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Hi Steven & All,
Good points. Aquatic plants typically have specialized tissues of=20
interconnected passages which are filled with gas when the plant is=20
active and I would expect this to be true of Lemna also. The relative=20
solubility of CO2 and O2 in water is no doubt relevant; CO2 is 16.4 and=20
31.5 times as soluble in water as O2 at 0o C and 10oC respectively. So=20
during the growing season, photosynthesis would keep such aerenchyma=20
filled with O2. With shorter days and less photosynthesis, CO2=20
production by respiration (year round in all living tissue night and=20
day) would gradually exceed O2 production and lead to a flooding of the=20
ballast tanks as the CO2 dissolves; submarine down.
No doubt details of the anatomy play a part but I would expect=20
stomata if present to be on the upper surface.
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>Hi Steven & All,</div><div>=C2=A0 =C2=A0 Good points. =C2=A0=
Aquatic plants typically have specialized tissues of interconnected passage=
s which are filled with gas when the plant is active and I would expect thi=
s to be true of Lemna also. The relative solubility of CO2 and O2 in water=
is no doubt relevant; CO2 is 16.4 and 31.5 times as soluble in water as O2=
at 0o C and 10oC respectively. So during the growing season, photosynthesis =
would keep such aerenchyma filled with O2. With shorter days and less phot=
osynthesis, =C2=A0CO2 production by respiration (year round in all living t=
issue night and day) would gradually exceed O2 production and lead to a flo=
oding of the ballast tanks as the CO2 dissolves; submarine down.=C2=A0</div=
><div>=C2=A0 =C2=A0 No doubt details of the anatomy play a part but I would =
expect stomata if present to be on the upper surface.</div><div>Yt, DW,Ken=
tville</div><div>=C2=A0 =C2=A0=C2=A0</div>
<div><br /></div>
<div>------ Original Message ------</div>
<div>From: "Stephen Shaw" <<a href=3D"mailto:srshaw@Dal.Ca">srshaw@Dal.C=
a</a>></div>
<div>To: "naturens@chebucto.ns.ca" <<a href=3D"mailto:naturens@chebucto.=
ns.ca">naturens@chebucto.ns.ca</a>></div>
<div>Sent: 3/10/2018 3:20:47 PM</div>
<div>Subject: Re: [NatureNS] Duckweed</div><div><br /></div>
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ode: space; -webkit-line-break: after-white-space;"><blockquote cite=3D"F3F