Dormancy Cycle Research Articles

Research progresses on the effects of light, temperature and water conditions on primary and secondary growth of trees.

Tree growth includes primary growth and secondary growth. The growth activity and dormancy cycle of trees can affect forest productivity and carbon sequestration capacity. Therefore, it is of great significance to examine the effects of environmental conditions (e.g., photoperiod, temperature and water) on tree growth for understanding the responses of trees to climate change and predicting forest productivity and carbon sequestration capacity under the background of global climate change. We reviewed the effects of photoperiod, temperature and water conditions on the primary and secondary growth of trees, and revealed the physiological mechanisms underlying their impacts on the synchronization or asynchronization between primary and secondary growth of trees. The shortcomings of the existing research were pointed out. For example, less attention had been paid to the enrionmental response and adaptation of root growth, as well as the physiological mechanism of the effect of light, temperature and water on tree growth. Research on the growth of underground roots should be strengthened in the future, and more attention should be paid to the physiological changes in the process of tree growth affected by environmental factors. Furthermore, the source and sink limitation theory and the process-based prediction model should be improved, aiming to provide a scientific basis for predicting forest productivity and carbon sequestration capacity and putting forward scientific policies of forest management.

The molecular regulation of pear bud dormancy cycle

The molecular regulation of pear bud dormancy cycle

Seed germination requirements of the rare Helosciadium repens (aka Apium repens) determine persistence of seeds in the soil seed bank.

The rare and threatened Heliosciadium repens grows in moist grasslands and has a distinct life cycle. Plants reproduce both clonally, although ramets tend to be short-lived, and sexually, with seeds that can form a persistent soil seed bank. The germination requirements of H. repens were investigated, yielding important information for its habitat management and conservation. We examined the soil seed bank in three populations and carried out germination experiments and embryo growth measurements with fresh seeds in laboratory, greenhouse and outdoor conditions. We also investigated the effects of storage and burial of seeds. H. repens formed a long-term persistent (>6 years) soil seed bank with very pronounced primary dormancy, but no secondary dormancy or dormancy cycles. Seeds can germinate throughout the growing season when temperatures are sufficiently high. Embryo growth and seed germination are triggered by light and, to a lesser extent, daily temperature fluctuations. Seeds of H. repens seem to have developed a unique germination syndrome with several strategies to remain dormant in the soil until optimal conditions are present for seedling establishment and survival. Both sexual reproduction and seed bank formation are crucial for the long-term survival of the populations.

Beyond floral initiation: the role of flower bud dormancy in flowering time control of annual plants.

The phenology of temperate perennials, including the timing of vegetative growth and flowering, is well known to be controlled by seasonal dormancy cycles. Dormant structures are known as buds and have specialized covering structures, symplastic isolation from the plant, and often autonomous stores of carbon and nitrogen reserves. In contrast, in annual plants, our current understanding of the control of the timing of flowering focuses on the mechanisms affecting floral initiation, the transition from a vegetative apical meristem to a inflorescence meristem producing flower primordia in place of leaves. Recently we revealed that annual crops in Brassicaceae exhibit chilling-responsive growth control in a manner closely resembling bud dormancy breakage in perennial species. Here I discuss evidence that vernalization in autumn is widespread and further discuss its role in inducing flower bud set prior to winter. I also review evidence that flower bud dormancy has a more widespread role in annual plant flowering time control than previously appreciated.

Identification of a key signaling network regulating perennating bud dormancy in Panax ginseng

BackgroundThe cycle of seasonal dormancy of perennating buds is an essential adaptation of perennial plants to unfavorable winter conditions. Plant hormones are key regulators of this critical biological process, which is intricately connected with diverse internal and external factors. Recently, global warming has increased the frequency of aberrant temperature events that negatively affect the dormancy cycle of perennials. Although many studies have been conducted on the perennating organs of Panax ginseng, the molecular aspects of bud dormancy in this species remain largely unknown. MethodsIn this study, the molecular physiological responses of three P. ginseng cultivars with different dormancy break phenotypes in the spring were dissected using comparative genome-wide RNA-seq and network analyses. These analyses identified a key role for abscisic acid (ABA) activity in the regulation of bud dormancy. Gene set enrichment analysis revealed that a transcriptional network comprising stress-related hormone responses made a major contribution to the maintenance of dormancy. ResultsIncreased expression levels of cold response and photosynthesis-related genes were associated with the transition from dormancy to active growth in perennating buds. Finally, the expression patterns of genes encoding ABA transporters, receptors (PYRs/PYLs), PROTEIN PHOSPHATASE 2Cs (PP2Cs), and DELLAs were highly correlated with different dormancy states in three P. ginseng cultivars. ConclusionThis study provides evidence that ABA and stress signaling outputs are intricately connected with a key signaling network to regulate bud dormancy under seasonal conditions in the perennial plant P. ginseng.

Genome-Wide Identification of Callose Synthase Family Genes and Their Expression Analysis in Floral Bud Development and Hormonal Responses in Prunus mume.

Callose is an important polysaccharide composed of beta-1,3-glucans and is widely implicated in plant development and defense responses. Callose synthesis is mainly catalyzed by a family of callose synthases, also known as glucan synthase-like (GSL) enzymes. Despite the fact that GSL family genes were studied in a few plant species, their functional roles have not been fully understood in woody perennials. In this study, we identified total of 84 GSL genes in seven plant species and classified them into six phylogenetic clades. An evolutionary analysis revealed different modes of duplication driving the expansion of GSL family genes in monocot and dicot species, with strong purifying selection constraining the protein evolution. We further examined the gene structure, protein sequences, and physiochemical properties of 11 GSL enzymes in Prunus mume and observed strong sequence conservation within the functional domain of PmGSL proteins. However, the exon-intron distribution and protein motif composition are less conservative among PmGSL genes. With a promoter analysis, we detected abundant hormonal responsive cis-acting elements and we inferred the putative transcription factors regulating PmGSLs. To further understand the function of GSL family genes, we analyzed their expression patterns across different tissues, and during the process of floral bud development, pathogen infection, and hormonal responses in Prunus species and identified multiple GSL gene members possibly implicated in the callose deposition associated with bud dormancy cycling, pathogen infection, and hormone signaling. In summary, our study provides a comprehensive understanding of GSL family genes in Prunus species and has laid the foundation for future functional research of callose synthase genes in perennial trees.

The Use of Transgenic Resistant Plums in Transgrafting Fails to Confer Plum Pox Virus Resistance in Prunus

In order to sustain productivity, growers are implementing fruit trees to provide plum pox virus (PPV) resistant varieties. Unfortunately, classical breeding approaches have failed to develop resistant varieties. RNA interference (RNAi) silencing, as an alternate strategy, has been shown to be an efficient approach to combat PPV disease (sharka). PPV resistance based on RNAi has been demonstrated in natural conditions for over 10 years. Experiments using graft inoculation in high-containment greenhouses show that silencing protects against PPV over multiple dormancy cycles. While the virus spreads throughout the vascular tissues of susceptible hosts, PPV movement is undetectable in resistant clones. However, in this work, when PPV-susceptible cultivars were grafted onto transgenic resistant plum rootstocks, the small interfering RNA (siRNA) stopped the systemic spread of PPV into the transgenic resistant shoots but did not appear to prevent the infection of susceptible scions. Thus, we do not have convincing evidence of siRNA transfer nor the provision of resistance to the susceptible scion. Notably, <em>Macrolophus</em> insects that were experimentally fed on plums could suck RNAi from the leaves of resistant clones. While the attempt to control PPV disease through transgrafting as a new breeding technology (NBT) cannot be confirmed, these studies provide insight into key regulatory interactions associated with RNAi accumulation and its environmentally safe use in perennial plants.

Light on perenniality: Para-dormancy is based on ABA-GA antagonism and endo-dormancy on the shutdown of GA biosynthesis.

Perennial para- and endo-dormancy are seasonally separate phenomena. Whereas para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, endo-dormancy is the short-day elicited arrest of terminal and AXBs. In hybrid aspen (Populus tremula x P. tremuloides) compromising the apex releases para-dormancy, whereas endo-dormancy requires chilling. ABA and GA are implicated in both phenomena. To untangle their roles, we blocked ABA biosynthesis with fluridone (FD), which significantly reduced ABA levels, downregulated GA-deactivation genes, upregulated the major GA3ox-biosynthetic genes, and initiated branching. Comprehensive GA-metabolite analyses suggested that FD treatment shifted GA production to the non-13-hydroxylation pathway, enhancing GA4 function. Applied ABA counteracted FD effects on GA metabolism and downregulated several GA3/4 -inducible α- and γ-clade 1,3-β-glucanases that hydrolyze callose at plasmodesmata (PD), thereby enhancing PD-callose accumulation. Remarkably, ABA-deficient plants repressed GA4 biosynthesis and established endo-dormancy like controls but showed increased stress sensitivity. Repression of GA4 biosynthesis involved short-day induced DNA methylation events within the GA3ox2 promoter. In conclusion, the results cast new light on the roles of ABA and GA in dormancy cycling. In para-dormancy, PD-callose turnover is antagonized by ABA, whereas in short-day conditions, lack of GA4 biosynthesis promotes callose deposition that is structurally persistent throughout endo-dormancy.

Seed dormancy revisited: Dormancy‐release pathways and environmental interactions

Abstract Many internal (inherent) and environmental (imposed) factors control seed dormancy and germination that we divide into three basic dormancy‐release pathways: Maternal structures and embryo physiology control inherent dormancy that is broken by various types of scarification and physiological changes, followed by imposed‐dormancy release when the prevailing environment is replaced by certain ‘standard’ conditions that stimulate germination (Pathway 1); imposed dormancy prevails even if inherent dormancy is broken or not applicable that is released when replaced by certain ‘standard’ environmental conditions which stimulate germination (Pathway 2); and release from inherent dormancy by light/dark or cold stratification is contingent on the pre‐existence of certain environmental conditions that stimulate germination (Pathway 3). On‐plant seed storage (serotiny) and frugivorous seeds are recognized here as representing special types of physical dormancy, as their properties are consistent with those of hard diaspores. Warm stratification does not require seeds to be moist as it is just a physical response. Heat may promote germination of non‐hard, as well as hard, seeds as it may increase their permeability further. Levels of germination gauge the net effect of inherent‐ and imposed‐dormancy release so that it is only possible to identify the extent of inherent‐dormancy release when conditions for germination are optimal (imposed dormancy has been annulled). While imposed dormancy may be protracted after inherent dormancy is broken by heat or chilling during the dry or cold seasons, release from both states may effectively coincide if smoke chemicals or light are received during the (wet) growing season. We suggest reserving the term secondary dormancy for seeds that return to (inherent or imposed) dormancy due to changed environmental conditions. Under seasonal climates, fluctuations in environmental conditions can lead to secondary dormancy and even dormancy cycling. We recognize four types of functional interactions between any two environmental factors that induce inherent‐dormancy release: binary interactions are ineffective, only one stimulus is effective, both are effective but non‐additive, or both are additive/synergistic. Two environmental stimuli that individually break dormancy but have no additive effect must be affecting the same process; this was demonstrated here for some interactions between heat and smoke. The three dormancy‐release pathways, together with internal, seasonal and stochastic interactions, are coordinated by the non‐dormant seed to ensure maximum germination under optimal conditions. To ignore any aspect outlined here leads to an impoverished understanding of the disparate seed ecology of species adapted to different stressful and disturbance‐prone habitats. Read the free Plain Language Summary for this article on the Journal blog.

A Model for Changes in Germination Synchrony and Its Implements to Study Weed Population Dynamics: A Case Study of Brassicaceae

In every agricultural system, weed seeds can be found in every cubic centimeter of soil. Weed seeds, as a valuable trait underlying the fate of weed populations, exhibit differing levels of seed dormancy, ensuring their survival under uncertain conditions. Seed dormancy is considered as an innate mechanism that constrains germination under suitable conditions that would otherwise stimulate germination of nondormant seeds. This work provides new insight into changes in germination patterns along the dormant to nondormancy continuum in seeds with physiological dormancy. Notable findings are: (1) germination synchrony can act as a new parameter that quantitatively describes dormancy patterns and, subsequently, weed population dynamics, (2) germination synchrony is dynamic, suggesting that the more dormancy decreases, the more synchrony is obtainable, (3) after-ripening and stratification can function as a synchronizing agent that regulates germination behavior. Freshly harvested seeds of Brassica napus with type 3 of non-deep physiological dormancy showed the most synchronous germination, with a value of 3.14, while a lower level of germination asynchrony was found for newly harvested seeds of Sinapis arvensis with type 1 of non-deep physiological dormancy, with an asynchrony value of 2.25. After-ripening and stratification can act as a synchronizing factor through decreasing the asynchrony level and increasing synchrony. There is a firm relationship between seed dormancy cycling and germination synchrony patterns, ensuring their survival and reproductive strategies. By germinating in synchrony, which is accompanied by cycling mechanisms, weeds have more opportunities to persist. The synchrony model used in the present study predicts germination behavior and synchrony along the dormant to nondormancy continuum in weed seeds with physiological dormancy, suggesting a useful method for the quantification of germination strategies and weed population dynamics.

The collaborative mode by PmSVPs and PmDAMs reveals neofunctionalization in the switch of the flower bud development and dormancy for Prunus mume.

Prunus mume (Rosaceae, Prunoideae) serves as an excellent ornamental woody plant with a large-temperature-range cultivation scope. Its flower buds require a certain low temperature to achieve flowering circulation. Thus, it is important to delve into the processes of flower bud differentiation and dormancy, which affected its continuous flowering. These processes are generally considered as regulation by the MADS-box homologs, SHORT VEGETATIVE PHASE (SVP), and DORMANCY-ASSOCIATED MADS-BOX (DAM). However, a precise model on their interdependence and specific function, when acting as a complex in the flower development of P. mume, is needed. Therefore, this study highlighted the integral roles of PmDAMs and PmSVPs in flower organ development and dormancy cycle. The segregation of PmDAMs and PmSVPs in a different cluster suggested distinct functions and neofunctionalization. The expression pattern and yeast two-hybrid assays jointly revealed that eight genes were involved in the floral organ development stages, with PmDAM1 and PmDAM5 specifically related to prolificated flower formation. PmSVP1-2 mingled in the protein complex in bud dormancy stages with PmDAMs. Finally, we proposed the hypothesis that PmSVP1 and PmSVP2 could combine with PmDAM1 to have an effect on flower organogenesis and interact with PmDAM5 and PmDAM6 to regulate flower bud dormancy. These findings could help expand the current molecular mechanism based on MADS-box genes during flower bud development and dormancy.

PpyABF3 recruits the COMPASS-like complex to regulate bud dormancy maintenance via integrating ABA signaling and GA catabolism.

Bud dormancy is essential for perennial trees that survive the cold winters and to flower on time in the following spring. Histone modifications have been reported to be involved in the control of the dormancy cycle and DAM/SVPs are considered targets. However, how the histone modification marks are added to the specific gene loci during bud dormancy cycle is still unknown. Using yeast-two hybrid library screening and co-immunoprecipitation assays, we found that PpyABF3, a key protein regulating bud dormancy, recruits Complex of Proteins Associated with Set1-like complex via interacting with PpyWDR5a, which increases the H3K4me3 deposition at DAM4 locus. Chromatin immunoprecipitation-quantitative polymerase chain reaction showed that PpyGA2OX1 was downstream gene of PpyABF3 and it was also activated by H3K4me3 deposition. Silencing of GA2OX1 in pear calli and pear buds resulted in a similar phenotype with silencing of ABF3. Furthermore, overexpression of PpyWDR5a increased H3K4me3 levels at DAM4 and GA2OX1 loci and inhibited the growth of pear calli, whereas silencing of PpyWDR5a in pear buds resulted in a higher bud-break percentage. Our findings provide new insights into how H3K4me3 marks are added to dormancy-related genes in perennial woody plants and reveal a novel mechanism by which ABF3 integrates abscisic acid signaling and gibberellic acid catabolism during bud dormancy maintenance.

Integrated transcriptome and small RNA sequencing in revealing miRNA-mediated regulatory network of floral bud break in Prunus mume.

MicroRNAs is one class of small non-coding RNAs that play important roles in plant growth and development. Though miRNAs and their target genes have been widely studied in many plant species, their functional roles in floral bud break and dormancy release in woody perennials is still unclear. In this study, we applied transcriptome and small RNA sequencing together to systematically explore the transcriptional and post-transcriptional regulation of floral bud break in P. mume. Through expression profiling, we identified a few candidate genes and miRNAs during different developmental stage transitions. In total, we characterized 1,553 DEGs associated with endodormancy release and 2,084 DEGs associated with bud flush. Additionally, we identified 48 known miRNAs and 53 novel miRNAs targeting genes enriched in biological processes such as floral organ morphogenesis and hormone signaling transudation. We further validated the regulatory relationship between differentially expressed miRNAs and their target genes combining computational prediction, degradome sequencing, and expression pattern analysis. Finally, we integrated weighted gene co-expression analysis and constructed miRNA-mRNA regulatory networks mediating floral bud flushing competency. In general, our study revealed the miRNA-mediated networks in modulating floral bud break in P. mume. The findings will contribute to the comprehensive understanding of miRNA-mediated regulatory mechanism governing floral bud break and dormancy cycling in wood perennials.

Winter dormancy in trees

Plants growing in temperate and boreal regions of the world have to face strikingly different environmental conditions during summer and winter. Being sessile organisms, plants have had to develop various strategies to adapt to these changes in light, temperature, and water availability, thereby optimizing their 'economy of growth'. While annual plants can endure unfavorable winter conditions in the form of a seed, or under a protective cover of thick snow, perennial plants such as trees adapt by going into a stage of deep sleep called winter dormancy. To enter dormancy, vegetative growth is stopped in the late summer or early autumn and the shoots are converted into buds, where the shoot apical meristems are protected by tightly closed and hardened bud scales (Figures 1 and 2). At the same time, cold hardiness develops and the need for water and nutrient uptake is drastically reduced. Deciduous trees also go through leaf senescence whereby the leaves develop their autumn colors and are shed (Figure 1A). The trees then spend the beginning of the winter in a state of deep sleep in which they are completely unreceptive to any environmental signals telling them to wake up. However, as winter progresses, the trees are gradually released from this slumber and will eventually flush their buds in the spring. Vegetative growth then resumes with the formation of new leaves and shoots during summer until the trees again go into growth cessation and the cycle is closed (Figures 1 and 2). This cycle of growth and dormancy is central for the ability of trees to adapt to growth at different latitudes and elevations. The further north, or the higher the elevation at which the trees grow, the earlier in the season the trees enter growth cessation and the later they flush their buds in the spring. This is because meteorological winter arrives earlier in the season and lasts longer into the spring. The trees therefore have to stop growth earlier in the season to ensure that they have enough time to complete bud formation and to develop cold hardiness and dormancy. They also have to be sure that winter is really over before flushing their buds. Winter dormancy is therefore a clear case of a trade-off between the length of the growing season and the protection against winter damage - a nice example of 'economy in biology', the theme of this special issue. This primer will briefly summarize what we know about the environmental signals that influence the annual growth cycle in trees, as well as our current understanding of the genetic pathways and molecular mechanisms regulated by these signals.

A quantitative analysis of temperature-dependent seasonal dormancy cycling in buried Arabidopsis thaliana seeds can predict seedling emergence in a global warming scenario.

Understanding how the environment regulates seed-bank dormancy changes is essential for forecasting seedling emergence in actual and future climatic scenarios, and to interpret studies of dormancy mechanisms at physiological and molecular levels. Here, we used a population threshold modelling approach to analyse dormancy changes through variations in the thermal range permissive for germination in buried seeds of Arabidopsis thaliana Cvi, a winter annual ecotype. Results showed that changes in dormancy level were mainly associated with variations in the higher limit of the thermal range permissive for germination. Changes in this limit were positively related to soil temperature during dormancy release and induction, and could be predicted using thermal time. From this, we developed a temperature-driven simulation to predict the fraction of the seed bank able to germinate in a realistic global warming scenario that approximated seedling emergence timing. Simulations predicted, in accordance with seedling emergence observed in the field, an increase in the fraction of the seed bank able to emerge as a result of global warming. In addition, our results suggest that buried seeds perceive changes in the variability of the mean daily soil temperature as the signal to change between dormancy release and induction according to the seasons.

Seed Germination and Seed Bank Dynamics of Eruca sativa (Brassicaceae): A Weed on the Northeastern Edge of Tibetan Plateau.

As a versatile cruciferous species, Eruca sativa is widely cultivated, but in some areas, it has become an invasive weed. There are few studies on its seed dormancy and soil seed bank. This research examined seed dormancy, germination, and dynamics of the soil seed bank of E. sativa, with a view to provide support for its prevention and control. We tested the effects of temperature, light, storage, water, and salinity stress on seed germination and burial depth on seedling emergence of E. sativa. Dynamics of the soil seed bank were determined with a 24 month in situ seed-burial study. Seeds of E. sativa can germinate in a temperature range of 5–35°C; moreover, they exhibited non-deep physiological dormancy (NDPD) at maturity, which can be broken by dry storage or exposure to low temperature in winter. Germination of E. sativa seeds was sensitive to water and salinity stress, and most seeds did not germinate at -0.3 MPa. When buried in soil in the field, seeds exhibited an annual dormancy/non-dormancy cycle and formed at least a short-term persistent soil seed bank. Seeds buried deeper than 5 cm can hardly emerge. Seeds of E. sativa have a wide germination temperature range and exhibited dormancy cycling, which promotes the formation of a persistent soil seed bank and enables it to better adapt to the harsh low-temperature climate of the Qinghai-Tibet Plateau. No-tillage would be a good management strategy for this species.

In-Depth Proteomic Analysis of the Secondary Dormancy Induction by Hypoxia or High Temperature in Barley Grains.

In barley, incubation of primary dormant (D1) grains on water under conditions that do not allow germination, i.e. 30°C in air and 15°C or 30°C in 5% O2, induces a secondary dormancy (D2) expressed as a loss of the ability to germinate at 15°C in air. The aim of this study was to compare the proteome of barley embryos isolated from D1 grains and D2 ones after induction of D2 at 30°C or in hypoxia at 15°C or 30°C. Total soluble proteins were analyzed by 2DE gel-based proteomics, allowing the selection of 130 differentially accumulated proteins (DAPs) among 1,575 detected spots. According to the protein abundance profiles, the DAPs were grouped into six abundance-based similarity clusters. Induction of D2 is mainly characterized by a down-accumulation of proteins belonging to cluster 3 (storage proteins, proteases, alpha-amylase inhibitors and histone deacetylase HD2) and an up-accumulation of proteins belonging to cluster 4 (1-Cys peroxiredoxin, lipoxygenase2 and caleosin). The correlation-based network analysis for each cluster highlighted central protein hub. In addition, most of genes encoding DAPs display high co-expression degree with 19 transcription factors. Finally, this work points out that similar molecular events accompany the modulation of dormancy cycling by both temperature and oxygen, including post-translational, transcriptional and epigenetic regulation.

A MADS‐box gene with similarity to FLC is induced by cold and correlated with epigenetic changes to control budbreak in kiwifruit

Temperate perennials require exposure to chilling temperatures to resume growth in the following spring. Growth and dormancy cycles are controlled by complex genetic regulatory networks and are governed by epigenetic mechanisms, but the specific genes and mechanisms remain poorly understood. To understand how seasonal changes and chilling regulate dormancy and growth in the woody perennial vine kiwifruit (Ac, Actinidia chinensis), a transcriptome study of kiwifruit buds in the field and controlled conditions was performed. A MADS-box gene with homology to Arabidopsis FLOWERING LOCUS C (FLC) was identified and characterized. Elevated expression of AcFLC-like (AcFLCL) was detected during bud dormancy and chilling. A long noncoding (lnc) antisense transcript with an expression pattern opposite to AcFLCL and shorter sense noncoding RNAs were identified. Chilling induced an increase in trimethylation of lysine-4 of histone H3 (H3K4me3) in the 5' end of the gene, indicating multiple layers of epigenetic regulation in response to cold. Overexpression of AcFLCL in kiwifruit gave rise to plants with earlier budbreak, whilst gene editing using CRISPR-Cas9 resulted in transgenic lines with substantially delayed budbreak, suggesting a role in activation of growth. These results have implications for the future management and breeding of perennials for resilience to changing climate.

Viral Reservoir Capacity of Wild Prunus Alternative Hosts of Plum Pox Virus Through Multiple Cycles of Transmission and Dormancy.

Plum pox virus (PPV) is a significant pathogen of Prunus worldwide and is known for having a broad experimental host range. Many of these hosts represent epidemiological risks as potential wild viral reservoirs. A comparative study of the PPV reservoir capacity of three commonly found native North American species, western choke cherry (Prunus virginiana var. demissa), black cherry (Prunus serotina), and American plum (Prunus americana) was conducted. Pennsylvania isolates of PPV-D were transmitted from the original host peach (Prunus persica cv. GF305) to all three species. Viral accumulation and transmission rates to alternative hosts and peach were monitored over the course of five vegetative growth and cold induced dormancy (CID) cycles. The three alternative host species demonstrated differences in their ability to maintain PPV-D and the likelihood of transmission to additional alternative hosts or back transmission to peach. Western choke cherry had low (5.8%) initial infection levels, PPV-D was not transmissible to additional western choke cherry, and transmission of PPV-D from western choke cherry to peach was only possible before the first CID cycle. Black cherry had intermediate initial infection levels (26.6%) but did not maintain high infection levels after repeated CID cycles. Conversely, American plum had a high level (50%) of initial infection that was not significantly different from initial infection in peach (72.2%) and maintained moderate levels (15 to 25%) of infection and PPV-D transmission to both American plum and peach through all five cycles of CID. Our results indicate that American plum has the greatest potential to act as a reservoir host for Pennsylvania isolates of PPV-D.

Fire Seasonality, Seasonal Temperature Cues, Dormancy Cycling, and Moisture Availability Mediate Post-fire Germination of Species With Physiological Dormancy.

Fire seasonality (the time of year of fire occurrence) has important implications for a wide range of demographic processes in plants, including seedling recruitment. However, the underlying mechanisms of fire-driven recruitment of species with physiological seed dormancy remain poorly understood, limiting effective fire and conservation management, with insights hampered by common methodological practices and complex dormancy and germination requirements. We sought to identify the mechanisms that regulate germination of physiologically dormant species in nature and assess their sensitivity to changes in fire seasonality. We employed a combination of laboratory-based germination trials and burial-retrieval trials in natural populations of seven species of Boronia (Rutaceae) to characterize seasonal patterns in dormancy and fire-stimulated germination over a 2-year period and synthesized the observed patterns into a conceptual model of fire seasonality effects on germination. The timing and magnitude of seedling emergence was mediated by seasonal dormancy cycling and seasonal temperature cues, and their interactions with fire seasonality, the degree of soil heating expected during a fire, and the duration of imbibition. Primary dormancy was overcome within 4–10 months’ burial and cycled seasonally. Fire-associated heat and smoke stimulated germination once dormancy was alleviated, with both cues required in combination by some species. For some species, germination was restricted to summer temperatures (a strict seasonal requirement), while others germinated over a broader seasonal range of temperatures but exhibited seasonal preferences through greater responses at warmer or cooler temperatures. The impacts of fires in different seasons on germination can vary in strength and direction, even between sympatric congeners, and are strongly influenced by moisture availability (both the timing of post-fire rainfall and the duration soils stay moist enough for germination). Thus, fire seasonality and fire severity (via its effect on soil heating) are expected to significantly influence post-fire emergence patterns in these species and others with physiological dormancy, often leading to “germination interval squeeze.” Integration of these concepts into current fire management frameworks is urgently required to ensure best-practice conservation. This is especially pertinent given major, ongoing shifts in fire seasonality and rainfall patterns across the globe due to climate change and increasing anthropogenic ignitions.