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No description available.

Taxodium distichum (L.) Rich. is a widely adaptable, long-lived tree species for landscape use. It is tolerant of substantial soil salt levels, but tends to defoliate in periods of extended or severe drought, when leaves come into contact with salty irrigation water, and tends to develop chlorosis on high pH soils. The purpose of this research was to identify provenances which may yield genotypes tolerant of these stresses. The appropriate name for baldcypress is Taxodium distichum (L.) Rich. var. distichum, for pondcypress is T. distichum var. imbricarium (Nutt.) Croom, and for Montezuma cypress is T. distichum var. mexicanum Gordon. A germination study of T. distichum var. mexicanum revealed that if immediate germination of ripe seed is desired, then the best treatments are a citric acid soak and hot water baths, however, if seeds can be stratified, then no pre-germination seed treatment is needed. Citric acid scarification and hot water baths produced the best germination. Stratification hastened germination rates and cumulative mean germination percentages. Stratification for 45 d appears to be sufficient, although for the best pre-germination treatments stratification requirements were less pronounced. Greenhouse screening studies of open-pollinated families for drought tolerance show genotypes from eastern localities were less tolerant than western genotypes. Taxodium distichum likely relies on both drought avoidance and drought tolerance strategies to deal with drought stress. A field screening for alkalinity tolerance showed a strong geographic component to the variation in tolerance of alkaline soils. When selecting plant material for an alkaline site, genotypes from Mexico and south Texas should be preferred, followed by central Texas genotypes. Greenhouse salinity screening showed that most genotypes tolerate moderate levels of soil salts, but at high soil salinities the tolerance appears to be highly genotype-dependent, rather than having a strong geographic pattern. Field evaluations demonstrated that T. distichum var. mexicana grew more rapidly on three Texas sites than the other varieties. These evaluations also suggest that when selecting plant material for an alkaline or xeric site, Mexican and south Texas genotypes should be preferred, followed by central Texas genotypes. Cold tolerance was not determined north of USDA hardiness zone 8.

Abstract Using thin-layer chromatography, 16 bacterial isolates from root nodules of 8 different Psoralea species were quantitatively assessed for their exudation of the metabolites lumichrome, riboflavin and IAA in response to pH, salinity and temperature. Our data showed that the bacterial strains tested differed in their levels of secretion of the three metabolites. For example, strain AS2 produced significantly greater amounts of lumichrome at both pH 5.1 and 8.1, while strains RTl and PI produced more lumichrome per cell at only pH 8.1. Strains API and RP2 also produced more riboflavin at pH 5.1 than at pH 8.1; conversely strain RTl secreted more riboflavin at pH 8.1 than at pH 5.1. Two P. repens strains (RPI and RP2) isolated from very saline environments close to the Indian Ocean produced significant levels of lumichrome and riboflavin at both low and high salinity treatments. However, strains ACI and Ll (from P. aculeata and P. laxa) even produced greater amounts of lumichrome and riboflavin at higher salinity (i.e. 34.2 mM NaCl) and probably originated from naturally saline soils. In this study, high acidity and high temperature induced the synthesis and release of high levels of IAA by bacterial cells. In contrast, there was greater strain secretion of lumichrome at lower temperature (10°C) than at high temperature (30°C). The variations in the secretion of lumichrome, riboflavin and IAA by bacterial strains exposed to different pH, salinity and temperature regimes suggest that genes encoding these metabolites are regulated differently by the imposed environmental factors. The data from this study also suggest that natural changes of pH, salinity and/or temperature in plant rhizospheres could potentially elevate the concentrations of lumichrome, riboflavin and IAA in soils. An accumulation of these molecules in the rhizosphere would have consequences for ecosystem functioning as both lumichrome and riboflavin have been reported to act as developmental signals that affect species in all three plant, animal, and microbial kingdoms.

Abstract Using thin-layer chromatography, 16 bacterial isolates from root nodules of 8 different Psoralea species were quantitatively assessed for their exudation of the metabolites lumichrome, riboflavin and IAA in response to pH, salinity and temperature. Our data showed that the bacterial strains tested differed in their levels of secretion of the three metabolites. For example, strain AS2 produced significantly greater amounts of lumichrome at both pH 5.1 and 8.1, while strains RTl and PI produced more lumichrome per cell at only pH 8.1. Strains API and RP2 also produced more riboflavin at pH 5.1 than at pH 8.1; conversely strain RTl secreted more riboflavin at pH 8.1 than at pH 5.1. Two P. repens strains (RPI and RP2) isolated from very saline environments close to the Indian Ocean produced significant levels of lumichrome and riboflavin at both low and high salinity treatments. However, strains ACI and Ll (from P. aculeata and P. laxa) even produced greater amounts of lumichrome and riboflavin at higher salinity (i.e. 34.2 mM NaCl) and probably originated from naturally saline soils. In this study, high acidity and high temperature induced the synthesis and release of high levels of IAA by bacterial cells. In contrast, there was greater strain secretion of lumichrome at lower temperature (10°C) than at high temperature (30°C). The variations in the secretion of lumichrome, riboflavin and IAA by bacterial strains exposed to different pH, salinity and temperature regimes suggest that genes encoding these metabolites are regulated differently by the imposed environmental factors. The data from this study also suggest that natural changes of pH, salinity and/or temperature in plant rhizospheres could potentially elevate the concentrations of lumichrome, riboflavin and IAA in soils. An accumulation of these molecules in the rhizosphere would have consequences for ecosystem functioning as both lumichrome and riboflavin have been reported to act as developmental signals that affect species in all three plant, animal, and microbial kingdoms.

Environmental effects on mutation have been documented for many years but have concentrated on agents that directly interact with DNA. Mutation research in its early history investigated a variety of more mundane environmental factors at levels that inhibited biological function and attempted to characterize their mutagenicity. This thesis revisits these old questions armed with more modern methods. It consists of one review chapter and three experimental chapters. The review chapter proposes that biological organization itself acts to direct mutation pressure, and that many mutations are context dependent within this organization. Experimentally, I performed an approximately 1,500-generation mutation accumulation (MA) experiment using the budding yeast Saccharomyces cerevisiae as an evolutionary genetic model. This thesis investigates the rates and distribution of effects of new mutations on fitness when they accumulate under a moderate salt stress. The first experimental section describes the production of the MA lines, measures the diploid fitness traits mitotic growth rate and sporulation, and uses changes in fitness among replicate lines to infer mutation parameters affecting these traits. Mutation rate estimates for these traits were roughly doubled in the salt stress treatment. The proportion of beneficial mutations was high for mutations affecting sporulation in both MA treatments but zero for growth rate. Measurements of haploid viability and haploid growth rate on strains derived from the diploid MA lines were used to infer mutation parameters. Mutation rates affecting haploid growth were ten-fold higher in our salt-line derivatives than those derived from the non-stress treatment. Variance component analysis identified a large fraction of genetic variation arising from differences among haploids within the same tetrad. This component was significantly larger in the salt MA treatment than the non-stress treatment. MA lines were subjected to a novel weak-acid stress. Mutation rate estimates were 38-fold higher in the salt MA treatment when lines were tested under acid stress. Cross-environmental genetic correlation for growth in acid stress versus standard media was significantly different between the two MA treatments suggesting that both MA environment and test environment are important factors when considering mutational effects on fitness.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

No description available.