[NatureNS] Duckweed

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From: Stephen Shaw <srshaw@Dal.Ca>
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Thread-Topic: [NatureNS] Duckweed
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Hi Dave,
I=92d forgotten about this, but isn=92t the primary observation to explain =
that the plants routinely sink in the winter and rise again in the spring? =
 What the mechanism is, is an interesting but secondary consideration.   In=
 your =91experimental=92 case with lighted duckweed, it should be producing=
 and accumulating some sort of gas mixture by September's photosynthesis, m=
aybe plus some concurrent respiration.  Your =91control', dark duckweed, sh=
ould have no photosythesic activity, so only respiration should produce any=
 gas at all, presumably CO2.

The problem is that neither =91expt' nor =91control' actually sank on Sept =
27.  Since the plant material including any dense starch should anyway have=
 an average density >1g/cc (i.e. greater than plain water*), excluding any =
gas spaces, both cases must still be floating according to some undissolved=
 gas inclusions, though not necessarily the same mix of CO2/O2 in the two c=
ases.  You get an A+ for industry and design, but Lemna gets an F for coope=
ration.

*Something I=92d not considered that you may know about but I don=92t is oi=
l/fat production in plants, which obviously is important for some land plan=
ts (canola, sesame, olives etc).  Oils have specific gravities <1, around 0=
.9 g/cc, so enough oil volume could be sufficient to float a water plant wi=
thout invoking gas spaces.  If something dense like starch gets converted t=
o an oil in spring, could that contribute, or is this just fantasy?

Is it possible that the trigger for sinking, whatever its mechanism, is a p=
articular drop in day length that had not yet been closely approached by Se=
pt 27, and especially in your toasty living room (temperature might be anot=
her factor)?   If you haven=92t flushed them in frustration, you should con=
vey both your test subjects to an unheated outhouse and extend your experim=
ent there at least to freeze-up.
Steve
---------------------------------------------------------
On Sep 27, 2018, at 12:35 PM, David <dwebster@glinx.com<mailto:dwebster@gli=
nx.com>> wrote:

Dear All,
    I have had my eye open this summer for some Lemna to test the gas compo=
sition 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 flooded air spaces 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 muc=
k of uncertain depth. So I set up a simple trial; some kept in darkness, ex=
cept for a few minutes daily for observation, and some kept on a windowsill=
.
    By Sept 27 those kept in darkness showed no tendency to sink but were n=
oticeably less green than those exposed to daylight and some artificial lig=
ht in the evening. This leads to the conclusion that flooding of intercellu=
lar gas spaces when CO2 predominates does not cause sinking of Lemna.  The =
converse explanation that Lemna rises to the surface in the spring when int=
ercellular spaces become filled by less soluble O2 is accordingly voided.
    The explanation given in the link posted by Nancy on Oct 3, 2017 is con=
sequently more sound. Starch accumulation in the fall. as photosynthesis ex=
ceeds respiration at lowering temperature, leads to sinking. Starch consump=
tion, as temperature warm in the spring and new buds start to grow, leads t=
o rise to the water surface. [density of starch is high; 1.5 g/cm^3]

Yt, DW, Kentville



------ Original Message ------
From: "Stephen Shaw" <srshaw@Dal.Ca<mailto:srshaw@Dal.Ca>>
To: "naturens@chebucto.ns.ca<mailto:naturens@chebucto.ns.ca>" <naturens@che=
bucto.ns.ca<mailto: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 lea=
f is bound to be somewhat denser than pond water, regardless of denser star=
ch, so will naturally sink at all times unless kept buoyant by the extracel=
lular gas bubbles or some other modification.  What seems to need explainin=
g is why the bubbles are maintained in summer (dissolved gases can pass thr=
ough lipid cell membranes quickly, so you=92d expect them to dissolve out i=
nto the surrounding pond), and why they disappear in winter.  He doesn=92t =
seem to know if it=92s CO2 or O2 in the bubbles, or both.  Perhaps they are=
 largely O2 in summer and dissolve and normally pass out of the leaf to oxy=
genate the pond and dissolved CO2 moves in, but leaf photosynthesis is suff=
iciently high in summer to maintain them, despite these losses.  In winter =
gas production simply stops, and the non-buoyant plants sink.  As winter en=
ds, respiration produces bubbles with a different gas, CO2, and they rise a=
gain then switch over to O2 as photosynthesis picks up.

It therefore seems likely that these leaves have some special surface coati=
ng that slows down gas exchange with the pond.  Maybe they still retain sto=
mata under the leaf which normally facilitate gas exchange for leaves in ai=
r, but these are modified to block or regulate gas exchange in water?  Mayb=
e this is already known, just not to us here?
Steve

On Mar 10, 2018, at 10:45 AM, David <dwebster@glinx.com<mailto:dwebster@gli=
nx.com>> wrote:

Hi Nancy & All,
    I admit not having wondered about this but I think he makes a simple pr=
ocess complicated. The air pockets to which he refers are presumably interc=
ellular spaces which in the absence of photosynthesis presumably can become=
 water filled. Starch has a specific gravity of 1.5 g/mL and, in cooler wea=
ther starch will accumulate when respiration slows more than photosynthesis=
 and the submarine will sink. With warming, growth resumes, respiration and=
 starch consumption rates increase and  the submarine rises.
Yt, DW, Kentville

------ Original Message ------
From: "nancy dowd" <nancypdowd@gmail.com<mailto:nancypdowd@gmail.com>>
To: naturens@chebucto.ns.ca<mailto: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 reapp=
earance in the Spring 2018 issue of Northern Woodlands Magazine.https://nor=
thernwoodlands.org/outside_story/article/duckweed-migration

I had never really thought about where it went in Fall or how it re-emerges=
 in Spring. Such an impo