How Salt Stresses Plants (Or Not)

5076907896 5740feb4e4Why 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….

3041145016 57fe4975cfSo, 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?

Here’s how:

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.

Good question!

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.

2477266021_17875c4ac3Whew! 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.

References

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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10 Comments

  1. In my science class we put an egg in water and added A LOT of salt. after the weekend we looked at the cell, and it grew from 80.8 to 87.4. Everyone in my class is wondering why/how it grew, and I was hoping you could provide an explanation.

    • Hmmm……
      I’m not sure that I understand your question.
      Was it a raw egg? What is the “cell”? You say “it grew from 80.8 to 87.4” Is this size (diameter), weight, or ? (Pro tip: Always assign units of measure to numerical values such as these.)
      My wild guess is that the “cell” had a lower water concentration than even the salty water, so water passively diffused into the “cell”, causing it to expand or “grow”.

    • WHY USE SALT FOR FERTILIZER?

      One of the myths that surround commercial fertilizers is that the salts they contain are “harsh” on the biology of the soil. The reality is that salt is essential to all of life. Either too much or too little can harm. Are fertilizers indeed too salty?
      Fertilizer salts form soluble ions in soil water. Increased concentration of ions increases osmotic pressure and decreases water potential, making it harder for plants to take up water. This is why plants affected by “fertilizer burn” look about the same as if they had been stricken by drought. They can’t get the water, because there’s too much salt in it.

      Fertilizer doesn’t have to burn, though. It’s all a matter of dosage. Plants can’t grow without salt, either. The nutrients they need are salts. The dissolved ions are exactly the form they take up. As long as the dosage is controlled, there is no harm applying a salt to the soil.

      The kind of salt is important. Specific salt ions have greater effects than others. The ammonium ion in particular can release free ammonia, especially at higher soil pH. Ammonia moves directly into plant cells. High concentrations can prevent root growth and even kill seedlings. On the other hand, phosphate ions hardly pose a salt hazard at all, since they never get to high concentrations in soil water.

      Salt is often associated with sodium, because common table salt is sodium chloride. Sodium ions can destroy soil structure and clog the flow of soil water. But there’s hardly any sodium in most commercial fertilizers.

      The chloride ion is one of the most soluble. Grapevines, woody trees, and many legumes are sensitive to it. Research in the southern states showed some soybean varieties to be sensitive to chloride. But research in many other places has found muriate of potash (potassium chloride) to be an effective source of potassium for soybeans grown in deficient soils. And crops like wheat and corn show great benefits from fertilizing with chloride.

      Several fertilizers aren’t truly salts. Urea, for example, is a soluble substance that isn’t a salt. Nevertheless, its solubility means it can have an osmotic effect, just like any other fertilizer. It also quickly decomposes to form a salt–the ammonium ion. Elemental sulfur is neither salt nor soluble–but it oxides into sulfate, a salt.

      Manures and composts contain inorganic salts, organic salts, and insoluble organic forms of nutrients. But their salt content per unit of nutrient may not differ much from fertilizer, since their nutrient content is a lot lower, and they contain salts not necessarily needed for the crop. Also, as they decompose, the nutrients turn into salts.

      How to avoid salt injury? Guidelines for safe rates are specific for each crop and are based on distance from the seedling, soil texture, soil moisture content, and the specific ions in the nutrient source. For ammonium, soil pH is an additional consideration.

      A high salt content is a consequence of a unique advantage commercial fertilizers possess. Using sources with concentrated soluble nutrients cuts transportation costs–less fuel is wasted in transporting non-nutritive material. In order to utilize that advantage, commercial fertilizers need to be applied at judicious rates.

      Why use salt for fertilizer? Because plants use salt for food.

  2. How is it that water from the Salton Sea is being used for irrigation, since it has nearly a 50% higher salt content than the ocean?

    • Salton Sea water is indeed too salty for irrigation.
      The crops growing adjacent to the Salton Sea have been irrigated for decades with water from the Colorado River.
      Actually, this is why the Salton Sea still exists. Without irrigation-water runoff from these fields of crops, the Salton Sea would likely have completely dried up years ago.
      Thanks for your comment. (BTW, I used to live in San Diego.)

      • Thanks for your reply and information. Why does it appear from satilite pics that canals carrying water to the crops in the Imperial and Coachella Valleys are connected directly to the Salton Sea? I was reading an article written by Sue McClurg with the San Diego State University extension office, and the article discusses all the agricultural benefits of the irrigation from the Salton Sea. Considering the Sae’s high salinity level, I am confused.

      • Thanks for your reply and information. Why does it appear from satilite pics that canals carrying water to the crops in the Imperial and Coachella Valleys are connected directly to the Salton Sea? I was reading an article written by Sue McClurg with the San Diego State University extension office, and the article discusses all the agricultural benefits of the irrigation from the Salton Sea. Considering the Sea’s high salinity level, I am confused.

        • No need for confusion, because there is no way Salton Sea water is being used for irrigation. (If so, then people would be irrigating crops with seawater.)
          Read McClurg’s article more carefully and I think you’ll discover that she means that the Salton Sea is of economic importance to irrigation because it serves as sort of large drainage pool. This allows farmers to flush their fields with Colorado River water so as to prevent salt build-up in the soil. The canals carry this drainage water from the fields to the Salton Sea.

  3. CHevere gracias para doinarma information para mi essay

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