How Salt Stresses Plants (Or Not) – Part 2: Halophytes

Atriplex argentea by Matt Lavin

Atriplex argentea (CC BY-SA 2.0) by Matt Lavin

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.

Mangrove (CC-BY-2.0) by jimw

Mangrove (CC-BY-2.0) by jimw

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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