Physiological plasticity of cardiorespiratory function in a eurythermal marine teleost, the longjaw mudsucker, Gillichthys mirabilis

An insufficient supply of oxygen under thermal stress is thought to define thermal optima and tolerance limits in teleost fish. When under thermal stress, cardiac function plays a crucial role in sustaining adequate oxygen supply for respiring tissues. Thus, adaptive phenotypic plasticity of cardiac performance may be critical for modifying thermal limits during temperature acclimation. Here we investigated effects of temperature acclimation on oxygen consumption, cardiac function and blood oxygen carrying capacity of a eurythermal goby fish, Gillichthys mirabilis, acclimated to 9, 19 and 26°C for 4 weeks. Acclimation did not alter resting metabolic rates or heart rates; no compensation of rates was observed at acclimation temperatures. However, under an acute heat ramp, warm-acclimated fish exhibited greater heat tolerance (CTmax=33.3, 37.1 and 38.9°C for 9°C-, 19°C- and 26°C-acclimated fish, respectively) and higher cardiac arrhythmia temperatures compared with 9°C-acclimated fish. Heart rates measured under an acute heat stress every week during 28 days of acclimation suggested that both maximum heart rates and temperature at onset of maximum heart rates changed over time with acclimation. Hemoglobin levels increased with acclimation temperature, from 35 g l(-1) in 9°C-acclimated fish to 60-80 g l(-1) in 19°C- and 26°C-acclimated fish. Oxygen consumption rates during recovery from acute heat stress showed post-stress elevation in 26°C-acclimated fish. These data, coupled with elevated resting metabolic rates and heart rates at warm temperatures, suggest a high energetic cost associated with warm acclimation in G. mirabilis. Furthermore, acclimatory capacity appears to be optimized at 19°C, a temperature shown by behavioral studies to be close to the species' preferred temperature.

Habitat Temperature and Precipitation of Arabidopsis thaliana Ecotypes Determine the Response of Foliar Vasculature, Photosynthesis, and Transpiration to Growth Temperature

Acclimatory adjustments of foliar vascular architecture, photosynthetic capacity, and transpiration rate in Arabidopsis thaliana ecotypes (Italian, Polish [Col-0], Swedish) were characterized in the context of habitat of origin. Temperatures of the habitat of origin decreased linearly with increasing habitat latitude, but habitat precipitation was greatest in Italy, lowest in Poland, and intermediate in Sweden. Plants of the three ecotypes raised under three different growth temperature regimes (low, moderate, and high) exhibited highest photosynthetic capacities, greatest leaf thickness, highest chlorophyll a/b ratio and levels of β-carotene, and greatest levels of wall ingrowths in phloem transfer cells, and, in the Col-0 and Swedish ecotypes, of phloem per minor vein in plants grown at the low temperature. In contrast, vein density and minor vein tracheary to sieve element ratio increased with increasing growth temperature - most strongly in Col-0 and least strongly in the Italian ecotype - and transpirational water loss correlated with vein density and number of tracheary elements per minor vein. Plotting of these vascular features as functions of climatic conditions in the habitat of origin suggested that temperatures during the evolutionary history of the ecotypes determined acclimatory responses of the foliar phloem and photosynthesis to temperature in this winter annual that upregulates photosynthesis in response to lower temperature, whereas the precipitation experienced during the evolutionary history of the ecotypes determined adjustment of foliar vein density, xylem, and transpiration to temperature. In particular, whereas photosynthetic capacity, leaf thickness, and foliar minor vein phloem features increased linearly with increasing latitude and decreasing temperature of the habitats of origin in response to experimental growth at low temperature, transpiration rate, foliar vein density, and minor vein tracheary element numbers and cross-sectional areas increased linearly with decreasing precipitation level in the habitats of origin in response to experimental growth at high temperature. This represents a situation where temperature acclimation of the apparent capacity for water flux through the xylem and transpiration rate in a winter annual responded differently from that of photosynthetic capacity, in contrast to previous reports of strong relationships between hydraulic conductance and photosynthesis in other studies.

In vivo isotope tracing reveals the versatility of glucose as a brown adipose tissue substrate

Active brown adipose tissue (BAT) consumes copious amounts of glucose, yet how glucose metabolism supports thermogenesis is unclear. By combining transcriptomics, metabolomics, and stable isotope tracing in vivo, we systematically analyze BAT glucose utilization in mice during acute and chronic cold exposure. Metabolite profiling reveals extensive temperature-dependent changes in the BAT metabolome and transcriptome upon cold adaptation, discovering unexpected metabolite markers of thermogenesis, including increased N-acetyl-amino acid production. Time-course stable isotope tracing further reveals rapid incorporation of glucose carbons into glycolysis and TCA cycle, as well as several auxiliary pathways, including NADPH, nucleotide, and phospholipid synthesis pathways. Gene expression differences inconsistently predict glucose fluxes, indicating that posttranscriptional mechanisms also govern glucose utilization. Surprisingly, BAT swiftly generates fatty acids and acyl-carnitines from glucose, suggesting that lipids are rapidly synthesized and immediately oxidized. These data reveal versatility in BAT glucose utilization, highlighting the value of an integrative-omics approach to understanding organ metabolism.

O-GlcNAc reports ambient temperature and confers heat resistance on ectotherm development

Effects of temperature on biological processes are complex. Diffusion is less affected than the diverse enzymatic reactions that have distinct individual temperature profiles. Hence thermal fluctuations pose a formidable challenge to ectothermic organisms in which body temperature is largely dictated by the ambient temperature. How cells in ectotherms cope with the myriad disruptive effects of temperature variation is poorly understood at the molecular level. Here we show that nucleocytoplasmic posttranslational modification of proteins with O-linked GlcNAc (O-GlcNAc) is closely correlated with ambient temperature during development of distantly related ectotherms ranging from the insect Drosophila melanogaster to the nematode Caenorhabditis elegans to the fish Danio rerio. Regulation seems to occur at the level of activity of the only two enzymes, O-GlcNAc transferase and O-GlcNAcase, that add and remove, respectively, this posttranslational modification in nucleus and cytoplasm. With genetic approaches in D. melanogaster and C. elegans, we demonstrate the importance of high levels of this posttranslational modification for successful development at elevated temperatures. Because many cytoplasmic and nuclear proteins in diverse pathways are O-GlcNAc targets, temperature-dependent regulation of this modification might contribute to an efficient coordinate adjustment of cellular processes in response to thermal change.

Growth temperature impact on leaf form and function in Arabidopsis thaliana ecotypes from northern and southern Europe

The plasticity of leaf form and function in European lines of Arabidopsis thaliana was evaluated in ecotypes from Sweden and Italy grown under contrasting (cool versus hot) temperature regimes. Although both ecotypes exhibited acclimatory adjustments, the Swedish ecotype exhibited more pronounced responses to the two contrasting temperature regimes in several characterized features. These responses included thicker leaves with higher capacities for photosynthesis, likely facilitated by a greater number of phloem cells per minor vein for the active loading and export of sugars, when grown under cool temperature as opposed to leaves with a higher vein density and a greater number of tracheary elements per minor vein, likely facilitating higher rates of transpirational water loss (and thus evaporative cooling), when grown under hot temperature with high water availability. In addition, only the Swedish ecotype exhibited reduced rosette growth and greater levels of foliar tocopherols under the hot growth temperature. These responses, and the greater responsiveness of the Swedish ecotype compared with the Italian ecotype, are discussed in the context of redox signalling networks and transcription factors, and the greater range of environmental conditions experienced by the Swedish versus the Italian ecotype during the growing season in their native habitats.

The Dynamic Nature of Hypertrophic and Fibrotic Remodeling of the Fish Ventricle

Chronic pressure or volume overload can cause the vertebrate heart to remodel. The hearts of fish remodel in response to seasonal temperature change. Here we focus on the passive properties of the fish heart. Building upon our previous work on thermal-remodeling of the rainbow trout ventricle, we hypothesized that chronic cooling would initiate fibrotic cardiac remodeling, with increased myocardial stiffness, similar to that seen with pathological hypertrophy in mammals. We hypothesized that, in contrast to pathological hypertrophy in mammals, the remodeling response in fish would be plastic and the opposite response would occur following chronic warming. Rainbow trout held at 10°C (control group) were chronically (>8 weeks) exposed to cooling (5°C) or warming (18°C). Chronic cold induced hypertrophy in the highly trabeculated inner layer of the fish heart, with a 41% increase in myocyte bundle cross-sectional area, and an up-regulation of hypertrophic marker genes. Cold acclimation also increased collagen deposition by 1.7-fold and caused an up-regulation of collagen promoting genes. In contrast, chronic warming reduced myocyte bundle cross-sectional area, expression of hypertrophic markers and collagen deposition. Functionally, the cold-induced fibrosis and hypertrophy were associated with increased passive stiffness of the whole ventricle and with increased micromechanical stiffness of tissue sections. The opposite occurred with chronic warming. These findings suggest chronic cooling in the trout heart invokes a hypertrophic phenotype with increased cardiac stiffness and fibrosis that are associated with pathological hypertrophy in the mammalian heart. The loss of collagen and increased compliance following warming is particularly interesting as it suggests fibrosis may oscillate seasonally in the fish heart, revealing a more dynamic nature than the fibrosis associated with dysfunction in mammals.

Acclimation of isoprene emission and photosynthesis to growth temperature in hybrid aspen: resolving structural and physiological controls

Acclimation of foliage to growth temperature involves both structural and physiological modifications, but the relative importance of these two mechanisms of acclimation is poorly known, especially for isoprene emission responses. We grew hybrid aspen (Populus tremula x P. tremuloides) under control (day/night temperature of 25/20 °C) and high temperature conditions (35/27 °C) to gain insight into the structural and physiological acclimation controls. Growth at high temperature resulted in larger and thinner leaves with smaller and more densely packed chloroplasts and with lower leaf dry mass per area (MA). High growth temperature also led to lower photosynthetic and respiration rates, isoprene emission rate and leaf pigment content and isoprene substrate dimethylallyl diphosphate pool size per unit area, but to greater stomatal conductance. However, all physiological characteristics were similar when expressed per unit dry mass, indicating that the area-based differences were primarily driven by MA. Acclimation to high temperature further increased heat stability of photosynthesis and increased activation energies for isoprene emission and isoprene synthase rate constant. This study demonstrates that temperature acclimation of photosynthetic and isoprene emission characteristics per unit leaf area were primarily driven by structural modifications, and we argue that future studies investigating acclimation to growth temperature must consider structural modifications.

Association between minor loading vein architecture and light- and CO2-saturated rates of photosynthetic oxygen evolution among Arabidopsis thaliana ecotypes from different latitudes

Through microscopic analysis of veins and assessment of light- and CO2-saturated rates of photosynthetic oxygen evolution, we investigated the relationship between minor loading vein anatomy and photosynthesis of mature leaves in three ecotypes of Arabidopsis thaliana grown under four different combinations of temperature and photon flux density (PFD). All three ecotypes exhibited greater numbers and cross-sectional area of phloem cells as well as higher photosynthesis rates in response to higher PFD and especially lower temperature. The Swedish ecotype exhibited the strongest response to these conditions, the Italian ecotype the weakest response, and the Col-0 ecotype exhibited an intermediate response. Among all three ecotypes, strong linear relationships were found between light- and CO2-saturated rates of photosynthetic oxygen evolution and the number and area of either sieve elements or of companion and phloem parenchyma cells in foliar minor loading veins, with the Swedish ecotype showing the highest number of cells in minor loading veins (and largest minor veins) coupled with unprecedented high rates of photosynthesis. Linear, albeit less significant, relationships were also observed between number and cross-sectional area of tracheids per minor loading vein versus light- and CO2-saturated rates of photosynthetic oxygen evolution. We suggest that sugar distribution infrastructure in the phloem is co-regulated with other features that set the upper limit for photosynthesis. The apparent genetic differences among Arabidopsis ecotypes should allow for future identification of the gene(s) involved in augmenting sugar-loading and -transporting phloem cells and maximal rates of photosynthesis.

Investigating the long-term legacy of drought and warming on the soil microbial community across five European shrubland ecosystems

We investigated how the legacy of warming and summer drought affected microbial communities in five different replicated long-term (>10 years) field experiments across Europe (EU-FP7 INCREASE infrastructure). To focus explicitly on legacy effects (i.e., indirect rather than direct effects of the environmental factors), we measured microbial variables under the same moisture and temperature in a brief screening, and following a pre-incubation at stable conditions. Specifically, we investigated the size and composition of the soil microbial community (PLFA) alongside measurements of bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth rates, previously shown to be highly responsive to changes in environmental factors, and microbial respiration. We found no legacy effects on the microbial community size, composition, growth rates, or basal respiration rates at the effect sizes used in our experimental setup (0.6 °C, about 30% precipitation reduction). Our findings support previous reports from single short-term ecosystem studies thereby providing a clear evidence base to allow long-term, broad-scale generalizations to be made. The implication of our study is that warming and summer drought will not result in legacy effects on the microbial community and their processes within the effect sizes here studied. While legacy effects on microbial processes during perturbation cycles, such as drying-rewetting, and on tolerance to drought and warming remain to be studied, our results suggest that any effects on overall ecosystem processes will be rather limited. Thus, the legacies of warming and drought should not be prioritized factors to consider when modeling contemporary rates of biogeochemical processes in soil.

Mathematical modeling of an oscillating gene circuit to unravel the circadian clock network of Arabidopsis thaliana

The Arabidopsis thaliana circadian clock is an interconnected network highly tractable to systems approaches. Most elements in the transcriptional-translational oscillator were identified by genetic means and the expression of clock genes in various mutants led to the founding hypothesis of a positive-negative feedback loop being the core clock. The identification of additional clock genes beyond those defined in the core led to the use of systems approaches to decipher this angiosperm oscillator circuit. Kinetic modeling was first used to explain periodicity effects of various circadian mutants. This conformed in a flexible way to experimental details. Such observations allowed a recursive use of hypothesis generating from modeling, followed by experimental corroboration. More recently, the biochemical finding of new description of a DNA-binding activity for one class of clock components directed improvements in feature generation, one of which revealed that the core of the oscillator is a negative-negative feedback loop. The recursive use of modeling and experimental validation has thus revealed many essential transcriptional components that drive negative arms in the circadian oscillator. What awaits is to more fully describe the positive arms and an understanding of how additional pathways converge on the clock.

Antecedent moisture and temperature conditions modulate the response of ecosystem respiration to elevated CO2 and warming

Terrestrial plant and soil respiration, or ecosystem respiration (R_eco ), represents a major CO₂ flux in the global carbon cycle. However, there is disagreement in how R_eco will respond to future global changes, such as elevated atmosphere CO₂ and warming. To address this, we synthesized six years (2007-2012) of R_eco data from the Prairie Heating And CO₂ Enrichment (PHACE) experiment. We applied a semi-mechanistic temperature-response model to simultaneously evaluate the response of R_eco to three treatment factors (elevated CO₂ , warming, and soil water manipulation) and their interactions with antecedent soil conditions [e.g., past soil water content (SWC) and temperature (SoilT)] and aboveground factors (e.g., vapor pressure deficit, photosynthetically active radiation, vegetation greenness). The model fits the observed R_eco well (R²  = 0.77). We applied the model to estimate annual (March-October) R_eco , which was stimulated under elevated CO₂ in most years, likely due to the indirect effect of elevated CO₂ on SWC. When aggregated from 2007 to 2012, total six-year R_eco was stimulated by elevated CO₂ singly (24%) or in combination with warming (28%). Warming had little effect on annual R_eco under ambient CO₂ , but stimulated it under elevated CO₂ (32% across all years) when precipitation was high (e.g., 44% in 2009, a 'wet' year). Treatment-level differences in R_eco can be partly attributed to the effects of antecedent SoilT and vegetation greenness on the apparent temperature sensitivity of R_eco and to the effects of antecedent and current SWC and vegetation activity (greenness modulated by VPD) on R_eco base rates. Thus, this study indicates that the incorporation of both antecedent environmental conditions and aboveground vegetation activity are critical to predicting R_eco at multiple timescales (subdaily to annual) and under a future climate of elevated CO₂ and warming.

Effects of seasonal change and experimental warming on the temperature dependence of photosynthesis in the canopy leaves of Quercus serrata

The effects of warming on the temperature response of leaf photosynthesis have become an area of major concern in recent decades. Although growth temperature (GT) and day length (DL) affect leaf gas exchange characteristics, the way in which these factors influence the temperature dependence of photosynthesis remains uncertain. We established open-top canopy chambers at the canopy top of a deciduous forest, in which average daytime leaf temperature was increased by 1.0 °C. We conducted gas exchange measurements for the canopy leaves of deciduous trees exposed to artificial warming during different seasons. The carbon dioxide assimilation rate at 20 °C (A₂₀) was not affected by warming, whereas that at 25 °C (A₂₅) tended to be higher in leaves exposed to warming. Warming increased the optimal temperature of photosynthesis by increasing the activation energy for the maximum rate of carboxylation. Regression analysis indicated that both GT and DL strongly influenced gas exchange characteristics. Sensitivity analysis revealed that DL affected A without obvious effects on the temperature dependence of A, whereas GT almost maintained constant A₂₀ and strongly influenced the temperature dependence. These results indicate that GT and DL have different influences on photosynthesis; GT and DL affect the 'slope' and intercept' of the temperature dependence of photosynthesis, respectively.

Air breathing in the Arctic: influence of temperature, hypoxia, activity and restricted air access on respiratory physiology of the Alaska blackfish Dallia pectoralis

The Alaska blackfish (Dallia pectoralis) is an air-breathing fish native to Alaska and the Bering Sea islands, where it inhabits lakes that are ice-covered in the winter, but enters warm and hypoxic waters in the summer to forage and reproduce. To understand the respiratory physiology of this species under these conditions and the selective pressures that maintain the ability to breathe air, we acclimated fish to 5°C and 15°C and used respirometry to measure: standard oxygen uptake (Ṁ(O₂)) in normoxia (19.8 kPa P(O₂)) and hypoxia (2.5 kPa), with and without access to air; partitioning of standard Ṁ(O₂) in normoxia and hypoxia; maximum Ṁ(O₂) and partitioning after exercise; and critical oxygen tension (P(crit)). Additionally, the effects of temperature acclimation on haematocrit, haemoglobin oxygen affinity and gill morphology were assessed. Standard Ṁ(O₂) was higher, but air breathing was not increased, at 15°C or after exercise at both temperatures. Fish acclimated to 5°C or 15°C increased air breathing to compensate and fully maintain standard Ṁ(O₂) in hypoxia. Fish were able to maintain Ṁ(O₂) through aquatic respiration when air was denied in normoxia, but when air was denied in hypoxia, standard Ṁ(O₂) was reduced by ∼30-50%. P(crit) was relatively high (5 kPa) and there were no differences in P(crit), gill morphology, haematocrit or haemoglobin oxygen affinity at the two temperatures. Therefore, Alaska blackfish depends on air breathing in hypoxia and additional mechanisms must thus be utilised to survive hypoxic submergence during the winter, such as hypoxia-induced enhancement in the capacities for carrying and binding blood oxygen, behavioural avoidance of hypoxia and suppression of metabolic rate.

Different Relationship between hsp70 mRNA and hsp70 Levels in the Heat Shock Response of Two Salmonids with Dissimilar Temperature Preference

The heat shock response (HSR) refers to the rapid production of heat shock proteins (hsps) in response to a sudden increase in temperature. Its regulation by heat shock factors is a good example of how gene expression is transcriptionally regulated by environmental stresses. In contrast, little is known about post-transcriptional regulation of the response. The heat shock response is often used to characterize the temperature tolerance of species with the rationale that whenever the response sets on, a species is approaching its lethal temperature. It has commonly been considered that an increase in hsp mRNA gives an accurate indication that the same happens to the protein level, but this need not be the case. With climate change, understanding the effects of temperature on gene expression of especially polar organisms has become imperative to evaluate how both biodiversity and commercially important species respond, since temperature increases are expected to be largest in polar areas. Here we studied the HSR of two phylogenetically related Arctic species, which differ in their temperature tolerance with Arctic charr having lower maximally tolerated temperature than Atlantic salmon. Arctic charr acclimated to 15°C and exposed to 7°C temperature increase for 30 min showed both an increase in hsp70 mRNA and hsp70 whereas in salmon only hsp70 mRNA increased. Our results indicate that the temperature for transcriptional induction of hsp can be different from the one required for a measurable change in inducible hsp level. The species with lower temperature tolerance, Arctic charr, are experiencing temperature stress already at the higher acclimation temperature, 15°C, as their hsp70 mRNA and hsp70 levels were higher, and they grow less than fish at 8°C (whereas for salmon the opposite is true). Consequently, charr experience more drastic heat shock than salmon. Although further studies are needed to establish the temperature range and length of exposure where hsp mRNA and hsp level are disconnected, the observation suggests that by measuring both hsp mRNA and hsp level, one can evaluate if a species is approaching the higher end of its temperature tolerance, and thus evaluate the vulnerability of an organism to the challenges imposed by elevated water temperature.

Seasonal influences on PCB retention and biotransformation in fish

There is extensive evidence that fish from waters with polychlorinated biphenyls (PCB)-contaminated sediments accumulate PCBs and related chemicals and that people who eat fish from contaminated waters have higher body burdens of PCBs and PCB metabolites than those who do not. PCBs and their metabolites are potentially toxic; thus, it is important to human health to understand the uptake, biotransformation, and elimination of PCBs in fish since these processes determine the extent of accumulation. The intestinal uptake of PCBs present in the diet of fish into fish tissues is a process that is influenced by the lipid composition of the diet. Biotransformation of PCBs in fish, as in mammals, facilitates elimination, although many PCB congeners are recalcitrant to biotransformation in fish and mammals. Sequential biotransformation of PCBs by cytochrome P450 and conjugation pathways is even less efficient in fish than in mammalian species, thus contributing to the retention of PCBs in fish tissues. A very important factor influencing overall PCB disposition in fish is water temperature. Seasonal changes in water temperature produce adaptive physiological and biochemical changes in fish. While uptake of PCBs from the diet is similar in fish acclimated to winter or summer temperatures, there is evidence that elimination of PCBs occurs much more slowly when the fish is acclimated at low temperatures than at warmer temperatures. Research to date suggests that the processes of elimination of PCBs are modulated by several factors in fish including seasonal changes in water temperature. Thus, the body burden of PCBs in fish from a contaminated location is likely to vary with season.

Tolerances and acclimation to low and high temperatures for cladodes, fruits and roots of a widely cultivated cactus, Opuntia ficus-indica

• Opuntia ficus-indica , a cactus widely cultivated for fruits and forage/fodder, has shoots composed of flattened stem segments (cladodes) that are relatively sensitive to freezing temperatures below -6°C but extremely tolerant of high temperatures up to 65°C. Based on the uptake of the vital strain neutral red, fruits and roots were damaged by 60 min below -7°C or above 55°C. • Young (6 wk old) and mature (1-yr-old) cladodes had 1.2°C greater low-temperature tolerance at day/night air temperatures of 20 : 10°C compared with 30 : 20°C and 2.8°C lower high-temperature tolerance. Fruits and roots (both 6 wk old) showed no such low-temperature acclimation; roots had high-temperature acclimation similar to that of cladodes, but fruits showed no high-temperature acclimation. • High-temperature tolerance did not change with age for cladodes and fruits up to 10 wk old nor did low-temperature tolerance for cladodes, but fruit low-temperature tolerance decreased by 2.6°C from 4.5 to 10 wk of age, a time when sugars, which can act as cryoprotectants, were increasing. • Cladodes showed 2.0°C greater tolerance of low temperatures with age up to 10 yr and 6.5°C greater high-temperature tolerance, which helps prevent the death of plants, especially during episodic freezing events.

Associations between the acclimation of phloem-cell wall ingrowths in minor veins and maximal photosynthesis rate

The companion cells (CCs) and/or phloem parenchyma cells (PCs) in foliar minor veins of some species exhibit invaginations that are amplified when plants develop in high light (HL) compared to low light (LL). Leaves of plants that develop under HL also exhibit greater maximal rates of photosynthesis compared to those that develop under LL, suggesting that the increased membrane area of CCs and PCs of HL-acclimated leaves may provide for greater levels of transport proteins facilitating enhanced sugar export. Furthermore, the degree of wall invagination in PCs (Arabidopsis thaliana) or CCs (pea) of fully expanded LL-acclimated leaves increased to the same level as that present in HL-acclimated leaves 7 days following transfer to HL, and maximal photosynthesis rates of transferred leaves of both species likewise increased to the same level as in HL-acclimated leaves. In contrast, transfer of Senecio vulgaris from LL to HL resulted in increased wall invagination in CCs, but not PCs, and such leaves furthermore exhibited only partial upregulation of photosynthetic capacity following LL to HL transfer. Moreover, a significant linear relationship existed between the level of cell wall ingrowths and maximal photosynthesis rates across all three species and growth light regimes. A positive linear relationship between these two parameters was also present for two ecotypes (Sweden, Italy) of the winter annual A. thaliana in response to growth at different temperatures, with significantly greater levels of PC wall ingrowths and higher rates of photosynthesis in leaves that developed at cooler versus warmer temperatures. Treatment of LL-acclimated plants with the stress hormone methyl jasmonate also resulted in increased levels of wall ingrowths in PCs of A. thaliana and S. vulgaris but not in CCs of pea and S. vulgaris. The possible role of PC wall ingrowths in sugar export versus as physical barriers to the movement of pathogens warrants further attention.

Minor loading vein acclimation for three Arabidopsis thaliana ecotypes in response to growth under different temperature and light regimes

In light of the important role of foliar phloem as the nexus between energy acquisition through photosynthesis and distribution of the products of photosynthesis to the rest of the plant, as well as communication between the whole plant and its leaves, we examined whether foliar minor loading veins in three Arabidopsis thaliana ecotypes undergo acclimation to the growth environment. As a winter annual exhibiting higher rates of photosynthesis in response to cooler vs. warmer temperatures, this species might be expected to adjust the structure of its phloem to accommodate greater fluxes of sugars in response to growth at low temperature. Minor (fourth- and third-order) veins had 14 or fewer sieve elements and phloem tissue comprised 50% or more of the cross-sectional area. The number of phloem cells per minor loading vein was greater in leaves grown under cool temperature and high light vs. warm temperature and moderate light. This effect was greatest in an ecotype from Sweden, in which growth under cool temperature and high light resulted in minor veins with an even greater emphasis on phloem (50% more phloem cells with more than 100% greater cross-sectional area of phloem) compared to growth under warm temperature and moderate light. Likewise, the number of sieve elements per minor vein increased linearly with growth temperature under moderate light, almost doubling over a 27°C temperature range (21°C leaf temperature range) in the Swedish ecotype. Increased emphasis on cells involved in sugar loading and transport may be critical for maintaining sugar export from leaves of an overwintering annual such as A. thaliana, and particularly for the ecotype from the northern-most population experiencing the lowest temperatures.

Biochemical photosynthetic responses to temperature: how do interspecific differences compare with seasonal shifts?

Plants show flexible acclimation of leaf photosynthesis to temperature that depends both on their prevailing growth environment and the climate where they originated. This acclimation has been shown to involve changes in the temperature responses of the apparent maximum rate of Rubisco carboxylation (Vcmax) and apparent maximum rate of electron transport (Jmax), as well as changes in the ratio of these parameters. We asked whether such changes in photosynthetic biochemistry attributable to climate of origin are similar in nature and magnitude to those attributable to growth environment. To address this question, we measured temperature responses of photosynthesis and chlorophyll fluorescence on six Eucalyptus species from diverse geographical and climatic regions growing in a common garden. Measurements were made in three seasons, allowing us to compare interspecific differences with seasonal changes. We found significant interspecific differences in apparent Vcmax and Jmax standardized to 25 °C, but there were no significant differences in the temperature responses of these parameters among species. Comparing data across seasons, we found significant seasonal changes in apparent Vcmax25, but not in Jmax25, causing a change in their ratio (J/V ratio). However, there were no seasonal changes in the temperature response of either parameter. We concluded that the growth environment had a much larger effect on temperature response than climate of origin among this set of species. Mean daytime temperature increased by 15 °C from winter to summer, whereas we estimated that the seasonal change in J/V ratio would cause a change in the optimum temperature (Topt) for gross photosynthesis of 3.6 °C. Use of a general relationship to describe photosynthetic temperature acclimation resulted in a strong underestimation of the Topt for photosynthesis for these species. Our results indicated that variation in photosynthetic temperature responses cannot be captured in one simple relationship with growth temperature. Further comparative research on species groups will be needed to develop a basis for modelling these interspecific differences in plant temperature acclimation.

Effects of Thermal Acclimation on the Tolerance of Bactrocera zonata (Diptera: Tephritidae) to Hydric Stress

Insects, similarly to other small terrestrial invertebrates, are particularly susceptible to climatic stress. Physiological adjustments to cope with the environment (i.e., acclimation) together with genetic makeup eventually determine the tolerance of a species to climatic extremes, and constrain its distribution. Temperature and desiccation resistance in insects are both conditioned by acclimation and may be interconnected, particularly for species inhabiting xeric environments. We determined the effect of temperature acclimation on desiccation resistance of the peach fruit fly (Bactrocera zonata, Tephritidae) – an invasive, polyphagous pest, currently spreading through both xeric and mesic environments in Africa and the Eurasian continent. Following acclimation at three constant temperatures (20, 25, and 30°C), the survival of adult flies deprived of food and water was monitored in extreme dry and humid conditions (<10 and >90% relative humidity, respectively). We found that flies acclimated at higher temperatures were significantly heavier, and contained more lipids and protein. Acclimation temperature significantly and similarly affected the survival of males and females at both high and low humidity conditions. In both cases, flies maintained at 30°C survived longer compared to 20 and 25°C – habituated counterparts. Regardless of the effect of acclimation temperature on survival, overall life expectancy was significantly shortened when flies were assayed under desiccating conditions. Additionally, our experiments indicate no significant difference in survival patterns between males and females, and that acclimation temperature had similar effects after both short (5–10 days) and long (11–20 days) acclimation periods. We conclude that acclimation at 30°C prolongs the survival of B. zonata, regardless of ambient humidity levels. Temperature probably affected survival through modulating feeding and metabolism, allowing for accumulation of larger energetic reserves, which in turn, promoted a greater ability to resist starvation, and possibly desiccation as well. Our study set the grounds for understanding the phenotypic plasticity of B. zonata from the hydric perspective, and for further evaluating the invasion potential of this pest.

Leaf-age effects on temperature responses of photosynthesis and respiration of an alpine oak, Quercus aquifolioides, in southwestern China

Temperature responses and sensitivity of photosynthesis (A(n_)T) and respiration for leaves at different ages are crucial to modeling ecosystem carbon (C) cycles and productivity of evergreen forests. Understanding the mechanisms and processes of temperature sensitivity may further shed lights on temperature acclimation of photosynthesis and respiration with leaf aging. The current study examined temperature responses of photosynthesis and respiration of young leaves (YLs) (fully expanded in current growth season) and old leaves (OLs) (fully expanded in last growth season) of Quercus aquifolioides Rehder and E.H. Wilson in an alpine oak forest, southwestern China. Temperature responses of dark respiration (R(dark)), net assimilation (A(n)), maximal velocity of carboxylation (V(cmax)) and maximum rate of electron transport (J(max)) were significantly different between the two leaf ages. Those differences implied different temperature response parameters should be used for leaves of different ages in modeling vegetation productivity and ecosystem C cycles in Q. aquifolioides forests and other evergreen forests. We found that RuBP carboxylation determined the downward shift of A(n_)T in OLs, while RuBP regeneration and the balance between Rubisco carboxylation and RuBP regeneration made little contribution. Sensitivity of stomatal conductance to vapor pressure deficit changed in OLs and compensated part of the downward shift. We also found that OLs of Q. aquifolioides had lower An due to lower stomatal conductance, higher stomatal conductance limitation and deactivation of the biochemical processes. In addition, the balance between R(dark) and A(n) changed between OLs and YLs, which was represented by a higher R(dark)/A(n) ratio for OLs.

Effect of growth temperature on photosynthetic capacity and respiration in three ecotypes of Eriophorum vaginatum

Ecotypic differentiation in the tussock‐forming sedge Eriophorum vaginatum has led to the development of populations that are locally adapted to climate in Alaska's moist tussock tundra. As a foundation species, E. vaginatum plays a central role in providing topographic and microclimatic variation essential to these ecosystems, but a changing climate could diminish the importance of this species. As Arctic temperatures have increased, there is evidence of adaptational lag in E. vaginatum, as locally adapted ecotypes now exhibit reduced population growth rates. Whether there is a physiological underpinning to adaptational lag is unknown. Accordingly, this possibility was investigated in reciprocal transplant gardens. Tussocks of E. vaginatum from sites separated by ~1° latitude (Coldfoot: 67°15′N, Toolik Lake: 68°37′, Sagwon: 69°25′) were transplanted into the Toolik Lake and Sagwon sites and exposed to either an ambient or an experimental warming treatment. Five tussocks pertreatment combination were measured at each garden to determine photosynthetic capacity (i.e., V_cmax and J_max) and dark respiration rate (R_d) at measurement temperatures of 15, 20, and 25°C. Photosynthetic enhancements or homeostasis were observed for all ecotypes at both gardens under increased growth temperature, indicating no negative effect of elevated temperature on photosynthetic capacity. Further, no evidence of thermal acclimation in R_d was observed for any ecotype, and there was little evidence of ecotypic variation in R_d. As such, no physiological contribution to adaptational lag was observed given the increase in growth temperature (up to ~2°C) provided by this study. Despite neutral to positive effects of increased growth temperature on photosynthesis in E. vaginatum, it appears to confer no lasting advantage to the species.

Seasonal changes in temperature dependence of photosynthetic rate in rice under a free-air CO(2) enrichment

BACKGROUND AND AIMS

Influences of rising global CO(2) concentration and temperature on plant growth and ecosystem function have become major concerns, but how photosynthesis changes with CO(2) and temperature in the field is poorly understood. Therefore, studies were made of the effect of elevated CO(2) on temperature dependence of photosynthetic rates in rice (Oryza sativa) grown in a paddy field, in relation to seasons in two years.

METHODS

Photosynthetic rates were determined monthly for rice grown under free-air CO(2) enrichment (FACE) compared to the normal atmosphere (570 vs 370 micromol mol(-1)). Temperature dependence of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (V(cmax)) and the maximum rate of electron transport (J(max)) were analysed with the Arrhenius equation. The photosynthesis-temperature response was reconstructed to determine the optimal temperature (T(opt)) that maximizes the photosynthetic rate.

KEY RESULTS AND CONCLUSIONS

There was both an increase in the absolute value of the light-saturated photosynthetic rate at growth CO(2) (P(growth)) and an increase in T(opt) for P(growth) caused by elevated CO(2) in FACE conditions. Seasonal decrease in P(growth) was associated with a decrease in nitrogen content per unit leaf area (N(area)) and thus in the maximum rate of electron transport (J(max)) and the maximum rate of RuBP carboxylation (V(cmax)). At ambient CO(2), T(opt) increased with increasing growth temperature due mainly to increasing activation energy of V(cmax). At elevated CO(2), T(opt) did not show a clear seasonal trend. Temperature dependence of photosynthesis was changed by seasonal climate and plant nitrogen status, which differed between ambient and elevated CO(2).

Membrane and calcium clock mechanisms contribute variably as a function of temperature to setting cardiac pacemaker rate in zebrafish Danio rerio

Here, we show that heart rate in zebrafish Danio rerio is dependent upon two pacemaking mechanisms and it possesses a limited ability to reset the cardiac pacemaker with temperature acclimation. Electrocardiogram recordings, taken from individual, anaesthetised zebrafish that had been acclimated to 18, 23 or 28°C were used to follow the response of maximum heart rate (f_Hmax ) to acute warming from 18°C until signs of cardiac failure appeared (up to c. 40°C). Because f_Hmax was similar across the acclimation groups at almost all equivalent test temperatures, warm acclimation was limited to one significant effect, the 23°C acclimated zebrafish had a significantly higher (21%) peak f_Hmax and reached a higher (3°C) test temperature than the 18°C acclimated zebrafish. Using zatebradine to block the membrane hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) and examine the contribution of the membrane clock mechanisms to cardiac pacemaking, f _Hmax was significantly reduced (by at least 40%) at all acute test temperatures and significantly more so at most test temperatures for zebrafish acclimated to 28°C vs. 23°C. Thus, HCN channels and the membrane clock were not only important, but could be modified by thermal acclimation. Using a combination of ryanodine (to block sarcoplasmic calcium release) and thapsigargin (to block sarcoplasmic calcium reuptake) to examine the contribution of sarcoplasmic reticular handling of calcium and the calcium clock, f _Hmax was again consistently reduced independent of the test temperature and acclimation temperature, but to a significantly lesser degree than zatebradine for zebrafish acclimated to both 28 and 18°C. Thus, the calcium clock mechanism plays an additional role in setting pacemaker activity that was independent of temperature. In conclusion, the zebrafish cardiac pacemaker has a limited temperature acclimation ability compared with known effects for other fishes and involves two pacemaking mechanisms, one of which was independent of temperature.

Head-tail-head neural wiring underlies gut fat storage in Caenorhabditis elegans temperature acclimation

Animals maintain the ability to survive and reproduce by acclimating to environmental temperatures. We showed here that Caenorhabditis elegans exhibited temperature acclimation plasticity, which was regulated by a head-tail-head neural circuitry coupled with gut fat storage. After experiencing cold, C. elegans individuals memorized the experience and were prepared against subsequent cold stimuli. The cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) regulated temperature acclimation in the ASJ thermosensory neurons and RMG head interneurons, where it modulated ASJ thermosensitivity in response to past cultivation temperature. The PVQ tail interneurons mediated the communication between ASJ and RMG via glutamatergic signaling. Temperature acclimation occurred via gut fat storage regulation by the triglyceride lipase ATGL-1, which was activated by a neuropeptide, FLP-7, downstream of CREB. Thus, a head-tail-head neural circuit coordinated with gut fat influenced experience-dependent temperature acclimation.