Lichens are poikilohydric symbioses, therefore they must cope with large fluctuations in their water content and consequent limitations in their metabolic activity. A model of water movement recently developed for the thallus of Flavoparmelia caperata suggests that the photobiont layer has a higher water content than the surrounding fungal pseudotissues during dehydration. Measurements of water potential (Ψ) and PSII quantum yield (Fv/Fm) performed on the same lichen and its isolated photobiont during dehydration supported this model. They showed that the decrease in Fv/Fm that occurred after the loss of cell turgor began at a more negative Ψ (i.e., at a lower water content) in the lichen than in the isolated photobiont. The hypothesis is that cavitation events in the fungal hyphae could cause water movement from the fungal pseudotissues toward the photobiont layer. We tested this hypothesis by measuring ultrasonic acoustic emissions (UAE), a technique that has been used for decades to detect cavitation events in the xylem of vascular plants, on desiccating lichens. UAE were measured on living and heat-devitalized marginal lobes of F. caperata and Lobaria pulmonaria during dehydration. No increase in UAE was observed at relative water contents (RWC) near the turgor loss point, weakening the cavitation hypothesis. Interestingly, a significant increase of UAE was observed at RWC of 7.8 (±0.3) % and 6.7 (±0.6) % in living lobes of F. caperata and L. pulmonaria, respectively, whereas at RWC of 5.2 (±0.7) % and 8.4 (±0.5) % in dead ones. These UAE were characterized by a higher amplitude than those occurring at higher RWCs, suggesting that they released higher energy. Notably, these energy-releasing phenomena arose at RWC below which cells make the transition from a rubbery to a glassy state. This implies that some as yet unknown processes occur inside a thallus even when its metabolism is shut down. Possible mechanisms leading to UAE will be discussed.
Ultrasonic acoustic emissions in desiccating lichens reveal energy release after metabolism shut down
Enrico Boccato;Francesco Petruzzellis;Andrea Nardini;Mauro Tretiach;Fabio Candotto Carniel
2022-01-01
Abstract
Lichens are poikilohydric symbioses, therefore they must cope with large fluctuations in their water content and consequent limitations in their metabolic activity. A model of water movement recently developed for the thallus of Flavoparmelia caperata suggests that the photobiont layer has a higher water content than the surrounding fungal pseudotissues during dehydration. Measurements of water potential (Ψ) and PSII quantum yield (Fv/Fm) performed on the same lichen and its isolated photobiont during dehydration supported this model. They showed that the decrease in Fv/Fm that occurred after the loss of cell turgor began at a more negative Ψ (i.e., at a lower water content) in the lichen than in the isolated photobiont. The hypothesis is that cavitation events in the fungal hyphae could cause water movement from the fungal pseudotissues toward the photobiont layer. We tested this hypothesis by measuring ultrasonic acoustic emissions (UAE), a technique that has been used for decades to detect cavitation events in the xylem of vascular plants, on desiccating lichens. UAE were measured on living and heat-devitalized marginal lobes of F. caperata and Lobaria pulmonaria during dehydration. No increase in UAE was observed at relative water contents (RWC) near the turgor loss point, weakening the cavitation hypothesis. Interestingly, a significant increase of UAE was observed at RWC of 7.8 (±0.3) % and 6.7 (±0.6) % in living lobes of F. caperata and L. pulmonaria, respectively, whereas at RWC of 5.2 (±0.7) % and 8.4 (±0.5) % in dead ones. These UAE were characterized by a higher amplitude than those occurring at higher RWCs, suggesting that they released higher energy. Notably, these energy-releasing phenomena arose at RWC below which cells make the transition from a rubbery to a glassy state. This implies that some as yet unknown processes occur inside a thallus even when its metabolism is shut down. Possible mechanisms leading to UAE will be discussed.Pubblicazioni consigliate
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