Question

why is the lumen of the thalakoid acidic while the stroma is alkaline in nature.?

Answer 1

Oxidation of two water molecules at PSII (in non-cyclic photophosphorylation), is accompanied by the release of 4 protons into the lumen of the thylakoid which is responsible for acidic nature of thylakoid lumen. And the oxidation and reduction of plastoquinone leads to transfer of 4 protons from stroma to lumen, which creates alkaline environment in stroma.