The point at which plants start to pearl depends on several factors -- the type & number of plants, the tank size, the amount of light , nutrient supply, the CO2 level, amount of surface agitation .... Water can only hold so much dissolved O2 (the exact amount depends on the temp.) -- when you go above this point (roughly 8-9 ppm), the water starts to become supersaturated and at the same time the O2 starts to escape in the form of bubbles. So basically when you supply all of a plants needs it will thrive and produce lots of O2. When the water can't hold any more you'll see pearling.
You may be confusing this with the target point or range for CO2, which is roughly 10-25 ppm, with 15 being the Krib's suggested target.
HTH,
Jared
