I think your making a ton of assumptions. I find it implausible that a single stem or a single strand of riccia would supersaturate a 4 foot tank.
Pretty sure @Greggz
has stated that he can force pearling by upping co2. If you don't see pearling in slower growing plants then in effect there is no pearling as far as the hobbyist is concerned. It's not about whether there is technically pearling for me, it's a matter of whether you see it.
If it was simply the o2 saturation of the tank, and the o2 can't be absorbed any more ALL plants would pearl. Pearling is an individual species thing. I don't think all plants produce the same amount of o2 within the same environment or at the same rate. Also if all closed-systems tended to go to oxygen saturation then you would expect to see pearling in tanks without co2 injection on a regular basis which just doesn't happen in the hobby.
I'm trying to figure out how to explain myself in a simpler manner, because I'll admit it's not intuitive. And you're rightabout photosynthetic rate playing an important role.
Let's say you have a kiddie pool that is perfectly level, and is filled all the way to the top. Maybe even some surface cohesion juuust keeping it from spilling. That pool is saturated
with water. If you add just one more drop of water to the pool, some water (maybe a drop's worth, maybe a little more or less depending on the metastability of the pool) will come out. I think you're thinking of it as "If I add a drop of water to a swimming pool it's not going to overflow" when you should think of it more in terms of adding a drop of water to a pool that is already completely full.
That's the equivalent of the single piece of riccia in a fish tank. The natural state of a body of water is gaseous equilibrium with the environment, which would be 100% oxygen saturation relative to the temperature, pressure, and amount of oxygen in the air (the completely full pool in this analogy). That single piece of riccia is not adding all
the oxygen to that body of water, it is adding oxygen to a body of water that is already full of oxygen.
A good way you can see supersaturation of water is just with water from your tap. Fill up a glass with cold water, put it on a stable surface and let it come up to room temperature. When you go and look at it at room temp, you'll notice small bubbles on the inside of the glass and when you give it a stir you should hear a light fizzing. That's because the water out of your tap is saturated with oxygen, and even a slight increase in temperature causes it to become supersaturated.
Of course O2 levels in a fish tank are a dynamic equilibrium. As mentioned before fish, snails, shrimp, bacteria, really anything non-photosynthetic in a fish tank will decrease O2 content through metabolic activity. If you think of those factors as driving DO down and photosynthesis/ diffusion as driving DO up, your hypothesis about photosynthetic rate is correct. When you have lots of fast-growing plants, you're more likely to reach the saturation point. Greggz' experience makes sense in this light - if the normal rate of photosynthesis is not fast enough to overcome DO depletion, there won't be pearling, but increasing the rate of photosynthesis through higher CO2 injection tips the balance towards oxygen accumulation to the point the water is saturated with oxygen.
That lovely photo you posted looks like a bucephalandra, and I'm sure you've seen anubias with bubbles accumulating under the leaves. These alone would seem to disprove that only growth rate causes pearling.