Part of the xanthophyll (or violaxanthin) cycle for controlling the concentration of zeaxanthin in chloroplasts. Catalyzes the two-step mono de-epoxidation reaction. Stereospecific for all-trans xanthophylls. Zeaxanthin induces the dissipation of excitation energy in the chlorophyll of the light-harvesting protein complex of photosystem II. all-trans-violaxanthin + 2 L-ascorbate = all-trans-zeaxanthin + 2 H2O + 2 L-dehydroascorbate Low expression in young leaves, but high expression in older leaves. Belongs to the calycin superfamily. Lipocalin family.