Ah, ha! Thanks to Joe Berry for explaining to me what the Mehler rxn is.
The Mehler Rxn reduces O2 from ferredoxin in a shunt from PSI. Electrons
from P700 travel to ferredoxin and then can go one of three different ways:
1-they reduce NADP+ in the normal noncyclic photophosphorylation pathway or
Z-scheme, 2-they flow into cyclic photophosphorylation via cytochrome b6, or
3-they reduce O2 in the Mehler rxn. and then the reduced O2 is converted to
H2O2 by SOD. The H2O2 is then converted back to O2 by catalase.