How to dry a bryophyte: A review and experimental test of four methods to induce desiccation tolerance

Abstract. A review of ∼290 research articles on bryophyte desiccation tolerance (DT) over the last century reveals four prominent methods that incorporate equilibrium drying. We compare these methods (called Conventional, Wetted substrate, Step-down and Partial drying) in terms of inducing the trait DT in four species of mosses occupying distinctly different evolutionary clades and known to exhibit an inducible strategy of DT (Phascum cuspidatum, Funaria hygrometrica, Bryum argenteum and Syntrichia obtusissima). Conventional=plants placed directly at 33% RH; Wetted substrate=plants dried at different rates (times) to 33% RH by wetting the substrate; Step-down=plants dried to equilibration in sequence from 100, 75, 54, then 33% RH; Partial drying=plants exposed to 100% RH prior to placement at 33% RH. Efficacy of each method was evaluated using postrehydration damage and recovery as assessed from chlorophyll fluorescence and leaf or tissue damage 7 days postrehydration. For each chlorophyll fluorescence measure, there was a significant three-way interaction between species, drying method and time. Three of the four methods produced good recovery after 24 h of rehydration, with the plants subjected to the Conventional method not recovering from desiccation. Photosynthetic damage immediately upon rehydration was reduced for the Partial dry method and similar for the Step-down and Wetted substrate methods. Tissue damage 7 d postrehydration was equivalent for the Wetted substrate, Step-down and Partial dry methods, and most plants died following the Conventional method of drying. Recovery following 24 h of rehydration was near control levels for all methods but the Conventional. Species differences were considerable among drying methods. The Subturgor Hypothesis is advanced to explain degree of induction of desiccation tolerance, and was generally supported, with the caveat that, among the three most successful methods tested, the Step-down method performed better than expected based upon time at subturgor.