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Re: Those bubbles and turnover of water in the substrate.
Ken Guin wrote, Tues, Nov. 4:
>I have noticed that right after I do a fifty percent water change, all of my
>plants start producing a lot of bubbles (very small ones in streams). I
>know they are probably oxygen bubbles, but why such an out pouring of them?
>
More nitrogen and oxygen has been "forced" into solution in the tap water
while it was under pressure in the city water system. When the water comes
out of the tap the pressure is reduced to one atmosphere and the water is
supersaturated with gas. These bubbles are from the excess gas comming out
of solution. It can diffuse into the air channels in aquatic plants and
make it look like they are doing a lot more photosynthesis than they really
are.
>Subject: Re: slow turnover of water in substrate
>
Roger Miller wrote, Tuesday, Nov. 4:
>
>....This is where I break down to speculation, and someone else could probably
>give a better answer. It seems inevitable to me that the plant's task of
>providing oxygen to its roots places an energy burden on the plant. If
>the roots are kept under aerobic conditions, then the plant doesn't have
>that burden, and the energy can be used for growth or fruiting. Of
>course, if the plant in question is a crop plant (corn, for example) that
>can't survive in an anoxic soil, then the problem is worse than any mere
>energy burden.........
From what I have read about it, all the plant does is have a system of air
channels extending throughout the leaves, stems and roots, and the oxygen
simply diffuses through the air channels into the roots and then into the
soil. Water lilies, as long as they have floating leaves, are able to
circulate the air utilizing differences in temperature and humidity, but
submerged aquatics apparently depend on diffusion of oxygen in the air
channels, and this requires no energy output from the plant. (See the
article, Water and Gas Transport in Aquatic Plants, by Ole Pedersen at
http://www.tropica.dk/aqh1.htm) Oxygen can diffuse 100,000 times faster
through air than it can diffuse through water. As long as the above-ground
portions of the plant are in oxygenated water, oxygen in the water can
diffuse into the air channels of the above-ground portions and then down
into the roots. Thus, the roots stay aerated during the night or on dark
days, because it isn't necessary for the plant to be photosynthesizing for
the roots to get oxygen. In my tanks with soil I can look up from
underneath at the bottom of the tank and see the roots next to the glass
and I can see a zone of precipitated iron about one to two centimeters wide
next to the roots. When I have large plants, such as swords, in pots, the
roots fill the soil and, after about 6 months of growth, oxygenate it so
thoroughly that all the iron becomes insoluble ferric forms and the plants
become iron deficient and show improvement in growth when I add chelated
iron to the water. You can tell the plants that are best suited for
growing in strongly anoxic soils by their thick white roots that quickly
rise to the surface when cut off. Crypts, swords, and Sagittaria have
roots like this. Anubias barteri, var. nana, on the other hand only has
minute air channels in its roots, and I grow it in mostly gravel with only
a very small amount of soil added.
I don't believe that movement of water through the substrate is necessary
to provide oxygen for roots. The air channels do that. Interestingly,
aquatic plants, even totally submersed ones, have a 'transpiration stream'
where water is taken up by the roots and moved out the tips of the leaves.
See the article on the Tropica site by Pedersen for more about that. This
activity moves water slowly through the substrate, and probably aids in
nutrient uptake. I have planted crypts in pudding-like mud with gravel on
top, and when I have pulled them out months later (They were grown
submersed) I find the mud has been compacted by the roots removing water.
Paul Krombholz in sunny, bright, Jackson, Mississippi,