’s all fine. Plants breathe out water. It evaporates, and needs to be replaced. However, transpiration is not simply a hazard of plant life. It is the “engine” that pulls water up from the roots to:

• supply photosynthesis (1%-2% of the total)
• bring minerals from the roots for biosynthesis within the leaf
• cool the leaf

So, since all the parts of the xylem are made up of non-living tissues, how does water get from the roots to the top of a 60-foot tree fern? The path taken is:

soil -> roots -> stems -> leaves

The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells). Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration. Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids (long tubular cells). What forces water through the xylem?

Here are some observations:

• The mechanism is based on purely physical forces because the xylem vessels and tracheids are lifeless.
• Roots are not needed. This was demonstrated over a century ago by a German botanist who sawed down a 70-ft oak tree and placed the base of the trunk in a barrel of picric acid solution. The solution was drawn up the trunk, killing nearby tissues as it went.
• However, leaves are needed. When the acid reached the leaves and killed them, the upward movement of water ceased.
• Removing a band of bark from around the trunk - a process called girdling - does not interrupt the upward flow of water. Girdling removes only the phloem, not the xylem, and so the foliage does not wilt. (In due course, however, the roots - and thus the entire plant - will die because the roots cannot receive any of the food manufactured by the leaves.)

In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above. As noted, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf. This is called the Transpiration-Pull theory. However, even the best vacuum pump can pull water up to a height of only 32 ft or so. This is because a column of water that high exerts a pressure (~15 lb/in2) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia or Douglas fir that may be 300 feet high?

Taking all factors into account, a pull of at least 150 lb/in2 is probably needed. The answer to the dilemma lies in the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form.

Polar molecules, such as water molecules, have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere (the hydrogen atoms in water). Thus, when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. The force of attraction is called a hydrogen bond. Each water molecule is hydrogen bonded to four others.

When water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. Tensions as great as 3000 lb/in2 are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires. Because of the critical role of cohesion, the transpiration-pull theory is also called the cohesion theory.

Water is held in the soil by a force. The force holding water in the soil is referred to as the soil moisture tension (SMT). To remove water from the soil, plants must exert a force greater than the soil moisture tension.

The plant doesn’t really care about the actual amount of water in the soil. It cares about the soil’s water-holding forces, how hard the soil is holding on to that water. For example, a clay loam may have an available water content of 1.2 inches/foot. And a sandy loam may have the same available water content of 1.2 inches/foot. A plant in the clay loam is probably feeling much more stress than the plant in the sandy loam. Finer soils will hold more water than coarse soils. But they will also hold on to it much tighter for any given water level.

To a plant, all other things being equal, it doesn’t care if it is growing in beach sand or black clay. If the moisture tension is equal, it feels the same amount of stress, regardless of the actual amount of water present. And it will develop at the same rate.

Temporary wilting can occur in any soil when the atmospheric demand for water exceeds the plants ability to extract water from the soil. This usually results from a temporary climatic condition. The plant will recover when conditions change. For example, the air usually cools down and increases in humidity after nightfall. Permanent wilting occurs when the soil moisture tension is too high for plants to extract water from the soil fast enough to sustain its life. This usually occurs when the soil moisture tension reaches 15 atmospheres. The condition is not reversible when climatic conditions change.

When plants remove water from the soil, oxygen is “pulled” into the soil to fill in the spaces. Overwatering can reduce the amount of air available in the root zone in either the field or greenhouse, although it is more common in soilless culture because of the reduced drainage and smaller rooting volume.

In either system, low oxygen levels (less than 3 gm l-1) reduce not only nutrient uptake, but growth and yield as well. In the yard, this is most frequently a problem with poorly-draining soils or those with sub-surface hardpans. In soilless culture, low oxygen levels generally occur in hot weather because the available oxygen in the solution decreases as the root zone temperature rises. It is also more likely to occur with peat substrates because they have a higher water-holding capacity, i.e. drain less freely, than rockwool and perlite.

Low levels of oxygen will stress your plant, making it more susceptible to disease, possibly drowning roots. With a reduced root ball, the plant cannot take up water and nutrients as well, and can display wilting symptoms just as if the were too little water in the soil. However, with a container plant [where good drainage is harder to create], the best way to tell if you are underwatering or overwatering is to knock the plant out of the pot and look at the soil and roots. If the soil is moist and not soggy and the roots look good, your plant is suffering from some other problem.

One last note. Salt burn, the yellowing and browning off of the edges of leaf tissue, is caused by too high a level of dissolved salt in the water (too much salt, too little water). That’s a good reason to ensure that you leach the salts out of the soil with a good thorough watering.