Browsing by Subject "Heat stress"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Assessment of various mechanisms involved in heat-stress induced reductions in orthostatic tolerance(2013-08) Lange, Andrew Peter; Brothers, Robert MatthewPurpose: This study aimed to expand our knowledge of the underlying mechanisms of orthostatic tolerance. First, cerebral perfusion was compared with reductions in orthostatic tolerance between normal thermic and heated conditions. The researchers' hypothesized that subjects with the greatest reduction in orthostatic tolerance will experience the largest drop in cerebral blood flow. Additionally, ANG II was measured in order to identify if during passive heating, the elevation in plasma ANG II is negatively correlated with heat-stress induced reductions in orthostatic tolerance. Lastly, orthostatic tolerance changes during the simulated hemorrhage between heat stress and normal thermic conditions will be compared to fitness level, measured by VO2 max. Results and Conclusion: Cerebral perfusion, as indexed by middle cerebral artery blood velocity, was reduced during heat stress compared with normothermia (P [less than] 0.001); however, the magnitude of reduction did not differ between groups (P = 0.51). In the initial stage of LBNP during heat stress (LBNP 20 mmHg), middle cerebral artery blood velocity and end-tidal PCO2 were lower; whereas, heart rate was higher in the large difference group compared with small difference group (P [less than] 0.05 for all). In opposition to the hypotheses, the large differences in tolerance to a simulated hemorrhage during normothermic and heat stress conditions are not solely related to the degree of heat stress-induced reduction in cerebral perfusion. Also, an individual's level of cardiorespiratory capacity (fitness) and/or the degree of heat stress-induced increase in plasma ANG II does not reliably predict the level of reduction in tolerance to a simulated hemorrhage challenge when heat stressed.Item Defining the Molecular and Physiological Role of Leaf Cuticular Waxes in Reproductive Stage Heat Tolerane in Wheat(2012-07-16) Mondal, SuchismitaIn wheat, cooler canopies have been associated with yield under high temperature stress. The objectives of this study were, i) to understand the role of leaf cuticular waxes as physiological adaptive mechanisms during reproductive stage high temperature stress ii) define quantitative trait loci (QTL) regulating leaf cuticular waxes and determine its link with the QTL for reproductive stage heat tolerance iii) define stable QTL associated with leaf cuticular waxes and yield stability across environments. For the first objective, thirteen wheat cultivars were subjected to a 2-day heat treatment at 38 degrees C at 10 days after pollination (DAP). Leaf cuticular waxes, canopy temperature depression and stomatal conductance were estimated during high temperature stress. At maturity the percent reduction in yield components in each cultivar was calculated. The wheat cultivars 'Kauz' and 'Halberd' had significantly high leaf cuticular wax content of 2.91mg/dm^-2 and 2.36mg/dm^-2 respectively and cooler canopies. Leaf cuticular waxes were significantly correlated with leaf temperature depression and reduction in yield components. A set of 121 recombinant inbred lines (RIL) population derived from the cross of heat tolerant wheat cultivar 'Halberd' and heat susceptible wheat cultivar 'Karl 92' was utilized for QTL mapping. The RIL population received a 2-day short-term high temperature stress at 38?C at 10DAP in 2008 and a long-term high temperature stress at 38 degrees C from 10DAP until maturity in 2009 in the greenhouse. The RIL population was also planted in College Station, Texas in 2009 and 2010 and in Uvalde, Texas in 2010. Leaf cuticular wax was estimated at 10DAP and leaf/spike temperatures were recorded during grain filling. Yield components were estimated after harvest. Heat susceptibility indexes for main spike yield components were estimated in the greenhouse. Overall ten significant QTL were identified for leaf cuticular waxes each explaining 8-19 percent of the variation respectively. Stable QTL for leaf cuticular waxes were located on chromosome 5A and 1B and co-localized with QTL for leaf/spike temperature depression and HSI for kernel weight and single kernel weight of main spike. Another QTL on chromosome 1B contributed by Karl92 was found in the greenhouse and field environments and co-localized with a previously identified QTL on 1B for spike non-glaucousness. The results suggest that leaf cuticular waxes may reduce leaf temperatures and improve adaptation during high temperature stress.Item Effects of High Nighttime Temperature and Role of Plant Growth Regulators on Growth, Development and Physiology of Rice Plants(2010-07-14) Mohammed, Abdul R.Seasonally high nighttime temperatures (HNT) along the United States Gulf Coast and in regions of similar climate, during the critical stages of development, could reduce rice yield and quality. To study the effects of HNT on plant physiology, a method for applying a controlled heating treatment to plant canopies was developed using overhead infrared heaters, which are relatively inexpensive and are accurate, precise and reliable in rapidly controlling the temperature. The apparatus successfully maintained air temperatures within the set points plus/minus 0.5 degrees C, and was used for all the experiments. Several experiments were conducted to determine the response of various physiological parameters during and following exposure of rice plants to HNT (32 degrees C) or ambient nighttime temperature (ANT) (27 degrees C) starting from 2000 h until 0600 h, and with or without plant growth regulator treatments. The plant growth regulator treatments included alpha-tocopherol (vitamin E), glycine betaine (GB), and salicylic acid (SA), which play different roles in inducing thermo-tolerance in plants. High nighttime temperature had no effect on plant height, number of tillers and panicles, or rice net leaf photosynthetic rates. However, HNT increased leaf respiration (dark respiration in the night) (21%) and decreased membrane thermo-stability (60%), pollen germination (20%), spikelet fertility (18% as a % of total spikelets), grain length (2%), and grain width (2%). The HNT also hastened plant development. The combinations of these effects decreased rice yield by 90%. Moreover, under HNT, there were decreases in leaf chlorophyll concentration (7%) and nitrogen concentration (18%). Application of GB and SA increased total antioxidant capacity of the rice plants by 17%, thereby decreasing the leaf respiration rates, increasing membrane thermo-stability, pollen germination, and spikelet fertility, thus increasing the yield. High nighttime temperature decreased leaf starch concentration (14%), grain total nonstructural carbohydrate (TNC) concentration (9%), and grain extractable invertase activity (20%). Vitamin E- or GB-treated plants had greater grain soluble-sugar concentrations, whereas SA-treated plants had greater leaf soluble-sugar concentrations and lower grain TNC concentrations. Invertase activity was shown to be not rate limiting or required for sucrose degradation for starch synthesis in grain of 'Cocodrie' rice under short-term high nighttime temperatures exposures during grain filling. In conclusion, HNT decreased rice yield by increasing plant respiration, rate of crop development, and decreasing membrane thermo-stability, pollen germination, spikelet fertility and grain dimensions. Exogenous application of GB and SA increased yields under HNT, possibly acting through increased antioxidant levels, which might have protected the membranes and enzymes against heat-induced ROS-mediated degradation.Item Effects of Strenuous Exercise on Stallion Sperm Quality(2012-10-19) Rosenberg, Jennifer L.Some stallions are expected to perform athletically and breed contemporarily. Athletic activity has the potential, especially during the summer months, to induce thermal stress to the testes, resulting in reduced reproductive capability due to decreased sperm quality and libido. There is concern in the horse industry about what level of exercise, if any, affects the reproductive capability of a stallion. Thermal stress associated with training and exercise may impact sperm quality and the future reproductive capability of the stallion. The goal of this study was to determine the effect of strenuous exercise on stallion sperm quality. The objectives were to measure changes in body and scrotal temperatures following strenuous exercise and sperm quality following strenuous exercise. Miniature Horse stallions (n = 7), implanted with subdermal thermosensory devices in the subcutaneous neck and scrotal tissue, were assigned to treatment group based on age and semen quality. Exercising stallions (EX; n = 3) were exercised 4 d/wk for 90 min for 12 wk, while non-exercising stallions (CN) were tied in the shade. Semen was collected from stallions for 5 consecutive days every 4 wk to evaluate semen quality (raw, 24 h and 48 h cooled). Subcutaneous scrotal (SQST), rectal (RT) and neck (NT) temperatures were recorded along with heart rate. Spermatozoa data were normally distributed; therefore, they were subjected to parametric analysis by repeated measures (wk) using the PROC MIXED procedure (SAS v 9.1; SAS Inst. Inc., Cary, NC). Model included treatment (CN or EX), time (wk 0, 4, 8, or 12), and stallion as the subject of the repeated measures. Compared to the CN group, EX stallions had elevated temperatures (avg RT 39.27 vs 37.07 degrees, NT 39.77 vs 37.44 degrees C, and SQST 34.90 vs 33.40 degrees C; P < 0.0001). There was no difference in sperm quality between treatment groups (P > 0.05). In this study, strenuous exercise in Miniature Horse stallions, did not affect sperm quality. This suggests that anecdotal reports of reduced sperm quality in stallions in training may have other causes other than elevated scrotal and body temperature. While previous studies have illustrated that prolonged insulation of the testes reduces semen quality, strenuously exercising stallions for up to 90 min under hot and humid ambient conditions may not be harmful to spermatogenesis.Item Influences of skin and core temperature on cardiovascular responses during exercise(2010-08) Lee, Joshua Floyd; Coyle, Edward F., 1952-; Tanaka, HirofumiThe cardiovascular effects of whole body heat stress during exercise are well established. However the independent contribution of elevated skin temperature (Tsk) or core temperature (Tc) on these responses remains unclear. The purpose of this study was to determine how increases in Tsk and Tc alone and in combination, impact cardiovascular responses during moderate intensity exercise. To accomplish this goal, eight healthy, recreationally active males were immersed to the neck in a cold (14 - 17°C) or hot (40 - 42.5°C) water bath for 20 to 25 min to alter Tc immediately prior to exercise with either cool Tsk (i.e. fans) or warm Tsk (i.e. heaters). Conditions during exercise were cool skin and cool core (CC), warm skin and cool core (WC), cool skin and warm core (CW), and warm skin and warm core (WW), and were conducted in a randomized crossover design. When data was combined (n=16), warm core conditions (CW and WW) were associated with significantly higher average heart rate (HR) and lower stroke volume (SV) during exercise compared to cool core conditions (CC and WC); 168.1 ± 3.2 vs. 152.2 ± 4.0 beats/min and 139.2 ± 7.3 vs. 147.7 ± 9.4 mL/beat, respectively. The approximate 9 mL/beat decline in SV and 16 beat/min increase in HR in warm core conditions tended to increase cardiac output (Q), 23.2 ± 0.6 vs. 22.2 ± 0.7 L/min, P=0.078. Similarly, warm Tsk conditions (WC and WW) were associated with significantly higher average HR and lower SV during exercise compared to cool Tsk conditions (CC and CW); 165.2 ± 3.3 vs. 155.1 ± 3.4 beats/min and 140.8 ± 7.8 vs. 146.0 ± 8.7 mL/beat, respectively. Additionally, there was also a trend for Q to be elevated with warm skin (23.0 ± 0.6 vs. 22.4 ± 0.6, P=0.075). Although combined data indicated that warm Tsk conditions significantly lowered average SV by ~6 mL/beat, there was no reduction in SV during exercise by warm Tsk, when Tes was cool (i.e. <37.0°C), as evidenced by identical values for SV in CC and WC, 147.7 ± 9.8 vs. 147.7 ± 9.0 mL/beat, respectively. In contrast, SV was significantly lower in WW compared with CW, 133.9 ± 7.0 vs. 144.4 ± 7.8 mL/beat, respectively. Therefore, the major reduction in SV by warm Tsk occurred during WW, when Tes was elevated (i.e. >38.0°C). Analyzing data independently for precooling and preheating conditions revealed that warm Tsk was associated with greater HR drift from 5 to 20 min of exercise, compared to cool Tsk, when esophageal temperature (Tes) was both cool or warm (23.9 ± 2.2 vs. 17.5 ± 2.3 and 12.3 ± 1.3 vs. 4.6 ± 1.7 beats/min, respectively). These observations demonstrate that both Tes and Tsk can directly influence cardiovascular responses during exercise, as indicated by elevations in HR during exercise with warm Tsk, with both warm and cool Tes. However SV is not compromised by warm Tsk if Tes is below 37.5°C. Furthermore, when both Tes and Tsk are elevated simultaneously, cardiovascular strain (i.e. increased HR and reduced SV) is much greater than when either is elevated alone. This is demonstrated by the finding that average HR was 175.8 ± 3.2 beats/min in WW, compared to 149.8 ± 4.0, 154.7 ± 4.1, and 160.3 ± 3.5 beats/min, in CC, WC, and CW, respectively, and the fact that SV was lowest during exercise in WW. In conclusion, individuals exercising in the heat should take measures to keep skin cool, especially when Tes is 39°C or greater to attenuate the cardiovascular strain that occurs with warm Tsk, when Tes is elevated.