Extended periods of hydration do not elicit dehardening to desiccation tolerance in regeneration trials of the moss syntrichia caninervis

Dehardening (deacclimation) to water stress is an integral phase of desiccation tolerance (DT) in bryophytes. Shoots of the desert moss Syntrichia caninervis were harvested and dehardened after exposure to field CO ₂ concentrations of 380 ppm (ambient) and 550 ppm (elevated). The dehardening period consists of three phases, with shoots from the first phase (3-7 d) the least able to recover from a rapid-drying event and the second (12-18 d) and third (21-27 d) phases characterized by shoots hardening to desiccation and approaching control levels. Response variables following this pattern include tissue chlorosis, protonemal emergence and area, shoot growth resumption, total regenerative shoot production, and probability of fungal attack. Dehydrin levels did not increase over time during the 27-d experimental hydrating period and thus could not account for the observed constitutive hardening to DT. This first experimental sequence on extended dehardening in bryophytes counters the assumption that field shoots hydrated longer than four consecutive days are entirely dehardened and provides evidence that mosses can harden to DT without relying on external cues such as partial hydration. We propose that the "dehardening phase" is a recovery phase during which plants are somewhat dehardened, and, following recovery, the plants enter a constitutive protective phase.