Redox potentials in photosynthesis light dependent stage
You are right about the conditions in the chloroplast stroma vs. standard conditions. The pH is high (i.e., low H+) in the chloroplast compartment where the reaction takes place (within the stroma), so the pE is shifted from the nominal "pE0", which is +1.23V (effectively you push the equation to the right by low concentration of H+).

See this for a textbook-like summary of the important reactions and this for a quick problem on the math behind pE in different conditions, as well as this for more on the stroma.