Dynamic DNA methylation differences between epitypes throughout the annual cycle, circannual clocks affect methylation levels, stable methylation marks in the promoters of 30 candidate genes, embryo–adult transmission of methylation marks. DNA methylation can change DNA properties, affecting chromatin accessibility, gene expression, and phenotypic variation. In clonal Norway spruce, warmer (WE) versus cooler (CE) embryogenic conditions produce phenotypically different trees. This climatic memory, induced during embryogenesis, remains stable in the resulting epitype trees, and the epigenetically altered timing of bud phenology persists between WE and CE epitypes even after decades under common garden conditions. We examined DNA methylation patterns in 14-year-old epitypes throughout the annual developmental cycle. Using targeted bisulfite sequencing, we screened for differential DNA methylation over a 3000 bp region in 2744 genes related to the epigenetic machinery, circadian clock, and phenology. Clustering DNA methylation differences in the CG context clearly separated epitype trees, confirming epigenetic mark differences. Differences in methylation of cytosines in all contexts were highly dynamic and varied markedly among annual developmental stages, suggesting the existence of circannual clocks affecting methylation levels in the studied genomic regions. Most stable methylation marks were identified in CG contexts, fewer in CHG and none in CHH contexts, consistent with differences in inheritance among methylation contexts. We identified stable CG and CHG methylation marks in the promoter regions of 30 specific genes. Two ARGONAUTE genes and 4 other genes exhibited stable marks across all time points for CE or WE, and putative embryo–adult transmission for some genes. These findings indicate that DNA methylation marks maintained in genomic regions throughout the annual cycle may contribute to an induced epigenetic memory established in embryos and later manifested as phenologically different epitype trees.

Risk assessment and protection of plant communities in contaminated ecosystems require in-depth understanding of differential sensitivity to chronic ionising radiation in plants. However, the contributing molecular factors to differential radiosensitivity among plant species are poorly understood. To shed light on this, we compared early events associated with protection, repair, and stress responses in gamma-irradiated (1–290 mGy h-1) seedlings of the radiosensitive conifer Norway spruce (Picea abies) and the radiotolerant Arabidopsis thaliana, by analysing growth, organelle and DNA damage, transcriptomes and the dynamics of antioxidant activities and expression of relevant genes. After 48 h of gamma radiation exposure, Norway spruce showed significantly reduced growth at 100–290 mGy h-1 and organelle damage, especially in mitochondria, at ≥ 1 mGy h-1 whereas A. thaliana showed normal vegetative growth at all dose rates, transiently delayed reproductive development at 290 mGy h-1 only, minor organelle damage only at ≥ 100 mGy h-1 and significantly less DNA damage than in Norway spruce at all dose rates. Comparative transcriptomics revealed that A. thaliana showed massive activation of genes related to DNA damage repair, antioxidants, and other stress responses at ≥ 1 mGy h-1 while Norway spruce mobilized transcription of such pathways only at ≥ 40 mGy h-1. The transcriptional activation of repair and protection responses at higher gamma dose-rates only and its absence in lower dose-rates, correlates with high radiosensitivity of Norway spruce, compared to the massive transcriptional activation from low dose-rates in the radiotolerant A. thaliana.

Regardless of DNA- and organelle damage and huge transcriptomic shifts towards stress management pathways after gamma irradiation at 1–100 mGy h−1, pluripotent stem cells of Norway spruce were able to retain their stemness. Conifers are among the most radiosensitive plant species. Elevated, sublethal levels of ionising radiation result in reduced apical dominance in conifers, indicating a negative effect on shoot apical meristems (SAMs). The SAMs, harbouring the pluripotent stem cells, generate all the cells of the shoot, enabling growth and reproduction. However, knowledge on the effects of ionising radiation on such stem cells is scarce, but important for risk assessment and radioprotection of plants in contaminated ecosystems. Here, we assessed the sensitivity of in vitro-grown stem cells of Norway spruce to 144 h of gamma irradiation at 1–100 mGy h−1, using such cells as a model for molecular toxicity of gamma radiation in conifers. Although there were no visible effects of the gamma irradiation on cell proliferation and subsequent embryo formation, dose rate-dependent DNA damage was observed at ≥ 10 mGy h−1, and comprehensive organelle damage at all dose rates. Massive dose rate-dependent transcriptome changes occurred, with downregulation of a range of genes related to cell division, DNA repair and protein folding but upregulation of stress-related hormonal pathways and several antioxidant-related genes. The upregulation of such genes, survival and continued proliferation of at least a subset of cells and the post-irradiation normalisation of expression of DNA repair and protein-folding genes together with somatic embryo formation suggest that stem cells are able to recover from gamma-irradiation-induced stress. Collectively, regardless of cellular abnormalities after gamma irradiation, and huge transcriptomic shifts towards stress management pathways, the pluripotent stem cell cultures were able to retain their stemness.

This study determines the functional role of the plant ultraviolet-B radiation (UV-B) photoreceptor, UV RESISTANCE LOCUS 8 (UVR8) under natural conditions using a large-scale 'synchronized-genetic-perturbation-field-experiment'. Laboratory experiments have demonstrated a role for UVR8 in UV-B responses but do not reflect the complexity of outdoor conditions where 'genotype × environment' interactions can mask laboratory-observed responses. Arabidopsis thaliana knockout mutant, uvr8-7, and the corresponding Wassilewskija wild type, were sown outdoors on the same date at 21 locations across Europe, ranging from 39°N to 67°N latitude. Growth and climatic data were monitored until bolting. At the onset of bolting, rosette size, dry weight, and phenolics and glucosinolates were quantified. The uvr8-7 mutant developed a larger rosette and contained less kaempferol glycosides, quercetin glycosides and hydroxycinnamic acid derivatives than the wild type across all locations, demonstrating a role for UVR8 under field conditions. UV effects on rosette size and kaempferol glycoside content were UVR8 dependent, but independent of latitude. In contrast, differences between wild type and uvr8-7 in total quercetin glycosides, and the quercetin-to-kaempferol ratio decreased with increasing latitude, that is, a more variable UV response. Thus, the large-scale synchronized approach applied demonstrates a location-dependent functional role of UVR8 under natural conditions.

Gymnosperms are long-lived, cone-bearing seed plants that include some of the most ancient extant plant species. These relict land plants have evolved to survive in habitats marked by chronic or episodic stress. Their ability to thrive in these environments is partly due to their phenotypic flexibility, and epigenetic regulation likely plays a crucial part in this plasticity. We review the current knowledge on abiotic and biotic stress memory in gymnosperms and the possible epigenetic mechanisms underlying long-term phenotypic adaptations. We also discuss recent technological improvements and new experimental possibilities that likely will advance our understanding of epigenetic regulation in these ancient and hard-to-study plants.

Hydrangea macrophylla 'Early Blue' was exposed to six different day (DT) and night (NT) and average daily temperatures (ADT) during flower bud formation (DT/NT & DEG;C: 12/12, 17/12, 17/17, 22/12, 22/17, 22/22). The aim was to evaluate the effect of ADT and DT/NT on the rate of bud formation and forcing time after a period of chilling. Furthermore, the role of callose in dormancy induction and release during cold storage was investigated. ADT & LE; 17 & DEG;C accelerated flower bud formation irrespective of DT and NT compared to an ADT > 17 & DEG;C. Floral buds developed at ADT 17 & DEG;C contained significantly lower level of callose compared to buds from the higher ADT regimes (ADT >17 & DEG;C). After 6 weeks of chilling (10 & DEG;C) the callose level was decreased to a similar level irrespective of the DT/NT regime during floral bud formation and/or floral stage at the start of the chilling treatment. We conclude that callose may play an important role in the dormancy release of H. macrophylla. However, floral buds do not have to be fully developed to remove the callose in response to chilling and no clear relationship was found between the levels of callose after chilling and forcing time.

An epigenetic memory of the temperature sum experienced during embryogenesis is part of the climatic adaptation strategy of the long-lived gymnosperm Norway spruce. This memory has a lasting effect on the timing of bud phenology and frost tolerance in the resulting epitype trees. The epigenetic memory is well characterized phenotypically and at the transcriptome level, but to what extent DNA methylation changes are involved have not previously been determined. To address this, we analyzed somatic epitype embryos of Norway spruce clones produced at contrasting epitype-inducing conditions (18 and 28°C). We screened for differential DNA methylation in 2744 genes related mainly to the epigenetic machinery, circadian clock, and phenology. Of these genes, 68% displayed differential DNA methylation patterns between contrasting epitype embryos in at least one methylation context (CpG, CHG, CHH). Several genes related to the epigenetic machinery (e.g., DNA methyltransferases, ARGONAUTE) and the control of bud phenology (FTL genes) were differentially methylated. This indicates that the epitype-inducing temperature conditions induce an epigenetic memory involving specific DNA methylation changes in Norway spruce.

Following nuclear events, risk assessment and protection of plant communities in contaminated ecosystems require in-depth understanding of radiosensitivity of plants. However, the physiological and molecular factors defining differential sensitivity to chronic ionising radiation exposure are poorly understood. In this study, we compared early molecular events associated with protection, repair, and stress responses as well as phenotypic and cellular effects in gamma-irradiated seedlings of the radiosensitive conifer Norway spruce ( Picea abies ) and the radiotolerant herbaceous Arabidopsis thaliana . After 48-h of irradiation, Norway spruce showed reduced growth at 290 mGy h -1 and severe organelle damage at ≥ 1 mGy h -1 whereas A. thaliana showed unaffected development, minor organelle damage at ≥ 100 mGy h -1 only and significantly less DNA damage at all dose rates. Comparative transcriptomics revealed that Norway spruce mobilized transcription of DNA damage repair and antioxidant genes at ≥ 40 mGy h -1 only while A. thaliana showed massive activation of genes related to DNA damage repair, antioxidants, and other stress responses as well as growth-promoting hormones and cell wall components at ≥ 1 mGy h -1 . Adverse effects on chloroplasts and mitochondria from low dose rates on and comprehensive downregulation of photosynthetic genes and activation of respiration genes at ≥ 40 mGy h -1 in Norway spruce but not in A. thaliana may reflect the higher energy demand in Norway spruce to simultaneously maintain its far larger genome and engage protection and repair systems. Hence, the absence of transcriptional response at lower gamma doses and activation of repair and protection at high dose rates only, when accumulated damage is high, is consistent with the high radiosensitivity of Norway spruce. Conversely, the more massive transcriptional activation of crucial repair and protection pathways even at low dose rates complies with the high radiotolerance of A. thaliana .

The Chernobyl Nuclear Power Plant (ChNPP) accident in 1986 resulted in extremely high levels of acute ionising radiation, that killed or damaged Scots pine (Pinus sylvestris) trees in the surrounding areas. Dead trees were cleared and buried, and new plantations established a few years later. Today, more than three decades later, gamma and beta-radiation near the ChNPP is still elevated compared with ambient levels but have decreased by a factor of 300 and 100, respectively. In the present work, Scots pine-trees growing at High (220 μGy h-1), Medium (11 μGy h-1), and Low (0.2 μGy h-1) total (internal + external) dose rates of chronically elevated ionising radiation in the Chernobyl Exclusion zone were investigated with respect to possible damage to DNA, cells and organelles, as well as potentially increased levels of phenolic and terpenoid antioxidants. Scots pine from the High and Medium radiation sites had elevated levels of DNA damage in shoot tips and needles as shown by the COMET assay, as well as increased numbers of resin ducts and subcellular abnormalities in needles. Needles from the High radiation site showed elevated levels of monoterpenes and condensed tannins compared with those from the other sites. In conclusion, more than three decades after the ChNPP accident substantial DNA damage and (sub)cellular effects, but also mobilisation of stress-protective substances possessing antioxidant activity were observed in Scots pine trees growing at elevated levels of ionising radiation. This demonstrates that the radiation levels in the Red Forest still significantly impact the plant community.

The ability to tolerate low freezing temperatures is an important component of winter survival and persistence of red clover. Cold acclimation (CA) allows plants to acquire higher levels of freezing tolerance. However, the biochemical responses to cold and the importance of such changes for the plant to acquire adequate freezing tolerance have not been investigated in red clover of Nordic origin, which has a distinct genetic background. To shed light on this, we selected five freezing tolerant (FT) and five freezing susceptible (FS) accessions and studied the effect of CA on the contents of carbohydrates, amino acids, and phenolic compounds in the crowns. Among those compounds which increased during CA, FT accessions had higher contents of raffinose, pinitol, arginine, serine, alanine, valine, phenylalanine, and one phenolic compound (a pinocembrin hexoside derivative) than FS accessions, suggesting a role for these compounds in the freezing tolerance in the selected accessions. These findings, together with a description of the phenolic profile of red clover crowns, significantly add to the current knowledge of the biochemical changes during CA and their role in freezing tolerance in Nordic red clover.

De-methyl esterification of homogalacturonan and subsequent cross-linking with Ca (2+) is hypothesized to enhance the freezing survival of cold acclimated plants by reducing the porosity of primary cell walls. To test this theory, we collected leaf epidermal peels from non- (23/18 degrees C) and cold acclimated (2 weeks at 12/4 degrees C) Japanese bunching onion (Allium fistulosum L.). Cold acclimation enhances the temperature at which half the cells survived freezing injury by 8 degrees C (LT50 = -20 degrees C) and reduces tissue permeability by 70-fold as compared with non-acclimated epidermal cells. These effects were associated with greater activity of pectin methylesterase (PME) and a reduction in the methyl esterification of homogalacturonan. Non-acclimated plants treated with 50 mM CaCl2 accumulated higher concentrations of galacturonic acid, Ca (2+) in the cell wall and a lower number of visible cell wall pores compared to levels observed in cold acclimated plants. Using cryo-microscopy, we observed that a 50 mM CaCl2 treatment did not lower the LT50 of non-acclimated cells but reduced the lethal intracellular ice nucleation to temperatures observed in cold acclimated epidermal cells. We postulate the PME-homogalacturonan-mediated reduction in cell wall porosity is integral to intracellular freezing avoidance strategies in cold acclimated herbaceous cells.

Light and temperature are crucial factors for the annual growth rhythm of tree seedlings of the boreal and temperate zone. Dormant, vegetative winter buds are formed under short days (SD) and altered light quality. In the conifer Norway spruce, expression of FTL2 increases and PaCOL1-2 and PaSOC1 decrease under light regimes, inducing bud set. Although temperature is known to modulate the timing of bud set, information about combined effects of light climate and temperature on bud phenology and gene expression is limited. We studied the interactive effects of temperature (18, 22/24 °C) and day extension with blue (B), red (R) or far-red (FR) light or different R:FR ratios compared to SD on growth–dormancy cycling and expression of FTL2, PaCOL1-2 and PaSOC1 in Norway spruce seedlings. Day-extension with B light and all treatments involving FR light sustained shoot elongation, with increased growth at higher temperature. The R light treatment resulted in delayed/prevented bud set compared to SD, with more delay/prevented bud set at 24 °C than 18 °C. This was associated with lower PaFTL2-transcript levels at 24 °C and more rapid subsequent bud burst. For the growth-sustaining treatments (long days, FR and B light), the PaFTL2-transcript levels were generally lower and those of PaCO1-2 and PaSOC1 higher compared with SD and R light. In conclusion, our results demonstrate more reduced/prevented bud set and faster bud burst with increased temperature under day extension with R light, indicating less deep dormancy than at lower temperature. Also, sustained shoot elongation under the B light treatment (27 µmol m−2 s−1) in contrast to the lower B light-irradiances tested previously (≤13 µmol m−2 s−1), demonstrates an irradiance-dependent effect of day extension with B light.

The signals from photoreceptors modify plant morphogenesis and regulate the timing of flowering. In the long-day plant petunia, flowering is accelerated under blue (B) and white (W) light compared to red (R) light. In Arabidopsis thaliana L., ELF genes are involved in circadian clock-associated regulation of flowering under different light conditions. In this study, we aimed to assess the involvement of ELF genes in control of flowering by light quality in petunia. Two ELF4-like genes, PhELF4-1 and PhELF4-2 with 76% and 70% similarity to orthologues in pepper but low overall similarity to ELF genes in A. thaliana L., were characterized in petunia and their expression patterns studied under different light qualities. Both genes showed a rhythmic expression pattern and higher expression under B light from light emitting diodes (LED) and W light from fluorescent lamps than under R LED light from LED. For both genes, the expression peaked towards the end of the day, 12 h after start of a 14 h photoperiod. Compared with PhELF4-2, PhELF4-1 expression showed higher amplitude with significantly higher peak expression. As investigated for PhELF4-1, such an expression rhythm was kept for two days after transfer of the plants to continuous lighting using B LED, indicating a circadian rhythm. PhELF4-1 also responded with a phase shift after transfer to short days of an 8 h photoperiod. These results indicate that PhELF4-like genes in petunia are under photoperiodic control involving a circadian clock and play a role in signal transduction from one or more B light photoreceptors.

UVB radiation caused irradiance-dependent and target-specific responses in non-UVB acclimated Lemna minor. Conceptual toxicity pathways were developed to propose causal relationships between UVB-mediated effects at multiple levels of biological organisation. Macrophytes inhabit waterways around the world and are used in hydroponics or aquaponics for different purposes such as feed and wastewater treatment and are thus exposed to elevated levels of UVB from natural and artificial sources. Although high UVB levels are harmful to macrophytes, mechanistic understanding of irradiance-dependent effects and associated modes of action in non-UVB acclimated plants still remains low. The present study was conducted to characterise the irradiance-dependent mechanisms of UVB leading to growth inhibition in Lemna minor as an aquatic macrophyte model. The L. minor were continuously exposed to UVB (0.008–4.2 W m−2) and constant UVA (4 W m−2) and photosynthetically active radiation, PAR (80 µmol m−2 s−1) for 7 days. A suite of bioassays was deployed to assess effects on oxidative stress, photosynthesis, DNA damage, and transcription of antioxidant biosynthesis, DNA repair, programmed cell death, pigment metabolism and respiration. The results showed that UVB triggered both irradiance-dependent and target-specific effects at multiple levels of biological organization, whereas exposure to UVA alone did not cause any effects. Inhibition of photosystem II and induction of carotenoids were observed at 0.23 W m−2, whereas growth inhibition, excessive reactive oxygen species, lipid peroxidation, cyclobutane pyrimidine dimer formation, mitochondrial membrane potential reduction and chlorophyll depletion were observed at 0.5–1 W m−2. Relationships between responses at different levels of biological organization were used to establish a putative network of toxicity pathways to improve our understanding of UVB effects in aquatic macrophytes under continuous UVB exposures. Additional studies under natural illuminations were proposed to assess whether these putative toxicity pathways may also be relevant for more ecologically relevant exposure scenarios.

Blue light (BL) affects different growth parameters, but information about the physiological effects of BL on conifer seedlings is limited. In northern areas, conifer seedlings are commonly produced in heated nursery greenhouses. Compared with Norway spruce, subalpine fir seedlings commonly show poor growth in nurseries due to early growth cessation. This study aimed to examine the effect of the BL proportion on the growth and development of such conifer seedlings in growth chambers, using similar photosynthetic active radiation, with 5% or 30% BL (400–500 nm) from high pressure sodium (HPS) lamps (300 μmol m−2 s−1) or a combination of HPS (225 μmol m−2 s−1) and BL-emitting diodes (75 μmol m−2 s−1), respectively. Additional BL increased transpiration and improved the growth of the Norway spruce seedlings, which developed thicker stems, more branches, and a higher dry matter (DM) of roots and needles, with an increased DM percentage in the roots compared with the shoots. In contrast, under additional BL, subalpine fir showed reduced transpiration and an increased terminal bud formation and lower DM in the stems and needles but no change in the DM distribution. Since these conifers respond differently to BL, the proportion of BL during the day should be considered when designing light spectra for tree seedling production.

Plants developed under constant high (>85%) relative air humidity (RH) have larger stomata that are unable to close completely in response to closing stimuli. Roses (Rosa x hybrida) developed in high RH have previously been shown to have high water loss during leaf dehydration and reduced dark-induced closure resulting in a shorter postharvest life. In this study, the effect of B-light on stomatal function under high RH conditions was investigated. The ability of rose leaves developed under continuous high (90%) or moderate (60%) RH to close their stomata in response to darkness and leaf dehydration assay was studied. Moreover, the level and regulation of ABA in light and darkness in relation to B-light was measured. Our results show that increased B-light proportion improved stomatal function and dark-induced stomatal closure under high RH conditions and that was associated with increased [ABA] in general and a dynamic ABA peak during darkness. Furthermore, increased B-light during the day was associated with the presence of high β-glucosidase activity during night. This indicates that B-light is important as a signal to activate the β-glucosidase enzyme and release ABA during night. Altogether, the improved stomatal function and reduced transpiration in combination with increased [ABA] indicate that preharvest B-light plays an important role in governing stomatal functionality and ABA homeostasis under high RH and can be a useful method to improve postharvest water balance of roses.

Persistent DNA damage in gamma-exposed Norway spruce, Scots pine and Arabidopsis thaliana, but persistent adverse effects at the organismal and cellular level in the conifers only. Gamma radiation emitted from natural and anthropogenic sources may have strong negative impact on plants, especially at high dose rates. Although previous studies implied different sensitivity among species, information from comparative studies under standardized conditions is scarce. In this study, sensitivity to gamma radiation was compared in young seedlings of the conifers Scots pine and Norway spruce and the herbaceous Arabidopsis thaliana by exposure to 60Co gamma dose rates of 1–540 mGy h−1 for 144 h, as well as 360 h for A. thaliana. Consistent with slightly less prominent shoot apical meristem, in the conifers growth was significantly inhibited with increasing dose rate ≥ 40 mGy h−1. Post-irradiation, the conifers showed dose-rate-dependent inhibition of needle and root development consistent with increasingly disorganized apical meristems with increasing dose rate, visible damage and mortality after exposure to ≥ 40 mGy h−1. Regardless of gamma duration, A. thaliana showed no visible or histological damage or mortality, only delayed lateral root development after ≥ 100 mGy h−1 and slightly, but transiently delayed post-irradiation reproductive development after ≥ 400 mGy h−1. In all species dose-rate-dependent DNA damage occurred following ≥ 1–10 mGy h−1 and was still at a similar level at day 44 post-irradiation. In conclusion, the persistent DNA damage (possible genomic instability) following gamma exposure in all species may suggest that DNA repair is not necessarily mobilized more extensively in A. thaliana than in Norway spruce and Scots pine, and the far higher sensitivity at the organismal and cellular level in the conifers indicates lower tolerance to DNA damage than in A. thaliana.

Natural changes in photoperiod, light quantity, and quality play a key role in plant signaling, enabling daily and seasonal adjustment of growth and development. Growing concern about the global climate crisis together with scattered reports about the interactive effects of temperature and light parameters on plants necessitates more detailed information about these effects. Furthermore, the actual light emitting diode (LED) lighting technology allows mimicking of light climate scenarios more similar to natural conditions, but to fully exploit this in plant cultivation, easy-to-apply knowledge about the natural variation in light quantity and spectral distribution is required. Here, we aimed to provide detailed information about short and long-term variation in the natural light climate, by recording the light quantity and quality at an open site in Switzerland every minute for a whole year, and to analyze its relationship to a set of previous tree seedling growth experiments. Changes in the spectral composition as a function of solar elevation angle and weather conditions were analyzed. At a solar elevation angle lower than 20°, the weather conditions have a significant effect on the proportions of blue (B) and red (R) light, whereas the proportion of green (G) light is almost constant. At a low solar elevation, the red to far red (R:FR) ratio fluctuates between 0.8 in cloudy conditions and 1.3 on sunny days. As the duration of periods with low solar angles increases with increasing latitude, an analysis of previous experiments on tree seedlings shows that the effect of the R:FR ratio correlates with the responses of plants from different latitudes to light quality. We suggest an evolutionary adaptation where growth in seedlings of selected tree species from high latitudes is more dependent on detection of light quantity of specific light qualities than in such seedlings originating from lower latitudes.

Exposure to ambient UV-B radiation may prime protective responses towards various stressors in plants, though information about interactive effects of UV-B and gamma radiation is scarce. Here, we aimed to test whether UV-B exposure could prime acclimatisation mechanisms contributing to tolerance to low-moderate gamma radiation levels in Scots pine seedlings, and concurrently whether simultaneous UV-B and gamma exposure may have an additive adverse effect on seedlings that had previously not encountered either of these stressors. Responses to simultaneous UV-B (0.35 W m −2 ) and gamma radiation (10.2–125 mGy h −1 ) for 6 days with or without UV-B pre-exposure (0.35 W m −2 , 4 days) were studied across various levels of organisation, as compared to effects of either radiation type. In contrast to UV-B, and regardless of UV-B presence, gamma radiation at ≥42.9 mGy h −1 caused increased formation of reactive oxygen species and reduced shoot length, and reduced root length at 125 mGy h −1 . In all experiments there was a gamma dose rate-dependent increase in DNA damage at ≥10.8 mGy h −1 , generally with additional UV-B-induced damage. Gamma-induced growth inhibition and gamma- and UV-B-induced DNA damage were still visible 44 days post-irradiation, even at 20.7 mGy h −1 , probably due to genomic instability, but this was reversed after 8 months. In conclusion, there was no evidence of a protective effect of UV-B on gamma-induced growth inhibition and DNA damage in Scots pine, and no additive adverse effect of gamma and UV-B radiation on growth in spite of the additional UV-B-induced DNA damage.

Key messageBud break in European beech is generally known to be under strong photoperiodic control, but our data indicate that bud break in the northernmost populations is temperature limited.AbstractEuropean beech (Fagus sylvatica) is among the dominant tree species in Europe, and is expected to increase its distribution northwards due to climate change. Although spring bud break in European beech is known to be under strong photoperiodic control, studies of populations from high elevations suggest a greater role of temperature in colder environments. This indicates that warming could yield advanced bud break for populations at high elevation and at high latitudes. Using live material collected in winter, spring and autumn from wild populations in Norway, representing the northernmost distribution range of European beech, we tested frost tolerance and effects of temperature and day length on bud break in controlled conditions. Furthermore, we observed bud break during spring in field conditions. Bud break in controlled conditions was faster with later sampling dates and higher temperatures, but showed little or no activity under short photoperiod. Field observations revealed earlier bud break in warmer sites, and remarkably, that bud break dates have advanced by more than 10days in less than three decades. Frost tolerance tests showed a gradual decrease in bud intactness from January to March, and a gradual increase from October to December. Our data indicate that bud break dates in the northernmost European beech populations are limited by temperature, similar to observations of populations at high elevation in Central Europe. Further warming can thus be expected to yield earlier bud break at the species' northernmost distribution range, possibly increasing the susceptibility of shoot tips to late spring frost events.