Really stupid question. Why does overwatering cause yellow leaves?

[inflorescence]

I know it sounds dumb, but exactly why/how do some lower leaves turn yellow when overwatered. I assume they aren't transfering their nitrogen to the rest of the plant because why would overwater cause a lack of N in the plant?
If it's N, why does the overwater signal the plant to sacrafice a lower leaf. Does the plant figure that if it sacrafices a lower leaf, this will save the newer growth. How, chemically, does the plant figure this out.

Is it from the lack of DO (dissolved oxygen) in the soil/medium and why would this make some leaves transfer their N out or otherwise remove their (green) chlorophyll pigment? I can understand that the plant may sense that it's choking, by why would it think that sacraficing lower leaves would rectify that situation. Why is a yellow color left when N and/or chlorophyll leaves a leaf. Are the plants cellulose cell walls naturally yellow without N?

I actually searched google and couldn't find the exact MECHANISM as to how this happens. me ->

 

[I.M. Boggled]

lack of oxygen availability...thats basically it in a nutshell

lack of oxygen availability...thats basically it in a nutshell

Quoting now from a posted thread by "Capt. Crip" called
"Easy Growing 101" @
http://www.icmag.com/ic/showthread.php?t=31181

As the plants get 2 or 3 sets of leaves you must start allowing the pots to dry some between waterings.

IMPORTANT STATEMENT
This is the most important thing you will see me write.
Once small plants get established you MUST let the soil dry between waterings.
Plants must have oxygen to the root system for rapid growth.
This is why I like to use extra perlite in my soil mixes because it allows the soil to dry faster which leads to faster growth!
Once plants are placed in 1 gallon pots you should be able to scratch down an inch or two before finding any signs of moisture before it is time to water again.
The pots will feel noticeably lighter when you pick them up.
If you pay close attention you will notice the bottom 2 leaves start to droop when a plant is desparate for water.
Once you figure out how dry they can get before drooping you will have figured out one of the hardest parts of growing for new growers.
The more time you spend with your plants the more you will begin to understand about the life cycles.
When you do water you want to make sure to let some drain out of the bottom.
This will keep nutrients from building up and keep your plants growing strong!
Continue feeding lightly until you transplant into the 1 gallon pots at which time you can go to full strength.

 

[inflorescence]

I appreciate your response I.M. Boggled.
What I want to know is, how (chemically) does low DO signal the plant to drop a lower leaf. Most things with plants are passive, ie give it too much N and it soaks it all up and becomes too green.

But HOW does low DO cause the lower leaf to drop. Obviously the plant has received a siganl to remove it's N from that leaf but what are the intermediary chemical steps from low DO--> x --> x --> x -->removal of n from the leaf. Does a hormone tell that leaf to evacuate it's N, or does the low DO actually cause the internal "plumbing" of the plant to choke, and the plant is then evolved to drop a leaf in response to this choking of it's internal DO levels.
In other words, if a plant cannot get enough DO at the root level, how does this chemical response travel up to the lower leaf and then cause it to evacuate it's N.

This question remends me of the question, "How do I measure 1 gallon of soil".

 

[BACKCOUNTRY]

OK, I'm no chemist so bare with me. Your question intrigued me, so I took a look around.

Here are some links I am sure will be more helpful, or at least point you in the right direction

http://www.plantstress.com/Articles/waterlogging_i/waterlog_i.htm

http://cropsoil.psu.edu/Courses/AGRO518/anoxia.htm#contents

 

[hoyt]

maybe sometimes you just gotta be happy with the hows (how does a plant respond to x) because as always, the hows are more important than the whys (why does a plant respond to x)!

 

[BACKCOUNTRY]

Basicly the way I understand it, the roots are unable to breathe oxygen, lack of oxygen means the plant can't metabolise nutrients, the plant strips and redirects the nurients in larger leaves(which are becoming useless as the plant is unable to supply the photosynth. processes occuring in them with nutrients), to the growing tips and seed baring areas(bud) of the plant. This is a survival reaction in the plant. I am sure every cell in the plant has a prepogrammed response to the various types of stress built into its plant "DNA".

The lower and larger leaves are the least likely to get light compared to the upper leaves, so they are eaten first. Also the higher parts of the plant are the places where reproduction(buds) will occur. The plant is attempting to last long enough to outlive whatever is causing the stress( in this case flooding), and reproduce .

The number of different reactions the plant has to this stress probably don't boil down to one equation. Although it looks like the primary one may be caused by the roots proucing ethyl alcohol from the plants starches rather than sugars, because of the lack of oxygen in a anareobic enviroment like waterlogged soil.

 

[inflorescence]

maybe sometimes you just gotta be happy with the hows (how does a plant respond to x) because as always, the hows are more important than the whys (why does a plant respond to x)!

Nah. If you know why a plant does something you will be able to predict how other different plants will react in a similar situation. If you only know how a certain plant responds to "x" then you only know about that plant. Every plant is different. Maybe you have the luxury of only growing one kind of plant, cannabis, but I grow any and all kinds of plants, because I love all plants equally.

 

[PazVerdeRadical]

hello inflorence, have you considered that the plant chooses to sacrifice the lower leaves because these are the ones who get the wetter and who get the less sunlight, which as we all know, is super-essential to life, so it gets rid of the least useful leaves... ?
peace.

 

[froth]

This is the kind of question you'd want to go to college to find out. Try and find a plant science major and i'll bet you'll get all the answers you want.

Look up your local college's plant department.. write a friendly email to a professor asking why one of your house plants keeps yellowing when you over water it. If you feel the need to ask why you are contacting the professor just say you have a friend who's kid goes to the school.

maybe it would work. but maybe i'm just high

edit:

I don't think nitrogen deficiency is exlcusively caused by over watering though.

 

[lifetime]

lack of oxygen and flushing the nutes out of the soil

 

[inflorescence]

http://www.plantstress.com/Articles/waterlogging_i/waterlog_i.htm

This link explains it all. I thought it was a simple question. This link blew my mind. I never imagined it was so complex.

 

[Guest]

why, old bean, youve done it again. splendid links

 

[Sante]

why, old bean, youve done it again. splendid links

Wonderful link!! Thanx even if now banned

 

[Stan G.]

If you haven't figured it out yet, over watering kills the delicate root hairs. Making uptake of nutrients difficult, then presenting itself as numerous nutrient deficiencies .