Gemma L. Hoyle

Adaptive plasticity and epigenetic variation in response to warming in an Alpine plant

Environmentally induced phenotypic plasticity may be a critical component of response to changing environments. We examined local differentiation and adaptive phenotypic plasticity in response to elevated temperature in half-sib lines collected across an elevation gradient for the alpine herb, Wahlenbergia ceracea. Using Amplified Fragment Length Polymorphism (AFLP), we found low but significant genetic differentiation between low- and high-elevation seedlings, and seedlings originating from low elevations grew faster and showed stronger temperature responses (more plasticity) than those from medium and high elevations. Furthermore, plasticity was more often adaptive for plants of low-elevation origin and maladaptive for plants of high elevation. With methylation sensitive-AFLP (MS-AFLP), we revealed an increase in epigenetic variation in response to temperature in low-elevation seedlings. Although we did not find significant direct correlations between MS-AFLP loci and phenotypes, our results demonstrate that adaptive plasticity in temperature response to warming varies over fine spatial scales and suggest the involvement of epigenetic mechanisms in this response.

Soil warming increases plant species richness but decreases germination from the alpine soil seed bank

Global warming is occurring more rapidly above the treeline than at lower elevations and alpine areas are predicted to experience above average warming in the future. Temperature is a primary factor in stimulating seed germination and regulating changes in seed dormancy status. Thus, plant regeneration from seed will be crucial to the persistence, migration and post disturbance recruitment of alpine plants in future climates. Here, we present the first assessment of the impact of soil warming on germination from the persistent alpine soil seed bank. Contrary to expectations, soil warming lead to reduced overall germination from the soil seed bank. However, germination response to soil temperature was species specific such that total species richness actually increased by nine with soil warming. We further explored the system by assessing the prevalence of seed dormancy and germination response to soil disturbance, the frequency of which is predicted to increase under climate change. Seeds of a significant proportion of species demonstrated physiological dormancy mechanisms and germination of several species appeared to be intrinsically linked to soil disturbance. In addition, we found no evidence of subalpine species and little evidence of exotic weed species in the soil, suggesting that the soil seed bank will not facilitate their invasion of the alpine zone. In conclusion, changes in recruitment via the alpine soil seed bank can be expected under climate change, as a result of altered dormancy alleviation and germination cues. Furthermore, the alpine soil seed bank, and the species richness therein, has the potential to help maintain local species diversity, support species range shift and moderate species dominance. Implications for alpine management and areas for further study are also discussed.

Pre- and Post-harvest Influences on Seed Dormancy Status of an Australian Goodeniaceae species, Goodenia fascicularis

BACKGROUND AND AIMS The period during which seeds develop on the parent plant has been found to affect many seed characteristics, including dormancy, through interactions with the environment. Goodenia fascicularis (Goodeniaceae) seeds were used to investigate whether seeds of an Australian native forb, harvested from different environments and produced at different stages of the reproductive period, differ in dormancy status. METHODS During the reproductive phase, plants were grown ex situ in warm (39/21 degrees C) or cool (26/13 degrees C) conditions, with adequate or limited water availability. The physiological dormancy of resulting seeds was measured in terms of the germination response to warm stratification (34/20 degrees C, 100 % RH, darkness). KEY RESULTS Plants in the cool environment were tall and had high above-ground biomass, yet yielded fewer seeds over a shorter, later harvest period when compared with plants in the warm environment. Seeds from the cool environment also had higher viability and greater mass, despite a significant proportion (7 % from the cool-wet environment) containing no obvious embryo. In the warm environment, the reproductive phase was accelerated and plants produced more seeds despite being shorter and having lower above-ground biomass than those in the cool environment. Ten weeks of warm stratification alleviated physiological dormancy in seeds from all treatments resulting in 80-100 % germination. Seeds that developed at warm temperatures were less dormant (i.e. germination percentages were higher) than seeds from the cool environment. Water availability had less effect on plant and seed traits than air temperature, although plants with reduced soil moisture were shorter, had lower biomass and produced fewer, less dormant seeds than plants watered regularly. CONCLUSIONS Goodenia fascicularis seeds are likely to exhibit physiological dormancy regardless of the maternal environment. However, seeds collected from warm, dry environments are likely to be more responsive to warm stratification than seeds from cooler, wetter environments.

Mimicking a Semi-arid Tropical Environment Achieves Dormancy Alleviation for Seeds of Australian Native Goodeniaceae and Asteraceae

BACKGROUND AND AIMS Seed physiological dormancy (PD) limits the use and conservation of some of Queenslands (Qld) native forb species. It was hypothesised that optimum dormancy-alleviating treatments would reflect environmental conditions that seeds experience in situ, and this premise was tested for PD seeds of four species native to south-west Qld. METHODS High temperatures and increased rainfall during summer are characteristic of this semi-arid tropical environment. Ex situ treatments were designed to mimic conditions that seeds dispersed in spring experience during the summer months before germinating in cooler autumn temperatures. Seeds received between 4 and 20 weeks of a dry after-ripening (DAR), warm stratification or dry/wet cycling treatment (DAR interspersed with short periods of warm stratification), in darkness, before being transferred to germination test conditions. In addition, natural dormancy alleviation of one of the Goodeniaceae species was investigated in situ. KEY RESULTS Dry/wet cycling resulted in higher levels of germination of Actinobole uliginosum (Asteraceae), Goodenia cycloptera and Velleia glabrata (Goodeniaceae) when compared with constant DAR or stratification, while Goodenia fascicularis (Goodeniaceae) responded better to short durations of warm stratification. Long durations of DAR partially alleviated PD of A. uliginosum; however, stratification induced and maintained dormancy of this species. Modifications to the dry/wet cycling treatment and germination test conditions based on data collected in situ enabled germination of G. cycloptera and V. glabrata to be further improved. CONCLUSIONS Treatments designed using temperature, relative humidity and rainfall data for the period between natural seed dispersal and germination can successfully alleviate PD. Differences between the four species in conditions that resulted in maximum germination indicate that, in addition to responding to broad-scale climate patterns, species may be adapted to particular microsites and/or seasonal conditions.

Predicting the impact of increasing temperatures on seed germination among populations of Western Australian Banksia (Proteaceae)

Temperature is a significant factor influencing seed germination and for many species temperature-mediated germination cues are vital for plant persistence. Rising temperatures forecast as a result of anthropogenic climate change may have a substantial influence on the population and range dynamics of plant species. Here, we report on the thermal constraints on seed germination in natural populations of four congeneric Banksia species collected from a longitudinal climate gradient in Western Australia. We investigated whether germination niche: (1) varied between species; (2) varied among populations of each species; and (3) varied in a consistent manner reflecting the climatic gradients of seed origin. We hypothesized that species would differ and that populations from warmer sites would have a broader temperature window for germination than populations from cooler sites. Species differed in the breadth of their germination niche, but temperatures that stimulated the most rapid and complete germination were similar across all species. A sharp reduction in germination percentage occurred above the optimum temperature, which coincided with significant delays in germination relative to the optimum. The temperatures causing these declines varied among populations. Across the species, there was a significant correlation between optimum germination temperature and mean annual temperature at seed source; however, there was no relationship at the population level for individual species. These data provide insight into the vulnerability of Banksia species to climate change, with those populations that require lower temperatures for germination, or have narrower optimal ranges for germination, likely to be most vulnerable to a warming climate.

Seed germination strategies: an evolutionary trajectory independent of vegetative functional traits

Seed germination strategies vary dramatically among species but relatively little is known about how germination traits correlate with other elements of plant strategy systems. Understanding drivers of germination strategy is critical to our understanding of the evolutionary biology of plant reproduction. We present a novel assessment of seed germination strategies focussing on Australian alpine species as a case study. We describe the distribution of germination strategies and ask whether these are correlated with, or form an independent axis to, other plant functional traits. Our approach to describing germination strategy mimicked realistic temperatures that seeds experience in situ following dispersal. Strategies were subsequently assigned using an objective clustering approach. We hypothesized that two main strategies would emerge, involving dormant or non-dormant seeds, and that while these strategies would be correlated with seed traits (e.g., mass or endospermy) they would be largely independent of vegetative traits when analysed in a phylogenetically structured manner. Across all species, three germination strategies emerged. The majority of species postponed germination until after a period of cold, winter-like temperatures indicating physiological and/or morphological dormancy mechanisms. Other species exhibited immediate germination at temperatures representative of those at dispersal. Interestingly, seeds of an additional 13 species “staggered” germination over time. Germination strategies were generally conserved within families. Across a broad range of ecological traits only seed mass and endospermy showed any correlation with germination strategy when phylogenetic relatedness was accounted for; vegetative traits showed no significant correlations with germination strategy. The results indicate that germination traits correlate with other aspects of seed ecology but form an independent axis relative to vegetative traits.

Pre- and post-harvest influences on physiological dormancy alleviation of an Australian Asteraceae species: Actinobole uliginosum (A. Gray) H. Eichler

The effects of maternal air temperature and soil moisture upon seed physiological dormancy (PD) alleviation of an Australian native Asteraceae were investigated. From the onset of flowering, Actinobole uliginosum plants growing ex situ were subjected to either a warm (mean 26 degrees C) or cool (mean 17 degrees C) temperature regime, with adequate or limited water availability. In the warm environment, the reproductive phase was accelerated, and plants yielded fewer seeds over a shorter, earlier harvest period, when compared to those in the cool environment. Initial germination of all seeds was low (< 20% at 15 degrees C) due to PD, which was gradually alleviated by a dry after-ripening (DAR) treatment (34/20 degrees C, 40% relative humidity, in darkness). Seeds from plants grown in the warm environment were more responsive to DAR than seeds from the cool environment, but maternal plant water availability had little effect on dormancy status. Germination was higher at 15 C than at 25/15 degrees C, reaching a plateau of c. 80% germination after 20 weeks DAR. Before DAR, application of GA(3) had little impact on seeds, which would consequently be classified as having deep PD if tested at the time of dispersal. However, DAR caused seeds to become increasingly responsive to GA(3), reaching 97% germination at 15 C following just 4 weeks of DAR, which would indicate non-deep PD if seeds were tested following a period of warm, dry storage. Maternal air temperature regulates PD status of A. uliginosum, such that seeds collected from a warmer environment are likely to be more responsive to DAR. Post-harvest storage in an environment suitable for DAR affects seed response to GA(3), which has implications for germination stimulation and dormancy classification.

Seeds at risk: How will a changing alpine climate affect regeneration from seeds in alpine areas?

Alpine areas are both regional water reservoirs and zones of high species endemism. Increasing temperatures and earlier snowmelt have already caused upward migration of species, changes in flowering phenology and increasing frost damage in plants. Thus, significant loss of diversity in alpine areas is imminent. Plant migration and distribution shifts occur mainly via seeds, which also provide the genetic diversity required for adaptation. The ability of plants to shift their distribution in response to climate change will depend on seed dispersal, germination and seedling success under new environmental conditions. Despite the critical importance of seeds and seedlings for species adaptation, migration and persistence, the majority of studies concerning climate change in alpine areas have mostly focused on the response of adult plants to warming. Temperature during seed development, as well as the temperature to which seeds are exposed post-dispersal, has been found to have strong effects on seed longevity, germination and seedling survival. Therefore, global change (particularly, warming) is expected to greatly impact regeneration of seeds in alpine areas. Despite evidence that climate change is advancing flowering phenology in several mountain ranges around the world, under natural and artificial warming, the cascade effects that early flowering can have on seeds and seedlings have been poorly studied. Indeed, while a literature search on Web of Science using the search terms “germination”, “alpine plants” and “climate change” revealed 50 studies, of which only 7 directly examined the effect of warming on germination and establishment of seeds of alpine plant species. Here, we discuss the findings of these studies. We identify critical questions regarding seeds and seedlings of alpine species that require urgent research and recommend experimental approaches. Answering these questions will assist in predicting the impacts of global warming and in conservation and management of plants in alpine areas.

The phenotypic response of co-occurring Banksia species to warming and drying

Projected warmer and drier climates are expected to impact heavily on plant diversity in Mediterranean-climate ecosystems, but experimental investigations of sensitivity and adaptive capacity are needed to better understand species responses. Here, we examine the effects of warming and drying on growth and allocation in seedlings from populations of two co-occurring Banksia shrub species from south-west Western Australia. We hypothesised that the species would show ecological divergence in functional traits reflecting niche differentiation. We expected to see tolerance to warming and drought correlated with position of the population on a climate gradient. We predicted that populations at the warmer/drier end of the gradient would show greater homeostasis of growth and allocation patterns in response to experimental treatment. Seedlings of the two species differed in leaf and allocation traits and in responses to experimental warming and drying. B. coccinea had smaller leaves with higher specific leaf area, and accumulated less overall biomass compared to B. baxteri when grown under cooler conditions. Under warmer conditions, B. coccinea could maintain growth, whereas B. baxteri suffered significant decline in biomass accumulation. Under water deficit conditions, both species showed significant reductions in leaf mass and area. Under combined warmer/drier conditions B. baxteri forfeited height growth and biomass and increased leaf allocation. The results support our hypothesis that seedlings of B. baxteri and B. coccinea are divergent in key functional traits and their sensitivity to warming and drying. However, we found no evidence for inter-population variation in traits being associated with position on a climate gradient.

Evidence of population variation in drought tolerance during seed germination in four Banksia (Proteaceae) species from Western Australia

Given the predicted changes in rainfall patterns for many Mediterranean climate regions, identifying seed tolerancetomoisturestressintheearliestphaseofplantdevelopmentisanimportantconsiderationforspeciesconservation, managementandrestoration. Here,weused polyethyleneglycol(PEG8000)toinduceplantwaterdeficit similartodrought stress ina fieldsituation.Seeds of fourWestern AustraliaBanksiaR.Br.(Proteaceae) species wereincubated at sevenlevels of moisture potential (0 to 1.5MPa) and three constant temperatures (10C, 15C and 20C). In the absence of moisture stress, germination was uniformly high, but increasing drought stress led to reduced and delayed germination in all species. Overall, the threshold moisture potential value for a significant decline, and delay, in germination was -0.25MPa. Results suggested that one species (B. coccinea) is likely to be most vulnerable to germination failure under predicted changes in rainfall patterns, whereas another (B. media) is likely to be less vulnerable. There was significant variation in populationresponsetodroughtstress.However,thisvariationcouldnotbeexplainedbyrainfallacrossspeciesdistributions. We discuss the PEG approach for assessing seed sensitivity to moisture stress, particularly in the context of shifting rainfall under climate change.