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Does anyone know if there's been further study on O2 transport in aquatic plants beyond what Ole Pedersen?

Specifically, Ole writes
...that a small group of aquatic plants, for example Lobelia dortmanna, have evolved an quite different carbon acquisition technique. They are able to utilize the very high CO2 concentrations of the sediment for photosynthesis in the leaves. The high CO2 levels in the sediment arise from the intensive mineralization of organic matter which takes place at the sediment surface. The aerenchyma of Lobelia is extremely well developed. The leaves possess two very large lacunae, which run continuously down through the short stem and out in one root each. The plant is short (less than 10 cm) so diffusion is still effective when carried out in gas phase. CO2 can, therefore, diffuse into the root and up through the stem and into the leaves.

In addition, the leaves are covered with a gas tight waxy cuticle, which assures that CO2 does not diffuse out into the surrounding water but is merely retained in the leaf until fixation in photosynthesis. In exchange, all the O2 evolved during photosynthesis diffuses down through the leaves and out into the roots and further out in the surrounding sediment. This, in particular, makes Lobelia an exceptional plant because all gas exchange takes place over the root surface and not primarily over the leaves as in all other plants. Lobelia inhabits nutrient poor sediments only, and the oxygen consumption is fairly low. Therefore, Lobelia sediments are often aerobic, due to the intensive O2 transport to the sediment, and an aerobic fauna should be able to inhabit those sediment - a theory which is untested so far.
I suspect this is also the case with certain Cryptocoryne species except that most Crypts to not have as developed a cuticle as Lobelia.

This is important because Ole argues our substrates should create an anaerobic condition in order to ensure nutrient availability.
In general however, an anaerobic sediment is to be preferred because important nutrients - phosphorus, iron and manganese - precipitate under oxic conditions and become unavailable for the plants which eventually may limit growth. Much help is also provided if we enrich the water with CO2. Hereby, we avoid carbon limitation of photosynthesis, which eventually will translate into a growth reduction, too. By CO2 enrichment of the water we actually mimic the natural conditions in many streams. CO2 rich groundwater often feeds the streams and natural CO2 concentrations up to several hundred times air equilibrium are common. So maybe all the fancy tricks we apply to our tank in the living room are more or less well-tested strategies in nature.
Should we therefore ensure anaerobic substrate conditions? Does this depend on whether the captive plant species have a well developed aerenchyma?
 

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Aerenchyma does not "develop" like it's something grows.
It forms after the roots, stem, leaf etc has forms and then the cells are lysed to make the passage way via ethylene and other mechanisms.

Crypts certainly have these, all wetland plants have some method of dealing with anaerobic substrates. This is a basic character of wetland soils.

Whether having an anaerobic or not substrate is any great advantage is debateable till the cows come home. It's dependent on a number of factors such as source and supply of nutrients etc.

You can add the nutrients in the water column or the roots.
This Lobelia has a very large thick cuticle, it's forced to obtain it's carbon in the substrate. It's a weird plant.

One thing needs to be dissected here, anaerobic is a general term and does not discuss nor quanatify the Redox levels which follow a pattern as the soil becomes more reductive.
Redox, sometimes refered to as Eh, is what determines the Fe3+=>Fe2+ states and is closely related to pH.

Fe and NO3 reduction occur at much higher redox levels than say sulfur reduction which would be a negative for the plant roots robbing them of needed O2 that would be quickly be neutralized by the O2+ H2S => SO4 + 2H+'s.

The issue of whether the O2 is the limiting issue or if the availabililty of nutrients is what is limiting growth are the two main issues here.

The nutrients can be added to the water column and the substrate and run the substrate aerobic(say at 400mv+), or at mild anaerobic levels and still get good results.

So how to get the best of both worlds, plenty of O2 but the lower redox values?

The micro sites in porous gravels like Flourite or Flora base etc allow you to have O2 on the outside and while having low redox on the inside or each GRAIN.

So you can replant and move things around and still maintain this separate layer.

That seems best and supplies the plant roots with plenty of O2 and plenty of anaerobic sites. Aerobic substrates process waste much better/faster.
If you are trying to maintain the peat, nutrients in the soil longer, deeper might help but ultimately the nutrients have to come from somewhere for the plants to grow.

So I think we already have some good methods/substrates around and this is a good reason why these products work so well.

Regards,
Tom Barr
 

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I have seen Lobelia dortmania in its natural habitat---soft water lakes---in Northern Wisconsin. There are a number of similar species that grow with L. dortmania, including Eriocaulon septangulare and Myriophyllum tenellum. The plants are all small and short. M. tenellum has only scale-like leaves and looks like a series of short, knobby two or three inch green stems coming up from a buried, horizontal rhizome.

CAM plants, instead of incorporating CO2 directly into the photosynthetic pathway, collect it separately by combining it with a three carbon acid to form a four carbon acid (malic acid) and then they remove it from the four carbon acid and feed it into the photosynthetic pathway. This extra collection step allows them to collect and accumulate CO2 when not doing photosynthesis. In plants that are not CAM plants CO2 collection is tied directly to photosynthesis, and there is no way to accumulate CO2 except by the process of photosynthesis. Cacti and succulents are CAM plants that keep their stomata open and collect CO2 at night and then keep their stomata closed during the day to save water and use up the stored CO2 in photosynthesis. They prevent the loss of a lot of water by collecting their CO2 during cool desert nights, instead of during the hot desert days.

Dave Huebert, a Canadian plant physiologist, has reported that L. dortmania is a CAM plant. It lives in soft water lakes where low CO2 is the the main factor limiting growth. Birge and Juday, in the 1940's did experiments on a soft water lake in northern Wisconsin, Lake Weber, where they added all the known mineral nutrients in varous combinations, but did not get any increase in plant growth. Only when they added CO2, did they get growth increases. With CO2 being the limiting nutrient, L. dortmania gets an advantage by being a CAM plant and being able to collect CO2 produced in the organic sediment day and night with its roots. It seems probable that the other soft water species, such as Myriophyllum tenellum, would also be CAM plants.

I wonder if Aponogeton undulatus is a CAM plant. If I don't give it CO2, the leaves are somewhat brownish in the morning but become green for most of the day. If I do give it CO2, the leaves get even more brown and stay brown all day if there is plenty of CO2 in the water. The brown color seems directly related to the amount of CO2 available, and the fact that they turn browner overnight, indicates that they are taking up CO2. Naomi Misumoto has told me that her Bacopa carolina varigated is reddish in the mornings and becomes green during the day. It could be a CAM plant, too.
 
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