How does root get their oxygen?

daniel_clSeptember 17, 2010


This has been bothering me for a long time. It should be a silly question for you:

Field-plants developed root rots when drowned by water. We know roots require oxygen to breath. But people, including myself, grow plants in the water and give this practice a cool name. Every time I look at those perfectly white, but totally submerged roots, I wonder how the roots survive?

My guesses:

(1) Far more plant pathogens live in the soil than in artificial water solution made from chlorine-treated tap water. The suffering submerged roots just lack the usual enemies to finish them off.

(2) The leaves, which makes oxygen during photosynthesis, can ship a small portion of oxygen products down to the root. As long as there is not too much root to feed, and there is a reasonable amount of oxygen dissolved in the water bath, the roots can survive.

(3) Some root sections are slightly above water. They breath liberally and sends down the oxygen.

(4) People lift up their hydroponic plant frequently to change/add water and to look at the roots. During these short seconds, the roots take a huge gasp.

Any ideas?

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My understanding is that when plants are getting too much water in the outdoor garden, and it is not draining, or draining slower than water is being added, it allows fungi to flourish, plus it lacks oxygen the plants need. If one was to stick a plant in a bucket of water and not aerate the water, once the oxygen is depleted, it will develop RR or the plant will die because it is not getting enough nutrients. It just takes longer because there are no fungi in the mix.


    Bookmark   September 17, 2010 at 4:52PM
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I have to disagree. Ever ask yourself why stems are round? It�s to start a hot cool cycle. While the sun hits one side the other is cool. The warm side draws water up by transpiration On the cool side water falls downward. .The plant nutrients it produces go down with it. Including air. The other thing is plants roots don�t die because of fungus. The fungus is just taking advantage of root death due to the change of environment. Not that the fungus is attacking healthy roots Air roots can't grow in water, water roots can't grow in medium.

    Bookmark   September 17, 2010 at 8:30PM
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Your post about "Air roots can't grow in water, water roots can't grow in medium." makes the same sense as the salesperson from Reimer seeds telling me the reason their seeds didn't germinate in a potting mix was because they knew they were destined to be grown hydroponically.

And the fungi do attack the roots. Look it up!


    Bookmark   September 17, 2010 at 9:55PM
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I know some plants do grow "air roots", but most of our hydroponic plants' "water roots" seems to be their only kind of roots.

It makes sense that oxygen could be dissolved/chelated in the plant's "body fluid" and gets sent down to the root. But there should be a limit capacity to it, or trees can never been root-drowned to death (they do).

I have a background in microbiology, so I may have a saying in plants' pathogens. The pathogens are constantly trying to attack the plant (and us), and the hosts are constantly using their various defense mechanisms to stay healthy. The lack of nutrient or oxygen slows down the hosts' immune mechanisms (like guns on the castle wall running out of ammo), making the hosts more susceptible to disease.

    Bookmark   September 17, 2010 at 11:48PM
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It makes perfect sense and I don't need to ask a saleperson either. What happens is the seed when it first germinate sense what kind of enviroment it's in an produces roots accordingly. But when you change that enviroment by letting it sit in water too long the roots died off and start growing the kind needed for the new enviroment. Many plants can go thru this many times others can't deal with it at all.

    Bookmark   September 18, 2010 at 12:06AM
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rhizo_1 (North AL) zone 7

If you are talking about terrestrial plants that are being grown in water, the roots are able to obtain dissolved oxygen in plentiful amounts from the water. Yes, the roots that develop in the water are quite different from those that would be a functioning root system on land.

You can't take an earth-bound plant (those trees that daniel referred to) and expect it to survive under water. As a matter of fact, their root system begin to die in a
very short time after submersion.

Now if you're talking about aquatic plants (hydrophytes)....that's another story.

    Bookmark   September 18, 2010 at 12:50AM
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"You can't take an earth-bound plant (those trees that daniel referred to) and expect it to survive under water. As a matter of fact, their root system begin to die in a
very short time after submersion. "

I'm finding out that yes you can. Any plant you can grown in ground you can grow hydroponicly. If you can keep the plant alive during the root change over form one type to the other. That large root system above is an earth bound plant that I changed over to water

    Bookmark   September 18, 2010 at 11:48PM
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This 2003 paper states in the discussion that plants with submerged roots transport oxygen from aerial parts (leaves) to the roots. I'm not sure if this was meant to be taken as a generality or the Amazon trees specifically. Later it examines some of the physiological changes in oxygen stressed roots and things get complicated.

This 1970 paper says more or less the same thing. That a flow of oxygen can occur between aerial parts and plant roots in mustard seedlings, depending on gas partial pressure and other factors. This experiment was done using an agar gel solution.

I think the mechanism for oxygen transport across the cell membrane of root tissue is simply diffusion. It's late and I'm kind of tired so I can only search so much on google scholar. It's worth remembering that a large fraction of "typical" soil is oxygen, thanks to soil being so porous.

My understanding is that oxygen starved plant, which are not adapted for such conditions, will create some biochemical garbage and then die. This probably creates the opportunity for pathogens to become established. Some plants prefer to grow in peat bogs where the roots are constantly submerged and those plants don't seem to be troubled by root rot, even if the water is so anoxic things won't substantially decompose for thousands of years. Each type of plant will have a varying ability to cope with conditions such as that. This next link is probably a decent review about the problems that can result for oxygen starved plants. Unfortunately my school isn't cool enough to have a subscription to this journal so I can't read anything more than the abstract (boooo!).

So, in summary (this may not be correct in all cases, I'm not a pro scholar yet) plants roots get their oxygen through diffusion. When submerged this process is going to depend on partial pressure of oxygen in the solution and the plant can transport oxygen from the above water parts to the root tissue, as you suggested. Some plants have special tricks and abilities that allow them to be better suited at surviving in these types of conditions than others, how they do this is fairly complicated and involves a lot of biochemistry and molecular biology.

    Bookmark   September 19, 2010 at 1:58AM
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Plant roots get oxygen in two ways. One is from actual air, the other is from dissolved oxygen. Dissolved oxygen is oxygen that has bound to the water molecules (in other words, directly adding oxygen to the water itself). That's one of the reasons it's beneficial to use a air pump and air stones (like the kind they use for fish tanks) in your nutrient reservoir.

Now depending on the type of system you are using, and how you water it, you may get lots of air to the roots. Like in a flood and drain system, with every cycle (flood and drain) when the system drains, it draws in fresh air down to the roots. A drip system will have air all the time, although it doesn't suck in fresh air like the way a flood and drain system will when it drains. Now with a water culture system, the roots are suspended in the water 24/7. That meens that all the oxygen the roots get is going to be from the dissolved oxygen, making it more important in that type of system. Note: that is refering to a true water culture system and not a combination (water culture/other system) like most typical so called DWC (deep water culture) systems, where only part of the root system is usually submerged, and part of the roots are always above the water line.

You can always add a small amount of Hydrogen Peroxide (H2O2) to the nutrient solution. Hydrogen Peroxide is nothing more than purified water and oxygen.

(H20) being water (2 hydrogen molecules and 1 oxygen molecules).
(H2O2) is 2 hydrogen molecules and "2" oxygen molecules.

You can get it in any pharmacy, grocery store etc., it's sold as an antiseptic (I get mine at Wal-Mart for $1.06). It's 3% hydrogen peroxide and 97% purified water. Now many may say there are impurities in Hydrogen Peroxide, but if you read the label the "active ingredients" should read 3% hydrogen peroxide, and the "inactive ingredients" should read purified water. I try to use 1 tsp (5mL) hydrogen peroxide to one gallon nutrient solution (per week). It dissipates over time so I try to add it weekly. I've used more (even though it wasn't recommended) with no ill effects that I noticed, but I try to stay in that range. I also know that it is not really recommended for seedlings.

    Bookmark   September 19, 2010 at 2:56AM
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So for a regular plant not specialized for an aquatic environment, submerged "regular" roots can survive because they get oxygen from (1) leaves (2) dissolved oxygen in the water (3) active aeration schemes. If the oxygen is sufficient for all roots to survive, they do well. Otherwise roots suffocate, don't grow, or get infected and eventually die.

So if one plants in container soil and wishes to "improve drainage", he/she is actually aiming at supplying more oxygen to the root. One can (1) give more light to encourage photosynthesis and the production of oxygen, (2) use a bamboo stick to poke a number of deep holes around the root ball, and (3) use pots that permits gas exchanged on its sides and bottom. Or (4), as the usual practice, use a soil that has larger particle size so they drain faster to allow more oxygen to come into the gaps.

    Bookmark   September 19, 2010 at 10:20PM
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I don't clam to be a botanist, and/or know all about plant biology. But I have a hard time thinking/beveling that plants can absorb oxygen through the leaves. From What I know plants "absorb CO2" through the leaves and "EXPEL OXYGEN" from the leaves. That's the element we all breathe, and why it's so important to quit cutting down so much of the rain forest's.

Oxygen may be a mobile element in the plant tissue (I don't know), but as far as I know it's only absorbed through the roots. Even if it's able to be absorbed through the leaves, I cant see it being in any significant amounts. Bottom line, from what I know get oxygen to the roots if it's of interest.

    Bookmark   September 20, 2010 at 4:34AM
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Well yes, plants take in/up Oxygen through all their pores, in fact mainly through the stomata on the undersides of their leaves- and yes, from the atmosphere.

Plants do actually both: they take up carbon dioxide and give off oxygen during the day but they do the opposite at night.

With photosynthesis, excess oxygen is given off by plants into the air. And as photosynthesis can only take place when there is light (UV), - at night plants are absorbers of oxygen, "on balance". In fact, during the day both processes occur, but photosynthesis proceeds more efficiently than respiration, and the carbon dioxide produced is immediately used in photosynthesis. In other words: at night, photosynthesis obviously ceases while respiration (of oxygen) CONTINUES.

    Bookmark   September 20, 2010 at 6:09AM
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I didn't read everyone's posts entirely, but it didn't seem the actual answer to the original question was given. Essentially, as I understand it, the question is really, "What's the difference between the hydro plant in water and the field plant that is drowned?"

1) Field plants that drown do so because the water goes stagnant. The water loses it's dissolved oxygen. Also, if the field has standing water, then that means it has poor drainage. Obvious, right? Well, then you should realize that it means the ground has become very compact in those areas, too. Especially in fields that have been tilled time and again, rely on chemical fertilization nearly exclusively, and do not make use of cover crops that keep the soil from compacting in the winter and rainy spring. In other words, the ground doesn't get enough oxygen as it is and when it gets wet it turns to mud, which will inhibit absorption of a lot of nutrients, including O2 and even water.

2) Hydroponics is no different. If the water goes stagnant, the plants will suffer and eventually die. They will "drown". You have to keep some dissolved oxygen in there. The more, the better. Or, you can keep the roots somewhat suspended above the water so that open air that is 100% humidified will keep in contact. Even though the plants adapt to different environments, the basics are the same. The roots adapt. They don't know anything exactly. They are triggered. The adaptations are responses to available resources. More oxygen available? It develops root hairs. No water? The roots recede or just don't develop. Pretty easy to see that in nursery plants. Some people do it on purpose as air pruning. Loose medium that is kept moist? Crazy root hairs. Dense medium, like a heavy clay? You will only find the root hairs in air pockets. You can test this by growing beans in a jar. Put the beans in mason jars with various media. Plant them at the side. Keep water, ambient temp, light, and bean species/age constant. So only water the same amount for each one. I recommend using course vermiculite, sand, heavy clay soil, compost, and a good mix of clay, sand, and compost. After the plants get two sets of leaves, pull the medium and plant all out gently to examine the roots. This is a project for elementary kids. We recently did this with ours. If you want to do it to compare flooded dirt versus hydro to see that there is little difference with survivability, I'm sure you can adapt the experiment. Just don't shake the jar. Agitation can add O2 to the water. Be sure to record data on the growth daily. Oh, I forgot to mention that you should wrap foil around the jars to keep light out. Only remove it to make short observations.

Point, there is no difference. You just aren't thinking of the field or the hydro in the right way. It's a little more than roots sitting in water. Does that make sense?

    Bookmark   September 21, 2010 at 10:49PM
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daniel cl,
I placed some tomato clones in a couple of jars of nutrient a while back, just to keep them happy over night. I got busy doing other things and forgot about them for 4 days. I would have stuck a couple of air hoses in the jars had I planed on leaving them that long. Did their roots adapt to the lack of dissolved oxygen in the standing solution? Did they become "Water Lily Roots"? Of course not. They died. If you've ever done much water gardening you would know that even Water Lilies need a slight amount of pond movement to supply a minimum amount of dissolved oxygen. Water absorbs oxygen at the point where the water contacts the oxygen, weather that point be the outside of a droplet, the inside of an air bubble, or simply at the surface of the water. This is why in hydroponics we either use bubblers, misters, sprayers or flow methods, to increase the area where the water molecules meet the oxygen. Static systems simply utilize mediums that maximize water to air contact. Roots will reach out for oxygen and dry out when they get too much, and grow down seeking moisture and rot when they stay too wet. Sure, the roots may look quite a bit different than the roots you pull out of the ground but the plant really isn't doing anything magical. A root still functions as roots do, stems still send moisture and Vitanium upwards, and leaves photosynthesize and function normally.

    Bookmark   September 22, 2010 at 11:02PM
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Oxygen is absorbed into the water in many ways. In nature, the water is seldom still, so its moving, and therefore able to pick up oxygen. In a container with the lid on (to keep out light) the roots consume the oxygen, and then the roots rot. Add an airstone, and problem solved.

    Bookmark   September 29, 2010 at 10:56PM
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My experience, at least for some tomato cuttings. I stuck them in a quart jars of tap water and 4 of 5 developed very nice roots, which was a first for me! Usually most of them would die but this time the temps were cooler and the jars were in the shade - that may have made a difference.


    Bookmark   October 3, 2010 at 4:47PM
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Yeah Mike, you're right - tomato cuttings root easily and don't need any artificial oxygenation if you put them in (a small or bigger) amount of water. If they die or wilt in a few days before taking roots, it's not because of a lack of oxygen in the water, but most likely because they suffer (ending up in cell damage) from heat or other unfavorable causes.

The mostly overseen aspect of water oxygenation and condition is probably that Hydrogen Molecules have a high affinity for Oxygen molecules anyway -> H2O. Which is definitely a good thing as "water" tends to catch Oxygen molecules when ever it can. In fact it can't be without them and that's why you actually never encounter H in it's pure form in nature ;-)

The other common error is to assume that one can saturate or hyper-charge water (or nutrient solution) with oxygen through mechanical methods. But as there is a limit to the dissolved oxygen water can absorb and hold, which is dependent on water (nutrient) temperature and to altitude to some extend - you can't exceed the physical maximum that can be dissolved under the existing conditions. At see level, water has indeed a bigger capacity of dissolved oxygen as water at a certain- or high altitude has. Hence, depending on temperatures and altitude (to some extend only for the later) water can (literally) only hold a certain amount of oxygen anyway.

And here is exactly the point where ideal water temperatures of nutrients make sense. But then again (and here I truly speak from experience) sometimes over-idealized or even overrated. Although a temperature of 24 degree C is commonly accepted as ideal and is probably the best compromise between dissolved oxygen CAPACITY and growing rate, - a slightly higher temperature, up to 28 degree C, may end up in a better growing rates with CERTAIN conditions. As long as the (indeed lower) capacity of dissolved oxygen at higher nutrient temps is replaced continuously, of course. One must add here that the higher the nutrient temperatures climb, the more likely fungal introduction and spread becomes.

There also is a lot of controversial debates and "experimentation" about if you can "add" any OXYGEN through air bubbles at all - and if, how much? And what would be the difference between finer and bigger bubbles? I've read a lot about the topic and couldn't conclude anything plausible out of it so far- which made me end up in not using any air bubblers at all. The only thing that I conclude, is that water movement of any sort (including the one created with bubblers) is accelerating and promoting oxygen replacement. Hence I generally use the downfall of any runback of nutrients for the purpose. With success I must say and apparently with sufficient re-oxygenation support. And that's the term I would use here, I guess: "re-oxygenation support" ;-)


    Bookmark   October 4, 2010 at 12:52AM
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