We posed two questions regarding the desiccation tolerance (DT) of the moss Syntrichia pagorum: (1) What is the ecological strategy of DT in three available life phases (propagula, juvenile and adult shoots)? and (2) which desiccating factor, rate of drying or equilibrating relative humidity, has the greater effect on plant survival and health in these three life phases? Plants of S. pagorum were regenerated from an herbarium specimen collected from the American southwest, subcultured to remove field effects, and grown in Petri dishes on native sand for up to 6 months, producing in sequence juvenile shoots, adult shoots and propagula. Whereas juvenile shoots exhibited an ecological strategy of inducible desiccation tolerance (IDT), the propagula and adult shoots largely exhibited the strategy of constitutive desiccation tolerance (CDT). This conclusion was reached by subjecting each life phase to two rates of drying (20 min and 48 hours at subturgor) in combination with two equilibrating relative humidities (RH, 50% and <1%), and monitoring germination rate (propagula), visual leaf damage, and chlorophyll fluorescence (propagula and shoots) upon rehydration. We show for the first time that (1) cultured, dehardened adult shoots and propagula of a bryophyte are inherently CDT at an equilibrating RH of 50%, (2) rate of drying has a greater effect on rehydrated plant health than equilibrating RH, and (3) a bryophyte shoot can transition during normal development from one ecological strategy of desiccation tolerance to the alternative strategy.
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