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[APD] Re: Aquatic vs Aquarium plants

Steve P asked:
"Do all of these borderline plants start growing leaves without a waxy
cuticle on
the leaves so that they can respire CO2 more easily?"

I don't think plants  (marsh, marginal etc) grow underwater leaves without a
cuticle to respire CO2, oxygen or take up nutrients more easily for that
matter. Terrestrial plants grow a cuticle and other similar structures to
conserve water. Why waste valuable energy in producing these substances if
you do not need them? With a cuticle, air-leaves were already respiring well
enough for the plant to grow and reproduce. So the increased availability of
CO2 would be an added bonus but not the prime reason for loosing the cuticle
in water.

If the plant lost its cuticle because it had a lowered CO2 availability and
was thus trying to make up for it, than it would be simple to prove/disprove
the hypothesis by simple experimentation. I do not think that experimental
terrestrial plants would loose the cuticle if the CO2 is lowered. (Both
experimental and control plants should be at high humidity to avoid
dessication.) And even if you increased the CO2 content in an aquarium I do
not think the plants would thicken their cuticle. So that would mean that
the plant looses its cuticle because of an increased humidity and not to
increase its CO2 availability. But if you do prove me wrong please let me

In some way the plant senses that the leaves are in a humid or dry
environment and thickens or thins the cuticle in new leaves accordingly. How
it does this I don't know, but its possibly the juvenile epidermis itself
which has this humidity sensory function.

"If so, how universal is this adaptation?"
All the plants I've tried under water so far loose their protective covering
after some time, but to different degrees. Samolus valerandi for example
dries out immediately if reexposed to air without a transition period, which
implies that it had no effective protective layer. On the otherhand, Phoenix
roebelenii, the dwarf date palm will only shrivel at its leaf tips if
exposed to very dry air after taking it out of the water, something that
would not have happened if it was not in water in the first place. (I have
germinated and grown this palm in my main display tank, but that's beside
the point). Why does'nt the Phoenix loose all its protective covering when
submersed? It may be that the plant epidermis slows down the cuticle
production in water but the mechanism does not switch off completely.

"Is it absolutely necessary for a plant to
survive underwater? I've suspected that some types of leaves such as
Anubias, could indeed be viable out of water too."

Anubias leaves are probably similar to the palm leaf I mentioned above. So
they will still dry out if placed in a dry room but will adapt readily to a
humid environment such as in a terrarium. If the atmosphere is dried out
slowly they would probably produce new leaves with thicker cuticles, so that
you can grow them as house plants.

Still, I don't know how this information, even if experimentally
proven/disproven, might help anybody grow plants better in aquariums though.

Anyway, Good luck

Best regards
in Malta

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