While regarded as a principal factor of desiccation tolerance in bryophytes (along with rate of drying, water content and duration dry), the rate of rehydration has been broadly neglected or inadequately assessed. Prior to receiving liquid water from a rain event, desiccated bryophytes are able to partially rehydrate their tissues by absorbing water vapor when ambient relative humidity is high, a process known as prehydration. A single clone of Syntrichia norvegica was cultured for 4–6 months in a suprasaturated condition. Shoot apices were used in experimental prehydrating (1–48 h), drying (5 min–300 h, equilibrating at ∼0% relative humidity), and duration dry (0–42 d continuously dry at 54% RH) series. Shoots were subsequently prehydrated 24 h prior to exposure to liquid water or directly immersed in liquid water and assessed for desiccation associated damage. Prehydration occurs over a 4-h period, after which shoot water content levels out. Shoots that normally die when equilibrated at low water contents were rescued with a prehydration treatment. At extremely rapid rates of drying (<20 min) prehydration effects were limited. However, at gradual rates of drying the prehydration treatment restored chlorophyll fluorescence and regeneration capacity to near control levels. Prehydration also improved the capacity of a shoot to recover following continuously dry episodes of several weeks. Without a prehydration treatment, shoot apices of the moss Syntrichia norvegica were killed by desiccation to equilibrium with 0% RH, and severely damaged upon rehydration from a 42-d dry period. With a prehydration treatment, both of these scenarios were mitigated. For the prehydration treatment to be effective, a rate of drying only of 30 or more minutes from full turgor without external free water to leaf curling achieves a significant lessening of damage.
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Vol. 121 • No. 2