Elevation provenance affects photosynthesis and its acclimation to temperature in the high-Andes alpine herb Phacelia secunda

• We analysed whether Phacelia secunda populations from different elevations exhibit intrinsic traits associated with diffusive and biochemical components of photosynthesis, and if they differ in acclimation of photosynthesis to warmer temperatures. We hypothesized that P. secunda will have similar photosynthetic performance regardless of altitudinal provenance and that plants from high elevations will have a lower photosynthetic acclimation capacity to higher temperature than plants from low elevations.
• Plants from 1600, 2800 and 3600 m a.s.l. in the central Chilean Andes were collected and grown under two temperature regimes (20/16 °C and 30/26 °C day/night). The following photosynthetic traits were measured in each plant for the two temperature regimes: A_N, g_s, g_m, J_max, V_cmax, Rubisco carboxylation k_cat^c.
• Under a common growth environment, plants from the highest elevation had slightly lower CO₂ assimilation rates compared to lower elevation plants. While diffusive components of photosynthesis increased with elevation provenance, the biochemical component decreased, suggesting compensation that explains the similar rates of photosynthesis among elevation provenances. Plants from high elevations had lower photosynthetic acclimation to warmer temperatures compared to plants from lower elevations, and these responses were related to elevational changes in diffusional and biochemical components of photosynthesis.
• Plants of P. secunda from different elevations maintain photosynthetic traits when grown in a common environment, suggesting low plasticity to respond to future climate changes. The fact that high elevation plants had lower photosynthetic acclimation to warmer temperature suggests higher susceptibility to increases in temperature associated with global warming.

     

Adaptive genetic differentiation in life-history traits between populations of <Emphasis Type="Italic">Mimulus guttatus</Emphasis> with annual and perennial life-cycles

The optimal allocation to sexual and vegetative reproduction as well as the optimal values of other life-history characteristics such as phenology, growth and mating system are likely to depend on the life-cycle of the organism. I tested whether plants of Mimulus guttatus originating from temporarily wet populations where the species has an enforced annual life-cycle have higher allocation to sexual reproduction, lower allocation to vegetative reproduction, more rapid phenology, faster growth, and floral traits associated with a self-fertilizing mating system than plants from permanently wet populations where the species has a perennial life-cycle. I grew a total of 1377 plants originating from three populations with an annual life-cycle and 11 populations with a perennial life-cycle in a greenhouse under permanently and temporarily wet conditions. Plants of M. guttatus in permanently wet conditions had significantly more vegetative reproduction and tended to have a faster growth than plants in the temporarily wet conditions, indicating plasticity in these life-history traits. Plants from populations with an annual life-cycle invested significantly more in sexual reproduction and significantly less in vegetative reproduction than the ones from populations with a perennial life-cycle. Moreover, this study showed that plants originating from populations with an annual life-cycle have a significantly faster development and floral traits associated with autonomous self-fertilization. In conclusion, this study suggests that there has been adaptive evolution of life history traits of M. guttatus in response to natural watering conditions that determine the life span of the species.     

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus)

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CT_max) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CT_max) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.     

Low night temperature acclimation of Phalaenopsis

The capability of Phalaenopsis to acclimate its photosynthetic capacity and metabolic activity to cool night temperature conditions is crucial for improving orchid production in terms of efficient greenhouse heating. The extent to which Phalaenopsis possesses acclimation potential and the mechanistic background of the metabolic processes involved, have, however, not been studied before. Plants were subjected to a direct and gradual shift from a day to night temperature regime of 28/28–28/16°C, the cold stress and cold acclimation treatment, respectively. In comparison with the cold stress treatment, the cold acclimation treatment led to a higher malate accumulation and a reduction in leaf net CO₂ uptake. Consistently, the contribution of respiratory CO₂ recycling to nocturnal malate synthesis was calculated to be 23.5 and 47.0% for the cold stress and cold acclimation treatment, respectively. Moreover, the lower levels of starch measured in the cold acclimated leaves confirmed the suggested enhanced respiratory CO₂ recycling, implying that Phalaenopsis CAM operation evolved towards CAM idling. It is, however, plausible that this adjustment was not an effect of the low night temperature per se but a consequence of cool-root induced drought stress. Apart from that, at the start of the photoperiod, membrane stability showed a depression which was directly counteracted by an increased generation of glucose, fructose and sucrose. From these observations, it can be concluded that the observed plasticity in CAM operation and metabolic flexibility may be recognized as important steps in the low night temperature acclimation of Phalaenopsis.     

Vegetative growth inRumex acetosella (Polygonaceae) originating from different geographic regions

Vegetative growth patterns ofRumex acetosella L. were investigated both in experimental conditions and in the field. Plants originating from geographically and ecologically contrasting areas expressed significant differentiation in height, weight, and leaf production, even at early stages. The plants belonging to subsp.angiocarpus grew thaller than other plants. The differences among populations grown in a uniform environment provide evidence for genetic differentiation in morphology. Plants in natural populations were considerably lighter than experimental plants, which intensively produced a large number of rosette leaves. These differences can be attributed to substantial phenotypic plasticity. When the correlation structure among the growth traits was analyzed, seed weight and early plant size appeared to influence future plant size.     

Genetic analysis of frost hardiness traits in tuber-bearingSolanum species

The inheritance of frost hardiness and cold acclimation potential traits was studied in three segregating populations derived from a cross betweenSolanum commersonii Dun. PI 243503 (cmm) andSolanum cardiophyllum Lindl., PI 184762 (cph), two parental genotypes with contrasting frost hardiness and cold acclimation potential. The levels of frost hardiness and cold acclimation potential were expressed as the LT₅₀, the temperature at which 50% of the cells in leaf discs were killed, as measured by the ion leakage method, following a controlled freeze test There was considerable variation in both frost hardiness and cold acclimation potential in all three segregating populations (F₁ F₁ xcmm, and F₁ xcph). Frost hardiness and cold acclimation potential were not correlated, suggesting that these two traits are under independent genetic control. The analysis of generation means indicated that the variation for both traits could be best explained by an additive-dominance model, with additive gene effects the most important Broad-sense heritability was 0.73 and 0.74 in the F₁ population, for frost hardiness and cold acclimation potential, respectively, and was 0.85 for either trait in the F₁ xcmm population, indicating that these two traits are highly inheritable. Our results suggest that it should be possible to incorporate the frost hardiness and cold acclimation traits from S.commersonii into cultivated potato species.     

Plasticity of Lythrum salicaria and Phragmites australis growth characteristics across a European geographical gradient

Plants of Lythrum salicaria and Phragmites australis originating from localities across the European north–south geographical gradient were cultivated in parallel in an outdoor tub experiment. A strong correlation was found between growth and morphometric characteristics related to plant size (plant height, basal diameter, aboveground- and belowground plant biomass, etc.) and the position of the respective populations along the north–south gradient. Plants of both L. salicaria and P. australis from the southern localities grew taller and more vigorously and flowered later than plants from relatively more northern localities. From this point of view, the plants originating from south European populations were comparable to invasive North American plants. Our study indicates that explanation of the competitive success of populations invading new geographical areas may involve the role of geographic gradients within the species native range.     

Effects of temperature and photoperiod on frost resistance of white clover (Trifolium repens) ecotypes

Effects of photoperiod and temperature on frost resistance of seedling populations and clones of white clover (Trifolium repens L.) originating from various latitudes (58°48'–69°54'N) and altitudes (up to 1100 m above sea level) were studied in a phytotron. Low-temperature-induced cold acclimation in all populations was significantly stimulated by short photoperiod and low temperature. The highest levels of frost resistance were found in the northernmost seedling populations and in clones from northern coastal locations.     

Cold acclimation in warmer extended autumns impairs freezing tolerance of perennial ryegrass (Lolium perenne) and timothy (Phleum pratense)

The effect of variable autumn temperatures in combination with decreasing irradiance and daylength on photosynthesis, growth cessation and freezing tolerance was investigated in northern‐ and southern‐adapted populations of perennial ryegrass (Lolium perenne) and timothy (Phleum pratense) intended for use in regions at northern high latitudes. Plants were subjected to three different acclimation temperatures; 12, 6 and 9/3°C (day/night) for 4 weeks, followed by 1 week of cold acclimation at 2°C under natural light conditions. This experimental setup was repeated at three different periods during autumn with decreasing sums of irradiance and daylengths. Photoacclimation, leaf elongation and freezing tolerance were studied. The results showed that plants cold acclimated during the period with lowest irradiance and shortest day had lowest freezing tolerance, lowest photosynthetic activity, longest leaves and least biomass production. Higher acclimation temperature (12°C) resulted in lower freezing tolerance, lower photosynthetic activity, faster leaf elongation rate and higher biomass compared with the other temperatures. Photochemical mechanisms were predominant in photoacclimation. The northern‐adapted populations had a better freezing tolerance than the southern‐adapted except when grown during the late autumn period and at the highest temperature; then there were no differences between the populations. Our results indicate that the projected climate change in the north may reduce freezing tolerance in grasses as acclimation will take place at higher temperatures and shorter daylengths with lower irradiance.     

Ecophysiological responses of Fagus sylvatica seedlings to changing light conditions. I. Interactions between photosynthetic acclimation and photoinhibition during simulated canopy gap formation

Natural regeneration of European beech (Fagus sylvatica L.) establishes under shade, but sudden exposure to high irradiance may occur due to openings in the canopy. To elucidate ecophysiological mechanisms associated with survival of European beech seedlings, the gas exchange, chlorophyll concentrations, and chlorophyll a fluorescence parameters of two different beech populations were studied under changing light conditions. Plants were grown both in a growth chamber and at a natural site (one population) where the seedlings were raised in containers placed in understory and in simulated canopy gaps. Upon exposure to high light in the growth chamber, photosynthetic rates of shade-acclimated leaves of seedlings from both populations increased severalfold and then decreased over several days to the rates of the low-light control seedlings. High-light seedlings always had the highest photosynthetic rates. Initial fluorescence displayed a trend opposite that of photosynthesis; it increased over time, and relative fluorescence and half-time rise declined continuously until the end of experiment to very low values. Exposure to high light of shade-acclimated seedlings resulted in a shift in chlorophyll concentrations to levels intermediate between high-light and low-light seedlings. The light treatment effects were statistically greater than population effects; however, seedlings from the Abetone population were found to be more susceptible to changing light conditions than seedlings from Sicily. Reciprocal light treatments on plants growing at the natural site confirmed the results obtained in the growth chamber experiment. Overall, beech seedlings grown in the field appeared to have a fairly large acclimation potential achieved by plasticity in the photosynthetic apparatus. The lack of pronounced acclimation to high light in seedlings grown in the growth chamber was ascribed to a threshold-type relationship between the acclimation capacity and the level of damage. These observations on the limited potential for acclimation to high light in leaves of European beech seedlings which show a clear capability to exploit sunflecks, are discussed in relation to regeneration following canopy gap formation and reinforce the view of the central role of gap formation in forest dynamics. We conclude that small forest gaps (in which sunflecks play a major role) may present a favorable environment for survival and growth of beech because of their limited ability to acclimate to a sudden increase in irradiance and because of the moderate levels of light stress found in small gaps.     

Acclimation of respiratory temperature responses in northern and southern populations of Pinus banksiana

Temperature acclimation of respiration may contribute to climatic adaptation and thus differ among populations from contrasting climates. Short-term temperature responses of foliar dark respiration were measured in 33-yr-old trees of jack pine (Pinus banksiana) in eight populations of wide-ranging origin (44-55 degrees N) grown in a common garden at 46.7 degrees N. It was tested whether seasonal adjustments in respiration and population differences in this regard resulted from changes in base respiration rate at 5 degrees C (R(5)) or Q(10) (temperature sensitivity) and covaried with nitrogen and soluble sugars. In all populations, acclimation was manifest primarily through shifts in R(5) rather than altered Q(10). R(5) was higher in cooler periods in late autumn and winter and lower in spring and summer, inversely tracking variation in ambient air temperature. Overall, R(5) covaried with sugars and not with nitrogen. Although acclimation was comparable among all populations, the observed seasonal ranges in R(5) and Q(10) were greater in populations originating from warmer than from colder sites. Population differences in respiratory traits appeared associated with autumnal cold hardening. Common patterns of respiratory temperature acclimation among biogeographically diverse populations provide a basis for predicting respiratory carbon fluxes in a wide-ranging species.     

Temperature acclimation and heat tolerance of photosynthesis in Norwegian Saccharina latissima (Laminariales,Phaeophyceae)

Kelps, seaweeds and seagrasses provide important ecosystem services in coastal areas, and loss of these macrophytes is a global concern. Recent surveys have documented severe declines in populations of the dominant kelp species, Saccharina latissima, along the south coast of Norway. S. latissima is a cold‐temperate species, and increasing seawater temperature has been suggested as one of the major causes of the decline. Several studies have shown that S. latissima can acclimate to a wide range of temperatures. However, local adaptations may render the extrapolation of existing results inappropriate. We investigated the potential for thermal acclimation and heat tolerance in S. latissima collected from three locations along the south coast of Norway. Plants were kept in laboratory cultures at three different growth temperatures (10, 15, and 20°C) for 4–6 weeks, after which their photosynthetic performance, fluorescence parameters, and pigment concentrations were measured. S. latissima obtained almost identical photosynthetic characteristics when grown at 10 and 15°C, indicating thermal acclimation at these temperatures. In contrast, plants grown at 20°C suffered substantial tissue deterioration, and showed reduced net photosynthetic capacity caused by a combination of elevated respiration and reduced gross photosynthesis due to lowered pigment concentrations, altered pigment composition, and reduced functionality of Photo‐system II. Our results support the hypothesis that extraordinarily high temperatures, as observed in 1997, 2002, and 2006, may have initiated the declines in S. latissima populations along the south coast of Norway. However, observations of high mortality in years with low summer temperatures suggest that reduced population resilience or other factors may have contributed to the losses.     

Coupling of respiration, nitrogen, and sugars underlies convergent temperature acclimation in Pinus banksiana across wide-ranging sites and populations

Patterns and mechanisms of short‐term temperature acclimation and long‐term climatic adaptation of respiration among intraspecific populations are poorly understood, but both are potentially important in constraining respiratory carbon flux to climate warming across large geographic scales, as well as influencing the metabolic fitness of populations. Herein we report on leaf dark respiration of 33‐year‐old trees of jack pine (Pinus banksiana Lamb.) grown in three contrasting North American common gardens (0.9, 4.6, and 7.9 °C, mean annual temperature) comprised of identical populations of wide‐ranging geographic origins. We tested whether respiration rates in this evergreen conifer acclimate to prevailing ambient air temperatures and differ among populations. At each of the common gardens, observed population differences in respiration rates measured at a standard temperature (20 °C) were comparatively small and largely unrelated to climate of seed‐source origin. In contrast, respiration in all populations exhibited seasonal acclimation at all sites. Specific respiration rates at 20 °C inversely tracked seasonal variation in ambient air temperature, increasing with cooler temperatures in fall and declining with warmer temperatures in spring and summer. Such responses were similar among populations and sites, thus providing a general predictive equation regarding temperature acclimation of respiration for the species. Temperature acclimation was associated with variation in nitrogen (N) and soluble carbohydrate concentrations, supporting a joint enzyme and substrate‐based model of respiratory acclimation. Regression analyses revealed convergent relationships between respiration and the combination of needle N and soluble carbohydrate concentrations and between N‐based respiration (R_N, μmol mol N^{? 1} s^{? 1}) and soluble carbohydrate concentrations, providing evidence for general predictive relationships across geographically diverse populations, seasons, and sites. Overall, these findings demonstrate that seasonal acclimation of respiration modulates rates of foliar respiratory carbon flux in a widely distributed evergreen species, and does so in a predictable way. Genetic differences in specific respiration rate appear less important than temperature acclimation in downregulating respiratory carbon fluxes with climate warming across wide‐ranging sites.     

List of members August, 1962

We investigated the acclimation of Chondrus crispus to growth at 5°C and 20°C in the laboratory. We were specifically interested in the responses of light-limited photosynthesis to temperature and the effects of short-term thermal changes (of the order of minutes). Thermal acclimation to constant temperatures over 3–4 weeks had significant effects on the light-use characteristics of this species such that in comparison with those grown at 5°C, 20°C-grown plants had higher concentrations of chlorophyll a and total phycobilins, which were associated with larger photosynthetic unit sizes. Plants grown at the higher temperature had greater photosynthetic efficiencies (α) and higher rates of light-limited photosynthesis at a given photon flux density than did plants acclimated to 5°C. Plants acclimated to 20°C were less sensitive to short-term temperature changes than were 5°C-acclimated plants. These results are discussed in terms of (1) the effects of growth temperature on light harvesting and (2) the implications of exposure to constant temperature for short-term thermal responses.     

Plant responses to past concentrations of CO2

The influence of recent historical changes in atmospheric CO₂ have been investigated by two methods: 1, the responses of leaf development and physiology as indicated by leaves stored in herbaria and 2, by investigating the differential growth responses of populations originating from naturally different CO₂ concentrations. Herbarium leaves indicate that stomatal density and leaf nitrogen have decreased over the last 150 to 200 years, while water use efficiency, estimated from leaf ¹³C and historical measurements of climate, has increased.Natural populations ofBoehmeria cylindrica were found growing at sites, in Florida, with CO₂ mole fractions varying naturally from 350 µmol mol^-1 to 505 µmol mol^-1. Plants were grown in the controlled environment, using seeds originating from populations occurring in the different CO₂ mole fractions. Plants from the different ambient CO₂ mole fractions showed different rates of growth and different non-linear responses of the shoot to root ratio in response to changes in the CO₂ mole fraction from 350 to 675 µmol mol^-1.The proposal that plants originating from high altitude will whow greater stimulations of growth with an increase in CO₂, than plants from low altitude, was rejected in experiments which simulated the atmospheric pressure at altitudes of 0 and 2000m, at CO₂ mole fractions of 350 and 700 µmol mol^-1 and on populations ofPlantago major originating from altitudes of 0 and 3335 m.     

Developmental acclimation affects clinal variation in stress resistance traits in Drosophila buzzatii

Patterns of clinal genetic variation in Drosophila are often characterized after rearing at constant temperatures. However, clinal patterns might change after acclimation if populations differ in their plastic response to fluctuating environments. We studied longevity, starvation and heat knock‐down resistance after development at either constant or fluctuating temperatures in nine Drosophila buzzatii populations collected along an altitudinal gradient in Tenerife, Spain. Flies that developed at fluctuating temperatures had higher stress resistance despite experiencing a slightly lower average temperature than those at constant temperatures. Genetic variation along the gradient was found in both stress‐resistance traits. Because Q_ST values greatly exceeded F_ST values, genetic drift could not explain this diversification. In general, differences among populations were larger after rearing at fluctuating temperatures, especially in heat knock‐down, for which clinal patterns disappeared when flies were reared at constant temperatures. This result emphasizes the importance of determining whether populations originating from different environments differ in their plastic responses to stress.     

Differentiation in populations of Hordeum spontaneum Koch along a gradient of environmental productivity and predictability: plasticity in response to water and nutrient stress

Plants from four populations of Hordeum spontaneum originating in distinct environments of Israel were compared for stress induced phenotypic plasticity. The environments ranged along a gradient of increasing rainfall amount and predictability from low (desert) to moderate (semisteppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The plants were exposed to a set of four treatments: no stress (optimum water and nutrients), water, nutrient and both water and nutrient stress. Plants from the four populations (or ecotypes) exhibited different patterns of plasticity in response to the different stresses (water and nutrients) and in different trait categories (reproductive, fitness and resource allocation). The importance of plasticity in response to water stress appears to decrease, and to nutrient stress appears to increase along the increasing rainfall gradient. The mountain ecotype, growing in an area with high potential productivity (amount of rainfall) but experiencing periodic frosts, was the most plastic among ecotypes in resource allocation under both water and nutrient stress, but exhibited low plasticity in other trait categories. In contrast, the desert ecotype had low plasticity in resource allocation under water stress and the lowest plasticity among the four ecotypes in all trait categories in response to nutrient stress. The ecotype originating in Mediterranean grassland, a predictable and most favourable environment, was highly plastic in fitness and allocation traits in response to low nutrient levels which is likely to occur due to competition in productive environment. We discuss the observed differences in ecotype plasticity as part of their environmentally induced adaptive ‘strategies’. We found no support for the hypothesis that plants originating in environments with greater variation and unpredictability are more plastic. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society 2002, 75 , 301–312.     

GENETIC CONSEQUENCES OF PASSIVE DISPERSAL IN POND-DWELLING COPEPODS

Pond-dwelling copepods have colonized habitats throughout North America after glaciers have receded. Most species are passively transported via resting eggs into new habitats. Colonists originating in a glacial refugium could lose some of the ancestral genetic diversity when they establish new populations and the attenuation may be substantial in populations far removed from the refugium due to multiple founder events. Genetic variation was measured in Heterocope septentrionalis from 27 populations at arctic sites near potential refugia and those more recently deglaciated to determine the effects of postglacial dispersal on patterns of genetic relatedness and diversity. Some populations were more distant, genetically, from others within the same site than those from other more distant sites. Eleven polymorphic enzyme loci were significantly more variable (F [1,294 df] = 5.94, P < 0.025) among individuals from populations near the Alaskan refuge than those at the eastern limit of their distribution. Because populations are typically extremely large and stable this loss of genetic diversity is attributed primarily to repeated founder events during colonization. This result suggests profound genetic changes may occur on a continental scale in passively dispersed copepods due to founder events alone. Their potential for divergence and speciation is greater than currently recognized.     

TEMPERATURE ACCLIMATION AND THE NERVOUS SYSTEM

1. The conduction velocity of the compound action potential of peripheral nerves shows compensatory acclimation to temperature in a fish, a snail, a crab, and probably also in the frog. The heat and cold tolerances of peripheral conduction are probably both increased by cold acclimation in the frog. 2. The properties of compound action potentials are not suitable for temperature acclimation studies, since different neuronal populations in the same nerve have been found to exhibit different temperature characteristics. 3. Single but septate giant nerve fibres of earthworms show compensatory temperature acclimation of the conduction properties, the form of the action potential and of the axonal cable properties, especially below 13–19 °C. 4. The fatty acids and the plasmalogen aldehydes of the phospholipids of the goldfish brain are more unsaturated at lower acclimation temperatures. 5. The Na⁺-K⁺ ATPase activity of the earthworm nerve cord shows compensatory acclimation at low temperatures. 6. The spontaneous activity of the central nervous system of insects is altered in a compensatory manner by temperature acclimation. In fish, the cold tolerance of simple and complex reflexes and of conditioning is adaptively altered by temperature acclimation. The role of the central nervous system, especially of the thermoregulatory centre, in the temperature acclimation of homeotherms is established. 7. There are adaptive isoenzymes of acetylcholinesterase in the brain of the rainbow trout. These isoenzymes differ from each other in respect of the temperature dependence of their enzyme-substrate affinity. The synthesis of acetylcholine receptor molecules may also be affected by temperature acclimation. 8. The metabolism of putative synaptic neurotransmitters (5-hydroxytryptamine, adrenaline, noradrenaline) is altered in the frog and mouse brains during the early phases of temperature acclimation. These changes may initiate the physiological processes connected with temperature acclimation. 9. The neuromuscular transmission in the frog shows after acclimation to cold, increased resistance to it and some indications of temperature compensation. 10. Changes in neurosecretion seem to be involved in temperature acclimation both in poikilotherms and homeotherms. The fast axonal transport of proteins shows compensatory acclimation to cold in the frog.     

Critical thermal maxima of juvenile alligator gar (Atractosteus spatula,Lacépède, 1803) from three Mississippi‐drainage populations acclimated to three temperatures

Local adaptation may cause thermal tolerance to vary between nearby but distinct populations of a species. During the summer of 2013, alligator gar Atractosteus spatula spawned from broodstock collected from three populations within the Mississippi River drainage separated by a 5° latitudinal gradient were acclimated to three temperatures (25, 30, and 35°C). Ten fish from each population were acclimated at each temperature. CT_Max was determined at each temperature for each population, using five fish for each population‐acclimation temperature pairing. CT_Max for each population‐acclimation temperature pairing was compared using two‐factor anova . CT_Max increased significantly with acclimation temperature (F_2,40 = 600.5, P < 0.001) but population had no significant effect (F_2,40 = 1.882, P = 0.166). Temperature tolerance appears to be consistent across populations of alligator gar, with no evidence of local adaptation.