Extreme low pH, not Al , is a key abiotic stressor for the extremophyte Carex angustisquama (Cyperaceae) in highly acidic solfatara fields.

Volcanic acidification creates extreme soil conditions, where rhizotoxicity from extremely low pH (2-3) and high Al strongly inhibit plant growth. C. angustisquama is a dominant extremophyte in highly acidic solfatara fields, where no other vascular plants can survive. Here we investigated the key abiotic stressor determining survival of this extremophyte. Soil analyses and topographic surveys were conducted to examine the effects of low pH and Al , two major abiotic stressors in acidic soils, on the occurrence of C. angustisquama in solfatara fields. Hydroponic culture experiments were also performed to test its growth responses to these stressors. In field surveys, the spatial distribution of soil pH was consistent with vegetation zonation within a solfatara field. In contrast, soil exchangeable Al content was overall low due to strong eluviation. Statistical analysis also supported the significant role of soil pH in determining the distribution of C. angustisquama in a solfatara field. Furthermore, hydroponic culture experiments revealed a higher tolerance of C. angustisquama to low pH than a sister species, especially in the range pH 2-3, corresponding to the pH values of the actual habitats of C. angustisquama. Conversely, no significant interspecific difference was detected in Al tolerance, indicating that both species had high Al tolerance. This study suggests that low pH is a critical abiotic stressor leading to formation of the extremophyte in highly acidic solfatara fields. In contrast, C. angustisquama displayed high tolerance to Al toxicity, probably acquired prior to speciation.