Suggested limits to the use of the hot tub and sauna by pregnant women

Because of reports of the potential risk of maternal hyperthermia to a developing embryo or fetus, studies were done to determine the length of time a woman must stay in a hot tub or sauna before her temperature reaches 38.9°C. The vaginal temperatures of 20 nonpregnant women of childbearing age were recorded while they sat in hot tubs set at 39.0°C or 41.1°C and in a sauna with an average temperature of 81.4°C. Five women were able to remain in the 39.0°C tub and six in the 41.1°C tub until their temperature reached 38.9°C, but in none did their temperature reach that level before 15 minutes in the 39.0°C tub or 10 minutes in the 41.1°C tub. The remainder left in discomfort while their body temperatures were lower. This indicates that the usual use of hot tubs is unlikely to raise a woman''s body temperature to potentially teratogenic levels, although prolonged use may. None of the women were able to remain in the sauna long enough for their temperature to reach 39.9°C.     

Limited heat tolerance in an Arctic passerine: Thermoregulatory implications for cold‐specialized birds in a rapidly warming world

1. Arctic animals inhabit some of the coldest environments on the planet and have evolved physiological mechanisms for minimizing heat loss under extreme cold. However, the Arctic is warming faster than the global average and how well Arctic animals tolerate even moderately high air temperatures (T _a) is unknown.
2. Using flow‐through respirometry, we investigated the heat tolerance and evaporative cooling capacity of snow buntings (Plectrophenax nivalis; ≈31 g, N = 42), a cold specialist, Arctic songbird. We exposed buntings to increasing T _a and measured body temperature (T _b), resting metabolic rate (RMR), rates of evaporative water loss (EWL), and evaporative cooling efficiency (the ratio of evaporative heat loss to metabolic heat production).
3. Buntings had an average (±SD) T _b of 41.3 ± 0.2°C at thermoneutral T _a and increased T _b to a maximum of 43.5 ± 0.3°C. Buntings started panting at T _a of 33.2 ± 1.7°C, with rapid increases in EWL starting at T _a = 34.6°C, meaning they experienced heat stress when air temperatures were well below their body temperature. Maximum rates of EWL were only 2.9× baseline rates at thermoneutral T _a, a markedly lower increase than seen in more heat‐tolerant arid‐zone species (e.g., ≥4.7× baseline rates). Heat‐stressed buntings also had low evaporative cooling efficiencies, with 95% of individuals unable to evaporatively dissipate an amount of heat equivalent to their own metabolic heat production.
4. Our results suggest that buntings’ well‐developed cold tolerance may come at the cost of reduced heat tolerance. As the Arctic warms, and this and other species experience increased periods of heat stress, a limited capacity for evaporative cooling may force birds to increasingly rely on behavioral thermoregulation, such as minimizing activity, at the expense of diminished performance or reproductive investment.

     

Acquisition of Thermotolerance in Soybean Seedlings : Synthesis and Accumulation of Heat Shock Proteins and their Cellular Localization

When soybean Glycine max var Wayne seedlings are shifted from a normal growth temperature of 28°C up to 40°C (heat shock or HS), there is a dramatic change in protein synthesis. A new set of proteins known as heat shock proteins (HSPs) is produced and normal protein synthesis is greatly reduced. A brief 10-minute exposure to 45°C followed by incubation at 28°C also results in the synthesis of HSPs. Prolonged incubation (e.g. 1-2 hours) at 45°C results in greatly impaired protein synthesis and seedling death. However, a pretreatment at 40°C or a brief (10-minute) pulse treatment at 45°C followed by a 28°C incubation provide protection (thermal tolerance) to a subsequent exposure at 45°C. Maximum thermoprotection is achieved by a 2-hour 40°C pretreatment or after 2 hours at 28°C with a prior 10-minute 45°C exposure. Arsenite treatment (50 micromolar for 3 hours) also induces the synthesis of HSP-like proteins, and also provides thermoprotection to a 45°C HS; thus, there is a strong positive correlation between the accumulation of HSPs and the acquisition of thermal tolerance under a range of conditions.

During 40°C HS, some HSPs become localized and stably associated with purified organelle fractions (e.g. nuclei, mitochondria, and ribosomes) while others do not. A chase at 28°C results in the gradual loss over a 4-hour period of the HSPs from the organelle fractions, but the HSPs remain selectively localized during a 40°C chase period. If the seedlings are subjected to a second HS after a 28°C chase, the HSPs rapidly (complete within 15 minute) relocalize in the organelle fractions. The relative amount of the HSPs which relocalize during a second HS increases with higher temperatures from 40°C to 45°C. Proteins induced by arsenite treatment are not selectively localized with organelle fractions at 28°C but become organelle-associated during a subsequent HS at 40°C.

     

Controlled Heat Stress Promotes Myofibrillogenesis during Myogenesis

Hyperthermia therapy has recently emerged as a clinical modality used to finely tune heat stress inside the human body for various biomedical applications. Nevertheless, little is known regarding the optimal timing or temperature of heat stress that is needed to achieve favorable results following hyperthermia therapy for muscle regeneration purposes. The regeneration of skeletal muscle after injury is a highly complex and coordinated process that involves a multitude of cellular mechanisms. The main objective of this study was to characterize the effects of hyperthermal therapy on the overall behavior of myoblasts during myogenic differentiation. Various cellular processes, including myogenesis, myofibrillogenesis, hypertrophy/atrophy, and mitochondrial biogenesis, were studied using systematic cellular, morphological, and pathway-focused high-throughput gene expression profiling analyses. We found that C2C12 myoblasts exhibited distinctive time and temperature-dependence in biosynthesis and regulatory events during myogenic differentiation. Specifically, we for the first time observed that moderate hyperthermia at 39°C favored the growth of sarcomere in myofibrils at the late stage of myogenesis, showing universal up-regulation of characteristic myofibril proteins. Characteristic myofibrillogenesis genes, including heavy polypeptide 1 myosin, heavy polypeptide 2 myosin, alpha 1 actin, nebulin and titin, were all significantly upregulated (p<0.01) after C2C12 cells differentiated at 39°C over 5 days compared with the control cells cultured at 37°C. Furthermore, moderate hyperthermia enhanced myogenic differentiation, with nucleus densities per myotube showing 2.2-fold, 1.9-fold and 1.6-fold increases when C2C12 cells underwent myogenic differentiation at 39°C over 24 hours, 48 hours and 72 hours, respectively, as compared to the myotubes that were not exposed to heat stress. Yet, atrophy genes were sensitive even to moderate hyperthermia, indicating that strictly controlled heat stress is required to minimize the development of atrophy in myotubes. In addition, mitochondrial biogenesis was enhanced following thermal induction of myoblasts, suggesting a subsequent shift toward anabolic demand requirements for energy production. This study offers a new perspective to understand and utilize the time and temperature-sensitive effects of hyperthermal therapy on muscle regeneration.     

Cooperation and opportunism in Galapagos sea lion hunting for shoaling fish

For predators, cooperation can decrease the cost of hunting and potentially augment the benefits. It can also make prey accessible that a single predator could not catch. The degree of cooperation varies substantially and may range from common attraction to a productive food source to true cooperation involving communication and complementary action by the individuals involved. We here describe cooperative hunting of Galapagos sea lions (Zalophus wollebaeki) for Amberstripe scad (Decapterus muroadsi), a schooling, fast swimming semipelagic fish. A group of 6–10 sea lions, usually females only, drove scad over at least 600–800 m from open water into a cove where, in successful hunts, they drove them ashore. Frequently, these “core hunters” were joined toward the final stages of the hunt by another set of opportunistic sea lions from a local colony at that beach. The “core hunters” did not belong to that colony and apparently were together coming toward the area specifically for the scad hunt. Based on the observation of 40 such hunts from 2016 to 2020, it became evident that the females performed complementary actions in driving the scad toward the cove. No specialization of roles in the hunt was observed. All “core hunters” and also opportunistically joining sea lions from the cove shared the scad by randomly picking up a few of the 25–300 (mean 100) stranded fish as did scrounging brown pelicans. In one of these hunts, four individual sea lions were observed to consume 7–8 fish each in 25 s. We conclude that the core hunters must communicate about a goal that is not present to achieve joint hunting but presently cannot say how they do so. This is a surprising achievement for a species that usually hunts singly and in which joint hunting plays no known role in the evolution of its sociality.     

Effect of Season and High Ambient Temperature on Activity Levels and Patterns of Grizzly Bears (Ursus arctos)

Understanding factors that influence daily and annual activity patterns of a species provides insights to challenges facing individuals, particularly when climate shifts, and thus is important in conservation. Using GPS collars with dual-axis motion sensors that recorded the number of switches every 5 minutes we tested the hypotheses: 1. Grizzly bears (Ursus arctos) increase daily activity levels and active bout lengths when they forage on berries, the major high-energy food in this ecosystem, and 2. Grizzly bears become less active and more nocturnal when ambient temperature exceeds 20°C. We found support for hypothesis 1 with both male and female bears being active from 0.7 to 2.8 h longer in the berry season than in other seasons. Our prediction under hypothesis 2 was not supported. When bears foraged on berries on a dry, open mountainside, there was no relationship between daily maximum temperature (which varied from 20.4 to 40.1°C) and the total amount of time bears were active, and no difference in activity levels during day or night between warm (20.4–27.3°C) and hot (27.9–40.1°C) days. Our results highlight the strong influence that food acquisition has on activity levels and patterns of grizzly bears and is a challenge to the heat dissipation limitation theory.     

Tropical bats counter heat by combining torpor with adaptive hyperthermia

Many tropical mammals are vulnerable to heat because their water budget limits the use of evaporative cooling for heat compensation. Further increasing temperatures and aridity might consequently exceed their thermoregulatory capacities. Here, we describe two novel modes of torpor, a response usually associated with cold or resource bottlenecks, as efficient mechanisms to counter heat. We conducted a field study on the Malagasy bat Macronycteris commersoni resting in foliage during the hot season, unprotected from environmental extremes. On warm days, the bats alternated between remarkably short micro-torpor bouts and normal resting metabolism within a few minutes. On hot days, the bats extended their torpor bouts over the hottest time of the day while tolerating body temperatures up to 42.9°C. Adaptive hyperthermia combined with lowered metabolic heat production from torpor allows higher heat storage from the environment, negates the need for evaporative cooling and thus increases heat tolerance. However, it is a high-risk response as the torpid bats cannot defend body temperature if ambient temperature increases above a critical/lethal threshold. Torpor coupled with hyperthermia and micro-torpor bouts broaden our understanding of the basic principles of thermal physiology and demonstrate how mammals can perform near their upper thermal limits in an increasingly warmer world.     

PULSATION FREQUENCY OF THE ADVISCERAL AND ABVISCERAL HEART BEATS OF CIONA INTESTINALIS IN RELATION TO TEMPERATURE

The frequency of pulsation of the heart of Ciona intestinalis increases with temperature in both advisceral and abvisceral direction, according to the Arrhenius equation. The increase in pulsation is the same in both directions. The following µ values were obtained: 8,000–, 12,000+, 16,000, in several combinations, with critical temperatures at 10°, 15°, and 20°C. The values found are comparable with earlier findings for activity of the heart in different animals. This quantitative correspondence suggests anew the conception that temperature characteristics may be employed for recognition of controlling processes. The fact that the µ''s and the critical temperature are the same for advisceral and abvisceral beats, indicates that the general metabolic condition of the two ends of the heart is the same in any one individual.     

Warm growth temperatures decrease soybean cholinephosphotransferase activity

The activity of cytidine 5′-diphosphate (CDP) choline: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) in developing soybean (Glycine max L. var Williams 82) seeds was 3 to 5 times higher in cotyledons grown at 20°C than in those grown at 35°C. Some characteristics of the enzyme from cotyledons cultured at 20 and 35°C were compared. In preparations from both growth temperatures, the enzyme showed a pH optimum of 7, K_m of 7.0 micromolar for CDP-choline, and an optimum assay temperature of 45°C. Both enzyme preparations were stimulated by increasing concentrations of Mg²⁺ or Mn²⁺, up to 10 millimolar and 50 micromolar, respectively, though Mn²⁺ produced lower activities than Mg²⁺. Enzymes from both 20 and 35°C show the same specificity for exogenous diacylglycerol. No metabolic effectors were detected by addition of heat treated extracts to the assay mixture. The above findings suggest that the higher enzyme activity at 20°C can be attributed to a higher level of the enzyme rather than to the involvement of isozymes or metabolic effectors. Enzyme activity decreased rapidly during culture at 35°C, indicating a rapid turnover of the enzyme. The level of temperature modulation was found to be a function of seed developmental stage.     

Variable Temperature Stress in the Nematode Caenorhabditis elegans (Maupas) and Its Implications for Sensitivity to an Additional Chemical Stressor

A wealth of studies has investigated how chemical sensitivity is affected by temperature, however, almost always under different constant rather than more realistic fluctuating regimes. Here we compared how the nematode Caenorhabditis elegans responds to copper at constant temperatures (8–24°C) and under fluctuation conditions of low (±4°C) and high (±8°C) amplitude (averages of 12, 16, 20°C and 16°C respectively). The DEBkiss model was used to interpret effects on energy budgets. Increasing constant temperature from 12–24°C reduced time to first egg, life-span and population growth rates consistent with temperature driven metabolic rate change. Responses at 8°C did not, however, accord with this pattern (including a deviation from the Temperature Size Rule), identifying a cold stress effect. High amplitude variation and low amplitude variation around a mean temperature of 12°C impacted reproduction and body size compared to nematodes kept at the matching average constant temperatures. Copper exposure affected reproduction, body size and life-span and consequently population growth. Sensitivity to copper (EC₅₀ values), was similar at intermediate temperatures (12, 16, 20°C) and higher at 24°C and especially the innately stressful 8°C condition. Temperature variation did not increase copper sensitivity. Indeed under variable conditions including time at the stressful 8°C condition, sensitivity was reduced. DEBkiss identified increased maintenance costs and increased assimilation as possible mechanisms for cold and higher copper concentration effects. Model analysis of combined variable temperature effects, however, demonstrated no additional joint stressor response. Hence, concerns that exposure to temperature fluctuations may sensitise species to co-stressor effects seem unfounded in this case.     

Heat stress enhances phytohemagglutinin synthesis but inhibits its transport out of the endoplasmic reticulum

In this study we examined the effect of heat stress (up to 6 hours at 43°C) on the biosynthesis and transport of phytohemagglutinin (PHA) in cotyledons of developing seeds of the common bean, Phaseolus vulgaris. Heat stress resulted in a decrease of total protein synthesis and an enhancement of the synthesis of heat shock proteins and PHA. Pulse chase experiments showed that a considerable proportion of the newly synthesized PHA was present in the endoplasmic reticulum (ER)/Golgi fraction and did not readily chase-out. Analysis with endoglycosidase H showed that the oligosaccharide sidechains of PHA were almost entirely in the high mannose configuration, indicating that most of the newly synthesized PHA was in the ER. However, some of the PHA became fucosylated at 43°C, indicating fucosyltransferase activity. That the biosynthesis and secretion of fucosyl-containing cell wall polymers proceeded normally at 43°C provided evidence that certain Golgi functions (i.e. transport to the cell wall) remained unaffected by heat stress. The ER obtained from these heat stress cotyledons had a greater density (1.16 g· cm⁻³ at 43°C instead of 1.14 g·cm⁻³ at 22°C) in sucrose gradients. Ultrastructural observations showed that the width of the lumen of the ER cisternae had increased from 20 nanometers at 22°C to 60 to 80 nanometers at 43°C; the lumen was filled with electrondense material presumed to be protein. The experiments are interpreted as evidence that heat stress imposes a block in the transport of PHA out of the ER. Whether heat stress affects the ER itself or alters the conformation of PHA, thereby preventing its transport, is not clear.     

Reaction Norms in Natural Conditions: How Does Metabolic Performance Respond to Weather Variations in a Small Endotherm Facing Cold Environments?

Reaction norms reflect an organisms'' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints.     

Heat stress reveals a specialized variant of the pachytene checkpoint in meiosis of Arabidopsis thaliana

Plant growth and fertility strongly depend on environmental conditions such as temperature. Remarkably, temperature also influences meiotic recombination and thus, the current climate change will affect the genetic make-up of plants. To better understand the effects of temperature on meiosis, we followed male meiocytes in Arabidopsis thaliana by live cell imaging under three temperature regimes: at 21°C; at heat shock conditions of 30°C and 34°C; after an acclimatization phase of 1 week at 30°C. This work led to a cytological framework of meiotic progression at elevated temperature. We determined that an increase from 21°C to 30°C speeds up meiosis with specific phases being more amenable to heat than others. An acclimatization phase often moderated this effect. A sudden increase to 34°C promoted a faster progression of early prophase compared to 21°C. However, the phase in which cross-overs mature was prolonged at 34°C. Since mutants involved in the recombination pathway largely did not show the extension of this phase at 34°C, we conclude that the delay is recombination-dependent. Further analysis also revealed the involvement of the ATAXIA TELANGIECTASIA MUTATED kinase in this prolongation, indicating the existence of a pachytene checkpoint in plants, yet in a specialized form.

Live cell imaging of plants exposed to different heat stresses provides a temporal framework of meiosis at high temperatures in wild-type and mutants for several meiotic recombination factors.     

Decreased growth temperature increases soybean stearoyl-acyl carrier protein desaturase activity

Developing soybean (Glycine max) seeds respond to a change in growth temperature by changing the level of stearoyl acyl carrier protein desaturase activity in the tissue. After 20 hours in liquid culture, seeds grown at 20°C show an increase in activity while seeds grown at 35°C show a decrease in activity, relative to their preculture levels. Analysis of the enzyme from both growth conditions shows the change not to be due to induction of kinetically distinct iosenzymes; desaturase activities from both 20°C and 35°C have identical behavior with regard to pH, temperature optimum, substrate concentration and cofactor requirements. Experiments with boiled extracts indicate that the modulation is not caused by induction of metabolic effectors. From these data, it appears that stearoyl-acyl carrier protein desaturase responds to changes in growth temperature by altering the level of active enzyme present in the tissue. The magnitude of this response is a function of the developmental stage of the seed and not a function of the growth conditions of the parent plant. Changing the age of the seeds from early late R5 changed the ratio of 20:35°C activity from 3.8:1 to 1.2:1. Changing the temperature at which the parent plants were grown over a range from 20/12°C to 34/28°C (day/night) produced only minor, and inconsistent, changes in the ratio of 20:35°C activities.     

Heat Stress Responses in Cultured Plant Cells : Heat Tolerance Induced by Heat Shock versus Elevated Growing Temperature

Using cultured pear (Pyrus communis cv Bartlett) cells, heat tolerance induced by heat shock was compared to that developed during growth at high temperature. After growth at 22°C, cells exposed to 38°C for 20 minutes (heat shock) showed maximum increased tolerance within 6 hours. Cells grown at 30°C developed maximum heat tolerance after 5 to 6 days; this maximum was well below that induced by heat shock. Heat shock-induced tolerance was fully retained at 22°C for 2 days and was only partly lost after 4 days. However, pear cells acclimated at 30°C lost all acquired heat tolerance 1 to 2 days after transfer to 22°C. In addition, cells which had been heat-acclimated by growth at 30°C showed an additional increase in heat tolerance in response to 39°C heat shock. The most striking difference between heat shock and high growth temperature effects on heat tolerance was revealed when tolerance was determined using viability tests based on different cell functions. Growth at 30°C produced a general hardening, i.e. increased heat tolerance was observed with all three viability tests. In contrast, significantly increased tolerance of heat-shocked cells was observed only with the culture regrowth test. The two types of treatment evoke different mechanisms of heat acclimation.     

Effects of heat shock on amino Acid metabolism of cowpea cells

When cowpea (Vigna unguiculata) cells maintained at 26°C are transferred to 42°C, rapid accumulation of γ-aminobutyrate (>10-fold) is induced. Several other amino acids (including β-alanine, alanine, and proline) are also accumulated, but less extensively than γ-aminobutyrate. Total free amino acid levels are increased approximately 1.5-fold after 24 hours at 42°C. Heat shock also leads to release of amino acids into the medium, indicating heat shock damage to the integrity of the plasmalemma. Some of the changes in metabolic rates associated with heat shock were estimated by monitoring the ¹⁵N labeling kinetics of free intracellular, extracellular and protein-bound amino acids of cultures supplied with ¹⁵NH₄⁺, and analyzing the labeling data by computer simulation. Preliminary computer simulation models of nitrogen flux suggest that heat shock induces an increase in the γ-aminobutyrate synthesis rate from 12.5 nanomoles per hour per gram fresh weight in control cells maintained at 26°C, to as high as 800 nanomoles per hour per gram fresh weight within the first 2 hours of heat shock. This 64-fold increase in the γ-aminobutyrate synthesis rate greatly exceeds the expected (Q₁₀) change of metabolic rate of 2.5- to 3-fold due to a 16°C increase in temperature. We suggest that this metabolic response may in part involve an activation of glutamate decarboxylase in vivo, perhaps mediated by a transient cytoplasmic acidification. Proline appears to be synthesized from glutamate and not from ornithine in cowpea cells. Proline became severalfold more heavily labeled than ornithine, citrulline and arginine in both control and heat-shocked cultures. Proline synthesis rate was increased 2.7-fold by heat shock. Alanine, β-alanine, valine, leucine, and isoleucine synthesis rates were increased 1.6-, 3.5-, 2.0-, 5.0-, and 6.0-fold, respectively, by heat shock. In contrast, the phenylalanine synthesis rate was decreased by 50% in response to heat shock. The differential effects of heat stress on metabolic rates lead to flux and pool size redistributions throughout the entire network of amino acid metabolism.     

Modeling the Recovery of Heat-Treated Bacillus licheniformis Ad978 and Bacillus weihenstephanensis KBAB4 Spores at Suboptimal Temperature and pH Using Growth Limits

The apparent heat resistance of spores of Bacillus weihenstephanensis and Bacillus licheniformis was measured and expressed as the time to first decimal reduction (δ value) at a given recovery temperature and pH. Spores of B. weihenstephanensis were produced at 30°C and 12°C, and spores of B. licheniformis were produced at 45°C and 20°C. B. weihenstephanensis spores were then heat treated at 85°C, 90°C, and 95°C, and B. licheniformis spores were heat treated at 95°C, 100°C, and 105°C. Heat-treated spores were grown on nutrient agar at a range of temperatures (4°C to 40°C for B. weihenstephanensis and 15°C to 60°C for B. licheniformis) or a range of pHs (between pH 4.5 and pH 9.5 for both strains). The recovery temperature had a slight effect on the apparent heat resistance, except very near recovery boundaries. In contrast, a decrease in the recovery pH had a progressive impact on apparent heat resistance. A model describing the heat resistance and the ability to recover according to the sporulation temperature, temperature of treatment, and recovery temperature and pH was proposed. This model derived from secondary mathematical models for growth prediction. Previously published cardinal temperature and pH values were used as input parameters. The fitting of the model with apparent heat resistance data obtained for a wide range of spore treatment and recovery conditions was highly satisfactory.     

Effect of temperature conditioning on chilling injury of cucumber cotyledons: possible role of abscisic Acid and heat shock proteins

Endogenous abscisic acid levels and induced heat shock proteins were measured in tissue exposed for 6 hours to temperatures that reduced their subsequent chilling sensitivity. One-centimeter discs excised from fully expanded cotyledons of 11-day-old seedlings of cucumber (Cucumis sativus L., cv Poinsett 76) were exposed to 12.5 or 37°C for 6 hours followed by 4 days at 2.5 or 12.5°C. Ion leakage, a qualitative indicator of chilling injury, increased after 2 to 3 day exposure to 2.5°C, but not to 12.5°C, a nonchilling temperature. Exposure to 37°C before chilling significantly reduced the rate of ion leakage by about 60% compared to tissue exposed to 12.5°C before chilling, but slightly increased leakage compared to tissue exposed to 12.5 or 37°C and held at the nonchilling temperature of 12.5°C. There was no relationship between abscisic acid content following exposure to 12.5 or 37°C and chilling tolerance. Five heat shock proteins, with apparent molecular mass of 25, 38, 50, 70, and 80 kilodaltons, were induced by exposure to 37 or 42°C for 6 hours, and their appearance coincided with increased chilling resistance. Heat shock treatments reduced the synthesis of three proteins with apparent molecular mass of 14, 17, and 43 kilodaltons. Induction of heat shock proteins could be a possible cause of reduced chilling injury in tissue exposed to 37 or 42°C.