Browsing by Subject "drought"
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Item A model for adaptive livestock management on semi-arid rangelands in Texas(Texas A&M University, 2006-08-16) Dube, SikhalazoA stochastic, compartmental Model for Adaptive Livestock Management (MALM) was developed for cow-calf enterprise for Rolling Plains of Texas from an existing model, Simple Ecological Sustainability Simulator (SESS). The model simulates forage and animal production. It runs on a monthly time step. Two stocking strategies, flexible and fixed, were evaluated at seven stocking levels for effects on forage and animal production, range condition, and net ranch income. Evaluation data were obtained from published and unpublished data from Texas A&M Agricultural Experimental Station at Vernon for Throckmorton. The model adequately simulated forage and animal production. Light fixed stocking rates and flexible stocking strategies resulted in cows of median body condition score (BCS) 5, compared to low BCS of 4 under moderate fixed stocking rate, and BCS of 3 under heavy fixed stocking. BCS declined from autumn to early spring and peaked in summer. Cows under light fixed stocking rates and under flexible stocking were heavier (460 kg) compared to those under heavy fixed stocking (439 kg). Replacement rates were lower under light stocking (22 %), compared to flexible (37 %) and heavy stocking (56 %). Calf crops were all above the reported 90 % expected for bred heifers because of the replacement policy. Flexible stocking strategy resulted in higher net income ($19.62 ha-1), compared to fixed light ($5.93 ha-1) or fixed heavy ($-17.35 ha-1) stocking strategies. Coefficient of variation (CV) in net income was highest under heavy stocking (90%) compared to light stocking (60%) and flexible stocking (50%). Maximum net income was obtained between 0.05 AUM??ha-1 and 0.13 AUM??ha-1 when fixed stocking strategy was used but when flexible stocking strategy was used maximum net income was obtained between 0.1 AUM??ha-1 and 0.17 AUM??ha-1. Range condition rapidly declined under fixed heavy stocking, increased under fixed and light flexible stocking, and remained constant under moderate flexible stocking. Heavy fixed stocking decreased range condition rapidly over a 20-year period. MALM was an effective tool to demonstrate effects of different management strategies. The model can function as a strategic or a tactical decision aid. It is concluded that there is potential for this model to assist managers in improving the sustainability of agriculture.Item Advanced Analysis of the Responses of Cotton Genotypes Growing Under Water Stress(2012-11-12) Maeda, Murilo Minekawa 1985-The ever-growing world population raises the concern and necessity of rational use and distribution of limited water resources. Water deficit is the single most dominant abiotic factor limiting cotton (Gossypium hirsutum L.) yield in drought-prone Texas croplands. Characterizing plant traits conferring drought tolerance to cotton genotypes and then transferring this information back to breeders and geneticists have the potential of significantly increasing and stabilizing production statewide. Although a plethora of physiological studies have been conducted and have demonstrated that drought tolerance in plants is likely to be conferred by a combination of plant traits rather than a single trait, this knowledge has not translated into improved breeding lines. Experiments were conducted in 2010 and 2011 in the Drought Tolerance Laboratory (Texas AgriLife Research and Extension Center in Corpus Christi, TX) to analyze the responses of cotton genotypes to different levels of water stress. This facility is equipped with computerized systems capable of continuously monitoring whole-plant water use as well as several environmental parameters. Sixteen cotton genotypes were provided by Monsanto Co. and the Texas AgriLife Cotton Improvement Programs at College Station and Lubbock. Seeds were pre-germinated in wet paper towels and then hand planted in large pots previously filled with fritted clay. A total of 3 and 8 (2010 and 2011, respectively) pots containing plants of each genotype were permanently placed on micro-lysimeters for continuous measurement of water use. Water regimes were imposed in 2010 (well-watered and water-stressed), and 2011 (water-stressed) when plants reached the early-flowering stage and were carried until plants reached maturity (100% open bolls). Data collected showed that genotypes have very distinct water use patterns. The water stress treatment imposed on the test plants negatively affected plant growth that was indicated by a lower plant height, total number of leaves, and main-stem nodes of stressed plants when contrasted to their well-watered counterparts. Stomatal density was remarkably different among genotypes and a higher density was found on the abaxial (lower) leaf surface for all genotypes studied. Root dry mass production had different responses depending upon the severity of the water stress. Highest root dry mass was observed when plants were exposed to a mild stress and lowest when a more severe water restriction was imposed.Item Analysis of Spatial Performance of Meteorological Drought Indices(2013-01-14) Patil, Sandeep 1986-Meteorological drought indices are commonly calculated from climatic stations that have long-term historical data and then converted to a regular grid using spatial interpolation methods. The gridded drought indices are mapped to aid decision making by policy makers and the general public. This study analyzes the spatial performance of interpolation methods for meteorological drought indices in the United States based on data from the Co-operative Observer Network (COOP) and United States Historical Climatology Network (USHCN) for different months, climatic regions and years. An error analysis was performed using cross-validation and the results were compared for the 9 climate regions that comprise the United States. Errors are generally higher in regions and months dominated by convective precipitation. Errors are also higher in regions like the western United States that are dominated by mountainous terrain. Higher errors are consistently observed in the southeastern U.S. especially in Florida. Interpolation errors are generally higher in the summer than winter. The accuracy of different drought indices was also compared. The Standardized Precipitation and Evapotranspiration Index (SPEI) tends to have lower errors than Standardized Precipitation Index (SPI) in seasons with significant convective precipitation. This is likely because SPEI uses both precipitation and temperature data in its calculation, whereas SPI is based solely on precipitation. There are also variations in interpolation accuracy based on the network that is used. In general, COOP is more accurate than USHCN because the COOP network has a higher density of stations. USHCN is a subset of the COOP network that is comprised of high quality stations that have a long and complete record. However the difference in accuracy is not as significant as the difference in spatial density between the two networks. For multiscalar SPI, USHCN performs better than COOP because the stations tend to have a longer record. The ordinary kriging method (with optimal function fitting) performed better than Inverse Distance Weighted (IDW) methods (power parameters 2.0 and 2.5) in all cases and therefore it is recommended for interpolating drought indices. However, ordinary kriging only provided a statistically significant improvement in accuracy for the Palmer Drought Severity Index (PDSI) with the COOP network. Therefore it can be concluded that IDW is a reasonable method for interpolating drought indices, but optimal ordinary kriging provides some improvement in accuracy. The most significant factor affecting the spatial accuracy of drought indices is seasonality (precipitation climatology) and this holds true for almost all the regions of U.S. for 1-month SPI and SPEI. The high-quality USHCN network gives better interpolation accuracy with 6-, 9- and 12-month SPI and variation in errors amongst the different SPI time scales is minimal. The difference between networks is also significant for PDSI. Although the absolute magnitude of the differences between interpolation with COOP and USHCN are small, the accuracy of interpolation with COOP is much more spatially variable than with USHCN.Item Breeding Maize for Drought Tolerance: Diversity Characterization and Linkage Disequilibrium of Maize Paralogs ZmLOX4 and ZmLOX5(2012-07-16) De La Fuente, GeraldMaize production is limited agronomically by the availability of water and nutrients during the growing season. Of these two limiting factors, water availability is predicted to increase in importance as climate change and the growing urban landscape continue to stress limited supplies of freshwater. Historically, efforts to breed maize for water-limited environments have been extensive; especially in the areas of root architecture and flowering physiology. As progress has been made and new traits have been discovered and selected for, the different responses to drought stress at specific developmental stages of the maize plant have been selected as a whole when drought tolerance is evaluated. Herein we attempt to define the characteristics of the maize drought response during different developmental stages of the maize plant that can be altered through plant breeding. Towards breeding for drought tolerance, 400 inbred lines from a diversity panel were amplified and sequenced at the ZmLOX4 and ZmLOX5 loci in an effort to characterize their linkage disequilibrium and genetic diversity. Understanding these characteristics is essential for an association mapping study that accompanies this project, searching for novel and natural allelic diversity to improve drought tolerance and aflatoxin resistance in maize. This study is among the first to investigate genetic diversity at important gene paralogs ZmLOX4 and ZmLOX5 believed to be highly conserved among all Eukaryotes. We show very little genetic diversity and very low linkage disequilibrium in these genes, but also identified one natural variant line with knocked out ZmLOX5, a variant line missing ZmLOX5, and five line variants with a duplication of ZmLOX5. Tajima's D test suggests that both ZmLOX4 and ZmLOX5 have both been under neutral selection. Further investigation of haplotype data revealed that ZmLOX12, a member of the ZmLOX family, showed strong LD that extends much further than expected in maize. Linkage disequilibrium patterns at these loci of interest are crucial to quantify for future candidate gene association mapping studies. Knockout and copy number variants of ZmLOX5, while not a surprising find, are under further investigation for crop improvement.Item Climate Warming and Drought Effects on Pinus and Juniperus Species: Contrasting Drought Tolerance Traits Limit Function and Growth in Tree Seedlings(2013-04-29) Lenoir, Katherine JudithJunipers and pines exhibit contrasting patterns of growth decline and mortality with climate change-type warming and drought; yet, the underlying physiological mechanisms are not fully understood. Does warming exacerbate the effects of drought on gas exchange physiology and growth? Do the combined effects of drought and warming differ for pines and junipers? To what extent do isohydric vs. anisohydric responses to water limitation in pines and junipers constrain net leaf CO2 exchange and plant growth response to drought and warming? To address these questions, we compared responses of leaf gas exchange and growth in seedlings of juniper (Juniperus scopulorum, J. virginiana) and pine (Pinus edulis, P. taeda) species of contrasting arid and mesic origin in a study of combined warming (ambient, +1.8 ?C) and enhanced summer drought (long-term mean, -40%). Warming and enhanced summer drought each reduced photosynthesis and growth and effects were largely independent, suggesting that warming exacerbates drought effects on growth. Enhanced summer drought and warming had distinct impacts on photosynthetic carbon gain that were differentially revealed depending upon soil water content. Warming reduced light-saturated net photosynthesis (Asat) under low soil water contents, whereas carry-over effects of drought treatment were evident under well-watered conditions. Short-term soil drying led to greater reduction of Asat in pines (-51%) rather than junipers (-30%). Under short-term water-limited conditions, Asat and gs were about two-times higher for junipers compared to pines. Relative growth rate of junipers declined with warming (-28%) and drought (-50%) treatments. In contrast, pine growth and Asat declined more with warming than drought. Only P. edulis exhibited increased mortality in response to warming and drought, reaching 75% in the combined warming and drought treatment. Diminished sensitivity of R to water limitations, coupled with steeper reductions in Asat with decreasing soil water content in isohydric pines compared to anisohydric junipers could account for the greater sensitivity of pines to warming and drought under climate change.Item Development of a screening method for drought tolerance in cotton seedlings(Texas A&M University, 2007-04-25) Longenberger, Polly SuzanneThe key to an efficient screening method is the ability to screen large amounts of plant material in the shortest time possible. Unfortunately, due to the complexity of drought tolerance, a quick and effective screen for this trait has yet to be established. The research reported herein was designed to evaluate a screening method for drought tolerance in cotton (Gossypium hirsutum L.) seedlings. Twenty-one converted race stocks (CRS) and two cultivars were evaluated for seedling drought tolerance on an individual plant basis. CRS are day-sensitive primitive lines derived from various wild race stocks that were converted to day neutrality for use in temperate region plant improvement programs (McCarty et al., 1993). Genotypes were evaluated October - November 2004 and February - March 2005 under greenhouse conditions at the Norman E. Borlaug Center for Southern Crop Improvement, College Station, TX. Seedlings were subjected to three sequential cycles of drought at 15 days after planting (DAP). Drought cycles consisted of withholding water until the moisture content of "indicator" cone-tainers, containing Deltapine 491 (DP 491), had an average volumetric water content of 0.07. Plants were then watered to field capacity and percent survival was recorded after 48 hours. Genotypes differed in their percent survival following three consecutive drought cycles. Drought cycles 2 and 3 did not contribute to the separation of genotypes. DP 491 was the most tolerant genotype evaluated. None of the CRS were more or less tolerant than Acala 1517-99. CRS M-9044-0165 was the most stable genotype across the two experiments.Item Diel Temperature and Dissolved Oxygen Patterns in Sites with and without Planktonic Life Stage of Thompsodinium intermedium in Comal Springs, TX(2012-07-16) Gilpin, CherylBetween July 2009 and October 2011, a new habitat was found for a rarely reported freshwater dinoflagellate species, Thompsodinium intermedium - Comal Springs (Comal County), Texas. In 2011, diel in-situ monitoring in monospecific blooms of this species revealed previously undetected negative impacts on endangered species habitat availability associated with conditions of low flow levels, recorded at the U.S. Geological Survey gage # 08169000 on Texas Commission on Environmental Quality river segment 1811 station 12655. During a period of low springflow in the summer of 2011, late afternoon and early morning measurements of dissolved oxygen and temperature and presence of dinoflagellate blooms were monitored at six sites. Significant differences in diel fluctuations were found in all of these parameters among sites with and without the planktonic blooms. These fluctuations increased risk of hypoxia and hyperthermia conditions at sites of planktonic bloom events. Arrays of in-situ continuous monitoring temperature/light probes were used inside and outside of blooms. Wildlife and human health implications are that hypoxia and hyperthermia are known to promote conditions favorable to harmful microbes which may be transported from springs to coastal bays. In-situ data demonstrated that T. intermedium blooms, hypoxia, and hyperthermia occurred in the upper Comal headwaters. These natural environmental stressors may be avoidable if adequate springflows are maintained to buffer against these impacts.Item Establishment and evaluation of a livestock early warning system for Laikipia, Kenya(Texas A&M University, 2005-08-29) Ryan, ZolaA new zone was added to the existing Livestock Early Warning System (LEWS), which is a subproject of the USAID Global Livestock Collaborative Research Support Program. LEWS uses the PHYGROW model and satellite imagery of weather and vegetation to estimate the availability of forage to livestock and wildlife. Drought advisories are then distributed to governments, development organizations, and pastoralists via the Internet, satellite radios, and written reports. The Laikipia zone was established in 2001 to provide drought early warning for the arid pastoral rangelands of the Ewaso Ngiro ecosystem in the Laikipia and southern Samburu Districts, Kenya. Field verification of PHYGROW estimates of standing crop was conducted in 2002. In addition, research was conducted to determine the ability of the warning system to provide significant advance notice of emerging drought conditions. Results of this study indicate that LEWS is capable of providing accurate estimates of forage availability on East African rangelands. There is also evidence that the use of LEWS advisories could accelerate drought response by pastoralists as much as three to seven weeks.Item Evaluation of chlorophyll fluorescence as a tool for the identification of drought tolerance in upland cotton(2009-05-15) Longenberger, Polly SuzanneA novel bioassay for the evaluation of plant water status was developed by Burke (2007). The research reported herein was designed to evaluate this new protocol as a tool for use in cotton breeding programs for the identification of drought tolerant genotypes. Twenty genotypes were selected to represent diverse germplasm pools for a two-year field evaluation. Replicated tests were performed in Lubbock, TX and College Station, TX in 2005, 2006, and 2007. Dryland and irrigated treatments were administered in a split plot arrangement of a randomized complete block design. Fluorescence measurements were taken at mid-bloom and late bloom growth stages of growth. Source leaf tissue was harvested at predawn and subjected to high temperature incubation with fluorescence measurements subsequently taken hourly for five hours. Drought stressed plants had not mobilized their carbohydrate reserves from their source leaves overnight and thus maintained cell viability and therefore higher chlorophyll fluorescence values throughout the incubation with the opposite being true for nonstressed plants. Fiber lint yield and fiber properties were measured at the conclusion of the 2005 season in College Station and the 2006 season in College Station and Lubbock for comparison with the fluorescence data. Five genotypes, ?Acala 1517-99?, ?Deltapine 491? (PVP no. 200100159), ?Tamcot CAMD-E?, ?Tamcot 22? and TAM 89E-51, an unreleased breeding line, were selected based on field evaluation results in a preliminary study in 2005 to be included in a diallel analysis to determine the heritability of fluorescence measurements. Genotype x treatment effects complicated the classification of genotypic responses to drought. Few and inconsistent correlations were found among fluorescence values and lint yield or fiber properties. The diallel analysis did not identify general combining ability or specific combining ability effects for chlorophyll fluorescence measurements. Thus this procedure provides little potential in selecting plants for drought tolerance when plants are grown under field culture. Selection among Tamcot 22 and TAM 89E-51 plants for high and low genotypes according to fluorescence values did not yield progeny different from unselected Tamcot 22 and TAM 89E-51.Item Freshwater Flow, Saltwater Intrusion, Paper Mill Effluent, and Fish Assemblage Structure in the Lower Neches River, Texas(2013-06-11) Pizano, Rebecca IIn 2011, Texas experienced the worst drought in recorded history. This has escalated concerns regarding environmental flows needed to sustain freshwater and estuarine systems as human needs are addressed during drought periods. In this thesis, I analyze fish assemblages and water quality variables in order to observe the effects of drought in the lower Neches River below the saltwater barrier located upstream from Beaumont, Texas. Fish and water quality samples were taken during drought conditions during fall 2011 and summer 2012, after a season of rain. During fall 2011, sites surveyed above the barrier had lower salinity but similarly low dissolved oxygen (DO) levels compared with sites surveyed below the barrier. Salinity levels during fall 2011 were relatively high (reaching up to 15 ppt), whereas salinity during summer 2012 never rose above 1.5 ppt. For gillnet samples obtained during fall 2011, fish species richness was higher in December following a series of rain events than during drought conditions in October and November. Although fish species richness was similar between fall 2011 and summer 2012, species composition varied greatly. For seine samples obtained during summer 2012, species richness was higher during May and July (when the barrier was open) than during June and August (when the barrier was closed). Species richness was lowest for sites in closest proximity to a paper mill effluent discharge pipe located below the barrier. Also, species richness was higher at sites above the barrier than at sites below the barrier regardless of whether or not the barrier was closed. Multivariate statistical analyses of gillnet samples revealed a large amount of compositional overlap among fish assemblages, regardless of time period and location; however, analyses of seine samples revealed that fish assemblages above the barrier were different than those from samples obtained below the barrier and that fish assemblages varied based on time period. Results indicate that, during periods of low flow, water quality deteriorates in the Lower Neches River below the saltwater barrier. During these periods of environmental degradation, fish assemblages have reduced diversity and sensitive freshwater species decline in abundance, with some absent from survey samples.Item Gene Expression and Association Analyses of Stress Responses in Loblolly Pine (Pinus taeda L.)(2012-02-14) Seeve, Candace MarieThe molecular mechanisms underlying disease-resistance and drought-resistance in forest trees are not well understood. Linking variation in gene expression with genetic polymorphisms and with variations in disease- and drought-resistance phenotypes can provide information about these complex traits. We used real-time quantitative polymerase chain reaction (PCR) to detect variations in the expression of 88 disease- and drought-responsive genes within an association population of 354 loblolly pine trees (Pinus taeda L.). Using association genetics approaches, we then linked 3,938 single nucleotide polymorphisms (SNPs) in candidate genes with gene expression phenotypes to identify novel disease- and drought-responsive genes. To further examine differences in gene expression induced by drought, Fusarium circinatum (responsible for pitch canker disease), and drought F. circinatum, the expression of 114 genes identified through comparative and association genetics approaches was analyzed on a subset of 24 loblolly pine trees possessing a range of pitch canker- and drought-resistance phenotypes. Significant differences in the uninduced expression of all 88 genes measured on the association population were observed among loblolly pine trees. Principal component analysis showed that some variation within the association population could be accounted for by population substructure of geographic origin. Hierarchical clustering of genes based on uninduced expression did not consistently group together functionally similar genes probably because expression was collected on unstressed stem tissue. This was supported in the smaller expression study as correlations between expression values of genes in the same functional networks were usually stronger when induced by a treatment compared with correlations between the uninduced expression of genes in the control group. Gene expression frequently changed by up to 4-fold in response to one or more treatments, but PtMYB12 was the only gene that exhibited a statistically significant change in response to treatments. ANOVA analyses of gene expression controlling for pitch canker resistance and for water use efficiency phenotypes identified differentially expressed genes suggesting that they may be contributing to these phenotypes. Finally, association genetics approaches detected 101 significant associations between SNPs in 94 candidate genes potentially involved in stress responses and 27 gene expression phenotypes.Item Influence of Newspaper Images on Student Perceptions of Agricultural Issues(2012-07-16) Dromgoole, AmyIn today's technological environment, there is constant competition for audience readership and viewership between various media outlets. News media provides a great deal of information to the general public through television, print, and web sources, especially in terms of agriculture. This study aimed to discover audience perceptions of two different natural disasters by examining the effects of photographic inclusion in print news articles including agricultural perceptions and content recall. Additionally, differences between self-perceived milk industry advocacy and a milk campaign story are also examined. Newspaper articles about the effects of the 2010-2011 drought in Texas and the aftermath of Tropical Storm Irene were also used. Students in the College of Agriculture and Life Sciences at Texas A&M University were surveyed in online pre and posttests. Student responses displayed a moderate relationship between photos and article content in regards to the Hurricane Irene article. A significant relationship was present between self-perceived non-advocates and their outlook on milk consumption and the dairy industry with the inclusion of photographs. There were differences seen between students who have family who work in agriculture, claim membership in an agriculture association, live on a farm or ranch, and were members of FFA as they viewed the drought article to be more positive than those who did not have these agricultural backgrounds. Furthermore, student responses show a relationship between the milk industry article in the pretest (photos included) and posttest (photos not included) by viewing the photos as positive, humorous, and shocking. This study found student perceptions of the two news articles related to the drought as well as the tropical storm to be the same regardless of photographic presence. This leads the researcher to conclude that photos had no effect on the overall perceptions of the news stories. However, students who received photographs did see a relationship between the photos associated with the flood article and the content presented in the story. This effect was not seen with the photographs of the drought story. Since the photos associated with the flood story were the original photos printed with the news story, it is probable to conclude that students properly associated photographic elements with that of the story's content.Item Interannual variability of summer precipitation in Texas and its implication to summer drought(2009-05-15) Myoung, BoksoonSince Texas normally receives most of its precipitation in the warm season, precipitation deficits in summertime may bring serious agricultural and hydrological disasters. While the underlying physical processes of summer precipitation deficit and drought are unclear, they can be understood in terms of convective instability. This research is designed to investigate how convective instability influences monthly mean precipitation in Texas in the summertime and to examine the modulation of convective instability and precipitation by upper-level circulations, soil moisture, vertical motion, and low-tropospheric warm air transport using NCEP/NCAR reanalysis data. Statistical approaches including correlation analysis, multiple linear regression analysis and back trajectory analysis were used to reveal the underlying dynamics of their linkage and causality. The results show that warming at 700 mb and surface dryness result in excessive convective inhibition (CIN), leading to precipitation deficits on a monthly time-scale. Temperature at 700 mb (Tlt) and surface dewpoint have little correlation suggesting different processes contribute to warming at 700 mb and surface dryness, respectively. Correlation analysis among the surface variables emphasizes the role of soil moisture on the dewpoint and thermodynamics at the surface. Back trajectory analysis indicates that a significant contributor to warming at 700 mb is the inversion caused by warm air transport from the Rocky Mountains and the Mexican Plateau where the surface potential temperature is greater than 307.5K rather than by subsidence. It was found that downward motion and warm air transport are enhanced in Texas when upper-level anticyclonic circulation develops in the southern US. Upper-level anticyclonic circulations in the southern US strongly affect Texas summertime precipitation by modulating the principal processes as follows. They increase CIN not only by enhancing warm air transport from the high terrain but also by suppressing occurrence of disturbances. The resulting reduced precipitation and dry soil significantly modulate surface conditions, which elevates CIN and decreases precipitation. The aforementioned chain-reaction of upper-level anticyclone influences can be understood in the context of CIN.Item Meteorological Factors Regulating the Population Expansion and Contraction of Amblyomma maculatum (Acari: Ixodidae) in Texas(2010-10-12) Coburn, Jordan McQuadeThe interaction of tick species and the microclimate that they inhabit is a subject that is frequently studied. The known biology of the Gulf Coast tick, Amblyomma maculatum Koch (Acari: Ixodidae), was used to conduct analyses to determine which meteorological factors regulate Gulf Coast tick survivorship in an area of Texas that is known for reduced occurrence of this tick species. Gulf Coast tick collection records, that indicated the collection of a single tick or multiple ticks from one animal or multiple animals at livestock markets, were obtained from the Texas Animal Health Commission. These records were used as an indicator of adult Gulf Coast tick abundance during each year in the 90 county study area and were used as the dependent variable in linear, quadratic, and cubic regression analyses. Independent variables used in these analyses were precipitation and differing drought thresholds during the peak activity time of the four life stages of the Gulf Coast tick and during combined life stage peak activity times. Linear, quadratic, and cubic regression analyses to measure the effect of precipitation during differing peak activity times of the Gulf Coast tick on adult Gulf Coast tick collection records were not statistically significant. These three regression analyses were also used to measure the effect of increasing drought thresholds, measured using a Keetch-Byram Drought Index, on adult Gulf Coast tick collection records. A determination was made that increasing drought thresholds during the peak activity time of differing Gulf Coast tick life stages reduce the number of Gulf Coast tick collection records the following year.Item Minimizing Biases in Radar Precipitation Estimates(2014-12-08) McRoberts, Douglas BThe demand for real-time drought information in recent years led to the development of a suite of objective drought indicators that relies on the high-resolution Stage IV precipitation estimates that are produced each day by the National Weather Service in near real-time. The drawback to using the Stage IV dataset for this purpose is the presence of numerous biases in the estimates, which lead to erroneous assessments of drought conditions. Among the types of biases in the Stage IV dataset are 1. Underestimation of precipitation due to beam blockage. 2. Range-dependent errors that originating from the measurement of reflectivity above the surface. 3. Mean-field biases resulting from radar calibration and measurement errors. A three stage bias correction procedure is developed and evaluated for minimizing the biases, methods used to produce an improved, bias-adjusted Stage IV precipitation dataset. The original Stage IV data are initially corrected by a beam blockage identification procedure and Kriging interpolation to replace the precipitation values in grid cells affected by blockage. Next, range-dependent and mean field biases are identified and corrected by use of a statistical model based on the vertical profile of reflectivity in mixed-phase precipitating systems. The last bias quantification procedure estimates and removes a two-dimensional field of residual biases using available gauges as an assumed unbiased estimate of the ground truth. Data withholding testing showed the bias-adjusted Stage IV dataset to have a significant reduction in the overall bias relative to the original Stage IV precipitation dataset. This includes a reduction in the overall bias at each of the three major steps. The bias-adjusted Stage IV dataset will be utilized in the drought indicators to enable a better objective assessment of real-time drought conditions.Item Overexpression of Tobacco Osmotin Protein in Carrot (Daucus carota L.) to Enhance Drought Tolerance(2012-10-31) Annon, Ali Hani HamzaLack of water is one of the most significant issues that already threaten world agriculture as many countries are unable to meet the demand for water to grow the crops. To make matters worse, the water availability is expected to fall by half by 2050, thus severely restricting agriculture production. Genetic engineering of crops to enhance their tolerance to such unfavorable environment represents one of the few approaches that can help us address this problem. Osmotin and osmotin-like proteins are stress proteins, belonging to the plant PR-5 group of proteins, which induced in response to various types of biotic and abiotic stresses in several plant species. Carrot plants were transformed with tobacco osmotin gene that encodes a protein lacking 20 amino-acid sequence at the C terminal end under the control of CaMV 35S promoter using the Agrobacterium-mediated transformation method. The gene integration and expression were confirmed by Southern and Western blot analyses and the transgenic plants were evaluated for their ability to tolerate drought stress. Under drought conditions, transformants exhibited slower rates of wilting compared to the wild-type and gained the ability to recover faster than their untransformed counterparts when the drought stress was alleviated. Under water stress, transformants showed lower levels of H2O2 accumulation, reduced lipid peroxidation and electrolyte leakage, and higher leaf water content. Taken together with some earlier reports, our results provide additional evidence for the protective ability of tobacco osmotin protein against drought stress and suggest a possible means to achieve tolerance against a serious type of abiotic stress.Item Perennial grass community response to severe drought, topo-edaphic variation, and long-term herbivory on the Edwards Plateau of Texas(Texas A&M University, 2005-11-01) Shackelford, Colin ScottPerennial grass vegetation dynamics of heavy grazing, moderate grazing and ungrazed treatments were analyzed during two extreme drought events: the drought of 1951 to 1956 and the drought events centered on the year 2000. Analysis of each drought event from weather records and Palmer Drought Severity Index values showed that the intensity, duration and pattern of each drought event were structurally unique. Grazing intensity was the primary driver of perennial grass species composition and community structure both during and between each drought event. Total basal area for each drought event was driven primarily by variation in precipitation pattern. Basal area per plant dynamics were significantly influenced by grazing intensity while plant density was driven by both precipitation and grazing intensity. Topo-edaphic variation significantly influenced the persistence and distribution of perennial grass species during an extreme drought event centered on the year 2000. The presence of large or abundant surface rock features facilitated the survival of perennial grasses during this drought event by creating a positive soil microenvironment. Sites with large surface rock features acted as drought refugia for perennial grass populations. Sites with abundant surface rock features had 10 times greater perennial grass basal area and 5 times greater plant density than sites with few surface rock features. Grazing intensity was the primary driver of species composition and community structure within both refugia and non-refugia sites. Grazing intensity had no effect on perennial grass basal area. However, grazed treatments had two times greater perennial grass density than non-grazed treatments. Species response to grazing intensity and surface rock cover was individualistic. Grazing response groups (intolerant, intermediate, and tolerant) separated along a gradient of surface rock cover and grazing intensity. Abundant surface rock features act as refugia for perennial grasses by facilitating their persistence during extreme drought events on the Edwards Plateau.Item Physiological applications for determining water use efficiency among cotton genotypes(2009-05-15) Bynum, Joshua BrianDrought stress can substantially alter plant metabolism by decreasing plant growth and photosynthesis. The lack of rapid and reliable screening criteria and measurement techniques for determining water use efficiency (WUE) of crop plants has greatly restricted progress in this critical area of crop improvement. In grain sorghum (Sorghum bicolor L.), WUE was associated with the transpiration ratio [CO2 assimilation (A) / transpiration rate (E), A:E] from leaf gas exchange measurements. Research is needed to identify drought effects on plant productivity and to exploit the use of this knowledge in breeding and agronomic efforts. Therefore, the objectives of this study were to determine if differences in A:E and other physiological parameters existed between two selected cotton (Gossypium hirsutum L.) genotypes and to evaluate the response of cotton genotypes experiencing water stress at two different growth stages on biomass production and yield. Two experiments were conducted using two cotton genotypes differing in drought tolerance. Each experiment was repeated three times in a randomized complete block design with six replications. In Experiment I, the water stress treatment was induced by withholding water when the plants reached the 4-node growth stage. The water stress treatment in Experiment II was imposed at early bloom. Gas exchange and chlorophyll fluorescence measurements were collected during dry-down and recovery periods to determine water stress effects on plant physiology. Biomass was partitioned following the recovery period, to examine phenotypic responses of plants exposed to water stress. The results of these experiments indicate that A:E is significantly increased as leaf water potential (?L) decreases with no differences in A:E between the two genotypes. Gas exchange measurements showed significant decreases with declining ?L and significant increases upon re-watering; yet, no differences were observed between the two genotypes. Chlorophyll fluorescence was not different between genotypes in either light- or dark-adapted leaves. In Experiment I TAM 89E-51 had a significantly greater seedcotton yield; however, in Experiment II TAMCOT 22 had the greater yield. These experiments suggest that the effects of water stress on cotton are a function of the intensity of the stress and the growth stage in which the stress is experienced.Item Responses of High Biomass Rice (Oryza sativa L.) to Various Abiotic Stresses(2011-10-21) Kondhia, Aditi NitinkumarRice produces a lot of biomass which is an important trait in increasing grain yield and it is a potential feedstock for bioenergy production. High biomass rice is important to meet the growing demands of grains and biomass for food, fodder and bio-fuel industries. Limited studies have been conducted to determine its response to unfavorable conditions. The main objectives of this study were to determine the response of selected high biomass rice to drought, rainfed and flooded conditions and identify best genotypes that can be grown in unfavorable areas. Two experiments were conducted in summer 2009 to evaluate biomass yield and agronomic traits of selected high biomass genotypes. A greenhouse study had genotypes grown under drought condition - different field capacity (FC) i.e. 100 percent, 75 percent and 50 percent FC, while the field study had rainfed and flooded environments. Most of the genotypes performed well under fully saturated soil conditions but some were less affected by drought. Limited water delayed first tiller emergence and reduced tiller count, rate of tiller production, plant height, rate of increase in height, shoot and root weight, root:shoot (R:S) ratio, percent dry matter (percent DM) and total biomass. The plant height, tiller plant-1, and total biomass at maturity were lower under rainfed conditions and their flowering was delayed compared to flooded conditions. Majority of these traits were correlated with high biomass yield. Genotype 11 which is tall and late maturing produced the highest number of tillers plant-1 and tillers/ 750 cm2 and had the highest biomass yield under both rainfed and flooded conditions. It performed equally well under drought conditions particularly in root and R:S ratio, but genotype 12 was the best in most parameters measured in the greenhouse. Although it was the shortest genotype, it was highest in biomass yield, earliest to tiller, had the highest shoot weight and tiller count, and had the fastest tiller production. The high biomass genotypes like conventional rice were affected by drought and performed better under flooded conditions. However, these two genotypes can produce optimum results under limited availability of water and hence be used for biomass production under stressed environments.Item The Drought Response of Physiological and Structural Traits in Loblolly Pine (P. taeda L.) Clones with a Focus on Mesophyll Conductance to CO_(2)(2014-07-10) Wilson, Elizabeth SusanClimate change will likely affect the productivity of forests through changes in precipitation and moisture availability. An important measure of a plant?s ability to assimilate carbon in photosynthesis with limited water loss, water use efficiency (WUE), is assessed by carbon stable isotopes using the Farquhar model. However, recent work has shown that mesophyll conductance to CO_(2) (g_(m)) is affected by environmental conditions, and the simplified model does not take into account this variability. Variation in this parameter could decrease the effectiveness of the stable isotope tool. A coupled gas exchange and carbon isotope system was developed and tested on well-watered loblolly pine seedlings. A cavity ring-down spectroscopy (CRDS) laser connected to a LI-6400XT gas exchange system allowed simultaneous measurements of photosynthesis and instantaneous carbon isotope discrimination. The standard deviation of two minute averaged delta over ten minute intervals ranged from 0.31? to 0.51?. Mesophyll conductance measured by this system on well-watered loblolly pine seedlings ranged from -0.74 to 0.99 mol m^(-2) s^(-1) and averaged 0.07 ? 0.04 mol m^(-2) s^(-1). A greenhouse study was conducted of the drought effects on g_(m) and other physical and biochemical traits in three clones of loblolly pine (P. taeda L.). Stomatal and hydraulic sensitivity to drought was assessed in the clones. Mesophyll conductance was estimated using several methods: gas exchange curve fitting, the variable J method of chlorophyll fluorescence, carbon isotope discrimination from leaf soluble carbohydrates, and instantaneous carbon isotope discrimination using a cavity ring-down spectroscopy laser. The three clones exhibited plasticity in stomatal conductivity and hydraulic conductivity in response to drought. The fastest growing clone also had the highest hydraulic conductivity and lowest WUE in well-watered conditions. There were no significant clonal or drought effects on g_(m), suggesting that the simplified Farquhar model is adequate for the use of stable isotopes as a proxy for WUE.