"Pearling" refers to small bubbles of oxygen forming on the leaves of
plants. Presumably because the bubbles of oxygen look like small
pearls resting on the plant.

The observed bubbles of O2 are indeed the result of photosynthesis. At
a slow rate of photosynthesis, the O2 produced by the plant dissolves
into the water and does not result in pearling. At a higher rate of
photosynthesis - either because the water has reached O2 saturation or
because the plant is producing O2 faster than it can dissolve (or
perhaps due to temporary local saturation if water circulation is
poor), the O2 will begin to form bubbles on the plant leaves, rather
than dissolving. This is when pearling is observed.

While pearling isn't completely diagnostic, it does indicate a certain
degree of health of the plants. Strong photosynthesis won't occur
without the correct conditions (ie enough light and CO2).

O2 saturation is strongly dependent on temperature. Like CO2, O2 is
not especially soluble in water. As the temperature increases, O2
solubility decreases (as is the case for CO2 and other gasses). The
effect of other water parameters (pH, KH, GH) is negligible on O2
solubility.

Hope that helps.

Jeff