Abstract
Dispersal is a fundamental biological process that can be divided into three phases: release, transportation, and deposition. Determining the mechanisms of diaspore release is of prime importance to understand under which climatic conditions and at which frequency diaspores are released and transported. In mosses, wherein spore dispersal takes place through the hygroscopic movements of the peristome, the factors enhancing spore release has received little attention. Here, we determine the levels of relative humidity (RH) at which peristome movements are induced, contrasting the response of species with perfect (fully developed) and specialized (reduced) peristomes. All nine investigated species with perfect peristomes displayed a xerochastic behavior, initiating a closing movement from around 50%–65% RH upon increasing humidity and an opening movement from around 90% RH upon drying. Five of the seven species with specialized peristomes exhibited a hygrochastic behavior, initiating an opening movement under increasing RH (from about 80%) and a closing movement upon drying (from about 90%). These differences between species with hygrochastic and xerochastic peristomes suggest that spore dispersal does not randomly occur regardless of the prevailing climate conditions, which can impact their dispersal distances. In species with xerochastic peristomes, the release of spores under decreasing RH can be interpreted as an adaptive mechanism to disperse spores under optimal conditions for long‐distance wind dispersal. In species with hygrochastic peristomes, conversely, the release of spores under wet conditions, which decreases their wind long‐distance dispersal capacities, might be seen as a safe‐site strategy, forcing spores to land in appropriate (micro‐) habitats where their survival is favored. Significant variations were observed in the RH thresholds triggering peristome movements among species, especially in those with hygrochastic peristomes, raising the question of what mechanisms are responsible for such differences.
Highlights
Because we focused on the dynamical response of peristomes to variations of relative humidity (RH), we did not implement here a range of fixed, stable RH conditions using, for instance, sulfuric acid solution at different concentrations
Starting at a relative humidity of 30%, each capsule was subsequently exposed to an increase in RH to 97%, and to a decrease in RH back to 30%, measuring the RH levels from which peristome movements started in each case
Johansson et al (2016) did not find any significant dif‐ ferences between fresh and herbarium specimens in B. rutabulum, our results show that part of the variation of the RH level triggering peristome movements was caused by age differences among speci‐ mens
Summary
Johansson et al (2016), showed that in Brachythecium rutabulum, a moss with a perfect peri‐ stome, peristome teeth start to open under decreasing RH already at 87% RH and are totally open at 69% RH. A xerochastic peristome movement is suggested to maximize the segregation of the spore mass and the chances of long‐distance dispersal under favorable wind conditions (Longton & Schuster, 1983; Medina & Estébanez, 2014). These findings suggest that the spore mass and the surrounding environment could be still quite moist when the capsule starts to open and clouds of spores are thrown out by the moving outer peristome teeth. Since the reduc‐ tion in peristomes represents independent evolutionary events, and might differ in the degree of specialization, we hypothesize that there would be a larger variation in the hygroscopic movements be‐ tween species with specialized than those that have perfect peri‐ stomes (H2)
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