| MadSci Network: Botany |
Eleanor Loveland asked: How does saltwater affect the plants on land? And our MAD Scientist Administrator added: A little beyond the scope of the question perhaps, but I've always wondered how the mangrove trees manage to survive in salt water.. what do they have that other terrestrial plants do not?
Eleanor asked a short, but very interesting, question. And, like many short, but interesting, questions, it has a rather long answer. In fact, to answer these questions, I'll actually answer three other questions:
I've also listed a few places with related information.
1. How does (too much) salt affect land plants?
As I (sort of) suggested in my Question 1, it's not salt which is the problem, it's too much salt. All living things (on Earth, anyway) need some salt. But too much salt is poisonous to plants, just as it is to animals (such as ourselves).
When there is too much salt in the soil, which happens if it is regularly flooded with salt-water (or if poor land management leads to salinization), then plants take too much in through their roots and too much gets into their tissues (that is, into the cells of their roots, stems and leaves). This excess salt interferes with the chemical reactions in cells which the plant needs to make food and to grow. As a result, the plant's growth is stunted and the plant may even die.
But salt has another effect: it starves the plant of water. In the poem The Rime of the Ancient Mariner by Samuel Taylor Coleridge are the famous lines:
Water, water, everywhere,
Nor any drop to drink.
Plants have a similar problem when their roots are bathed in salty water: they can actually die of thirst!
This happens because the salt interferes with the way in which plant roots take in water. Plants rely on a process called osmosis to get water from the soil.The tissue around the tiny hairs on plant roots allows water to pass through easily (it is very permeable to water) but it only allows salts and other chemicals through very slowly (it is less permeable to these). When water in the soil is fresh, it tends to flow into the roots, then is sucked up the stem to the leaves. When the water in the soil is salty, water tends to be sucked out of the roots into the soil. [You can demonstrate this by doing a simple experiment. Simply peel a potato and put it in a jar of very salty water. After a few hours it will start to shrivel up as the salt (sort of) sucks the water out of the potato (actually out of the potato cells).]
2. How does (too much) water affect land plants?
It is often not only the salt in salt water which harms plants; strangely it can also be the water. Most plants take up some oxygen through their roots (the root cells need oxygen to respire, just as animals do). Land plants can do this because soil usually has many tiny pockets of air. Soil which is frequently flooded by water—be it salt or fresh—can become waterlogged; the tiny pockets get filled up with water. If this happens, the plant roots suffocate from lack of oxygen and die.
3. How do some plants—such as mangroves—manage to grow when other plants can't?
Mangroves are an example of a type of plant called a halophyte by botanists. Microsoft® Encarta has this definition:
Halophyte, term for salt-resistant plants, coined by the U.S. geologist Oscar E. Meinzer (1876-1948). Found in salt marshes and mangrove swamps, many halophytes could thrive in well-watered lands except for their inability to compete with the other plants found there. Generally fleshy and with gray-green foliage, many halophytes deal with the salt in their environment by eliminating it through special salt-secreting glands in their leaves. Others store the salt in leaves and stems that are shed at the end of the growing season, while the remaining halophytes keep salt from entering their cells by means of semipermeable membranes in their roots. One typical genus is that of the saltbush, Atriplex.
As this suggests, these plants have special features (adaptations) which allow them to cope with their difficult environment. The definition also lists the three main ways that mangroves deal with the problem of too much salt:
The reason that all plants don't do this, is that doing this costs the plant energy (so mangroves may grow more slowly than other plants).
But, as I said earlier, the salt is only one of several problems that these plants have. Mangroves also have to use energy to get water into their roots, and their are several different ways in which they do this. One way they do this is by accumulating chemicals (sometimes even salt) inside the cells of the roots so that there is a greater concentration of materials in the root than in the soil and water again flows into the root (this is rather complicated and difficult to explain).
To deal with the problem of waterlogging mangroves, particularly those lower down the shore, develop special roots. Some have projections which stick up out of the soil into the air and allow the roots to get oxygen. Others, including the stilt-root mangrove, have projections which grow out from the trunk into the air. These roots can help with another problem mangroves have, which is that the soil they grow in is often very soft (it is usually mud): the extra roots help stabilise the plant.
I hope this answers your questions!
I couldn't find anything much on the Web about plants and salt, but you can learn more about those fascinating plants, mangroves, and the ecosystem they create: