Spores from open capsules of Physcomitrium patens tolerate up to seven years of desiccation and do not respond to prehydration

The desiccation tolerance of the spores of the model moss Physcomitrium patens is largely unknown. An opportunity arose to test spore longevity in P. patens capsules stored in near-darkness under laboratory conditions at low relative humidity (RH) levels for up to 11 years. Most of the capsules had slowly dried over a period of weeks as intact cultures in a growth chamber at high humidity. After this initial drying, the cultures with capsules were placed in the dark or at very dim lighting at (1) ambient relative humidity averaging 35% for periods up to 7 years, or (2) at 0% RH (one culture and a few separated sporophytic shoots) for up to 11 years. Germination was high (≥ 92%) for spores stored at ~35% for up to 5 years, dropping precipitously to 68% and < 15% in years 6 and 7, respectively. After experiencing periods of 5 and 11 years desiccation at equilibrating RHs of ~0%, spores of P. patens became nonvi­able except for a small percentage (< 2%) of spores only from closed capsules desiccated continuously for 11 years. Although spores of P. patens are on the large side for mosses with presumably greater storage reserves, toleration of dry dark periods longer than 7 years in open capsules is unlikely. Prehydration (exposing spores to humid air prior to added water), had no effect on spore germination through 7 years of desiccation, although for the oldest spores tested with open capsules (7 years), prehydration improved viability from 6 to 22%.