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[APD] Re: Aquatic-Plants Digest, Vol 15, Issue 7


I agree that net carbon gain could be one way to define growth, but I don't think it is really the most useful definition for this situation. I think many would consider plant growth to be an increase in size and mass of a plant.  Size increases come from cell division and especially cell elongation.  The carbon required in making new cells comes from stored carbon, not CO2 that drifts into the the cells from the air.  And cell elongation is essentially pumping water into the cell vacuoles and stretching them out.  The water uptake needed for this increases mass and the stretching of the cell and thereby increases the size of the stem (or leaf or other structure).  I seriously doubt that someone reporting that they have a lot of plant growth would say that they've seen a lot of carbon getting stored, they would say the stems are getting longer and the leaves are getting bigger.  

While the exact timing of carbon uptake may be useful, or at least worth debating amongst physiologists, I think for the average aquatic gardener (and even a plant systematist like myself!) a physical increase in size (due to cell elongation and division) is the ideal definition.  

If nothing else, keep in mind what Tom Wood's original question was.  I think your definition of growth applied to his observation would lead him to think he was imagining things when the plants were actually increasing in size.

I'm curious about your statement on O2: "Some also define growth or primary production as O2 evolved, especially in aquatic photosynthesis."
Could you clarify that a bit further?  I'm not disagreeing with it, I just don't understand what you are referring to.



Message: 4
Date: Tue, 2 Nov 2004 09:57:44 -0800 (GMT-08:00)
From: Thomas Barr 
Subject: [APD] Re: growth at night
To: aquatic-plants at actwin_com

>Yes, plants do grow at night. Keep in mind that CO2 is used in photosynthesis. Photosynthesis is the plants >method of creating food (i.e. sugars) which is then stored for use during growth (by the way fertilizers are not >food, they are nutrients). Oxygen is a byproduct of the reaction.

>Here is the basic photosynthesis reaction:

>6CO2+ 12H2O +light energy--> C6H1206+ 6O2+ 6H2O

>C6H12O6 is glucose and is stored until needed for respiration. Photosynthesis only takes place during the day, >but photosynthesis is NOT responsible for growth in a plant, respiration is (well, actually it provides the energy >directly used for growth, but I don't want this to be too confusing). 

But there is no "net carbon gain" at night. Some/many define that as growth. If CO2 is not being reduced, then there is no net carbon gain. Plants don't take in CO2 at night except for CAM plants. 

Some also define growth or primary production as O2 evolved, especially in aquatic photosynthesis.

Neither of these occur at night.

So you could say plants don't "grow" at night.
They only respire. 
Finding research on plant respiration only is more problematic.

Tom Barr

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