Freezing of isolated thylakoid membranes in complex media. V. Inactivation and protection of electron transport reactions.

When chloroplast thylakoid membranes isolated from spinach leaves (Spinacia oleracea L. cv. Monatol) were frozen in media containing the predominant inorganic electrolytes of the chloroplast stroma, linear photosynthetic electron transport became progressively inhibited. After onset of freezing, both PSII- and PSI-mediated electron flow were inactivated almost to the same extent. Prolonged storage of the membranes in the frozen state increased damage to PSII relative to PSI activity. Under these conditions, a preferential injury of the water oxidation system was not observed. In thylakoids stored at 0 °C, PSI activity remained fairly unimpaired but inactivation of PSII occurred with strongest inhibition at the oxidizing side.The addition of low-molecular-weight cryoprotectants such as glycerol, sugars, certain amino acids and carbonic acids to thylakoid suspensions prior to freezing provided almost complete preservation of PSI activity and considerable but incomplete stabilization of PSII.