Why would leaves in a K+ solution have more open stoma than leaves in a Na+ solution?

rowkem

1. The problem statement, all variables and given/known data

So we did an experiment where we put a leaf section in KCl and another leaf section in NaCl. There was a marked difference in the percent of stomata that opening between the two. We let the two sections sit in solution for 60 mins each.

The leaves in KCl had almost twice as many stoma open as the leaves in NaCl. Both were the same pH and had equal light exposure. The measurement areas were of roughly equal size.

That said, the key difference was the presence of K+ ions as opposed to Na+ ions. Now - I can't figure out why the two ions would produce such different results. Any ideas?

3. The attempt at a solution

I think it has something to do with on being positive and the other ion being negative. Though, I can't explain it in terms of the osmotic potential between the extracellular and intracellular regions, which cause the water to flow to regulate the guard cells of the stoma. Does K+ create higher osmotic potential which then means more water movies into the guard cells - thereby increasing the stomatal opening and therefore CO2 uptake, which means a higher photosynthetic rate? Or have I got this completely wrong? I've looked for lit on this topic and it's very limited, if at all existent.

alxm

I assure you, there's plenty of literature on what potassium ions do to stomata.

rowkem

Yes, I realize there is much literature on what potassium does, seeing as how that's what typically moves through the transport protein. There isn't lit though, on why sodium is less effective is opening stomata.

Could it be that plants have simply evolved to use potassium, and therefore there's more transport proteins? Could it then be said that plants haven't evolved to use sodium and therefore there aren't as many transport proteins to pump the sodium into the intracellular region?

Are transport proteins that selective in the ions they move?

alxm

Yes they are. K+ channels have a set of four box-shaped 'cages' stacked on top of each other, where the corner of each 'cage' is an keto-oxygen from the protein backbone. The geometry is so precise that only potassium will coordinate to all six corners, and ultimately pass through.


MacKinnon got the 2003 Nobel prize in Chemistry for figuring out how it worked.

rowkem

OK - that has help immensely. I realize now that sodium channels are highly selective and I did a quick read up on the topic. Thanks.

So - last question. Have plants evolved to have more potassium ion channels than sodium channels? If this is the case, then we can make some conclusions.

alxm

I don't actually know the answer to that. But potassium is, as you see, a signalling substance. (both in plant and animal cells), so its presence is fairly tightly and actively regulated.