Re: What plant gives off the most oxygen per volume of plant?

C4  plants will give off the most oxygen per volume of plant. In general,
a plant will generate one mole of oxygen per mole of carbon dioxide that is 
taken up during photosynthesis. This ratio is a function of photosynthetic
rate and leaf structure and orientation. The former is dependent upon 
light, temperature, and water status. Leaf structure, among other things, 
refers to leaf thickness, chloroplast number and chloroplast orientation. 
It is difficult to pinpoint a specific plant because photosynthetic rate
is adaptive to the plant's particular environment. Chloroplast number and
orientation are also adaptive. C4 plants are the most likely candidates for
higher oxygen generation because, in general, they are the most efficient  
at carbon dioxide uptake and photosynthesis. 
   
As you are probably aware, C4 plants include wheat, maize, and many 
subtropical shrubs and low-lying plants within a semi-arid type of 
environment.