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Ecophysiological measurements on two pendulous forest mosses from Uganda, Pilotrichella ampullacea and Floribundaria floribunda

Abstract Pressure-volume curves from thermocouple psychrometer measurements show water-relations parameters for Pilotrichella ampullacea and Floribundaria floribunda in the same range as for other epiphytic and forest-floor bryophytes. The estimated osmotic potential at full turgor was floribunda. Both can hold large amounts of external capillary water, up to an equivalent relative water content of ~12 in P. ampullacea and ~6 in F. floribunda. Both species recovered from 11 months' dry storage at 5ºC, but this required a period of days; recovery was slower and less complete in F. floribunda. P. ampullacea showed very rapid initial recovery of FV/FM after periods of a few days' desiccation, but this became progressively less complete, and subsequent full recovery slower, as desiccation time increased from 20 h to 12 days. Simple compensation-period experiments suggested that P. ampullacea can regain a positive carbon balance within 30-60 min after 20 h air-dry (-37 MPa), and within 2-2½ h after 6 days dry. Photosynthetic electron transport in both species is saturated at PPFD ~100 µmol m-2 s-1; both show rather low NPQ (to ~2.5), and 1 (400 µmol m-2 s-1). The measurements show P. ampullacea to be well adapted to intermittent drying at the ambient humidity and light levels likely to be encountered in a humid tropical forest, and suggest that F. floribunda requires somewhat shadier and more continuously moist conditions. The results are consistent with the limited published data, and underline the importance of a large capacity for holding external capillary water and for rapid recovery from short periods of drying in the maintenance of a positive carbon balance by these pendulous mosses.