Overwintering of herbaceous plants in a changing climate. Still more questions than answers.

Plant Science 225 (2014) 34–44

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Overwintering of herbaceous plants in a changing climate. Still more questions than answers Marcin Rapacz a,∗ , Åshild Ergon b , Mats Höglind c , Marit Jørgensen d , Barbara Jurczyk a , Liv Østrem e , Odd Arne Rognli b , Anne Marte Tronsmo b a

˙ University of Agriculture in Kraków, Faculty of Agriculture and Economics, Department of Plant Physiology, ul. Podłuzna 3, 30-239 Kraków, Poland Norwegian University of Life Sciences, Department of Plant Sciences, Box 5003, N-1432 Ås, Norway c Bioforsk - Norwegian Institute for Agricultural and Environmental Research, Særheim, Postvegen 213, 4353 Klepp, Norway d Bioforsk - Norwegian Institute for Agricultural and Environmental Research, Holt, Postboks 2284, 9269 Tromsø, Norway e Bioforsk - Norwegian Institute for Agricultural and Environmental Research, Fureneset, 6967 Hellevik i Fjaler, Norway b

a r t i c l e

i n f o

Article history: Received 24 March 2014 Received in revised form 13 May 2014 Accepted 15 May 2014 Available online 27 May 2014 Keywords: Climate change Forage grasses Freezing tolerance Plant breeding Snow-mould fungi Winter hardiness

a b s t r a c t The increase in surface temperature of the Earth indicates a lower risk of exposure for temperate grassland and crop to extremely low temperatures. However, the risk of low winter survival rate, especially in higher latitudes may not be smaller, due to complex interactions among different environmental factors. For example, the frequency, degree and length of extreme winter warming events, leading to snowmelt during winter increased, affecting the risks of anoxia, ice encasement and freezing of plants not covered with snow. Future climate projections suggest that cold acclimation will occur later in autumn, under shorter photoperiod and lower light intensity, which may affect the energy partitioning between the elongation growth, accumulation of organic reserves and cold acclimation. Rising CO2 levels may also disturb the cold acclimation process. Predicting problems with winter pathogens is also very complex, because climate change may greatly influence the pathogen population and because the plant resistance to these pathogens is increased by cold acclimation. All these factors, often with contradictory effects on winter survival, make plant overwintering viability under future climates an open question. Close cooperation between climatologists, ecologists, plant physiologists, geneticists and plant breeders is strongly required to predict and prevent possible problems. © 2014 Elsevier Ireland Ltd. All rights reserved.

Contents 1. 2. 3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simulation models for understanding climate change impact on the vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cold acclimation during future autumns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Temperature, light, photoperiod and growth cessation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Soil water excess and cold acclimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The effects of elevated CO2 on cold acclimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The impact of future winter climate on plant survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Deacclimation and reacclimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Ice encasement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Resistance to low temperature fungal plant pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Developing crops for the future winter climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abbreviations: IPCC, The Intergovernmental Panel on Climate Change; RCPs, Representative Concentration Pathways. ∗ Corresponding author. Tel.: +48 124253301; fax: +48 124253320. E-mail address: [email protected] (M. Rapacz). http://dx.doi.org/10.1016/j.plantsci.2014.05.009 0168-9452/© 2014 Elsevier Ireland Ltd. All rights reserved.

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M. Rapacz et al. / Plant Science 225 (2014) 34–44

1. Introduction The average temperature of the Earth surface has increased by 0.85 ◦ C since 1880, largely as a result of human activity, mainly the production of greenhouse gasses [1]. It is predicted that even if greenhouse gas emissions could be fixed at the current level, the temperature would continue to rise before it eventually starts to fall again. The Intergovernmental Panel on Climate Change (IPCC) mentions four Representative Concentration Pathways (RCPs) in its fifth assessment report. They describe four possible climate future scenarios. RCP2.6, RCP4.5, RCP6.0, and RCP8.5, are named after a possible range of radiative forcing values (+2.6, +4.5, +6.0, and +8.5 W m−2 , respectively) in the year 2100. RCP2.6 represents a very optimistic pathway with stringent climate policies to limit emissions, whereas RCP6.0 and RCP8.5 represent more pessimistic pathways with limited or not climate policies. According to these RCPs, the average global surface temperature is expected to rise by 1.0 ◦ C, 1.8 ◦ C, 2.2 ◦ C and 3.7 ◦ C, respectively between 1986–2005 and 2081–2100. The models were run with an assumption of CO2 concentrations reaching 421 ppm, 538 ppm, 670 ppm, and 936 ppm, respectively, by the year 2100 [1]. Greater temperature increases are predicted at higher latitudes and in winter, rather than in summer [1]. Moreover, the daily and seasonal minimum temperature is predicted to increase more than the maximum temperatures [1]. All in all, this would suggest a decreased risk of exposure to extreme low temperatures for temperate grassland and crop in the future. However, the frequency, degree, and length of extreme winter warming events, leading to partial or complete snowmelt during winter, increased during the last 50-years [2,3]. If this situation persists, plants may be at a greater risk of frost-induced damage due to lack of snow cover, and of anoxia damage due to accumulation of ground ice. Snow cover is a good insulation, and a 10–20 cm layer is often enough to smooth outmost of the fluctuations in air temperature and maintain the temperature around the plant is close to the freezing [4], except for areas with more extreme low temperatures, where a 30–40 cm layer of snow is needed [5]. As global warming will decrease both the duration of snow cover and snow depth [1], resulting in plants being exposed to freezing temperatures in the future due to decreased snow cover. This can only be assessed locally through a comparison of air temperature fluctuations and snow cover data in the climate change projections [4]. Historical observations indicate that the land area covered by snow in spring in the Northern hemisphere has decreased by 7% since 1922. Projections of spring snow covered land area for the Northern hemisphere by the end of the 21st century predict a further decrease between 7% (RCP2.6) and 25% (RCP8.5) [1]. Regional projections for 2100, compared with current conditions in Norway, indicate quicker snowmelt by up to 2–3 months for low-land regions and up to 1 month for high-altitude regions [6]. For eastern Canada, a decrease in the duration of snow cover by 1.5 month is expected within the next 50 years [7]. For the Swiss Alps, it has been predicted that the snow line would move 150 m up for each increase of 1 ◦ C. The increased winter temperatures will be generally accompanied by increased precipitation in autumn and winter at higher latitudes [1]. This will change the risk of water-logging, snow and ice accumulation on the fields. Moreover, the amount of light that is already low due to the sun inclination in this period, will be further reduced because of increased cloud cover. For example, the greater winter precipitation will result in an increase in seasonal maximum snow depth at high-altitude locations in Norway, but not in low-land locations, where much of the precipitation will fall as rain [6]. On a larger scale, increases in maximum monthly snow depths are expected for Siberia and northern Canada, while decreases are expected for most other regions of the Northern hemisphere [1]. Extreme precipitation events are likely to become more intense

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and more frequent by the end of the 21st century for mid-latitude land areas, and likely for most land areas at other latitudes [1]. While it is well established that the average temperature has increased over the last century and will continue to rise even in the most optimistic RCP, there is still large uncertainty concerning the temperature variability within a given year and between years [1]. With the perspective of winter survival, the day-to-day variation is of special interest. A common misconception among biologists and agronomists, concerned with plant winter survival in the future, seems to be the assumption that the global warming will lead to an increased frequency of freezing and thawing cycles. Temporary increases of such cycles may occur at locations going through a transition from winters with stable sub-zero temperatures to milder conditions [8], such as for many locations in continental Canada [9]. Still, the general trend seems to be a decrease in the frequency of freezing-thawing cycles, as shown for example in the regional down-scaling studies for Norway [6] and in statistical modeling studies for Germany [10]. Several studies have also indicated a trend of reduced day-to-day temperature variability in the winter season during the last 50–100 years [11,12]. Furthermore, a brief analysis of daily temperature extremes described in the Fifth Coupled Model Intercomparison project also suggests that there will be a decrease in the intra-seasonal variability of European winter temperatures [13]. However, it should be noted that the occurrence of cold extremes is dependent on a number of factors that the current climate models cannot predict with great confidence, such as blocking events and atmospheric flow of cold air to warmer regions [8]. Given the large uncertainty concerning future winter temperature variability, biologists should neither overemphasize nor exclude the possibility of increased variability.

2. Simulation models for understanding climate change impact on the vegetation Simulation models are necessary tools for understanding climate changes and their impact on the vegetation, including the performance of agricultural crops. Several process-based models are available that simulate the yield-building processes in grasslands. Each model has a unique representation of plant processes in the form of mathematical functions. Examples of the models available for temperate grasslands include LINGRA, CATIMO, PaSIM or STICS. Several of these models have been used to simulate the impact of climate change on the yields of grassland in temperate regions. Most simulation studies indicate increased annual grass yields at high latitudes, where temperature is the major growth limiting factor [14–16], and decreased grass yields in many regions at lower latitudes, where growth is predicted to become increasingly limited by water availability [17]. All these studies are limited due to being carried out with models that do not account for the possible effect of climate change on the winter survival of grasses. No model yet available simulates the effect of the major winter stress factors on the winter survival of grasslands. There are, however, models available for other crops [18,19], including perennial grasses [20], that simulate freezing tolerance as a function of air temperature around a plant and plant development stage which can be used for the assessment of frost injury risk under different climate conditions. The impact of climate change on cold hardening in the autumn and of risk of frost and ice related damage during winter was assessed using the freezing tolerance model in timothy and perennial ryegrass [20]. This was later followed by a similar study for a larger region in the Northern Europe [15]. Simulations of the future conditions (2040–2065) were compared with the baseline period of 1960–1990. The simulations indicated that the risk of frost and ice related injury would remain low at the most locations in the

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future for the winter hardy timothy species. A geo-climatic gradient was revealed for less winter hardy perennial ryegrass, with decreased risk of winter frost injury in some coastal locations in the west, and increased risk of winter frost injury in a larger number of locations in both east and west, based on the risk of plant exposure to the temperatures below their actual freezing tolerance level. In addition, an increased risk of spring frost injury was predicted for some locations in the west where warm temperatures led to early growth start with loss of frost tolerance after which the temperature dropped to levels that the growing plants had difficulties to handle. The effect of such events on the winter survival and yield, i.e. linking the frost injury to yield is yet to be investigated. There is also a need to simulate frost and ice related winter injury for different RCPs and time periods. It is necessary to develop models that consider the effects of winter stress factors on winter survival other than frost and ice, such as water logging and fungal diseases. The effects of climate change on the cold acclimation process that affect winter survival should also be included in future models. The major challenge of the simulation-based studies of crop performance under future climate conditions is the scarcity of models that simulate the effects of pathogens on yield. 3. Cold acclimation during future autumns The development of freezing tolerance in response to low non-freezing temperatures is called cold acclimation. In order to understand cold acclimation in the future climate it is necessary to take a closer look at the interactions between temperature and other factors concerning cold acclimation. Herbaceous plants must be able to tolerate many stresses to survive winter: freezing temperatures, limited production of photoassimilates, snow cover, snow mould fungi, lack of water, hypoxia or anoxia caused by water-saturated soil or ice cover (ice-encasement), as well as soil movement due to freeze-thaw cycles. However, plants also become more tolerant to other winter stresses during the cold acclimation, and as a result positive correlations were found after cold acclimation between freezing and ice encasement tolerance [21], as well as between freezing tolerance and snow mould resistance [22]. 3.1. Temperature, light, photoperiod and growth cessation Cold acclimation in herbaceous plants is initiated at around 10 ◦ C and it accelerates as the temperature falls to 5–0 ◦ C. Thus, the increase in the temperature predicted for autumn, will directly reduce the effectiveness of cold acclimation (Fig. 1A). The effect of temperature is modified by other factors, such as photoperiod, irradiance, PSII redox state, soil water potential and nutrient status [23]. Thus, the rate of cold acclimation and the maximum cold tolerance achieved vary from season to season as a result of a large number of interacting factors. More research studies aimed at understanding these interactions are needed to predict the effects of climate change on cold acclimation. For example, we can assume that cold acclimation under shorter day length may increase freezing tolerance, but only as long as the intercepted light intensity does not fall under a critical level. Cold acclimation is also highly dependent on light [24], and the chloroplasts may be central elements here not only as photosynthesis site but also as the sensors of the environmental signals and as the site of the acclimation processes [25]. Photostatis the energy balance of the chloroplasts seems to be very important in the early autumn (pre-acclimation) not only for the photosynthetic acclimation to cold, but also for the growth cessation [26]. Growth cessation is a prerequisite for proper cold acclimation as confirmed and described at the physiological [27] and molecular [28] levels. The temperature rise in

Fig. 1. Variable effects of climate change on freezing tolerance and overwintering. (A) Temperature increase during autumn makes cold-acclimation less efficient. (B) Increased clouding combined with higher temperatures alter photostasis and may decrease the efficiency of prehardening/cold acclimation. (C) Decrease in light amount and shorter day at cold-acclimating temperature, which occur later in the autumn may decrease photosynthetic rate in plants requiring active photosynthesis for cold acclimation. Temperature vs. photoperiod interactions may also be important. (D) Increasing water content in soil may either decrease or increase freezing tolerance. (E) Atmospheric CO2 increase may either increase or decrease freezing tolerance. (F) The temperature rise during winter will decrease freezing damages but dehardening under non-stable temperatures and shallower snow cover will contradict this effect (G). (H) Apart from freezing tolerance different winter stresses may change their effects on winter survival. Increasing problems with flooding and ice-encasement may be predicted after snow melt during warm spell depending on local conditions.

late summer and early autumn predicted for the next decades may decrease cold acclimation ability, growth cessation, as well as photosynthetic acclimation to cold [29]. In high-latitude regions these processes are exacerbated by decreased PSII reduction, when low temperatures will occur later: at lower light radiation and shorter photoperiod. It can be expected that under such conditions combined with increasing temperatures the effectiveness of cold acclimation will decrease (Fig. 1B and C). However, the decrease in cold acclimation efficiency may vary between species. Pre-acclimation temperatures did not affect the photosynthetic acclimation in red clover per se, but did so in perennial ryegrass and timothy [30]. The north-adapted accessions of these grasses were generally more freezing tolerant than south-adapted ones. Cold acclimation efficiency was more reduced at warmer temperatures in the northern entries of timothy and ryegrass compared to the southern ones. Southern ecotypes of Arrhenatherum elatius, whose growth cessation is less sensitive to photoperiod, can increase their biomass for a longer time during autumn. This, in combination with sufficient freezing tolerance during warmer winters, results in higher biomass accumulation after winter, when compared to northern ecotypes [31]. Still, there is no surety that winters will become milder and provide optimal survival conditions. If not, the northern species, in which growth cessation depends more on the shorter photoperiod than PSII overreduction, as observed recently for perennial ryegrass and Festulolium [32], are more winter hardy even in more southern locations. Cold acclimation depends on the temperature and duration [33], and thus the short time span for hardening in late autumn, accompanied by low light conditions and approaching frost, may also contribute to suboptimal cold acclimation. More research is necessary to establish minimum light requirements for well-functioning cold acclimation in different species and to investigate the variation among genotypes with respect to light requirements.


Growing seasons are expected to become longer, as a result of climate change, particularly in the north. Acclimation will be shifted to a time of the year with shorter photoperiods and lower total irradiance (Fig. 1C). Although air temperature is more important than a photoperiod for triggering cold acclimation in herbaceous plants, latitudinal differences in response to photoperiod occur in ecotypes, e.g. of the grass A. elatius [31], in line with responses found in trees [34]. This effect is particularly noticeable at very high latitudes, where the most rapid decrease in day length from summer to winter occurs and where the sun is absent in mid-winter. The effects of these temperature-photoperiod interactions may depend on the plant species and seems to be most important for conifers. The interactive effects of photoperiod and elevated temperatures in the autumn was suggested as an important factor for predict the effect of climate change on the productivity of boreal forests [35]. In that case a detrimental role of photostasis (photoacclimation mechanism in chloroplast which enable to maintain a constant balance between light energy absorption and utilization – PSII redox state) was emphasized. 3.2. Soil water excess and cold acclimation The increased precipitation expected during autumn and winter at higher latitudes [1] may disturb cold acclimation under certain conditions. Temperature is an important factor controlling the effect of water surplus or anoxic conditions on cold acclimation and winter survival [36]. When plants are grown at high temperature, water is necessary for respiration, and low water potential under such conditions is stressful to the plants and results in reduced reserves and freezing tolerance [36]. Winter hardy species such as timothy (Phleum pratense) and winter hardy cultivars of red clover (Trifolium pratense) are less sensitive to water excess in autumn than the less winter hardy species such as perennial ryegrass (Lolium perenne) [21,36]. Oxygen deficiency also affects carbohydrate reserves. Winter hardy species with low respiration rate, such as timothy, may endure anoxic conditions better than the species with higher respiration rate [37]. This is facilitated by their greater ability to economize on reserves, together with minimized accumulation of toxic products of anaerobic respiration. The effect of low temperature flooding on freezing tolerance is not well understood, and some contradictory reports have appeared (Fig. 1D). Decreased freezing tolerance observed during flooding at low temperature is linked to increased hydration of tissues, as demonstrated by a strong correlation between cold hardiness and crown water content [38,39]. The shift from aerobic to anaerobic processes during flooding is accompanied by the accumulation of ethanol, CO2 , and to a lesser extent, lactic acid. However, the direct relationship between accumulation of these compounds under low temperature flooding and freezin tolerance has not been established. Flooding at low temperatures is not as damaging as flooding at higher temperatures, because higher temperature limits oxygen solubility in water and enhances enzyme activity [40]. Thus, although the reduced freezing tolerance during flooding has been confirmed [41], a positive effect on the cold acclimation capacity was also observed [42]. This effect was transient or stable, depending on the Festuca pratensis genotype tested. A stable increase in freezing tolerance was related to higher carbohydrate concentration observed under flooding [42] is consistent with other results [37], where higher carbohydrate levels under oxygen deficiency favored winter survival and spring regrowth in timothy. The transient increase in freezing tolerance was linked to the upregulation of genes encoding transcription factors involved in cold acclimation [42]. The low temperature flooding in that experiment was preceded by a two-week flood at moderately high (+15 ◦ C) temperatures. It is possible that more unfavorable conditions during flooding at a higher temperature would induce an

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acclimation response that increases the efficiency of subsequent cold acclimation. The effect of water excess during winter will probably have major consequences on plant freezing tolerance. However, the nature of the changes seems to be more complex than initially hypothesized. Comprehensive metabolic and genetic studies, investigating both short and long term flooding, should be performed. This would answer the questions about global consequences of soil water excess for winter survival. In addition to this, the effect of high-temperature flooding pre-treatment should be ascertained. 3.3. The effects of elevated CO2 on cold acclimation Various reports claim that freezing tolerance may decrease, increase or remain unaffected under elevated atmospheric CO2 (Fig. 1E). The decrease was reported for native temperate grasslands [43], in Eucalyptus pauciflora [44] and F. pratensis leaves [45], whereas contrasting reports were published for Pinus sylvestris [46] and Picea mariana [47]. The lack of influence of CO2 atmospheric concentration on the freezing tolerance of Triticum aestivum was described under elevated and ambient temperature conditions in open-top chamber experiments [48]. Some reports indicate that under twice elevated CO2 concentration the signal for acclimation may be delayed or altered, at least in trees [49]. This may be due to higher daytime leaf temperatures, resulting from reduced transpiration and evaporative cooling under elevated CO2 [50]. Furthermore, there are at least two other mechanisms that possibly explain a decrease in freezing tolerance under high CO2 concentration. The ice nucleation temperature was higher under elevated CO2 , resulting in greater incidence of frost damage in snow gum (E. pauciflora) [51]. A decrease in freezing tolerance under CO2 enrichment to 0.08% in F. pratensis was linked to downregulation of transcription factor genes regulating a set of COR genes essential for CA [45]. These two mechanisms may all be elements of a common reaction. The chloroplast redox signal which triggers the cold acclimation, may be disturbed either by temperature increase or by increased oxidation of PSII, coming from a higher CO2 assimilation rate that decreased PSII reduction [24]. A weaker signal disturbs the expression of transcription factor genes essential for cold acclimation and in turn increases the temperature at which ice nucleation occurs in plant tissues. Conversely an increase in freezing tolerance under elevated CO2 , observed in P. mariana, was related to the changes in bud phenology [47]. The greater accumulation of carbohydrates under elevated CO2 [52] may also contribute to the improved freezing tolerance. The stability of the effect of changing the acclimation state by high CO2 level can be questioned. The effect of enhanced cold acclimation at elevated temperatures and CO2 was observed only in the first year of the experiment with Scots pine (P. sylvestris) plants [46]. The use of different plant species, ages and tissues, different methodology and duration of CO2 exposure, have led to contrasting results concerning freezing tolerance under elevated CO2 . Speciesdependent differences in the cold acclimation under elevated CO2 have been observed. Possibly this may affect the future change in species mix in natural ecosystems or a change in what crops will be preferred in future agroecosystems. Such differences are probably due to specific mechanisms involved in developing the freezing tolerance in different species. Further experiments should be performed, based on unified methodologies, to examine the effect of elevated CO2 on the cold acclimation and to investigate the direction of the reaction and the stability of this effect. Nevertheless, there is some uncertainty on how the reduced incidence and severity of frost, due to climate warming combined with changed vulnerability to frost-induced damage under

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elevated CO2 , may affect plants in frost prone areas. The overall effect is hard to predict. 4. The impact of future winter climate on plant survival Although a general increase in temperature reduces the risk of plant exposure to extreme low temperatures in the future (Fig. 1F), many other factors may have negative effects on overwintering. 4.1. Deacclimation and reacclimation Plants cold-deacclimate (lose freezing tolerance) gradually towards the end of winter and in the transition from winter to spring when they resume growth and development, which is better understood in woody than herbaceous species [53,54]. The maximum freezing tolerance is often found to occur in mid-winter [55]. A gradual deacclimation from then on appears regardless of temperature and may be related to depletion of organic reserves and a general weakening of plants. In addition, plants also deacclimate upon higher temperatures in spring or due to mild spells in mid-winter. This deacclimation may be reversible or irreversible, depending on the physiological status of the plant and the exposure temperature [29]. Deacclimation can cause problems when there is a rapid drop to freezing temperatures in spring or after a mild weather spell [56]. The expected higher winter temperatures potentially result in more winter kill through deacclimation (Fig. 1G). Therefore, the resistance to deacclimation and/or ability to quickly reacclimate upon low temperatures is expected to be critical for winter survival. In order to make predictions and adapt to future conditions, more information is needed on: on (1) the kinetics of deacclimation and reacclimation in response to temperature (thresholds and rates); (2) the effects of a plant physiological condition, determined by e.g. cold acclimation conditions during the previous autumn, as well as stress exposure, respiratory rates and exhaustion of organic reserves during winter; (3) interactions between temperature, photoperiod and PSII redox state; and (4) genetic variations among and within species. Conversely, the effects of a warm spell underreal field conditions on plant survival are uncertain. It is still possible that frost episodes following mild weather spell will also be warmer, which might actually lead to less rather than more plant injuries. Mild weather episodes during winter lead to snow melt, anoxia, and ice formation, which in turn may increase the risk of frost exposure and ice encasement stress (Fig. 1H) as has been observed under both natural and experimental winter warming conditions [57]. We should assumed that the final effect of warmer winters on freezing tolerance is difficult to predict without further studies and model development. Deacclimation is mainly temperature-driven. In general, it is a function of both the temperature and the duration of exposure. While it takes weeks to build up maximum freezing tolerance, it can take only days to lose it [33,58–60]. The effect of deacclimating temperatures on freezing tolerance in ryegrass and timothy depends on the initial level of freezing tolerance. Thus, the varieties or species with high initial freezing tolerance deacclimated rapidly, but did not lose the entire tolerance, due to their higher initial level, contrary to the species and varieties with lower initial freezing tolerance [60]. Deacclimation may be also controlled by PSII redox state [29,61]. In oilseed it is faster and becomes irreversible when warmer temperatures operate at lower light intensities, which seems to fit perfectly the scenarios of winter climate change. However, more data are needed about this phenomenon in grasses and other perennial crops. Deacclimation may occur as a simple consequence of other physiological processes, such as depletion of organic reserves,

growth and production of sensitive tissues. Such mechanisms of deacclimation are associated with the organic resource economy of the plant [54]. Winter survival in grasses and legumes is associated with accumulation and usage of organic reserves [62]. Storage materials are accumulated during cold acclimation and are gradually depleted during winter. When cold acclimation will be shifted to the late autumn with less or almost no light at higher latitudes, the limited availability of storage materials accumulation will be a problem during winter. Plants with limited reserves towards the end of the winter may not have the sufficient amount of energy to maintain freezing tolerance. Moreover, at higher winter temperatures this type of deacclimation will occur faster due to higher rates of respiration. Warmer temperatures can also stimulate growth, particularly towards spring when days are longer. Lower freezing tolerance of actively growing plants, at the tissue or whole plant level, is attributed to higher water content, little organic storage, as well as less investment in freezing tolerance mechanisms or all together. Resuming growth while maintaining freezing tolerance involves a compromise between spending and keeping the organic reserves. Winter rye (Secale cereale) seedlings can grow and cold acclimate at the same time [63]. In contrast this situation is likely to be different in plants such as wheat that survive the winter and are vernalized (see below). Deacclimation is partly a reversal of the cold acclimation process. Most of A. thaliana genes up-regulated during cold acclimation were down-regulated during deacclimation, and vice-versa [64]. However, there are some differences: deacclimation is a faster process and the increased resistance to pathogen infection observed in cold acclimated plants is not lost as rapidly upon warm temperatures as freezing tolerance is [58,59]. The process of reacclimation is not exactly the same as the primary cold acclimation. Due to limited light and damaged leaf tissue, light-dependent processes of accumulation of photosynthates and ROS (reactive oxygen species) signaling may be less important in reacclimation than during cold acclimation. Also, winter both alters the physiological condition of the plant (due to stress and exhaustion of organic reserves) and developmentally reprograms it through vernalization. These effects of winter, in addition to growth and other developmental processes initiated by warmer temperatures and longer days, may affect the ability of a plant to reacclimate. Plants with initiated generative development lost their ability to cold acclimate [65,66]. The relationships between vernalization, photoperiod and freezing tolerance have been particularly studied in cereals. In these species, the expression of genes involved in cold acclimation are down-regulated when vernalization is saturated, but before any generative development of the apex is visible under the microscope [67,68]. An interaction between vernalization and photoperiod is observed during deacclimation: in the cultivars requiring a long day for flowering the effect of vernalization on freezing tolerance is stronger when the plants are vernalized under long days than under short days, whereas vernalization- and photoperiodinsensitive cultivars are not able to develop much FT at all [69–71]. After deacclimation at 20 ◦ C, wheat plants grown under a 20 h photoperiod were much less able to reacclimate than plants grown under an 8 h photoperiod [72]. Hence, vernalization is associated both with a tendency to deacclimate and a lower ability to reacclimate, but while its effect on deacclimation is not due to actual development of reproductive tissues, the effect on reacclimation ability might be. It was suggested that in the regions with long, mild winters, the mechanisms extending the vegetative phase (vernalization and/or photoperiod requirements) may actually be more important for winter survival than the maximum attainable freezing tolerance [69,70]. The same argument applies to the climate change. Recent evidence indicates that vernalization and transition to reproductive growth stage may be not as tightly linked with freezing tolerance in perennial grasses as in cereals [73,74]. This


calls for further research comparing these two groups of plants with respect to control mechanisms for deacclimation resistance and reacclimation ability. There is a genetic variation among and within species in the dynamics of cold acclimation, deacclimation and reacclimation (i.e. concerning the rate of change in freezing tolerance, maximum tolerance achieved, threshold temperatures, etc.), as well as genotype × environment interactions. These different traits are often not correlated, and therefore they appear to be under different genetic and environmental control. This holds promise for the development of cultivars that are fine-tuned to local conditions in a climate change scenario [53]. 4.2. Ice encasement Ice encasement is a major winter stress factor for high-latitude vegetation [75]. Ice crusts develop that encapsulate the plant and limit the gas exchange with the atmosphere. Little is known about the risk of ice encasement under future climate conditions, as this is largely a local (field scale) phenomenon, while most climate projections have a large scale (global to regional) resolution. Ice damage to cultivated crops has almost disappeared in many places in Iceland during the last decade where it used to dominate before or has moved to new areas [75]. Results from simulations with a grassland model concerning the risk of ice damage in Norway and elsewhere in Northern Europe are presented in Section 2 above showing that the future risk will change in both direction depends on species and area. 4.3. Resistance to low temperature fungal plant pathogens Perennial and winter annual plants in the subarctic regions are exposed to attack from low temperature pathogens, mainly so-called snow moulds. Snow moulds are fungal plant pathogens that have the ability to grow at low temperatures; they are either psychrophilic (capable of growth and reproduction in temperatures ranging from −20 ◦ C to +10 ◦ C) or psychrotrophic (capable of growth at temperatures at or less than 7 ◦ C) organisms. The longer the duration of snow cover, the more severe the damage by snow moulds [76]. The most opportunistic snowmould, Microdochium nivale, however, has a temperature span from −6 to +28 ◦ C, with optimum around 20 ◦ C, and can attack plants both with and without the snow cover [77]. During the mesophilic stage M. nivale can inflict diseases (symptoms) like root and foot rot and head blight on host plants. Climate change would result in a significant decrease in the prevalence and severity of snow moulds [78,79]. This seems plausible, because reduced snow cover duration, brought upon by a warmer climate in the Northern Hemisphere, will result in shorter incubation period for the fungi. Under this scenario, snow moulds may become almost extinct in some regions due to the lack of permanent snow cover. However, other scenarios predict episodic extremes of weather, including years of heavy snowfall and colder winters [80,81]. Since some snow mould fungi can live saprophytically in the soil for years, and the inoculum of sclerotia-forming species can survive long periods in the soil in the absence of conducive environmental conditions [82], the inoculum will be available to attack crops despite a long absence of snow. In this scenario, less frequent but more severe overall attacks by snow moulds will likely occur as a result of climate change. Moreover, warmer temperatures, more precipitation and hence less irradiation during autumn may counteract the natural cold acclimation of plants that is essential for the development of maximum resistance to snow moulds in grasses and cereals [83]. Climate change is expected to affect various snow mould species differently, since different strategies in the adaptation to the cold

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environments were observed in different taxa [84]. The distribution patterns of snow mould species have changed recently in Hokkaido, Japan [85]. In some regions, T. ishikariensis seems to have replaced S. borealis as the main snow mould species, indicating that prevailing environmental conditions have changed for sufficiently long periods that permit this shift in the species distribution. Since M. nivale also has a mesophilic stage, this may permit accumulation of sufficient inoculum in the years that are not conducive to snow mould development, and replace other strictly psycrophilic fungi. ˛ Moreover, there are reports from Poland (Maria Wedzony, Krakow, personal communication, 2010) and the Netherlands (Gerard van’t Klooster, Nijmegen, personal communication, 2007) indicating that this species actually seems to have become more abundant during in the regions with much milder winters than Scandinavia. Its ability to grow at the temperatures from sub-zero to 28 ◦ C may be a contributing factor in the apparent increase in the area of adaptation of this fungus and its spreading to “warmer” areas where it had not been previously observed, including golf courses in Hawaii and California with abnormally high rainfall and cooler growing season temperatures (F.P. Wong, University of California, unpublished, 2006). Most fungal species have a great capacity for environmental adaptation. The effects of a projected increase in average temperatures in the Northern Hemisphere on the pathogens such as Microdochium species will likely be translated into the production of additional generations of asexual spores (conidia) during spring and late summer/autumn. Increased inoculum production facilitates the evolution of environmental adaptation and the overcoming of host resistance [86]. The sexual stage of M. nivale var. nivale is not commonly found. Environmental stress is likely to promote sexual reproduction in this fungi [87]. Microorganisms that can reproduce either sexually or asexually seem to preferentially express the sexual stage when exposed to environmental stresses [88]. Genotypes of Aspergillus nidulans reverted to sexual reproduction in stressful conditions in which their fitness was the lowest [89]. Variability within species of microorganisms has been attributed to the presence of transposable elements, specifically retrotransposons that are stimulated by environmental stresses [90]. These studies suggest that fungi react to the environmental stresses by boosting their capacity for sexual reproduction. High reproductive capacity, exhibited by most plant pathogens, leads to a large number of mutations that can accumulate in the population. Sexual reproduction under stress may lead to a recombination of advantageous mutations (i.e. new virulence combinations, higher aggressiveness, increased resistance to fungicides) that can lead to new and fitter genotypes. Climatic change, by augmenting the stress levels encountered by plant pathogens, may ultimately expand the opportunities for spreading and exploitation of new habitats for some pathogens, including M. nivale. In such a scenario, the likeliness for snow mould species of becoming extinct does not seem highly probable. Highly opportunistic microorganisms will rather predominate, exerting a negative impact on crop species.

5. Developing crops for the future winter climate New plant breeding activities should be established to secure active germplasm for many species in order to have available genetic resources to use in any possible future climate scenario [91]. In the light of uncertain future climates it is important to maintain genetic variation. The more phenotypic and genotypic knowledge we have about our germplasm, the easier it will be to “breed upon demand”. Development of genomic selection as a breeding tool for outbreeding forage crops could facilitate efficient cultivar development in the future [92]. It must be emphasized that increasing genetic variation without losing short-term productivity should be

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the main target for breeding, because it enables long-term adaptation in semi-permanent systems. Due to the very complex problem of future overwintering described earlier it is hard to predict the future winter-hardy plant ideotype. However, some possible directions can be indicated. Timothy (P. pratense) will remain the main grass species at higher latitudes in the foreseeable future due to its persistency [93]. Increasing the regrowth capacity of timothy will be a major breeding goal to meet the requirements of a prolonged growing season, which may enable an additional cutting during the summer/autumn. With a prolonged growing season it might be desirable to make better use of the growth conditions in spring/early summer for the elongation growth and production, however too early start of the vegetation may increased the risk of freezing damage in the late winter. Both meadow fescue (F. pratensis) and smooth meadow-grass (Poa pratensis) are well-adapted grass species and they are frequently sown in mixtures together with timothy. Meadow fescue is, however, prone to fungal attack during the regrowth phase, and the deeper rooted tall fescue (Festuca arundinacea) might become a more important species at both lower and high latitudes, also due to possible drought incidences in spring/early summer [1]. As a parent species in ×Festulolium cultivars tall fescue has proved to be a winter hardy alternative at higher latitudes [93]. Introduction of grass species, such as perennial ryegrass (L. perenne) and ×Festulolium [94], to higher latitudes may gain increased interest due to their high yielding capacity and nutritional quality [95]. ×Festulolium has often been described as hybrids that might be particularly designed for specific tasks, due to the vast genetic variation within the Lolium-Festuca complex [94,96]. In the far north, grassland species will need to be particularly bred for the unique combination of warmer temperatures and photoperiod during autumn (see Section 3.1). In addition, the problems caused by deacclimation may increase in areas where the mean temperature during mid-winter approaches 0 ◦ C and the elongation growth may start when warm spells occur. Breeding of more snow-mould tolerant plants also needs special attention. Screening for resistance to snow mould fungi after artificial inoculation has revealed significant genetic variation in the response to M. nivale in both forage grasses and winter rye [97–99]. Since the broad sense heritability of resistance to M. nivale is sufficiently high, (0.79 in winter rye [97] and 0.49 in cocksfoot (Dactylis glomerata) [99], respectively), a selection based improvement of resistance should be feasible. Current cultivars of perennial grasses and winter cereals develop maximal snow mould resistance only after the CA. The ability to cold acclimate, however, varies among genotypes, as well as the ability to develop “cold-induced” snow mould resistance. Snow mould resistance in a detached leaf test was unaffected or repressed by cold hardening, and thus the existence of different types of snow mould resistance was proposed [100]. Some resistance mechanisms are present in wheat plants prior to the cold acclimation [101]. Non-acclimated older plants of wheat [102] and perennial ryegrass [103] are more resistant to snow mould than the young ones. These findings clearly indicate the possibility of identifying and enhancing snow mould resistance that does not rely on cold acclimation in the autumn. Isolates/populations of M.nivale vary in their preference for different host species of grasses and cereals [104]. This indicates that the species M. nivale may be grouped into several different varieties, and resistance to one variety does not necessary confer resistance to another. But due to the heterogeneity of this fungal species, to obtain cold hardening independent resistance in different species of grasses and cereals, it may be necessary to identify fungal isolates with preference for each grass species for screening purpose. However, the genes conferring the ability to develop “cold-induced” snow mould resistance should not be discarded, since this may still be the most crucial component of snow mould resistance.

When considering developing new crops for the future winter climate we should also discuss genes whose action may be especially important for overwintering under changing climates and the use of genetic engineering for developing new, better adapted cultivars. This is important because anthropogenic climate change seems to be faster than natural selection. During autumn and winter diverse environmental stresses induce changes in the expression of a huge number of genes with concomitant whole plant physiological changes. Is it likely that manipulating a single or a few genes will be successful in improving winter survival? Can we predict some changes in the signaling crosstalk between different environmental factors controlling winter hardiness which will change in the future? We should remember that before we start searching genes involved in abiotic stress tolerance, we should: (1) create reliable models of the future environment conditions, (2) simulate these conditions during precise phenotyping, (3) find contrasting genotypes. At present the plant scientists are at the end of the first stage, but some elements of the complex response may be discussed. Freezing tolerance is only one component of winter survival but it is the component that best correlates with winter survival in the field [105]. A range of transcription factors and regulators involved in low temperature response/cold acclimation are obvious targets for the genetic manipulations. The ICE1-CBF/DREB1 (C-repeat binding factor/dehydration responsive element binding) signaling pathway is induced by low temperature and activates a large number of COR/LEA/DHN (cold regulated/late embryogenesis abundant/dehydrin) protein coding genes, the expression of which results in cold acclimation and improved freezing tolerance. Several signaling components of this pathway, which is best described in Arabidopsis thaliana, but seems to be highly conserved, have been described [106]. ICE1, constitutively expressed in all tissues, is localized in the nucleus, and becomes functional after LT induced post-transcriptional sumoylation and phosphorylation by binding to CBF3 promoter. ICE1, a protein well known to stress physiologists as a regulator of CBF expression connected with temperature sensing is also a key regulator in stomata biogenesis [107]. In a recent study, H1.3 histone expressed in stomata cells of A. thaliana leaves was not only showed to be involved in stomata biogenesis but also when induced by a combination of low light and drought, as a remodeling factor in the expression of hundreds of genes, including those connected with cold acclimation (Rutowicz et al., 2014, unpublished). This may suggest that combinations of different stress factors may alter the expression of genes in a very different way from that when only a single stressor is present. Thus, it seems that links between different systems are poorly understood, closely related and may be imbalanced by just a small change in the environment. Three Arabidopsis CBF/DREB1 transcription factors are characterized by complex interactions, i.e. CBF2 down-regulates CBF1 and CBF3, while CBF3 may negatively regulate CBF2 [106]. Other negative regulators of CBF3 expression are MYB15, HOS1, and ZAT12/ZAT10. MYB15 binds to the promoter regions of CBF3/DREB1 and represses the expression, while HOS1 negatively regulates ICE1 by ubiquitination and proteasome degradation. ZAT12/ZAT10 are cold-induced C2H2 zinc finger transcription factor genes. LOS2, a bifunctional enolase, is possibly a negative regulator of ZAT10 during cold acclimation. Repression of MYB15, which leads to the expression of CBF3/DREB1, can happen through SIZ1-dependent sumoylation of ICE1 that activates and/or stabilizes ICE1 protein [108]. In cereals and grasses the CBF3/4 subfamilies have undergone extensive duplications, and an involvement of tandem repeats of CBF genes in the freezing tolerance has been demonstrated [109–111]. This system is regulated by low temperature; however a link to other triggering factors has also been reported. For example PSII over-reduction may regulate CBF expression [24], and thus we can expect some changes in their function when cold


acclimation happens at low light intensity during late autumn. CBFs are reported as integrators of PSII over-reduction, phytochrome and hormone signaling during cold acclimation [112]. Cold-acclimation under a shorter photoperiod, as well as warm spells during winter, will affect the expression of the photoperiod sensing genes and vernalization related genes. It is possible that both photoperiodic response and vernalization control may be connected with CBFs, which has also been shown to affect the dormancy, leaf senescence [113], leaf anatomy, plant architecture, and photosynthetic activity [114]. LOS2, mentioned before as a protein involved in CBFs regulation was reported to be upregulated under low-temperature flooding [42], which may indicate its wider role in adaptation to the changing winter climate. Several studies in Arabidopsis and other dicots have demonstrated that freezing tolerance of non-acclimated plants can be increased by constitutive expression of CBFs by a genetic transformation [115]. However, pleiotropic effects like reduced growth, later flowering and reduced seed production have been observed in most cases, and they can be alleviated or reduced by using inducible promoters [116,117]. The comparisons of multiple transgenic A. thaliana lines overexpressing CBFs in cold and control conditions have demonstrated that fitness costs and benefits vary with transgene and environment; overexpression of CBF2 and CBF3 caused reduced growth and seed production in both environments, while there was no or marginal effects of CBF1 overexpression [118]. The frequency of spring frosts might increase in the northern regions due to an earlier onset of spring associated with the climatic changes. A constitutive expression of CBF transcription factors might be a viable strategy for improving the freezing tolerance of non-acclimated spring cereals, as was demonstrated in spring barley expressing the wheat transcription factors TaCBF14 and TaCBF15 [119]. The transgenic lines demonstrated better freezing tolerance (measured as ion leakage), more efficient photosystem II (Fv /Fm ), and upregulation of specific COR and DHN genes. The transgenic lines exhibited slower growth and slightly later flowering time. A very interesting observation was an increased level of GA2ox5 transcript that deactivates specific gibberellins and thus reduces stem elongation, most likely by accumulation of DELLA proteins [28]. The intimate crosstalk between the cold-acclimation, vernalization, circadian clock and photoperiod flowering time signaling pathways is crucial for fine-tuning the seasonal pattern of growth, winter survival and phenological development. Retardation of growth by controlling the transition to generative growth and apex/stem elongation is central for winter survival, as it is clear that freezing tolerance decreases rapidly when the stems start to elongate. Another group of transcription factors that may be more important for overwintering in the future are MYB family transcription factors, which are also induced during anoxia [120]. This family of transcription factors is involved also in cold tolerance and ABA signaling [121]. Excess water in the soil may also change the ethylene signaling [122]. Ethylene has also been shown to induce the expression of pathogenesis-related proteins with antifreeze activity [123]. As mentioned before, maintaining carbohydrate resources may be crucial for the survival under conditions of warmer winters. High degree of polymerization fructans accumulate in grasses during the cold acclimation and high levels of these fructans are linked to the freezing tolerance and winter survival. However, many enzymes are involved in the synthesis and degradation of fructans and thus the targets of genetic manipulation can be difficult to determine. In winter wheat there is a positive correlation between fructan content and snow mould resistance [124] however, a molecular mechanism of this correlation is not known. Constitutive overexpression of two wheat fructosyltransferase genes, wft1

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and wft2, in perennial ryegrass led to a significant increase in fructan content in the transgenic plants and to improved freezing tolerance at the cellular level (electrical conductivity test) [125]. Brachypodium distachyon, a model grass species that does not contain fructosyltransferase genes [126], was transformed with wheat wft1 gene and its homolog, PpFT1 from timothy (P. pratense) encoding sucrose:fructan 6-fructosyltransferase responsible for the increase in fructan polymerization degree [127]. This transformation resulted in retarded growth in the transgenic lines, especially in the lines expressing PpFT1 gene, but they showed better freezing tolerance after the cold acclimation than the lines expressing the wft1 gene, and accumulated higher DP fructans. Based on these studies, it seems that the genetic manipulation of fructan biosynthesis is a promising avenue for improving the freezing tolerance in cereals and grasses. However, since the freezing tests are based on in vitro methods, it remains to be demonstrated that the result will translate into improved winter survival in the field. This is a general problem with nearly all the reported transgenic studies. They are basic studies using a short term cold acclimation, indirect laboratory methods, e.g. electrolyte conductivity, for measuring freezing tolerance, and relatively few transgenic lines. Short and long term exposure to the freezing temperatures impose very different stresses on plants. Long term freezing stress is the most relevant for the predicting of winter survival, and poor correlation was found between LT50 (lethal temperature for 50% plant kill) and LD50 (lethal duration time for 50% plant kill) values among winter canola cultivars [128].

6. Conclusions The fact that future winters are expected to be milder does not mean that the problems with winter survival of winter and perennial crops will disappear, especially at higher latitudes. Moreover, in the future climate a relative importance of different factors affecting winter survival may change, forcing us to breed new cultivars. The effectiveness of future plant breeding will depend on the introduction of new testing methods and understanding the genetic control of complex plant × winter environment interactions. Each possible effect of climatic event on plants overwintering needs to be well proven. Thus, many problems are open for research. For instance, there are no convincing data indicating future detrimental effects of mid-winter warming on freezing tolerance in the field, which was observed for example during winter 2013/14 in Norway and in Poland. A new and increasing problem with ice cover has also been observed by Polish farmers and breeders, but this aspect has remained unexplored, mainly due to the lack of appropriate prediction models. Golf-course greenkeepers in Norway have reported recently that the greens were severely attacked by Microdochium after warm spells that thawed the snow, yet too cold for plants to resume growth. As plant scientists concerned with winter survival in a changing climate, we should ask the climate researchers for more precise predictions on how the climate will change, and investigate the consequences of the expected changes. This could be done by comparing historical records of weather and plant winter survival, by carrying out experiments simulating the expected climate changes, and by modeling plant responses. We should also point out which climate variables are of special concern with respect to winter survival, and ask the climatologists for more information on these variables when we find gaps in the existing climate projections. Data on temperature, radiation and precipitation should be used for climate simulation around the plants (e.g. soil temperature, snow and ice depth), using soil physical/micro meteorological models, and scale-up from field to regional or national level, if more general statements for larger regions are expected.

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Acknowledgments The author’s research is supported by the The Research Council of Norway in the frame of the project VARCLIM: Understanding the genetic and physiological basis for adaptation of Norwegian perennial forage crops to future climates, National Science Centre in Poland, Ministry of Agriculture and Rural Development in Poland as well as public funds for science appropriated to The University of Agriculture in Krakow. References [1] T.F. Stocker, et al. (Eds.), Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom/New York, 2013. [2] C. Johansson, V.A. Pohjola, C. Jonasson, T.V. Callaghan, Multi-decadal changes in snow characteristics in sub-Arctic Sweden, Ambio 40 (2011) 566–574. [3] A. Shabbar, B. Bonsal, An assessment of changes in winter cold and warm spells over Canada, Nat. Hazards 29 (2003) 173–188. [4] S.M. 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