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Evergreen conifers are champions of winter survival, based on their remarkable ability to acclimate to cold and develop cold hardiness. Counterintuitively, autumn cold acclimation is triggered not only by exposure to low temperature, but also by a combination of decreasing temperature, decreasing photoperiod and changes in light quality. These environmental cues control a network of signaling pathways that coordinate cold acclimation and cold hardiness in overwintering conifers, leading to cessation of growth, bud dormancy, freezing tolerance and changes in energy metabolism. Advances in genomic, transcriptomic and metabolomic tools for conifers have improved our understanding of how trees sense and respond to changes in temperature and light during cold acclimation and the development of cold hardiness, but there remain considerable gaps deserving further research in conifers. In the first section of this review, we focus on the physiological mechanisms used by evergreen conifers to adjust metabolism seasonally and to protect overwintering tissues against winter stresses. In the second section, we review how perception of low temperature and photoperiod regulate the induction of cold acclimation. Finally, we explore the evolutionary context of cold acclimation in conifers and evaluate challenges imposed on them by changing climate and discuss emerging areas of research in the field.

Thermal macrophysiology is an established research field that has led to well-described patterns in the global structuring of climate adaptation and risk. However, since it was developed primarily in animals, we lack information on how general these patterns are across organisms. This is alarming if we are to understand how thermal tolerances are distributed globally, improve predictions of climate change, and mitigate effects. We approached this knowledge gap by compiling a geographically and taxonomically extensive database on plant heat and cold tolerances and used this dataset to test for thermal macrophysiological patterns and processes in plants. We found support for several expected patterns: Cold tolerances are more variable and exhibit steeper latitudinal clines and stronger relationships with local environmental temperatures than heat tolerances overall. Next, we disentangled the importance of local environments and evolutionary and biogeographic histories in generating these patterns. We found that all three processes have significantly contributed to variation in both heat and cold tolerances but that their relative importance differs. We also show that failure to simultaneously account for all three effects overestimates the importance of the included variable, challenging previous conclusions drawn from less comprehensive models. Our results are consistent with rare evolutionary innovations in cold acclimation ability structuring plant distributions across biomes. In contrast, plant heat tolerances vary mainly as a result of biogeographical processes and drift. Our results further highlight that all plants, particularly at mid-to-high latitudes and in their nonhardened state, will become increasingly vulnerable to ongoing climate change.

Summary Thermal limits of ectotherms have been studied extensively and are believed to be evolutionarily constrained, leaving ectotherms at risk under future climate change. Phenotypic plasticity may extend the thermal limits, but we lack detailed characterizations of thermal limit reaction norms as well as an understanding of the interspecific variation in these reaction norms. Here, we investigated the interspecific variation in phenotypic plasticity of thermal limits in 13 Drosophila species. We obtained high‐resolution reaction norms for upper and lower thermal limits across the permissive developmental thermal range (12·5–30 °C). The estimated phenotypes were then associated (while accounting for phylogeny) with climatic parameters from the species’ distributional range. All species showed linear reaction norms for cold tolerance (CTmin) and heat tolerance (CTmax) across developmental acclimation temperatures. We observed strong beneficial cold acclimation to lower temperatures in all species. Conversely, the heat acclimation response was non‐existent in some species, and decreasing or increasing with increasing developmental acclimation temperatures in other species. The degree of phenotypic plasticity of CTmin and CTmax was related neither to the basal thermal limits (trade‐off hypothesis) nor to climatic parameters connected to latitudinal distributions (latitudinal hypothesis). A substantial and linear developmental plasticity of lower thermal limits is a general characteristic of Drosophila species, which allows for straightforward application in species distribution models. In general, upper thermal limits also show linear norms of reaction, but their adaptive significance is limited and highly variable among species, making general predictions across species rather impossible. High‐resolution estimates of norms of reaction of thermal limits can considerably increase our understanding of the capacity of ectotherms to acclimate to different thermal environments. However, our understanding of the environmental drivers of the evolution of phenotypic plasticity and thus of the interspecific differences remains ambiguous, potentially constrained by limited microclimate information. A lay summary is available for this article. Lay Summary

Extreme environmental conditions, such as heat, salinity, and decreased water availability, can have a devastating impact on plant growth and productivity, potentially resulting in the collapse of entire ecosystems. Stress-induced systemic signaling and systemic acquired acclimation play canonical roles in plant survival during episodes of environmental stress. Recent studies revealed that in response to a single abiotic stress, applied to a single leaf, plants mount a comprehensive stress-specific systemic response that includes the accumulation of many different stress-specific transcripts and metabolites, as well as a coordinated stress-specific whole-plant stomatal response. However, in nature plants are routinely subjected to a combination of two or more different abiotic stresses, each potentially triggering its own stress-specific systemic response, highlighting a new fundamental question in plant biology: are plants capable of integrating two different systemic signals simultaneously generated during conditions of stress combination? Here we show that plants can integrate two different systemic signals simultaneously generated during stress combination, and that the manner in which plants sense the different stresses that trigger these signals (i.e., at the same or different parts of the plant) makes a significant difference in how fast and efficient they induce systemic reactive oxygen species (ROS) signals; transcriptomic, hormonal, and stomatal responses; aswell as plant acclimation. Our results shed light on how plants acclimate to their environment and survive a combination of different abiotic stresses. In addition, they highlight a key role for systemic ROS signals in coordinating the response of different leaves to stress.

Leaf photosynthetic capacity (light-saturated net assimilation rate, A A ) increases from bottom to top of plant canopies as the most prominent acclimation response to the conspicuous within-canopy gradients in light availability. Light-dependent variation in A A through plant canopies is associated with changes in key leaf structural (leaf dry mass per unit leaf area), chemical (nitrogen (N) content per area and dry mass, N partitioning between components of photosynthetic machinery), and physiological (stomatal and mesophyll conductance) traits, whereas the contribution of different traits to within-canopy A A gradients varies across sites, species, and plant functional types. Optimality models maximizing canopy carbon gain for a given total canopy N content predict that A A should be proportionally related to canopy light availability. However, comparison of model expectations with experimental data of within-canopy photosynthetic trait variations in representative plant functional types indicates that such proportionality is not observed in real canopies, and A A vs. canopy light relationships are curvilinear. The factors responsible for deviations from full optimality include stronger stomatal and mesophyll diffusion limitations at higher light, reflecting greater water limitations and more robust foliage in higher light. In addition, limits on efficient packing of photosynthetic machinery within leaf structural scaffolding, high costs of N redistribution among leaves, and limited plasticity of N partitioning among components of photosynthesis machinery constrain A A plasticity. Overall, this review highlights that the variation of A A through plant canopies reflects a complex interplay between adjustments of leaf structure and function to multiple environmental drivers, and that A A plasticity is limited by inherent constraints on and trade-offs between structural, chemical, and physiological traits. I conclude that models trying to simulate photosynthesis gradients in plant canopies should consider co-variations among environmental drivers, and the limitation of functional trait variation by physical constraints and include the key trade-offs between structural, chemical, and physiological leaf characteristics.

To investigate the effect of diel variation in pCO₂ on coral calcification, branches of Acropora hyacinthus were collected in 2 habitats (upstream and downstream in a unidirectional flow) in a shallow back reef in Moorea, French Polynesia, where different diel amplitudes of pCO₂ oscillation were expected. Corals were maintained for 6 wk under different pCO₂ regimes (constant versus oscillatory), each delivered in 3 configurations: constant conditions of 400 μatm, 700 μatm, and 1000 μatm pCO₂, or oscillatory conditions varying daily from 280 to 550 μatm, 550 to 1000 μatm, or 400 to 2000 μatm, with minima and maxima during the day and night, respectively. Calcification rates in all treatments tended to increase over time, and the interaction between Time and pCO₂ regime (i.e. constant versus oscillating) was significant (or close to significant) for upstream corals due to higher calcification in oscillatory pCO₂. A significant pCO₂ regime effect was detected in the highest pCO₂ for downstream corals, with higher calcification in the 400 to 2000 μatm oscillatory pCO₂ treatment compared to the 1000 μatm constant pCO₂ treatment. After 6 wk, calcification of A. hyacinthus was affected significantly by habitat, the pCO₂ level, and the pCO₂ regime. Calcification generally was reduced by high pCO₂ and was ≥21% greater in 400 to 2000 μatm oscillatory pCO₂ versus 1000 μatm constant pCO₂ treatment. Increased calcification in the 400 to 2000 μatm oscillatory pCO₂ treatment suggests that natural diel oscillations in pCO₂ could play a role by reducing the locally negative effects of rising pCO₂ associated with ocean acidification on coral calcification.

Evergreen species are widespread across the globe, representing two major plant functional forms in terrestrial models. We reviewed and analysed the responses of photosynthesis and respiration to warming in 101 evergreen species from boreal to tropical biomes. Summertime temperatures affected both latitudinal gas exchange rates and the degree of responsiveness to experimental warming. The decrease in net photosynthesis at 25°C (A net25) was larger with warming in tropical climates than cooler ones. Respiration at 25°C (R 25) was reduced by 14% in response to warming across species and biomes. Gymnosperms were more sensitive to greater amounts of warming than broadleaved evergreens, with A net25 and R 25 reduced c. 30–40%with > 10°C warming. While standardised rates of carboxylation (Vcmax25) and electron transport (J max25) adjusted to warming, the magnitude of this adjustment was not related to warming amount (range 0.6–16°C). The temperature optimum of photosynthesis (T optA) increased on average 0.34°C per °C warming. The combination of more constrained acclimation of photosynthesis and increasing respiration rates with warming could possibly result in a reduced carbon sink in future warmer climates. The predictable patterns of thermal acclimation across biomes provide a strong basis to improve modelling predictions of the future terrestrial carbon sink with warming.

Unpredicted variability in temperature is associated with frequent extreme low-temperature events. Wheat is a leading crop in fulfilling global food requirements. Climate-driven temperature extremes influence the vegetative and reproductive growth of wheat, followed by a decrease in yield. This review describes how low temperature induces a series of modifications in the morphophysiological, biochemical, and molecular makeup of wheat and how it is perceived. To cope with these modifications, crop plants turn on their cold-tolerance mechanisms, characterized by accumulating soluble carbohydrates, signaling molecules, and cold tolerance gene expressions. The review also discusses the integrated management approaches to enhance the performance of wheat plants against cold stress. In this review, we propose strategies for improving the adaptive capacity of wheat besides alleviating risks of cold anticipated with climate change.

A common practice in thermal biology is to take individuals directly from the field and estimate a range of thermal traits. These estimates are then used in studies aiming to understand broad scale distributional patterns, understanding and predicting the evolution of phenotypic plasticity, and generating predictions for climate change risk. However, the use of field-caught individuals in such studies ignores the fact that many traits are phenotypically plastic and will be influenced by the thermal history of the focal individuals. The current study aims to determine the extent to which estimates of upper thermal limits (CTmax), a frequently used measure for climate change risk, are sensitive to developmental and adult acclimation temperatures and whether these two forms of plasticity are reversible. Examining a temperate and tropical population of Drosophila melanogaster we show that developmental acclimation has a larger and more lasting effect on CTmax than adult acclimation. We also find evidence for an interaction between developmental and adult acclimation, particularly when flies are acclimated for a longer period, and that these effects can be population specific. These results suggest that thermal history can have lasting effects on estimates of CTmax. In addition, we provide evidence that developmental and/or adult acclimation are unlikely to contribute to substantial shifts in CTmax and that acclimation capacity may be constrained at higher temperatures.

Significance Mitigating thermal stress through evolutionary adaptation or physiological plasticity is critical for species’ persistence in changing climates. Sparse knowledge of genetic and physiological architectures of thermal plasticity hampers our ability to predict organismal resilience to climate change. Understanding the independence of short- and long-term plasticity and constraints of basal thermotolerance on plasticity is important for understanding responses to climate change. We show heritable genetic variation for basal cold tolerance and plasticity in a midlatitude Drosophila melanogaster population. High long-term plasticity predicted high short-term plasticity, and basal cold tolerance constrained both plasticity measures. There was no overlap in SNPs associated with either plasticity type. Overlapping molecular function of SNPs suggests shared physiology between long- and short-term plasticity, despite distinct genetic architectures. Seasonal and daily thermal variation can limit species distributions because of physiological tolerances. Low temperatures are particularly challenging for ectotherms, which use both basal thermotolerance and acclimation, an adaptive plastic response, to mitigate thermal stress. Both basal thermotolerance and acclimation are thought to be important for local adaptation and persistence in the face of climate change. However, the evolutionary independence of basal and plastic tolerances remains unclear. Acclimation can occur over longer (seasonal) or shorter (hours to days) time scales, and the degree of mechanistic overlap is unresolved. Using a midlatitude population of Drosophila melanogaster , we show substantial heritable variation in both short- and long-term acclimation. Rapid cold hardening (short-term plasticity) and developmental acclimation (long-term plasticity) are positively correlated, suggesting shared mechanisms. However, there are independent components of these traits, because developmentally acclimated flies respond positively to short-term acclimation. A strong negative correlation between basal cold tolerance and developmental acclimation suggests that basal cold tolerance may constrain developmental acclimation, whereas a weaker negative correlation between basal cold tolerance and short-term acclimation suggests less constraint. Using genome-wide association mapping, we show the genetic architecture of rapid cold hardening and developmental acclimation responses are nonoverlapping at the SNP and corresponding gene level. However, genes associated with each trait share functional similarities, including genes involved in apoptosis and autophagy, cytoskeletal and membrane structural components, and ion binding and transport. These results indicate substantial opportunity for short-term and long-term acclimation responses to evolve separately from each other and for short-term acclimation to evolve separately from basal thermotolerance.