Understanding desiccation tolerance and sex-specific responses in Bryum argenteum: insights from phenological phases and physiological analyses.

BACKGROUND AND AIMS: Desiccation tolerance (DT) is crucial for survival in arid environments, where organisms develop strategies in reproduction, maintenance, and defense to cope with water scarcity. Therefore, investigating the relationship between reproduction and DT is essential to understand the ecology and adaptive strategies of species. This study explores the connection between the development of male and female gametangia in the moss Bryum argenteum and the decrease in DT during the progression of phenological phases in gametangia and protonema. METHODS: Samples collected from a dry tropical forest in Brazil were cultivated, cloned, and subjected to desiccation. Subsequently, the physiological parameters of shoots and protonemata were analysed. Shoot and protonema regeneration were monitored for 28 days after the physiological analyses. Both phases were subjected to control and desiccation treatments. KEY RESULTS: Significant effects of desiccation and sex on the physiological parameters and regeneration capacity of shoots and protonemata were found. Male shoots generally exhibited lower values in Fv/Fm, and ϕPSII, while females demonstrated higher values and better recovery after desiccation. Protonemata also showed variation in Fv/Fm over time and sex, with no significant differences in ϕPSII between them. Desiccated male shoots had higher mortality rates and produced fewer new shoots. Regarding the females, the regeneration patterns varied between the desiccation-exposed groups and the control, with decreased shoot production, and some protonemata growing into filaments without forming shoots. CONCLUSION: These findings improve our understanding of bryophytes ecological responses to desiccation stress and provide insights into their adaptive strategies in challenging environments, such as the possible rarity of males in dioicous mosses populations.

Strategies of desiccation tolerance vary across life phases in the moss Syntrichia caninervis.

PREMISE: Desiccation tolerance (DT) is a widespread phenomenon among land plants, and variable ecological strategies for DT are likely to exist. Using Syntrichia caninervis, a dryland moss and model system used in DT studies, we hypothesized that DT is lowest in juvenile (protonemal) tissues, highest in asexual reproductive propagules (gemmae), and intermediate in adults (shoots). We tested the long-standing hypothesis of an inherent constitutive strategy of DT in this species. METHODS: Plants were rapidly dried to levels of equilibrating relative humidity (RHeq) ranging from 0 to 93%. Postrehydration recovery was assessed using chlorophyll fluorescence, regeneration rates, and visual tissue damage. For each life phase, we estimated the minimum rate of drying (RoDmin ) at RHeq = 42% that did not elicit damage 24 h postrehydration. RESULTS: DT strategy varied with life phase, with adult shoots having the lowest RoDmin (10-25 min), followed by gemmae (3-10 h) and protonema (14-20 h). Adult shoots exhibited no detectable damage 24 h postrehydration following a rapid-dry only at the highest RHeq used (93%), but when dried to lower RHs the response declined to <50% of control fluorescence values. Notably, immediately following rehydration (0 h postrehydration), shoots were damaged below control levels of fluorescence regardless of the RHeq, thus implicating damage. CONCLUSIONS: Life phases of the moss S. caninervis had a range of strategies from near constitutive (adult shoots) to demonstrably inducible (protonema). A new response variable for assessing degree of DT is introduced as the minimum rate of drying from which full recovery occurs.

Morphological and physiological traits in relation to carbon balance in a diverse clade of dryland mosses.

Plant functional trait analyses have focused almost exclusively on vascular plants, but bryophytes comprise ancient and diverse plant lineages that have widespread global distributions and important ecological functions in terrestrial ecosystems. We examined a diverse clade of dryland mosses, Syntrichia, and studied carbon balance during a precipitation event (C-balance), a functional trait related to physiological functioning, desiccation tolerance, survival, and ecosystem carbon and nitrogen cycling. We examined variability in C-balance among 14 genotypes of Syntrichia and measured an additional 10 physiological and 13 morphological traits at the cell, leaf, shoot, and clump level. C-balance varied 20-fold among genotypes, and highest C-balances were associated with long, narrow leaves with awns, and small cells with thick cell walls, traits that may influence water uptake and retention during a precipitation event. Ordination analyses revealed that the axis most strongly correlated with C-balance included the maximum chlorophyll fluorescence, Fm , indicating the importance of photosystem II health for C exchange. C-balance represents a key functional trait in bryophytes, but its measurement is time intensive and not feasible to measure on large scales. We propose two models (using physiological and morphological traits) to predict C-balance, whereby identifying simpler to measure traits for trait databases.

Moss antheridia are desiccation tolerant: Rehydration dynamics influence sperm release in Bryum argenteum.

PREMISE OF THE STUDY: Free-living sperm of mosses are known to be partially desiccation tolerant. We hypothesized that mature moss antheridia should also tolerate desiccation and that rehydration to partial turgor (prehydration) or rehydration to full turgor (rehydration) before immersion in water is required for full recovery from any damaging effects of prior desiccation. METHODS: Bryum argenteum (silvery-thread moss) was grown in continuous culture for several months, produced mature perigonia (clusters of antheridia), and these were subjected to a slow rate of drying (∼36 h from full turgor to desiccation) and equilibration with 50% relative humidity. Perigonia were prehydrated (exposed to a saturated atmosphere) or rehydrated (planted upright in saturated media) for 0, 45, 90, 135, 180, and 1440 min, then immersed in sterile water. Time to first sperm mass release, number of antheridia releasing sperm masses, and the integrity of the first sperm mass released were assessed. KEY RESULTS: Rehydration of dried antheridia for at least 3 h before immersion in water resulted in antheridia functioning similar to control undried antheridia. Compared with rehydration, prehydration was not effective in the recovery of antheridia from desiccation. CONCLUSIONS: For the first time, moss antheridia are shown to be fully desiccation tolerant at a functional level, capable of releasing fully functional sperm following a slow drying event provided the antheridia are allowed to rehydrate at least 3 h before immersion in water.

Embryos of a moss can be hardened to desiccation tolerance: effects of rate of drying on the timeline of recovery and dehardening in Aloina ambigua (Pottiaceae).

BACKGROUND AND AIMS: Embryonic sporophytes of the moss Aloina ambigua are inducibly desiccation tolerant (DT). Hardening to DT describes a condition of temporary tolerance to a rapid-drying event conferred by a previous slow-drying event. This paper aimed to determine whether sporophytic embryos of a moss can be hardened to DT, to assess how the rate of desiccation influences the post-rehydration dynamics of recovery, hardening and dehardening, and to determine the minimum rate of drying for embryos and shoots. METHODS: Embryos were exposed to a range of drying rates using wetted filter paper in enclosed Petri dishes, monitoring relative humidity (RH) inside the dish and equilibrating tissues with 50% RH. Rehydrated embryos and shoots were subjected to a rapid-drying event at intervals, allowing assessments of recovery, hardening and dehardening times. KEY RESULTS: The minimum rate of slow drying for embryonic survival was ∼3·5 h and for shoots ∼9 h. Hardening to DT was dependent upon the prior rate of drying. When the rate of drying was extended to 22 h, embryonic hardening was strong (>50% survival) with survival directly proportional to the post-rehydration interval preceding rapid drying. The recovery time (repair/reassembly) was so short as to be undetectable in embryos and shoots desiccated gradually; however, embryos dried in <3·5 h exhibited a lag time in development of ∼4 d, consistent with recovery. Dehardening resulted in embryos incapable of surviving a rapid-drying event. CONCLUSIONS: The ability of moss embryos to harden to DT and the influence of prior rate of drying on the dynamics of hardening are shown for the first time. The minimum rate of drying is introduced as a new metric for assessing ecological DT, defined as the minimum duration at sub-turgor during a drying event in which upon rehydration the plant organ of interest survives relatively undamaged from the desiccating event.

The rate of drying determines the extent of desiccation tolerance in Physcomitrella patens.

The effect of differential drying rates on desiccation tolerance in Physcomitrella patens (Hedw.) Bruch & Schimp. is examined. In order to provide more evidence as to the status of desiccation tolerance in P. patens, a system was designed that allowed alteration of the rate of water loss within a specific relative humidity. An artificial substrate consisting of layers of wetted filter paper was used to slow the drying process to as long as 284h, a significant increase over the commonly used method of exposure (saturated salt solution). By slowing the rate of drying, survival rates and chlorophyll fluorescence parameters improved, and tissue regeneration time was faster. These results indicate a trend where the capacity for desiccation tolerance increases with slower drying, and reveal a much stronger capacity for desiccation tolerance in P. patens than was previously known.

The desert moss Pterygoneurum lamellatum (Pottiaceae) exhibits an inducible ecological strategy of desiccation tolerance: effects of rate of drying on shoot damage and regeneration.

PREMISE OF THE STUDY: Bryophytes include clades that incorporate constitutive desiccation tolerance, especially terrestrial species. Here we test the hypothesis that the opposing ecological strategy of desiccation tolerance, inducibility, is present in a desert moss, and address this hypothesis by varying rates of drying in a laboratory study. Desiccation tolerance is arguably the most important evolutionary innovation relevant to the colonization of land by plants; increased understanding of the ecological drivers of this trait will eventually illuminate the responsible mechanisms and ultimately open doors to the potential for the application of this trait in cultivated plants. METHODS: Plants were cloned, grown in continuous culture (dehardened) for several months, and subjected to rates of drying (drying times) ranging from 30 min to 53 h, rehydrated and tested for recovery using chlorophyll fluorescence, leaf damage, and regeneration of protonema and shoots. KEY RESULTS: Rate of drying significantly affected all recovery responses, with very rapid drying rates severely damaging the entire shoot except the shoot apex and resulting in slower growth rates, fewer regenerative shoots produced, and a compromised photosynthetic system as inferred from fluorescence parameters. CONCLUSIONS: For the first time, a desert moss is shown to exhibit an ecological strategy of desiccation tolerance that is inducible, challenging the assumption that arid-land bryophytes rely exclusively on constitutive protection. Results indicate that previous considerations defining a slow-dry event in bryophytes need reevaluation, and that the ecological strategy of inducible desiccation tolerance is probably more common than currently understood among terrestrial bryophytes.