How Plants Work

The Increasing Need for More Salt-Tolerant Crop Plants

The salinization of soils is a very serious agricultural problem. Because saline soils can adversely affect the establishment, growth, and development of crop plants, leading to huge losses in yield, the significance of soil salinity in reducing agricultural productivity is enormous.

Unfortunately, the salinity of croplands is increasing steadily in many parts of the world, especially in arid and semiarid regions. And the effects of “global weirding” will likely exacerbate the situation.

Despite efforts to address this problem using better soil management and cropping practices, it remains a growing threat to agriculture worldwide.

It’s clear that these improved management practices must also be accompanied by a better array of salt-tolerant crop plants available to farmers.

But where will such plants come from?

Breeding, Grafting, and Genetically-Engineering Crop Plants for Increased Salt Tolerance

Some salt-tolerant crop plants already exist, such as some varieties of barley. Traits that help confer salt tolerance in these barley varieties may be bred into other cereals, such as wheat, for example. Plant breeders may also use wild relatives of existing crop species as sources of “salt-tolerance” genes.

A recent report regarding the evolution of salt tolerance in grasses may explain why it’s been so difficult to breed salt tolerance into cereals despite the fact that there are many naturally salt-tolerant grasses in the world. Briefly, this study indicates that wild grasses may be able to adapt to saline conditions relatively quickly (from an evolutionary perspective), but then discard these adaptations if not needed. This suggests that plant breeders probably have a good chance of being able to generate salt tolerant cereal crop plants, but it also says that these adaptations likely come with significant costs to the plant.

Another approach that may be viable, using crops such as tomato, is grafting salt-sensitive, but desirable, varieties onto salt-tolerant rootstocks. (e.g., see ref. 2 below)

As plant biologists learn more precisely about how salt tolerance works at the cellular and molecular levels in plant cells, specific genes may also be introduced into crop plants by genetic engineering.

Such examples include proteins involved in the membrane transport of salts in plant cells. (e.g., see Ref. 3 below)

Domesticating Halophytes

Instead of trying to make current crop plants more salt tolerant, why not domesticate plants that are already salt tolerant to use as crop plants?

As described in Ref. 4 below: “Our approach has been to domesticate wild, salt-tolerant plants, called halophytes, for use as food, forage and oilseed crops. We reasoned that changing the basic physiology of a traditional crop plant from salt-sensitive to salt-tolerant would be difficult and that it might be more feasible to domesticate a wild, salt-tolerant plant. After all, our modern crops started out as wild plants.”

Unfortunately, this approach has not proved to be successful so far, except in limited cases of use as forage plants.

Can Mycorrhizae Help Save the Day?

Mycorrhizae are symbiotic relationships that form in the soil between certain fungi and most native plants. One of the benefits plants may get by forming mycorrhizal associations is improved salt tolerance, which has been well documented. (for example, see Ref. 5 below)

It may be possible to improve salt tolerance in crop plants by encouraging mycorrhizal associations under cultivated conditions.

Also, by learning how mycorrhizal fungi help improve plant salt tolerance may help in developing salt-tolerant crops.

Seawater Greenhouses in the Desert?

Finally, it’s worth mentioning that there are people who are using seawater in a much different approach in trying to grow crop plants in deserts.

Briefly, this approach is to use solar power to generate electricity to convert seawater into freshwater. This freshwater is then used to irrigate plants in greenhouses. (For example, please see references 6, 7 & 8 below.)

Bottom line: At least two things seem clear. Firstly, we need to know a lot more about the basic biology of salt-tolerance mechanisms in plants. Secondly, solutions to the problem of ever-increasing saline soils will likely come from multiple approaches.

References

1. Barrett-Lennard, E. G. and T. L. Setter (2010) “Developing saline agriculture: moving from traits and genes to systems.” Functional Plant Biology, Vol. 37 , pp. iii-iv. (PDF)

2. Sanchez-Bel, P., Egea, I., Flores, F. B. and Bolarin, M. C. (2012) “Tomato: Grafting to Improve Salt Tolerance.”, in Improving Crop Resistance to Abiotic Stress, Volume 1 & Volume 2 (eds N. Tuteja, S. S. Gill, A. F. Tiburcio and R. Tuteja), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. doi: 10.1002/9783527632930.ch42 (Abstract)

3. Mian, A. A., et al. (2010) “Improving crop salt tolerance: anion and cation transporters as genetic engineering targets.” Plant Stress 5 (PDF)

4. Glenn, E. P., J. J. Brown and J. W. O’Leary (1998) “Irrigating crops with seawater.” Scientific American, August 1998, pp. 76-81. (PDF)

5. Evelin, H., R. Kapoor, and B. Giri (2009) “Arbuscular mycorrhizal fungi in alleviation of salt stress: a review.” Annals of Botany, Vol. 104 pp. 1263-1280. (PDF)

6. Jonathan Margolis (2012) “Growing food in the desert: is this the solution to the world’s food crisis?.” The Observer, Saturday 24 November 2012. (Full Text)

7. Sundrop Farms

8. Seawater Greenhouse

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Yes, You Can Water Some Plants With Seawater

Although salty water and saline environments are usually harmful to most plants, why should anybody care about salt stress on plants?

Here’s a good answer to that question:
“Ubiquitously, no toxic substance restricts plant growth more than does salt. Salt stress presents an increasing threat to plant agriculture. Among the various sources of soil salinity, irrigation combined with poor drainage is the most serious, because it represents losses of once productive agricultural land. The reason for this so-called ‘secondary salinization’ (as opposed to primary salinization of seashore salty marshes) is simple: water will evaporate but salts remain and accumulate in the soil.” (from Ref. 1 below)

So, it’s clear that soil salinity is of serious concern to people involved in agriculture, and the evidence suggests that this problem is getting worse. What are some possible solutions?

Well, the obvious solution is to try to make crop plants more salt tolerant. But how?

One approach plant scientists have used is to study plants that are naturally salt tolerant and see how they are able to grow, and even thrive, under saline conditions. Such plants are called “halophytes”, and some halophytes can even grow in seawater. (By the way, plants unable to resist salts to the same degree as halophytes are sometimes called glycophytes, “sweet plants”.)

The two most well-known examples of halophytes are mangroves and saltbush (genus Atriplex). Mangroves usually grow at the intersection of the land and the sea, in the tropics and the subtropics. In contrast, saltbush is typically found in desert environments, with alkaline soils.

How do such plants tolerate such high salt levels? Plant scientists have been investigating this question for decades and have found three common strategies that halophytes typically use in order to tolerate saline conditions: exclusion, sequestration, and excretion.

Perhaps the best strategy is exclusion. In this case, the roots actually filter out sodium chloride by using waxy layers around cells in order to force the saline solution to have to pass across semipermeable cell membranes. Such membranes allow water to flow through, but resist the passage of the highly charged sodium and chloride ions.

In highly saline environments, some sodium chloride is able to leak through the exclusion barriers and into the roots and up to the shoots and the leaves along with the flow of water. Halophytes can use two other strategies in order to deal with this problem.

What do people often do with stuff that they no longer want? Answer: They stick it in the garage. And that’s how many halophytes deal with toxic sodium; they stick it in the plant cell’s garage, a.k.a., the vacuole.

Most, if not all, enzymes in halophytes are just as sensitive to sodium as glycophytes. So most halophytes have special sodium “pumps” in the vacuole that use cellular energy to transport sodium from the cytoplasm into the vacuole. (Think of a sump pump, pumping out any water that leaks into a basement.)

The cytoplasm contains a lot of enzymes that are very sensitive to sodium. In contrast, the inside of the vacuole contains relatively few enzymes, and thus is metabolically inactive compared the the rest of the cell. So, sodium can accumulate inside the vacuole without harming the rest of the cell.

The third strategy that halophytes may use to tolerate saline environments is to simply excrete any sodium that leaks into the plant. Saltbush, for example, is well-known for doing this. Indeed, the leaves of saltbush often appear silvery due to salt crystals on the surface of the leaves. (e.g., see photo above right)

Although halophytes may employ three different strategies in order to tolerate saline conditions, can any of these strategies be used to render major crop plants more salt tolerant?

Next time: We’ll explore some current strategies that people are using in order to deal with the growing problem of salt stress in agriculture.

References

1. Zhu, J.-K. (2007) “Plant Salt Stress.” (PDF)

2. The Mangrove Action Project (Website)

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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?

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.

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.

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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