Why Not Irrigate Plants With Seawater?
A recent report about how salt stops plant growth got me wondering about what’s the latest news regarding plant salt tolerance.
But first, let’s see why watering most plants (including all the major crop plants) with seawater would be harmful, and likely be lethal.
You can’t water most plants with seawater for pretty much the same reason you can’t drink seawater to keep you from dehydrating.
It’s because of the salt, primarily, the sodium chloride. More specifically, it’s because the level of sodium chloride is so high.
Seawater has a salinity of about 3.5% dissolved salts – predominantly sodium (Na+) and chloride (Cl-) ions. I did the math (so that you don’t have to), and it works out to be about 4.7 ounces of sodium chloride per gallon of water, or about a half a cup of table salt dissolved in a gallon of water.
I won’t go in to why it’s harmful for humans to drink seawater – you can read all about it here.
Let’s focus on why high salinity, specifically high sodium chloride levels, is harmful to most plants.
“Water, water, everywhere,…”
Back in the day, when I was teaching botany lab, one of the experiments we did in this class was to water beans and corn seedlings with artificial seawater and to see what happened. It didn’t take very long for the leaves of these plants to start wilting, as if they were drying out, even though the plants were in soil that was literally saturated with water.
This little experiment effectively demonstrated the first harmful effect of high salinity on plants, namely, the osmotic effects.
I should pause here and remind you that under normal conditions water moves from the soil into the roots, and from the roots up to the shoots, etc., passively, via osmosis.
Warning: The word “osmosis” is one of those unfortunate terms that induces the “MEGO” response in most people, especially students in class. (MEGO= “my eyes glaze over”.) So, let’s try to avoid the “O” word, shall we? And proceed….
So, putting it another way, the reason that plants can take up water from the soil and move it up to the leaves is NOT because they have little water pumps actively pumping the water into the roots and up the plant. There are no such little water pumps in plants. So how do land plants get and move water?
First, it’s important to keep in mind that water tends to move passively (diffuse) from a place of relative high water concentration to a place of relative low water concentration. Think of opening a bottle of smelly perfume at one end of the room. Pretty soon you’ll be able to smell the perfume at the other end of the room. Why is that? Because the smelly perfume has moved passively (diffused) from an area of relatively high concentration – the bottle – to the other end of the room, where perfume concentration is relatively very low. Given enough time, the perfume smell will eventually fill up the entire room, reaching what some might call a diffusion equilibrium.
What would be an example of high water concentration? Answer: pure water; pure distilled water, with no dissolved salts (solutes). And an example of relatively low water concentration? Answer: water with a lot of dissolved salts; classic example = seawater.
So, the reason watering plants with seawater causes them to wilt (draws water out of the plant) is because the seawater has a lower water concentration than the plant. And because the water molecules will passively diffuse from relatively high concentration to relatively low concentration, the seawater will draw the water right out of the poor plants.
But, you might ask, why doesn’t the saltwater just diffuse into the plant so that some kind of diffusion equilibrium is reached, sort of like the perfume smell eventually filling the empty room.
For the answer, we have to go back to osmosis. The key to osmosis is the presence of a semipermeable membrane, which allows water to pass through it, but NOT dissolved solutes, especially salts. All living cells, including plant cells, are surrounded by semipermeable membranes. So, water can easily flow in and out of the cells osmotically, but not dissolved salts. Indeed, to move dissolved solutes across the membranes, cells typically have to make little pores or transporters in the membrane in order to do so.
Whew! It can take a lot of time to try to explain why high-salt conditions tends to draw water out of plants. (For a more thorough discussion, please see Ref. 2 below.) But let’s get back to seeing what the most harmful effects of osmotic (salinity) stress are on plants.
Perhaps most important point is to note that that plant osmotic stress caused by saline soils results in many of the same physiological effects on plants as does drought (i.e., water stress).
Briefly, since plants rely on water uptake for growth, one of the first the observable effects of high salinity conditions on most plants is the inhibition or even cessation of growth.
Although less sensitive to water stress than plant growth, both protein synthesis and photosynthesis are also inhibited by plant water stress caused by saline environments.
The Toxic Effects of Too Much Salt
In addition to the osmotic effects on plants, the second problem when most plants are exposed to high salinity conditions (e.g., saline soils) is that sodium, and certain other ions, are toxic to plants when their concentrations are relatively high.
Despite the semipermeable membranes, under high salinity conditions, sodium chloride and other dissolved salts can leak into the cells.
Under normal conditions, the cytoplasm of plant cells typically contains a lot more (10x to 50x) potassium ions (K+) than sodium ions.
Abnormally high amounts of Na+ and high concentrations of total salts can inactivate some enzymes and inhibit protein synthesis.
At a high concentration, Na+ may displace calcium ions (Ca++) from the cell membranes, causing them to become “leaky”, that is, to lose their semipermeable nature. This can have disastrous, even lethal, effects on plant cells. Photosynthesis is also inhibited when high concentrations of Na+ and/or Cl- accumulate in chloroplasts.
These are just a few of the harmful effects that salt can have on plants. Although most plants, and virtually all crop plants, are sensitive to high levels of salt, there are some plants called halophytes that grow in water of high salinity, even seawater. How do they do it?
We’ll see how such plants “work” next time.
1. Zhu, J.-K. (2007) “Plant Salt Stress.” (PDF)
2. Bray, E. A. (1997) “Plant responses to water deficit.” Trends in Plant Science, Vol. 2, pp. 48-54. (PDF)
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