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

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 -1.82 MPa in P ampullacea and -1.43 MPa in E floribunda. Both can hold large amounts of external capillary water, up to an equivalent relative water content of similar to12 in P. ampullacea and similar to6 in F.floribunda. Both species recovered from 11 months' dry storage at 5degreesC, but this required a period of days; recovery was slower and less complete in E,floribunda. P. ampullacea showed very rapid initial recovery of F-V/F-M 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 It air-dry (-37 MPa), and within 2-2(1)/(2) h after 6 days dry. Photosynthetic electron transport in both species is saturated at PPFD similar to100mumol m(-2) s(-1); both show rather low NPQ (to similar to2.5), and 1 - q(P) rising to similar to0.8 at saturating PPFD (400 mumol 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.