Heat stress poses a serious threat to plant survival and productivity, and has a direct influence on crop yield stability. Plants response to high temperature is tightly controlled by complex genetic networks. Plants can be acclimated through gradual pre-exposure to increasing temperatures and that in turn causes higher survival in subsequent and otherwise lethal heat stress conditions. To investigate the physiological and molecular processes underlying heat acclimation and recovery, we examined changes in Arabidopsis thaliana transcriptome throughout the acclimation and the subsequent heat shock treatment. Groups of differentially expressed genes and enriched biological pathways that constitute the heat transcriptional memory were identified. The function of flavonoids in plant heat stress were further explored experimentally. In addition, we observed altered stomata density and aperture responses in heat acclimated plants, and this might be partially controlled by AGAMOUS-LIKE16 (AGL16) transcription factor and its negative regulator microRNA824 (miR824). By utilizing an automated non-invasive phenotyping facility, we have developed a protocol to record plant growth and photosynthetic performance after heat stress in wild type Arabidopsis thaliana and mutant lines at daily intervals. Through an imaging-based analysis of plants growth, we confirmed impaired thermotolerance of hsp101 compared to wild type plants by a time-series growth, morphology and chlorophyll responses. This offers a novel experimental setup for thermotolerance screenings in Arabidopsis, with defined digital markers linking the function of selected genes in heat stress responses to phenotypic traits.

It is well established that older adults display marked impairments in the heat loss responses of sweating and skin blood flow relative to young adults which can exacerbate body heat storage by compromising whole-body heat loss (evaporative + dry heat exchange). Similarly, young women display reductions in whole-body heat loss relative to young men during exercise in dry heat. As such, it is possible that the age-related decline in whole-body heat loss will be greater among women relative to men. To examine whether the age-related decline in whole-body heat loss would be greater in women relative to men during exercise in dry heat, and whether this response would be more pronounced with a greater elevation in the level of heat stress, whole-body heat loss (evaporative ± dry heat exchange) was evaluated in 80 individuals (46 men, 34 women) aged between 18-70 years. Participants completed an incremental exercise model involving three, 30-min bouts of semi-recumbent cycling at fixed rates of metabolic heat production (150, 200, 250 W/m2), each separated by a 15-min recovery period in hot-dry conditions (40˚C, ~15% relative humidity). Whole-body heat loss was measured using direct calorimetry whereas metabolic heat production was measured using indirect calorimetry. Whole-body heat loss declined with age (across men and women) during moderate- and vigorous-intensity exercise by 4.2 and 6.6 W/m2 (both P < 0.050), respectively, however, these relationships were not modified by sex (both P > 0.050). Nonetheless, whole-body heat loss was lower in women relative to men during moderate- and vigorous-intensity exercise by 8.4 and 12.1 W/m2 (both P < 0.05), respectively. Therefore, the results of this thesis demonstrate that the age-related decline in whole-body heat loss is not dependent on sex.

Periconceptional heat stress is known to reduce the likelihood of establishing pregnancy; however, some conceptuses will survive. Of the pregnancies that continue to term, a proportion will be heifers which are retained as replacement animals. Alterations that occur as a result of exposure to thermal stress during such critical stages in development likely result in differential performance between the heat stress-conceieved (HSC) cows and thermoneutral-conceived (TNC) cows. National Dairy Herd Improvement Association data was obtained from Dairy Records Management Systems. Records (n =14,189,891) included cows born between 1977 and 2010 in FL, GA, SC, MS, LA, AL, and TX. Records were edited to include only Holsteins born between 2000 and 2010 (n = 704,419). Conception dates were calculated by subtracting 276 d from the recorded birth date. Records for cows conceived within the months of June, July, and August were retained as HSC cows; cows conceived within the months of December, January, and February were retained as TNC contemporaries. Significant differences (P<0.01) in mature-equivalent milk yield were observed in all first lactation cows, and in cows that were retained within one herd for three lactations. In the latter group alterations in milk compositions were statistically significant (P<0.01), but not biologically so. Furthermore, significant differences (P<0.01) in days open were observed in cows retained within one herd for three lactations. The effects of periconceptional heat stress were particularly noticeable during seasonal comparisons, with HSC cows seemingly having an advantage in subsequent episodes of heat stress. / Master of Science

This thesis set out to investigate how heat stress may affect the oxidative balance and influence the activity patterns of animals. The use of wild animals has been scarce in testing the effects of heat stress on the oxidative balance of animals, especially under a simulated heat wave condition. Animals are expected to differ in their oxidative stress in response to heat stress, depending on their thermotolerance and the life history trait of activity is tested here as an indirect measure of thermotolerance. Therefore, two rodent species within the same family were used for experiments, one with a nocturnal activity rhythm and the other with a crepuscular/diurnal activity rhythm. Animals were tested firstly in a laboratory whole-body hyperthermia experiment, followed by a 3-day heat stress test with extreme temperatures. Following these experiments, animals were subjected to a simulated heat wave to determine how ecologically relevant temperatures may influence not just the oxidative balance of individuals, but also their activity patterns and water drinking in the absence of a microclimate. In order to determine the oxidative balance of individuals under control and heat treatment conditions, markers of oxidative damage and antioxidant defense were used. Markers for lipid and protein oxidation and a marker of enzymatic antioxidant activity and total non-enzymatic antioxidant activity were investigated in the liver, kidney, brain and heart. During the simulated heat wave, activity was measured with video recordings, with the videos analysed for water drinking behaviour as counts and time spent drinking water. During the whole-body hyperthermia experiment without the extreme temperatures, both species demonstrated no significant changes in liver oxidative markers except for high oxidative damage in the kidney for R. dilectus and high oxidative damage in the brain for M. namaquensis. During the 3-day acclimation experiment with extreme temperatures, both species were oxidatively stressed in the heart, with significant oxidative damage in the kidney for R. dilectus and significant oxidative damage in the liver for M. namaquensis. During the simulated heat wave, both species showed no significant changes in liver oxidative balance. The kidney was oxidatively stressed for M. namaquensis, with both species exhibiting signs of oxidative stress in the brain. Animals did not shift their activity during the heat wave, but rhythms demonstrated signs of disruption during the simulated heat wave. Activity increased during the day for both species during the heat wave. This was due to animals being uncomfortable and drinking more water. For the nocturnal species, the frequency of drinking water increased during the day and during the night during the simulated heat wave. The crepuscular species only increased in the amount of time spent drinking water during the day during the simulated heat wave. Overall, the nocturnal species was more susceptible to heat stress inducing either increased oxidative damage or reduced antioxidant defense compared to the crepuscular species, but this was tissue specific and also dependant on the experimental condition. It is recommended that future studies directly measure the reduced: oxidised state of individuals in addition to markers of defense and damage. / Thesis (PhD (Zoology))--University of Pretoria, 2019. / Zoology and Entomology / PhD (Zoology) / Unrestricted

Light and temperature are two of the most important environmental factors regulating plant development. Although heat stress has been well studied, little is known about the interaction between light and temperature. In this study, we performed phenotypic assays comparing seedling responses to heat under light and dark conditions. Seedlings exposed to heat in the dark show lower survival rates than seedlings stressed in the light. To identify transcriptional changes underlying light-dependent heat tolerance, we used RNA-sequencing. The light-dependent heat stress responses involved a plethora of genes which could be potential candidate genes for light-induced heat tolerance, including transcription factors (bHLH) and genes commonly associated with biotic stress. By using the latest high-throughput phenotyping facility, we found that the light-dependent heat tolerance is reflected more on the maintenance of photosynthetic capacity, rather than leaf temperature. These results provide insights into how light increases heat stress tolerance in Arabidopsis seedlings and suggest its underlying mechanisms.

The purpose of this research was to identify nonwoven protective fabrics designed to have improved comfort properties over a fabric currently used in limited-use chemical protective clothing by using physiological and perceptual dimensions of comfort. Fabrics that are both waterproof and breathable have potential for use in chemical protective clothing by repelling liquid while allowing the diffusion of moisture vapor from the body. Microporous laminated fabrics used in chemical protective clothing allow moisture to evaporate from the body and move through the fabric while preventing liquid chemicals from coming in contact with the skin. Five fabric types were evaluated by wear testing coveralls and measuring selected physiological and perceptual responses of five subjects performing moderate-intensity exercise in thermoneutral and hot, humid environments. Physiological responses included skin temperature, body temperature, heart rate, sweat rate, and fluid loss. Perceptual responses included moisture sensation, thermal sensation, Rating of Perceived Exertion, and overall comfort. In the thermoneutral environment, results show heart rate to be the only variable affected by the different fabric types, while time effect was significant within all dependent variables. In the hot, humid environment, skin and body temperatures were significantly affected by fabric type, while time effect was significant within all variables except skin temperature. The environment effect was significant within all dependent variables except RPE and heart rate. Based on the statistical insignificance of subjects' overall comfort responses in both environments, it can be said that the differences in the physical characteristics of the fabrics may not be great enough to affect the wearer's comfort level. However, two of the four fabrics were determined to have potential for further investigation. Further research comparing experimental fabrics with fabrics used widely in industry will continue the effort of improving the comfort of limited-use protective clothing. / Master of Science

Developing foliar growth of ivy geraniums (Pelargonium peltatum) bleaches white after exposure to temperatures greater than 30°C. This study investigated chloroplast development in ivy geraniums under heat stress comparing a heat sensitive cultivar, Temprano™ Lavender, and a heat tolerant cultivar, Contessa™ Red. Using transmission electron microscopy and spectrophotometry, foliar bleaching under heat stress was found to be due to an absence of developed chloroplasts within the bleached new growth accompanied by lower chlorophyll content. To determine whether heat stress related foliar bleaching could be prevented, cytokinin and gibberellins were applied in combination, at different rates before, during or after a heat stress event. Applying 50 to 100 ppm gibberellins before heat stress reduced bleaching in new growth. Gibberellins applied at 50 ppm within a week of a heat stress event decreased bleaching. Net photosynthesis and chlorophyll fluorescence was greater in non-heat stressed plants than heat stressed plants.

<p> Body temperature regulation at high ambient temperatures was compared in pregnant and non-pregnant female albino rats. At an ambient temperature of 40°C, pregnant rats maintained lower body temperatures than non-pregnant rats without added expenditure of moisture for evaporative cooling. This was surprising because of the increased heat load on the pregnant animal resulting from a large weight gain and increased food intake. The maintenance of lower body temperatures in the heat by pregnant rats was possible because (1) pregnant rats produce less heat; i.e. consume less oxygen during exposure to 40°C than do non-pregnant rats and thus need to dissipate less heat; and (2) pregnant rats have a lower body temperature threshold for increased submaxillary salivary gland output in the heat, which makes more water for evaporative cooling available to them at lower body temperatures.</p> <p> The maintenance of lower body temperatures in the heat by pregnant rats suggests that pregnant rats have a need or a preference for lower body temperatures in the heat. The change in body temperature regulation during pregnancy is likely related to physiological and anatomical changes in the body which accompany pregnancy. Alterations in body temperature regulation to meet the changed physiological state of pregnancy provide another example of the body's remarkable ability to maintain homeostasis.</p> / Thesis / Doctor of Philosophy (PhD)

The development of whitening of the youngest leaves of actively growing ivy geranium (Pelargonium peltatum L.) has been observed as the season changes from late spring to summer. This study was conducted to determine the specific environmental causes of whitening, if micronutrients deficiencies cause similar whitening, whether low night temperatures can reverse whitening, and whether salicylic acid affects growth and whitening in ivy geraniums. Two cultivars, ‘Beach’ and ‘Butterfly’, with different susceptibility to whitening were chosen for this study. Elevated air temperature, but not elevated root-zone temperature, was found to be the environmental cause of whitening in ivy geranium. Elevated air temperatures severely reduced plant growth, leaf area, fresh weight, and dry weight in both cultivars. Elevated air temperature reduced photosynthetic pigments and their ratios in ivy geranium. Carotenoids and pheophytins decreased in ‘Butterfly’ at elevated air temperature. Foliar total Fe levels indicated no inhibition of Fe-uptake at elevated temperatures. Applications of Fe-chelate at elevated temperatures helped chlorophyll synthesis in ivy geraniums. Deficiency treatments of all micronutrients, Fe, Mn, Zn, S or Mg did not result in whitening in either cultivar of ivy geraniums. Salicylic acid did not affect whitening of ivy geraniums. It did not affect growth, leaf area, fresh or dry (stem, leaf or total) weight, fresh: dry weight ratio, leaf area ratio, specific leaf area or foliar nutrient (Fe, Mn, Zn, Mg and S) content in either cultivar. Cultivars varied in their response to low night temperature. ‘Beach’ reduced its plant growth and fresh to dry weight ratio whereas ‘Butterfly’ did not. Fe-chelate application did not reduce growth, leaf area, fresh weight, dry weight or fresh:dry weight ratio of either cultivar. Although Fe-application did not reduce whitening in ivy geraniums, it helped to preserve chlorophyll, particularly chlorophyll b as indicated by Chl a:b ratio in ‘Beach’. Whitening in ivy geranium is a heat stress response initially exhibited by young, developing leaves and is caused by elevated air temperatures. Whitening is the result of impaired photosynthetic pigments synthesis and/or degradation.

<p> Learning processes have been demonstrated to play an integral role in drug tolerance. More recently, researchers have found that associative mechanisms also play an important role in the adaptation to cold exposure (Riccio, MacArdy & Kissinger, 1991). The present study investigated the effect of contextual stimuli on temperature response to repeated heat stress. Rats receiving repeated heat exposures (56°C, 10 min, 6 trials) demonstrated adaptation to the heat as measured by a decrease in hyperthermia. The tolerance to the heat stress was not disrupted by changing the contextual cues associated with the heat. These findings demonstrate tolerance to repeated heat stress but do not provide evidence of associative learning in this adaptation. Future experiments should assess the question of thermoregulation and associative processes using highly discriminant conditioning environments.</p> / Thesis / Master of Science (MSc)