Heat acclimation increases the local heat loss responses of sweating and skin blood flow which is thought to persist for up to 3 weeks post-acclimation. However, the extent to which increases in local heat loss affect whole-body heat loss as a function of increasing levels of heat stress remains unresolved. Using direct calorimetry, we examined changes in whole-body evaporative heat loss (EHL) during progressive increases in metabolic heat production 1) prior to (Day 0), during (Day 7) and following a 14-day heat acclimation protocol (Day 14) – Induction phase, and; 2) at the end of a 1-week (Day 21) and 2-week decay period (Day 28) – Decay phase. Ten males performed intermittent exercise (3 x 30-min (min) bouts of cycling at 300 (Ex1), 350 (Ex2), and 400 watts•meters2 (W•m2) (Ex3) separated by 10 and 20 min rest periods, respectively). During the induction period, EHL at Day 7 was increased at each of the three exercise bouts (Ex1: + 6%; Ex2 +8%; Ex3: +13%, all p≤0.05) relative to Day 0 (EHL at Ex1: 529 W; Ex2: 625 W; Ex3: 666 W). At Day 14, EHL was increased for all three exercise bouts compared to Day 0 (Ex1: 9%; Ex2: 12%; Ex3: 18%, all p≤0.05). As a result, a lower cumulative change in body heat content (ΔHb) was measured at Day 7 (-30%, p≤0.001) and Day 14 (-47%, p≤0.001). During the decay phase, EHL at Day 21 and 28 was only reduced in Ex 3 (p≤0.05) compared to Day 14. In parallel, ΔHb increased by 39% (p=0.003) and 57% (p≤0.001) on Day 21 and Day 28 relative to Day 14, respectively. When Day 28 was compared to Day 0, EHL remained elevated in each of the exercise bouts (p≤0.05). As such, ΔHb remained significantly lower on Day 28 compared to Day 0 (-16%, p=0.042). We show that 14 days of heat acclimation increases whole-body EHL during exercise in the heat which is maintained 14 days post-acclimation.

Heat acclimation increases the local heat loss responses of sweating and skin blood flow which is thought to persist for up to 3 weeks post-acclimation. However, the extent to which increases in local heat loss affect whole-body heat loss as a function of increasing levels of heat stress remains unresolved. Using direct calorimetry, we examined changes in whole-body evaporative heat loss (EHL) during progressive increases in metabolic heat production 1) prior to (Day 0), during (Day 7) and following a 14-day heat acclimation protocol (Day 14) – Induction phase, and; 2) at the end of a 1-week (Day 21) and 2-week decay period (Day 28) – Decay phase. Ten males performed intermittent exercise (3 x 30-min (min) bouts of cycling at 300 (Ex1), 350 (Ex2), and 400 watts•meters2 (W•m2) (Ex3) separated by 10 and 20 min rest periods, respectively). During the induction period, EHL at Day 7 was increased at each of the three exercise bouts (Ex1: + 6%; Ex2 +8%; Ex3: +13%, all p≤0.05) relative to Day 0 (EHL at Ex1: 529 W; Ex2: 625 W; Ex3: 666 W). At Day 14, EHL was increased for all three exercise bouts compared to Day 0 (Ex1: 9%; Ex2: 12%; Ex3: 18%, all p≤0.05). As a result, a lower cumulative change in body heat content (ΔHb) was measured at Day 7 (-30%, p≤0.001) and Day 14 (-47%, p≤0.001). During the decay phase, EHL at Day 21 and 28 was only reduced in Ex 3 (p≤0.05) compared to Day 14. In parallel, ΔHb increased by 39% (p=0.003) and 57% (p≤0.001) on Day 21 and Day 28 relative to Day 14, respectively. When Day 28 was compared to Day 0, EHL remained elevated in each of the exercise bouts (p≤0.05). As such, ΔHb remained significantly lower on Day 28 compared to Day 0 (-16%, p=0.042). We show that 14 days of heat acclimation increases whole-body EHL during exercise in the heat which is maintained 14 days post-acclimation.

Soil salinity and heat stress are two major abiotic stresses affecting plant growth and yield. Transcription factors (TFs) are key regulators in stress responses. They link stress sensing with many tolerance mechanisms by translating stress signals into changes in gene expression that ultimately contribute to stress tolerance. The NAC (NAM, ATAF and CUC) TF family have been found to be involved in responses to biotic and abiotic stresses. In this PhD project, the role of NAC TFs in response to heat and salt stress was studied in the model system Arabidopsis thaliana (Arabidopsis), and in two agriculturally relevant species, Solanum lycopersicum (tomato) and Chenopodium quinoa (quinoa). Plants have the ability to acquire thermotolerance if they are pre-exposed to a mild, non-lethal high temperature. The maintenance of acquired thermotolerance for several days is known as thermomemory. Here we investigated the role of NAC TFs in thermotolerance. The expression profiles of 104 Arabidopsis NAC TFs were measured and compared between primed and unprimed plants. Some NACs with a distinctive expression pattern in response to thermopriming were selected for further phenotypic analysis. Knock-out (KO) mutants of the ATAF1 gene showed an enhanced thermomemory phenotype compared with wild type plants (WT) and from this work, the functions of the ATAF1 gene were studied further. RNAseq co-expression analyses of ATAF1 overexpressor and ataf1 KO plants found that ANAC055 expression was co-regulated with that of ATAF1. JUBGBRUNNEN1 (JUB1) is another NAC TF involved in responses to heat, drought and salinity. In this study, the role of AtJUB1 overexpression in salinity was investigated in tomato plants. AtJUB1 overexpression resulted in higher proline levels and improved maintenance of water content and biomass in AtJUB1-overexpressing plants grown hydroponically under salinity compared with WT plants. Quinoa has recently gained much attention because of its high nutritional value and high tolerance to several stresses including drought and salinity. NAC TFs are hypothesized to play a major role in quinoa’s tolerance to abiotic stresses. In this study, the NAC TFs family were identified and investigated in the genome of quinoa. 107 NAC TF genes were identified and their transcriptional responses to different stresses including salt, drought and heat were investigated.

xvii, 245 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / There has been a lot of research investigating the effects of heat stress and exercise on the physiological adaptations to heat acclimation. It is well documented that heat acclimation improves heat tolerance and performance in a hot environment; however, some of the mechanisms of adaptation are not clear. Furthermore, the role of heat acclimation on exercise performance in cool environments is currently unknown. Therefore, in Chapter IV we aimed to determine the effects of heat acclimation on lactate threshold and maximal oxygen uptake (VO 2max ) in cool and hot conditions. We also sought to investigate the effects of heat acclimation on leg blood flow and oxygen delivery during a single-leg knee extensor exercise. We found that heat acclimation improved lactate threshold and VO 2max in cool and hot environments but did not alter the leg blood flow and oxygen delivery during the leg kicking exercise. In Chapter V we investigated the heat acclimation effects on performance during a 1-hour time trial in hot and cool environmental conditions and the potential mechanisms by which this occurs. A secondary objective was to study whether the pacing strategy was modified during the time trial post-heat acclimation. The results demonstrated that heat acclimation improved time trial performance in both thermal environments by approximately 7% but pacing strategy was not altered. The purpose of the studies in Chapter VI were twofold. First, we sought to investigate how heat acclimation affects the skin blood flow and sweating responses to pharmacological treatment with specific dosages of the muscarinic receptor agonist acetylcholine. Second, we examined the maximal skin blood flow responses to a period of heat acclimation by locally heating the forearm with a water spray device for 45 minutes and measured brachial artery blood flow via ultrasound. We found that heat acclimation increased sweat rate and skin blood flow responses to given concentrations of acetylcholine, suggesting a role for peripheral mechanisms. On the other hand, maximal skin blood flow remained unchanged after heat acclimation. / Committee in charge: Christopher Minson, Chairperson, Human Physiology; John Halliwill, Member, Human Physiology; Andrew Lovering, Member, Human Physiology; Michael Sawka, Member, Not from U of 0; Scott Frey, Outside Member, Psychology

De multiples événements sportifs majeurs vont se dérouler prochainement dans des environnements chauds voire tropicaux, justifiant l’intérêt scientifique actuel pour les questions associées à l’effet de la chaleur sur la performance en endurance. Il est admis que l’effort est subjectivement perçu comme plus difficile, en parallèle de la dégradation de la performance, à mesure que la température ambiante s’élève au cours d’une épreuve prolongée. Contrecarrer les effets délétères de la chaleur passe notamment par une exposition répétée dans des conditions écologiques ou simulées d’exercice (i.e. stratégie d’acclimatation). Toutefois, le décalage actuel entre les recommandations d’application issues d’études scientifiques et les conditions réelles de préparation et d’organisation des athlètes de haut niveau est à l’origine d’un faible engouement actuel pour de telles méthodes. Par conséquent, ce travail de thèse a eu pour ambition de répondre à certaines inconnues relatives à l’application de contenus d’entraînement classiques en ambiance chaude, en particulier (i) leur conséquence sur la capacité de performance de l’athlète et (ii) les moyens d’optimiser le contrôle et la régulation de la charge d’entraînement. Dans ce contexte, nous avons proposé à des sujets entraînés de soutenir la production de seuils de RPE (i.e. exercice à RPE fixe) dans un cadre expérimental puis d’entraînement. Au-delà de la validité de ce modèle, nous soutenons que la régulation volontaire de l’intensité dépend, au-delà de la perception de l’effort, de l’état émotionnel et de la motivation de l’individu. Lors d’une première étude, nous avons recherché à comparer les performances de solutions techniques pour le suivi de la température centrale en conditions écologiques (i.e. température gastro-intestinale vs. température frontale par capteur à annulation de flux). Les résultats obtenus ont validé l’usage du capteur à annulation de flux comme alternative pendant l’exercice, en dépit de l’absence de corrélation avec les mesures gastro-intestinales. Ceci suggère d’éventuelles perspectives en matière de contrôle de la température corporelle pendant l’exercice. La seconde étude s’est intéressée aux possibles conséquences de la répétition de sessions d’entraînement exigeantes et des contraintes logistiques d’un stage d’acclimatation (i.e. accumulation de fatigue mentale) sur la perception de l’effort et la performance. En dépit de l’absence d’effets combinés de la tâche cognitive préexercice et de la chaleur ambiante, les résultats tendent à démontrer le rôle-clé de la température cutanée et de la sensation de chaleur sous-jacente dans la régulation de la puissance soutenue à RPE-15 (chaud vs. tempéré : -0,022 vs. -0,008 W.kg-1.min-1). Enfin, la troisième étude suggère un potentiel intérêt de l’application de hautes intensités autorégulées, associée à une diminution du volume d’entraînement (-23%), lors d’une période d’acclimatation de courte durée (i.e. 5 jours). Le moindre effet observé, en comparaison d’un protocole à intensité fixe, sur la performance au cours d’un exercice de contre-la-montre (i.e. expérimental vs. fixe : 1,4 vs. 2,8 %) soulève toutefois l’importance du rapport volume-intensité dans la construction d’un protocole d’acclimatation. De manière générale, l’ensemble des résultats de cette thèse offrent des perspectives pour une individualisation et une adaptation spécifique à l’activité sportive des protocoles d’acclimatation à la chaleur. / The increasing number of major sport events that will take place in hot and/or tropical environments justify the current scientific interest in the effects of heat strain on endurance performance. During a prolonged self-paced exercise, it is well known that the subjectively perceived effort is higher as the ambient temperature increases and the performance level decreases. A repeated exposure to the heat in ecological and/or simulated exercise conditions may counteract the subsequent deleterious effects. However, the discrepancy between guidelines from scientific research and training priorities of well-trained athletes causes a lack of interest in these methods. Therefore, we aimed through the current thesis work to improve the current knowledge about heat acclimation strategies, more precisely about i) its effects on athlete’s performance capacity and ii) the optimization of training load monitoring and building. To shed light on these issues, we proposed to our trained and/or well-trained subjects to cycle at a fixed RPE first in an experimental framework, and then during a training program. We submit that the self-regulation of fixed-RPE exercise work rate depends not only of perceived exertion but also on emotional and motivational parameters. The first study aimed to compare the performance of technical devices for core temperature monitoring in ecological conditions (i.e. gastrointestinal point vs. forehead point from a zero-heat-flux sensor). Results showed that zero-heat-flux measurements might be considered as relevant during exercise. In this way, some opportunities may be considered for the monitoring of body temperature during field-based exercise. The second study investigated the subsequent effects of repeated strenuous training sessions and logistical constraints during a heat camp (i.e. higher mental fatigue) on perceived exertion and endurance performance. Despite the lack of combined effects from the prior cognitive task and the ambient condition during exercise, skin temperature and underlying heat sensation impact the linear decrease of power output at RPE-15 (hot vs. neutral: -0,022 vs. -0,008 W.kg-1.min-1). Lastly, the third study suggested an interest for self-regulated high intensities, associated with a decrease of the total exposure duration (-23%), during a short-term heat acclimation protocol (i.e. 5 days). However, the slighter effect on the improvement of performance in comparison with a classic constant-power training program, (HIT vs. constant-power: 1,4 vs. 2,8 %) provides scope for the building of training load (i.e. volume vs. intensity) in this context. In summary, the overall results of this thesis work open some perspectives for individualizing or adapting heat acclimation strategies to sport-specific conditions.