Germination Niche Research Articles

Seed germination of exotic and native winter annuals differentially responds to temperature and moisture, especially with climate change scenarios

AbstractSeeds of winter annuals require a summer after‐ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co‐occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After‐ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after‐ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear‐cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.

Seed germination traits shape community assembly along a hydroperiod gradient.

Hydroperiod drives plant community composition in wetlands, resulting in distinct zonation patterns. Here, we explored the role of seed germination traits in shaping wetland community assembly along a hydroperiod gradient. Specifically, we tested the hypothesis that seeds of reed, mudflat, swamp, shallow- and deep-water communities only germinate under a specific set of environmental factors characterized by the community-specific optimal conditions for seedling survival and growth. In a three-factorial experiment, we tested the seed germination response of 50 species typical for temperate wetlands of Europe to temperature fluctuations (constant vs. fluctuating temperature), illumination (light vs. darkness) and oxygen availability (aerobic vs. hypoxia). Phylogenetic principal component analysis, cluster analysis and phylogenetic linear regressions were used to confirm the community-specific seed germination niches. Our study revealed the presence of five distinct, community-specific seed germination niches that reflect adaptations made by the study communities to decreasing light intensity, temperature fluctuations and oxygen availability along the hydroperiod gradient. Light as a germination trigger was found to be important in mudflats, swamps and shallow water, whereas the seeds of reed and deep-water species were able to germinate in darkness. A fluctuating temperature is only required for seed germination in mudflat species. Germination of species in the communities at the higher end of the hydroperiod gradient (reed and mudflat) demonstrated a strict requirement for oxygen, whereas swamp, shallow- and deep-water species also germinated under hypoxia. Our study supports the recent argument that the inclusion of seed germination traits in community ecology adds significant insights to community response to the abiotic and biotic environment. Furthermore, the close relationship between seed germination adaptations and community assembly could help reach a better understanding of the existing patterns of wetland plant distribution at local scales and wetland vegetation dynamics, as well as facilitate nature conservation measures and aquatic habitat restoration.

Optimizing physiological dormancy break of understudied cold desert perennials to improve seed-based restoration

In degraded drylands, post-disturbance native plant community re-establishment is almost exclusively conducted using seeds. These restoration efforts are challenging, in large part as a result of a limited understanding of seed dormancy and germination traits. In this paper, we characterize these traits for four herbaceous perennials (Asteraceae) that are widely distributed and highly desired in the restoration of ecologically imperiled drylands of North America. We assess baseline germination and the effects of chemical stimulants across a suite of temperatures and quantify the influence of (1) cold stratification, (2) dry after-ripening, and (3) dry cold storage on germination.Three months of cold stratification achieved near-maximum dormancy alleviation for all species. Exposure to warmer temperatures following stratification increased germination further, reducing the required stratification length. Across all species, at least a third of seeds maintained at 3 ± 2 °C germinated, suggesting that in situ germination occurs in late winter or early spring. Changes in the thermal germination envelope were species-dependent, indicating different germination niches and strategies for population persistence. Dry after-ripening and cold storage improved germination, but responses were highly variable across species, temperature, and time. Chemical stimulants promoted germination in three species and may be an alternative to cold stratification or after-ripening.

Germination niche breadth and potential response to climate change differ among three North American perennials

Responses of species to environmental change are largely dependent on their niche breadth. To investigate the relationship between germination niche breadth and sensitivity to climate change, we selected three North American perennials: Physalis longifolia, Asclepias syriaca and Penstemon digitalis. Dormancy loss and germination requirements were determined for seeds from populations along a Midwest US latitudinal gradient. Fresh seeds were incubated at 1, 5, 9, 15/6, 20/10, 25/15 and 30/15°C, and seeds cold stratified at 1, 5 and 9°C for 4–12 weeks were incubated at 15/6, 20/10, 25/15 and 30/15°C. Germination niche breadth (Levins’ Bn) was calculated from final germination proportion. In addition, a sequence of temperatures evaluated the effect of future warming on germination phenology. Germination differed significantly among populations and collection latitudes (P < 0.001), but variation did not have a latitudinal pattern. Niche breadth was widest for Physalis and Asclepias and narrowest for Penstemon (P ≤ 0.05), with implications for germination phenology. Germination shifted to autumn for Physalis and Asclepias under future warming in northern collection regions, and shifted earlier in spring for Penstemon regardless of region. Due to limited stratification and germination requirements, resulting in the narrowest niche breadth, we initially predicted Penstemon to be most at-risk, as future warming would fall outside its stratification envelope. However, species with wide niche breadths (Physalis and Asclepias) may be more vulnerable to climate change due to maladaptive shifts in germination phenology.

Seed ecology of Saxifraga italica: effects of light, temperature and gibberellic acid

Knowledge about seed germination ecology is necessary for development of protocols for in ex situ conservation and in situ restoration of rare species. Here we investigate the effect of temperature, light, GA3 and cold storage in the germination niche of Saxifraga italica (Saxifragaceae), and endemic species of Central Apennines (Italy). Several temperatures, both in dark and light conditions, and the effect of pre-chilling (60 days at 5°C) were investigated. Light was varied by changing photon flux, both in intensity and duration. The intensity of light was tested from 1.28 to 26.87 μmol (photon)·m−2 ·s−1 , and pre-treatment with light was carried out by exposing seeds for 1, 2, 4, 8, 24 and 48 h at the beginning of the germination test. Finally, the effects of GA3 at two different concentrations. 250 ppm and 500 ppm, was tested. The results show that the species under study produce small non-dormant seeds germinating only with long-term light stimulation or with GA3. The requirement for light could be substituted by gibberellins but not by pre-chilling, as this species germinating only when on the soil surface was exposed to light and without competition. Thus, in accordance with the species distribution and life strategies, the germination niche highlights that for this species light is a fundamental factor promoting germination.

Seed germination niche across habitats: an introduction to this special issue

Seed germination niche across habitats: an introduction to this special issue

Effects of temperature on germination in eight Western Australian herbaceous species

Germination is a high-risk phase in a plant’s life cycle and is directly regulated by temperature. Seeds germinate over a range of temperatures within which there is an optimum temperature, with thresholds above and below which no germination occurs. Rapid changes in temperature associated with global warming may cause a disconnect between temperatures a seed experiences and temperatures over which germination can occur. This paper explores the temperature dimension of the germination niche of eight herbaceous species from South West Western Australia as part of a broader assessment of endemic native species at risk of decline under global warming. The data obtained from germination studies on a temperature gradient plate were used to populate models to predict optimum germination responses (mean time to germination, germination timing and success) under current (1950–2000 averages) and future (2070 high greenhouse gas emission) temperature scenarios. The species exhibited a mix of germination responses, often with high tolerance to high diurnal temperatures. A number of species did not reach their full potential within the experimental period, indicating the presence of dormancy not overcome due to temperature alone. Modelling revealed that for some species the opportunity for germination may decline due to rising temperatures, but for others there would be little change, though a shift in germination timing may be expected. This approach to identifying extinction risk contributes tangibly to efforts to predict plant responses to environmental change and can help prioritize species for management actions, direct limited resources towards further investigations and supplement bioclimatic modelling.

Seed germination niche for common ragweed (Ambrosia artemisiifolia L.) populations naturalized in Turkey

Common ragweed (Ambrosia artemisiifolia L.) is an invasive species, and is currently spread in different habitats over several regions in Turkey. However, no information exists on the seed germination biology of Turkish populations. Therefore, the seed germination niche (upper and lower limits of environmental conditions for seed germination) was determined for common ragweed populations in two climatic regions (Thrace and Black Sea) of Turkey. Seed germination was evaluated under different photoperiods, constant temperatures and levels of water stress, salinity and pH. Seedling emergence was recorded from various seed burial depths. Significant differences were observed among populations for the seed germination niche and seedling emergence. Seed germination niche helps explain the occurrence of the species in the diverse environmental conditions found across Turkey. Seeds from the Thrace populations were more tolerant to water and salinity stress and germinated under relatively higher pH levels than those from the Black Sea populations. Further, seeds from the Thrace region germinated to higher percentages at all temperatures than those from the Black Sea populations. Seeds from all populations germinated to higher percentages in light/dark than in dark. Seedling emergence was significantly retarded by a seed burial depth >4 cm. The results suggest that the species has extensive range expansion potential in the country and that immediate management of the naturalized populations is needed. In agricultural habitats, the species can be managed by shallow tillage and subsequent management of emerging seedlings to reduce the soil seed bank. Management strategies such as appropriate mowing time and mowing frequency need to be developed for control of roadside populations.

Germination niches and seed persistence of tropical epiphytic orchids in an urban landscape.

Urbanisation has contributed to significant biodiversity loss, yet, urban areas can facilitate biodiversity conservation. For instance, there is evidence of urban trees supporting natural establishments of orchids, the most species-rich plant family on Earth. However, the germination niches-which include both suitable biophysical conditions and orchid mycorrhizal fungus/fungi (OMF)-are not sufficiently known for most species, especially tropical epiphytic orchids. The fate of their dispersed seeds is poorly understood as well. We conducted fungal baiting and seed sowing experiments, next-generation sequencing, generalised linear models, and seed viability tests to detect and identify potential OMF, investigate biophysical factors that influenced OMF availability and orchid germination, and assess seed longevity. Ceratobasidiaceae- and Serendipitaceae-associated OMF were successfully detected in three of four orchid species. In general, orchid species and humus presence had significant effects on OMF availability. Orchid species and temperature were predictive of germination. Post-experiment viability tests revealed that one orchid species, Grammatophyllum speciosum Blume, may produce long-lived seeds. The results suggest that urban trees can support OMF and orchid germination, but both processes are limited by biophysical factors. This study also indicates the possibility of seed persistence among epiphytic species. As orchid germination niches are complex and tend to be unique to individual species, we do not encourage generalisations. In contrast, species-specific information can help formulate useful recommendations towards conservation.

Development of Ependymal and Postnatal Neural Stem Cells and Their Origin from a Common Embryonic Progenitor

The adult mouse brain contains an extensive neurogenic niche in the lateral walls of the lateral ventricles. This epithelium, which has a unique pinwheel organization, contains multiciliated ependymal (E1) cells and neural stem cells (B1). This postnatal germinal epithelium develops from the embryonic ventricular zone, but the lineage relationship between E1 and B1 cells remains unknown. Distinct subpopulations of radial glia (RG) cells in late embryonic and early postnatal development either expand their apical domain >11-fold to form E1 cells or retain small apical domains that coalesce into the centers of pinwheels to form B1 cells. Using independent methods of lineage tracing, we show that individual RG cells can give rise to clones containing E1 and B1 cells. This study reveals key developmental steps in the formation of the postnatal germinal niche and the shared cellular origin of E1 and B1 cells.

Progenitor Cell Heterogeneity in the Adult Carotid Body Germinal Niche.

Somatic stem cells confer plasticity to adult tissues, permitting their maintenance, repair and adaptation to a changing environment. Adult germinal niches supporting somatic stem cells have been thoroughly characterized throughout the organism, including in central and peripheral nervous systems. Stem cells do not reside alone within their niches, but they are rather accompanied by multiple progenitor cells that not only contribute to the progression of stem cell lineage but also regulate their behavior. Understanding the mechanisms underlying these interactions within the niche is crucial to comprehend associated pathologies and to use stem cells in cell therapy. We have described a stunning germinal niche in the adult peripheral nervous system: the carotid body. This is a chemoreceptor organ with a crucial function during physiological adaptation to hypoxia. We have shown the presence of multipotent stem cells within this niche, escorted by multiple restricted progenitor cell types that contribute to niche physiology and hence organismal adaptation to the lack of oxygen. Herein, we discuss new and existing data about the nature of all these stem and progenitor cell types present in the carotid body germinal niche, discussing their role in physiology and their clinical relevance for the treatment of diverse pathologies.

Broad Seed Germination Niche as an Adaptation to Heterogeneous Aquatic Habit—a Case Study of Four Potamogeton species

Aquatic habitats represent an extremely heterogeneous media for the inhabiting its plants, because key environmental factors vary there rapidly at relatively small scales. In this study, we explore the seed germination strategies of four pondweed species (Potamogeton compressus L., P. natans L., P. pectinatus L. and P. vaginatus Turcz.) in relation to the habitats they occupy. We hypothesised that, due to the high heterogeneity of aquatic environments, none of the environmental factors, such as light, temperature fluctuations or oxygen would play the role of a reliable trigger for seed germination of these species. The results of the germination experiments confirm the proposed hypothesis: under almost all conditions – fluctuating and constant temperatures, light and darkness, aerobic, flooded and hypoxia conditions – germination percentages ranged from 88.5 to 100%. An interspecific comparison of germination patterns also evidenced that all the investigated species showed the same response to all the studied environmental factors. We speculate that similar adaptations present in other aquatic macrophytes may point out the broader occurrence of this phenomenon in aquatic plants.

The carotid body: a physiologically relevant germinal niche in the adult peripheral nervous system.

Oxygen constitutes a vital element for the survival of every single cell in multicellular aerobic organisms like mammals. A complex homeostatic oxygen-sensing system has evolved in these organisms, including detectors and effectors, to guarantee a proper supply of the element to every cell. The carotid body represents the most important peripheral arterial chemoreceptor organ in mammals and informs about hypoxemic situations to the effectors at the brainstem cardiorespiratory centers. To optimize organismal adaptation to maintained hypoxemic situations, the carotid body has evolved containing a niche of adult tissue-specific stem cells with the capacity to differentiate into both neuronal and vascular cell types in response to hypoxia. These neurogenic and angiogenic processes are finely regulated by the niche and by hypoxia itself. Our recent data on the cellular and molecular mechanisms underlying the functioning of this niche might help to comprehend a variety of different diseases coursing with carotid body failure, and might also improve our capacity to use these stem cells for the treatment of neurological disease. Herein, we review those data about the recent characterization of the carotid body niche, focusing on the study of the phenotype and behavior of multipotent stem cells within the organ, comparing them with other well-documented neural stem cells within the adult nervous system.

Proliferative cells in the rat developing neocortical grey matter: new insights into gliogenesis.

The postnatal brain development is characterized by a substantial gain in weight and size, ascribed to increasing neuronal size and branching, and to massive addition of glial cells. This occurs concomitantly to the shrinkage of VZ and SVZ, considered to be the main germinal zones, thus suggesting the existence of other germinative niches. The aim of this study is to characterize the cortical grey matter proliferating cells during postnatal development, providing their stereological quantification and identifying the nature of their cell lineage. We performed double immunolabeling for the proliferation marker Ki67 and three proteins which identify either astrocytes (S100β) or oligodendrocytes (Olig2 and NG2), in addition to a wider panel of markers apt to validate the former markers or to investigate other cell lineages. We found that proliferating cells increase in number during the first postnatal week until P10 and subsequently decreased until P21. Cell lineage characterization revealed that grey matter proliferating cells are prevalently oligodendrocytes and astrocytes along with endothelial and microglial cells, while no neurons have been detected. Our data showed that astrogliogenesis occurs prevalently during the first 10 days of postnatal development, whereas contrary to the expected peak of oligodendrogenesis at the second postnatal week, we found a permanent pool of proliferating oligodendrocytes enduring from birth until P21. These data support the relevance of glial proliferation within the grey matter and could be a point of departure for further investigations of this complex process.

Seed germination niche of the halophyte Suaeda maritima to combined salinity and temperature is characterised by a halothermal time model

Sensitivity to salinity during seed germination potentially serves as a niche specifier for halophytes. Quantification of germination niche with respect to salinity, and any interaction with temperature, is therefore important to understand, particularly regarding the impacts of sea level rise in response to climate change. Population-based germination models quantify the response to temperature and water potential. However, there is limited applicability to salinity. Here, we develop a new approach considering thermal time (for temperature), halo time (for salinity) and halothermal time (for combined temperature and salinity), to quantify the germination of Suaeda maritima, a model halophyte with a worldwide distribution. We find salt concentration has a profound effect on the estimated base temperature for germination, Tb, being c. −4 °C in 600 mM NaCl compared with c. 1 °C in water. The predicted maximum concentration of NaCl for 50% germination to occur (NaClmax50) is 1280 mM at 5 °C and decreases (lower concentration) as the temperature increases. Overall, germination becomes slower as the concentration of salt increases (i.e., longer halo time) or the temperature decreases (i.e., longer thermal time). Combining all parameters into a halothermal time model is most successful at 5 and 10 °C. However, Na+ accumulation in the seed which coincides with maintained seed moisture content, regardless of salt concentration, influences the model fit. The models reveal two different germination niches: at cold temperatures, germination is slow but temperature and salinity are not necessarily limiting; and at warmer temperatures, germination is fastest but constrained by NaClmax50.

Germination niche breadth varies inconsistently among three Asclepias congeners along a latitudinal gradient.

Species responses to climate change will be primarily driven by their environmental tolerance range, or niche breadth, with the expectation that broad niches will increase resilience. Niche breadth is expected to be larger in more heterogeneous environments and moderated by life history. Niche breadth also varies across life stages. Therefore, the life stage with the narrowest niche may serve as the best predictor of climatic vulnerability. To investigate the relationship between niche breadth, climate and life stage we identify germination niche breadth for dormant and non-dormant seeds in multiple populations of three milkweed (Asclepias) species. Complementary trials evaluated germination under conditions simulating historic and predicted future climate by varying cold-moist stratification temperature, length and incubation temperature. Germination niche breadth was derived from germination evenness across treatments (Levins Bn ), with stratified seeds considered less dormant than non-stratified seeds. Germination response varies significantly among species, populations and treatments. Cold-moist stratification ≥4weeks (1-3°C) followed by incubation at 25/15°C+ achieves peak germination for most populations. Germination niche breadth significantly expands following stratification and interacts significantly with latitude of origin. Interestingly, two species display a positive relationship between niche breadth and latitude, while the third presents a concave quadratic relationship. Germination niche breadth significantly varies by species, latitude and population, suggesting an interaction between source climate, life history and site-specific factors. Results contribute to our understanding of inter- and intraspecific variation in germination, underscore the role of dormancy in germination niche breadth, and have implications for prioritising and conserving species under climate change.

A Role for Nrf2 Expression in Defining the Aging of Hippocampal Neural Stem Cells

Redox mechanisms are emerging as essential to stem cell function given their capacity to influence a number of important signaling pathways governing stem cell survival and regenerative activity. In this context, our recent work identified the reduced expression of nuclear factor (erythroid-derived 2)-like 2, or Nrf2, in mediating the decline in subventricular zone neural stem progenitor cell (NSPC) regeneration during aging. Since Nrf2 is a major transcription factor at the heart of cellular redox regulation and homeostasis, the current study investigates the role that it may play in the aging of NSPCs that reside within the other major mammalian germinal niche located in the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus. Using rats from multiple aging stages ranging from newborn to old age, and aging Nrf2 knockout mice, we first determined that, in contrast with subventricular zone (SVZ) NSPCs, Nrf2 expression does not significantly affect overall DG NSPC viability with age. However, DG NSPCs resembled SVZ stem cells, in that Nrf2 expression controlled their proliferation and the balance of neuronal versus glial differentiation particularly in relation to a specific critical period during middle age. Also, importantly, this Nrf2-based control of NSPC regeneration was found to impact functional neurogenesis-related hippocampal behaviors, particularly in the Morris water maze and in pattern separation tasks. Furthermore, the enrichment of the hippocampal environment via the transplantation of Nrf2-overexpressing NSPCs was able to mitigate the age-related decline in DG stem cell regeneration during the critical middle-age period, and significantly improved pattern separation abilities. In summary, these results emphasize the importance of Nrf2 in DG NSPC regeneration, and support Nrf2 upregulation as a potential approach to advantageously modulate DG NSPC activity with age.

Seed germination ecology in Trapa natans L., a widely distributed freshwater macrophyte

Trapa natans is an aquatic annual plant that grows in stagnant water and occupies an extremely wide, yet discontinuous native range across temperate Europe, Asia, and Africa, and is naturalized in North America and Australia. Despite its wide distribution and invasion success, its seed germination ecology is poorly known. We hypothesized that, due to its wide distribution range, T. natans seeds should germinate under a broad spectrum of environmental conditions. In the present study, freshly matured seeds were cold stratified and tested for germination over a range of temperatures in light/dark, and dark under aerobic and hypoxia conditions. Seed and seedling tolerance to freezing and desiccation also were determined. Freshly matured seeds were physiologically dormant, requiring 9 weeks of cold stratification to overcome the dormancy. Seeds tolerated freezing down to −14 °C, whereas those with an emerged hypocotyl exhibited a high sensitivity to freezing down to −4 °C. The non-dormant seeds germinated (75–100%) under almost all environmental conditions, i.e. constant 22 °C and alternating temperature of 14/6 °C to 30/22 °C and 30/10 °C, light and darkness, oxic and hypoxia, thus supporting our proposed hypothesis. Finally, seeds were found to be highly desiccation-sensitive, even at high relative humidity. Our findings suggest that the wide germination niche could help explain its broad distribution and invasion success at local and global scales.

The seed germination niche limits the distribution of some plant species in calcareous or siliceous alpine bedrocks

Functional plant traits are used in ecology to explain population dynamics in time and space. However, because the germination niche is an essential stage in alpine plant life cycles and is under strong environmental pressure, we hypothesised that inter-specific variability in germination traits might contribute to alpine plant distributions. Germination traits of seven closely related species from calcareous and siliceous habitats were characterised in the laboratory, including base, optimum and ceiling temperatures (Tb, To, Tc, respectively), base water-potential (Ψb) and the pH range of the germination phenotype. Species’ vegetative traits (specific leaf area, leaf area and leaf dry matter content) were obtained from the TRY-database. Traits and habitat similarities and dissimilarities were assessed. Species were plotted in a multivariate space using two separate principal component analyses: one each for germination and vegetative traits. Species from calcareous habitats showed significantly higher Tb, lower Ψb and lower capacity to germinate under acidic pH than species from siliceous habitats. Moreover, high To and Tc, a narrow temperature range for germination at dispersal, and vegetative traits values were similar across both habitats. Whilst plant traits seem to have adapted to the shared environmental conditions of the two alpine habitats, some germination traits were affected by the habitat differences. In conclusion, species occurrence in two habitats (calcareous, siliceous) appears to be limited by some germination constraints and provide greater differentiation of species habitat preference than that defined by vegetative traits.

Development of C-lignin with G/S-lignin and lipids in orchid seed coats – an unexpected diversity exposed by ATR-FT-IR spectroscopy

AbstractMembers of the orchid family occupy many germination niches, in terrestrial, epiphytic and epilithic environments. How orchid seeds attach to their substrate and survive after dispersal is largely unknown. C-lignin is a recently discovered specialized lignin, found in seed coats of some plants, including orchid species, but its functional and biological significance is obscure. We studied seed coat ontogenesis in three species (Neuwiedia veratrifolia,Cypripedium formosanumandPhalaenopsis aphrodite) that represent basal and advanced branches in orchid phylogeny and divergent life forms. From each species, controlled pollination yielded several stages of seed development, from which seed coats (testa) were isolated and analysed by ATR-FT-IR spectroscopy. The use of the ATR set-up ensured that the chemical information originated only from the integral outer seed surface layers. The FT-IR bands of C-lignin are presented here for the first time, and distinguished from bands of G/S-lignin. In the seed coats, C-lignin developed after G/S-lignin inN. veratrifoliaandC. formosanum, while only G/S-lignin developed inP. aphrodite. We discuss C-lignin properties and possible function in relation to seed coat properties. The species differed with respect to sequence and amounts of deposition, not only of lignins but also lipids, resulting in differences in mature seed coat compositions. Thus we revealed an unexpected and marked diversity among orchids with respect to seed surface chemistry, with possible implications for seed and germination ecology.