Re: Compensation point for aquatic plants

Mick Nally wrote, Friday, March 22:

....>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 respiration
>are entirely used up in photosynthesis. The plant is in an optimum state of
>equillibrium and is evolved to reach this point for its given growing

       ..........<rest snipped>

The compensation point is an intensity of light where photosynthesis just
balances out respiration.  It is not a particularly desirable level of
light because the plant is unable to grow.  It helps to think of plants as
little businesses.  Photosynthesis can be considered total income, and
respiration would then be expenses.  In light, respiration and
photosynthesis occur together, and, if the plant is to grow, photosynsis
must exceed respiration.  In fact, photosynthesis must exceed respiration
over a 24 hour period, if the plant is to grow.  The plant, like a
business,  must have a net profit in order to grow.  Income over the day,
week, month, or whatever, must be greater than expenses.  At the
compensation point, plants merely exist.  Actually, they probably grow at
one end and die at the other and maintain the same overall weight of living
tissue.  If the light level were at the compensation point during the day,
the plant would die  in the long run because there would be expenses during
the night not recvered during the day when income and expenses equal each

Nearly all aquatic plants are shade tolerant, and that means that they have
relativly low respiration rates (reduced expenses) compared to sun-loving
plants.  They also have greater efficiency in capturing light at low levels
than do sun-loving plants.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174

Where I am wondering what happened to my spring break!