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Dear All,
I have had my eye open this summer for some Lemna to test the gas=20
composition idea; the more soluble CO2 predominating as days shorten=20
leading to flooding of gas spaces and sinking; photosynthesis=20
predominating in spring as days lengthen leading to flooded air spaces=20
being filled by less soluble O2 and rise to the surface.
And I was able to collect some on Sept 21 without having to wade in=20
muck of uncertain depth. So I set up a simple trial; some kept in=20
darkness, except for a few minutes daily for observation, and some kept=20
on a windowsill.
By Sept 27 those kept in darkness showed no tendency to sink but=20
were noticeably less green than those exposed to daylight and some=20
artificial light in the evening. This leads to the conclusion that=20
flooding of intercellular gas spaces when CO2 predominates does not=20
cause sinking of Lemna. The converse explanation that Lemna rises to=20
the surface in the spring when intercellular spaces become filled by=20
less soluble O2 is accordingly voided.
The explanation given in the link posted by Nancy on Oct 3, 2017 is=20
consequently more sound. Starch accumulation in the fall. as=20
photosynthesis exceeds respiration at lowering temperature, leads to=20
sinking. Starch consumption, as temperature warm in the spring and new=20
buds start to grow, leads to rise to the water surface. [density of=20
starch is high; 1.5 g/cm^3]
Yt, DW, Kentville
------ Original Message ------
From: "Stephen Shaw" <srshaw@Dal.Ca>
To: "naturens@chebucto.ns.ca" <naturens@chebucto.ns.ca>
Sent: 3/10/2018 3:20:47 PM
Subject: Re: [NatureNS] Duckweed
>Interesting puzzle, perhaps not so simple. Any cell system like a live=20
>leaf is bound to be somewhat denser than pond water, regardless of=20
>denser starch, so will naturally sink at all times unless kept buoyant=20
>by the extracellular gas bubbles or some other modification. What=20
>seems to need explaining is why the bubbles are maintained in summer=20
>(dissolved gases can pass through lipid cell membranes quickly, so=20
>you=E2=80=99d expect them to dissolve out into the surrounding pond), and=
why=20
>they disappear in winter. He doesn=E2=80=99t seem to know if it=E2=80=99s =
CO2 or O2 in=20
>the bubbles, or both. Perhaps they are largely O2 in summer and=20
>dissolve and normally pass out of the leaf to oxygenate the pond and=20
>dissolved CO2 moves in, but leaf photosynthesis is sufficiently high in=20
>summer to maintain them, despite these losses. In winter gas=20
>production simply stops, and the non-buoyant plants sink. As winter=20
>ends, respiration produces bubbles with a different gas, CO2, and they=20
>rise again then switch over to O2 as photosynthesis picks up.
>
>It therefore seems likely that these leaves have some special surface=20
>coating that slows down gas exchange with the pond. Maybe they still=20
>retain stomata under the leaf which normally facilitate gas exchange=20
>for leaves in air, but these are modified to block or regulate gas=20
>exchange in water? Maybe this is already known, just not to us here?
>Steve
>
>On Mar 10, 2018, at 10:45 AM, David <dwebster@glinx.com> wrote:
>
>>Hi Nancy & All,
>> I admit not having wondered about this but I think he makes a=20
>>simple process complicated. The air pockets to which he refers are=20
>>presumably intercellular spaces which in the absence of photosynthesis=20
>>presumably can become water filled. Starch has a specific gravity of=20
>>1.5 g/mL and, in cooler weather starch will accumulate when=20
>>respiration slows more than photosynthesis and the submarine will=20
>>sink. With warming, growth resumes, respiration and starch consumption=20
>>rates increase and the submarine rises.
>>Yt, DW, Kentville
>>
>>------ Original Message ------
>>From: "nancy dowd" <nancypdowd@gmail.com>
>>To: naturens@chebucto.ns.ca
>>Sent: 3/10/2018 9:08:02 AM
>>Subject: [NatureNS] Duckweed
>>
>>>This is an interesting article on Duckweed seasonal disappearance and=20
>>>reappearance in the Spring 2018 issue of Northern Woodlands=20
>>>Magazine.https://northernwoodlands.org/outside_story/article/duckweed-mi=
gration
>>>
>>>I had never really thought about where it went in Fall or how it=20
>>>re-emerges in Spring. Such an important floating pond plant in=20
>>>productive fresh water
>>>
>>>Nancy D
>>>
>>>Sent from my iPad
>
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<body><div>Dear All,</div><div>=C2=A0 =C2=A0 I have had my eye open this su=
mmer for some Lemna to test the gas composition idea; the more soluble CO2=
predominating as days shorten leading to flooding of gas spaces and sinking=
; photosynthesis predominating in spring as days lengthen leading to floode=
d air spaces being filled by less soluble O2 and rise to the surface.=C2=A0=
</div><div>=C2=A0=C2=A0=C2=A0=C2=A0And I was able to collect some on Sept 2=
1 without having to wade in muck of uncertain depth. So I set up a simple t=
rial; some kept in darkness, except for a few minutes daily for observation=
, and some kept on a windowsill.=C2=A0</div><div>=C2=A0 =C2=A0 By Sept 27 t=
hose kept in darkness showed no tendency to sink but were noticeably less g=
reen than those exposed to daylight and some artificial light in the evenin=
g. This leads to the conclusion that flooding of intercellular gas spaces w=
hen CO2 predominates does not cause sinking of Lemna. =C2=A0The converse ex=
planation that Lemna rises to th