Influence of prehydration events on revival of the dryland moss Syntrichia caninervis desiccated for 17 years: does water content matter?

Desiccation-tolerant bryophytes often encounter humid air prior to liquid forms of precipitation during rehydration in nature, although much of our understanding of desiccation physiology does not take this into account. We sought to examine the implications of such conditions on regeneration in 17-year desiccated shoots of the desert moss Syntrichia caninervis. Moss shoot tips were rehydrated using either liquid water directly or after a prehydration period (exposure to humid air) from <1 hour up to 7 days followed by liquid water. About 50% of shoot tips rehydrated with liquid water directly were viable, compared to ∼85% of shoot tips prehydrated prior to the addition of liquid water. This increase in viability occurred due to exposure to humid air during the first hour of prehydration, when shoot water content reached 20% dry mass (DM) and relative humidity of the surrounding air reached 91%. Shoot water content increased from 5% to ∼100% after 24-48 hours of prehydration, with a full turgor water content of 350% DM. Leaf damage was reduced by prehydration periods of 8 hours or longer, except at the longest prehydration period (7 days). A prehydration period of at least 4 hours allowed shoots to regenerate 4.5 days earlier and to produce 2.5× the total number of regenerative points, compared with control (direct liquid water) shoots. Results suggest that a maximum water content during prehydration of 100% is not required for the benefits of prehydration on survival and regeneration following an extended period of continuous desiccation, and water content as low as 20% DM reached through prehydration can result in increased shoot viability.