Ascorbate in thylakoid lumen as an endogenous electron donor to Photosystem II: Protection of thylakoids from photoinhibition and regeneration of ascorbate in stroma by dehydroascorbate reductase

Photoinhibition of the electron transport activity from tyrosine Z (Y_Z) in PS II to NADP⁺in Tris-treated thylakoids was suppressed by electron donation with either diphenylcarbazide or ascorbate (AsA) during the photoinhibition treatment. This suggests that AsA prevents donor side-induced photoinhibition in vivo as an endogenous donor. AsA in the lumen is photooxidized to monodehydroascorbate (MDA) in Tris-treated thylakoids, as detected by electron spin resonance spectrometry, but not in oxygenic thylakoids. Redox analysis of pyridine nucleotide in the presence of either MDA reductase or dehydroascorbate (DHA) reductase showed that the MDA photoproduced in the lumen is disproportionated to AsA and DHA, and the DHA leaking into the stroma is reduced to AsA by DHA reductase. No leakage of MDA through the thylakoid membrane was observed. Thus, the DHA-reducing enzyme system is indispensable in maintaining AsA concentrations in chloroplasts.