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My question is basically out of curiosity and comes from observing how certain plants (such as mangroves or salt cedar) can grow in seawater. If this gives the plant an advantage, why haven't all plants that grow in coastal areas developed a salt tolerance? How can the plants keep from intaking salt, while absorbing other soluble nutrients through the roots?

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  • $\begingroup$ It wouldn't be an advantage if there was no salt water near by. $\endgroup$
    – canadianer
    Commented Jun 22, 2014 at 0:20

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A few things you have to realize when you ask 'why hasn't species x evolved trait y':

  1. What is the cost of evolving this trait? If you gain one trait, you lose another (or others). Everything in evolution is a tradeoff: that's why you're not nine feet tall, you'd be able to beat up all your enemies great, but it would take so much food to sustain you, it wouldn't be worth it.

  2. Not every trait can be easily evolved. When you're starting as a freshwater plant, there are many, many, many, many mutations that have to occur for you to become salt-tolerant. Every one of those mutations would have to give a fitness advantage to be positively selected. The chances of this happening are low, which means if it happens at all, it would take a long time.

  3. Who knows, perhaps these plants are, slowly, becoming more and more salt-tolerant and you're seeing them at an intermediate point in their evolution. Everything is evolving all the time, if these plants were not in a salty environment until recently, mutation and selection haven't had time to work their magic.

Update from comment:

Can you flesh out a little on "How can the plants keep from intaking salt, while absorbing other soluble nutrients through the roots?"

Sure! I'll give you the simple answer first: there are different mechanisms for absorbing salt and for absorbing nutrients, so they can function largely independently.

Now the longer explanation: Salt and nutrients don't just diffuse (much) through cell walls, they have to be transported across by what are called transmembrane proteins. Think of the cell membrane (a layer inside the cell wall) as basically something like a layer of oil (it's called a lipid bilayer) -- you know oil and water don't mut, and for the different ions in salt (sodium ion and chloride), neither do oil and salt or oil and many nutrients so they get sort of escorted across that barrier by big proteins that form sort of a tunnel through it.

There's a different transport protein for different substances; the salt ones will only let salt through, and in fact there are different ones for letting salts in and letting salts out, and for the different ions (sodium and chloride) in salt.

Update from comment:

This makes me wonder how salt harms most plants, if it can't get in except by 'invitation'

Quite a bit of salt can diffuse in, the molecules are small enough (unlike most nutrients) to pass through the membrane via the channels that let water in. Every living creature has a quite sophisticated setup of "ion channels" to pump salt out. If suddenly its cells are in a very salty environment, these pumps can't keep up.

Update from comment:

So if salt can diffuse in, what do salt-tolerant plants have that other plants don't?

Mostly, if I recall correctly, three things: They will have enzymes that have evolved to work in higher salt concentrations, they will have cellular functions to "sequestrate" salt, i.e. store it in organelles called vacuoles instead of leaving it where it can harm other organelles, and of course they will expend more resources on ion pumps to get the salt concentration in the cell to an acceptable level.

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  • $\begingroup$ Can you flesh out a little on "How can the plants keep from intaking salt, while absorbing other soluble nutrients through the roots?" $\endgroup$
    – J. Musser
    Commented Jun 24, 2014 at 22:33
  • $\begingroup$ @proofreader Nice. Thanks for the info. This makes me wonder how salt harms most plants, if it can't get in except by 'invitation'. $\endgroup$
    – J. Musser
    Commented Jun 25, 2014 at 14:39
  • $\begingroup$ @proofreader So if salt can diffuse in, what do salt-tolerant plants have that other plants don't? $\endgroup$
    – J. Musser
    Commented Jun 28, 2014 at 17:50
  • $\begingroup$ @J.Musser Salt still enters the plant, they have the abilty to get rid of it. . and the term is Halophyte plants. $\endgroup$
    – John
    Commented May 13 at 0:55
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The white mangrove (Avicennia marina) does take up some salt but deals with this by excreting it from growing leaves and cramming it into older leaves that are about to drop.

Also, some fleshy maritime plants may exploit a high internal salt concentration as a way to hold on to extra water in dry periods.

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  • $\begingroup$ Please add reputable sources to your claims so others can background read on your topic $\endgroup$
    – AliceD
    Commented May 13 at 8:12
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    $\begingroup$ Thanks for adding a source. Wikipedia is volatile, however, in the sense that its content can change quickly. In addition, it is notoriously badly referenced, and full of circular citations. Anyway, if you can back that second statement up too, that would be great; it isn't mentioned in the wikipedia entry. $\endgroup$
    – AliceD
    Commented May 13 at 11:13
  • $\begingroup$ PS, above answer didn't cite sources either, I know (I just dropped a banner), but criteria for posting were a lot looser back then $\endgroup$
    – AliceD
    Commented May 13 at 11:16

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