bubbles on plants

Where does the oxygen produced by aquatic plants go?

It must originate within the leaf cells which contain the
chloroplasts. At that point it's in solution within the
protoplasm of the cell. I suppose that the cell walls
and surrounding specialized cells may be designed to
conduct oxygen and other gases in gaseous form (as opposed to
in solution) throughout the structures of the plant. Some
plants have well developed air tubes (forget the name)
which help carry oxygen into the substrate in order to
modify the environment for their benefit. Normally these
air channels are not emitting bubbles into the water.

I suspect that a good many plants permit oxygen (and CO2)
to pass out (and into) through the leaf surface. This
would depend on the leaf structure of the plant being
considered. I imagine that O2 passing out of the plant by
this mechanism will be in solution rather than as gaseous

When you observe O2 bubbles forming on plants and the other
items in the aquarium, I believe that these are the result
of oxygen diffusing out of solution at a site of chemical
"instability". By that I mean, there is an electro-chemical
reason for the oxygen to change states at that site just as
rain drops and snow crystals require a tiny speck of dust to
make the transition from a gas to a liquid or a solid.

Different plants employ highly specialized metabolic strategies
(look for past articles in the APD re growth strategies)
Plants photosynthesize at different rates although I expect
that most are capable of producing enough oxygen under ideal
circumstances to achieve the ~9 milligrams / liter H2O which
is the saturation point for oxygen at room temperature and
pressure. The ideal for your plants and fish is between 5-7 
mg/L O2 (according to "Aquarium Plants Manual" by Scheurmann)
but this would only occur in a tank with clean water, strong
light and few fish.

The plant leaves or other objects where you observe bubbling
may not actually be the ones responsible for contributing
the larger proportion of oxygen into solution!