Mick Nally wrote:

[snipped a bunch]

>I have a distant memory from school biology of a concept known as "The
>Compensation Point". This (if I remember correctly) is a state that terrestial
>plants strive to achieve where the O2 + sugars produced by photosynthesis
>are entirely used up in respiration and the water + CO2 produced by
>>respirationare entirely used up in photosynthesis.

The compersation point is generally taken to be that condition where CO2
produced by respiration and CO2 consumed by photosynthesis are in balance,
so that one measures a NET photosynthetic rate of zero. The actual "stuff"
produced and consumed are not necessarily (or even probably) the same
molecules. Just the opposing rates themselves are in balance. The
compensation point is not a desireable condition since there is no net gain
of carbon for growth and development. Plants need to EXCEED the
compensation point in order to actually gain carbohydrate. You are correct
that terrestrial plants, at least, can vary their compensation point based
on preconditioning (your forest floor example would tend to result in a
plant better able to exist in shade, but this would be because this plant
is better able to exceed its compensation point under the low light
conditions it has been grown under).

>Does this apply to aquatic plants? If so is it the state we should be striving
>to achieve for our plants?

We should be attempting to get our plants to exceed their compensation
point, or perhaps better stated, we should be selecting plants that can
exceed their compensation points under the conditions we are providing in
our tanks.


    John Shane                                        (802) 656-2907
University of Vermont                       jshane at moose_uvm.edu
 Forestry Department                        FAX --   (802)656-8683