Abstract. Using fluorescence measurements at 77 K, the susceptibility to photoinhibition was determined for variously dehydrated leaves of the desiccation–resistant fern Polypodium polypodioides. As expected, water loss increased the potential danger of light-induced damage. However, the long-term consequences of that damage differed depending on the hydration state of the leaves during the light treatment, suggesting that different targets were affected. This became evident when the rehydrated leaves were subjected to conditions conducive to recovery from typical photoinhibition. After an exposure to high photon flux densities, full restoration of normal photochemical efficiencies occurred only when the photoinhibitory treatment had occurred while the leaves were fully hydrated, or when the dehydration had reached a stage at which only the primary charge separation and storage were operative. On the other hand, lesions caused by high light intensities in slightly dehydrated leaves which had retained some electron transport activity between the two photosystems, or in desiccated leaves incapable of any measureable photosynthetic functions, were very inefficiently or not at all repaired. One unexpected hazard of high light intensities for dehydrated leaves turned out to be further desiccation which could reach a threshold at which extensive cellular damage began to ensue. In nature, P. polypodioides avoids light–induced damage by rolling its leaves during dehydration so that only the reflectant abaxial leaf surface is exposed to ambient light, and by occupying shaded habitats.
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