Temperature-Induced Changes of Variable Fluorescence-Yield in Intact Leaves

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Abstract
Variable fluorescence (Fv) of intact leaves was measured when the temperature was lowered at a rate of 1–2°C per mn, from 20°C to −20°C. The quantum flux density of the exciting light was 1–2 μE m−2 sec−1 in order to sensitize F only at 20°C. The fluorescence yield decreased rapidly at the freezing point of the leaf and upon further cooling the fluorescence yield increased again. Fm was obtained a few degrees below the freezing point. Repeated freeze-thaw cycles caused successively increased damage to the thylakoid membranes on either the oxidizing or the reducing side of photosystem II. An eventual loss of Fv over Fo was typical for damage on the water splitting side of photosystem II, whereas damage after the primary electron acceptor Q of photosystem II was characterized by an invariable fluorescence yield at Fm over the temperature range examined.

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