Some Reflections on Ebb and Flow and Plant Response to Hypoxia in Nature

A discussion of ebb and flow situations led me to some speculation. It has always been an open question with me as to how long it might take a plant to adapt to flooding. Then I realized that ebb and flow can be a model of a situation that could easily occur in nature. It turns out, though, that ebb and flow operates too rapidly for the adaptation, and the adaptation requires time provided by circumstances in nature that our ebb and flow does not emulate.

Still, how plants survive seasonal changes in environment is worth our attention, because it directly applies to issues we may encounter when our system fail or when we are thinking about a radical modification.

If I’ve lost you already, this is what I’m talking about. Most of our plants live in nature taking free air circulating in soil. They send visible rootlets out into the soil seeking air. We have them do it too in wicking. And of course, Kratky, Dutch bucket and NFT leave plants in their native air acquisition mode, just without the soil.

And roots in submerged root systems, like DWC, have structures that can take dissolved oxygen. But one of the most interesting of plant structures is those that adapt when roots are submerged and air is absent in a solution, when it is hypoxic, by developing the structures called aerenchyma to take atmospheric air and move it down to roots so they can continue using oxygen. They start developing them under stress, such as being submerged in stagnating water where the oxygen content is falling. Aerenchyma are structures that form channels through the plant, root, leaf and stem. In the diagram, they are the holes in the outer part of the root cross-section.

 

Look at how they work. They’re like a plants own aerators. They’re not quite like air stones. They can’t aerate the water they are in. Instead, they move air from the upper plant to the roots where it is needed. Plant parts above the roots can take in air, but they normally leave the roots to get their own. Remember that the leaves process gasses differently from roots. Roots need oxygen. That’s what the aerenchyma do when the roots are flooded with hypoxic water. They move air down from parts of the plant that have access to the atmosphere.

It is easy to prove that they move oxygen downward. When researchers blocked aerenchyma just above the root crown in a plant that was depending upon them, oxygen levels in the root tissues fell dramatically.

Plants develop aerenchyma in response to the stress of flooding. But we know soil plants with soil watered to saturation and unable to drain will die. How do they live long enough to adapt to things like seasonal flooding in nature?

I think we have to look to nature to help find some answers, although some things are known from studies. There are waterlogging-tolerant soybean genotypes of soy beans that are so good at this that they were able to form more than 20% aerenchyma after 7 days of a waterlogging treatment. So, we might reasonably expect that the time to develop an effective network of air transport structures is measured in days. Knowing what happens in our submerged root systems supports that. When our nutrient aeration fails, plants are affected within hours and do not have time to adapt and recover. Very likely they die before enough time passes with them in viable condition to develop enough aerenchyma to support the roots.

But the situation is sometimes not so dire in nature where sudden flooding is almost always the result of overflowing waterways. This is fast-moving water, likely roiling more or less violently, and quite possibly fed by falling rain. So it is likely well-aerated at first. This aerated water will flow around the plant for a time before eventually either receding or settling down to stay for a while.

If it recedes, the plant is fine with things as they were before. If it stays for a while, there is a longish period during which the water is less and less agitated and aerated and finally becomes stagnant and loses even more oxygen to biological processes, much of them death processes. And it is in that time that the plant can invoke the aerenchyma adaptation and complete enough of it for the plant to survive. The evolutionary force here is obvious. Plants that are better at developing the adaptation get to live and reproduce after floods more often than less able plants. There are, of course, places and times when water flows in, becomes immediately still and stays, like poor, ill-draining garden soil that is overwatered from a hose. In many of those situations plants simply die. (There are exceptions. Rice carries the adaptation all the time.)

As we operate ebb and flow, it becomes apparent that plants don’t need constant free air to remain in the default mode of using rootlets to get free air. And our relatively brief episodes of flooding with hypoxic water do not create enough stress to induce the plant to try to switch to bringing air down from above. It would require much longer flood times to begin the process. Plants would not want to expend the energy to adapt just because of even frequent heavy rains, so long as the water drained promptly.

So, although it floods, our ebb and flow system is almost all of the time a roots-in-free-air system, along with Kratky and NFT. This has some implications. One is that, since the plants get plenty of free air during the long ebb periods, aerating the reservoir of an ebb and flow system is largely wasted effort. The brief floods and relatively long ebb periods wouldn’t give the plant much time to use it. It would do no harm, of course.

There is really a continuum of possible ebb and flow cycles between the extremes of always flooded and always dry, both of which would kill the plants. The roots must have both states, flooded and ebbed, just as in Kratky and NFT they must have some root in air and some in nutrient. Simple experience has taught that the flood periods need only be relatively short and that ebb times must be longer, but not so long that roots dry out. Roots can afford interrupted periods of nutrient, but they need essentially continuous access to oxygen.

Plants seem to be quite efficient feeders when given even some small chance. Krayky systems are routinely left with nutrient down to the last few inches. Dutch bucket roots get all they need “in passing” so to speak. NFT feeds them well from the very thin nutrient “film.” Relatively brief flooding in nutrient solution is enough. And nutrient doesn’t entirely go away in a properly cycled ebb and flow. Media typically holds some. And even in bare tanks, trailing roots have residual shallow pools left in the bottom to feed on.

But I think it does suggest that an ebb and flow operator should be prepared from some short-term pH swings. Plants are being tossed back and forth between feast and famine and good aeration and none. And we know plants themselves take actions like altering pH locally to reach the optimal pH range for the nutrient then want at the moment. Operators should be tolerant of natural rapid shift in an ever-changing ebb and flow environment and not intervene with buffers unless the system goes well outside of normal range, and even then, wait a day to see if it comes back.

To see just how complex all that is, see the blog post, Why is My pH Doing That – And Does it Matter?  https://redwing-farm.blogspot.com/2021/11/why-is-my-ph-doing-that-and-does-it.html 

I should add that my description may make it sound like plants have only one way to respond to lack of oxygen in the growing medium. But the reality is that there is a whole array of complex physical adjustments and adaptations and metabolic responses that can come into play, exactly which and to what degree varying with the plant species. Adventitious roots are another important adaptation, but it wasn’t necessary to muddy the water with a discussion of it, since they perform a similar function. Plants bring quite a collection of strategies to the game of survival. But our system failures tend to be catastrophic, not giving plants the time the need in diminishing oxygen environments to grow the adaptations.

I also want to say that in preparing this, I recognized that I had made incorrect statements elsewhere about the role of aerenchyma and left some wrong impressions about why plants suffered loss of aeration and exaggerated the risks of moving plants between submerged root systems and roots-in-air systems. Those who have my book should download a new free copy with the correction

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