In fact during photosynthesis plants actually release O2 into the water, then during the dark period(which is when they actually make roots, But they will still do dark time things in a 24/7 cycle just not as efficiently)when they resperate in the dark they use the O2 in the water...
http://bcn.boulder.co.us/basin/data/COBWQ/info/DO.html
What i mean about the dark cycles is that if you dont give your plants a dark cycle they will do the same things in constant light just not as well or quickly! You can clone in 24/7 just not as fast!
I learned that one in the ezcloner onb 24/7 nothing was rooting for weeks, turned the lights to 18/6 and roots over night!
http://sgs-hydroponic.com/?page_id=179 Clones grown in rockwool or other soil like mediums have plenty of O2 available to the roots because the water going over the media when watered pulls DO down into the medium and the multiple surfaces and pores hold the O2 in there thats why you can use heat mats and really make the roots spring out...In water based mediums the warmer the H2O the less DO is in it and the less your plants get...The optimum water temp/Do is 68 but this is a little cold for cloning quickly..the ultimate areocloner room to water temp is room 70 water 72