Answer:
The lumen would have the lower pH because it have greater number of protons.
Explanation:
In the light reaction, the excited electrons in photosystem II releases its energy in Electron transport chain by moving from higher potential to lower potential.
This energy is utilized by proton pump to move the electron from the stroma to Thylakoid lumen against the concentration gradient. There are free protons present in stroma as well as in lumen. so, if the light reaction continues, it will keep accumulating the protons in lumen. These protons are used to synthesize ATP by ATP synthase.
Higher proton concentration means lesser the pH and lower proton concentration mean higher the pH. So pH will be lower in lumen.
Hoping this might be helpful...!
