Browsing by Subject "drought tolerance"
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Item Physiology and Genetics of Drought Tolerance in Cowpea and Winter Wheat(2012-02-02) Verbree, David 1977-In the wake of rising temperatures, erratic rainfall, and declining ground water table, breeding for drought tolerance in food crops has become a top priority throughout the world. Phenotyping a large population of breeding lines for drought tolerance is time-consuming and often unreliable due to multiple possible mechanisms involved. In cowpea (Vigna unguiculata L. Walp), a box-screening method has been used to partition the confounding effects that shoot and root traits have on drought tolerance by restricting root growth and providing a homogeneous soil moisture environment across genotypes. Nonetheless, multiple mechanisms of shoot drought tolerance have been reported which further complicate phenotyping. In winter wheat (Triticum aestivum L.), canopy temperature depression (CTD) has been proposed as a good indicator of drought tolerance. The recent development of low-cost thermal imaging devices could enable high-throughput phenotyping of canopy temperature. While CTD can be an indicator of overall plant water status, it can be confounded by high stomatal resistance, which is another seemingly contradictory mechanism of drought tolerance. The objectives of this study were to explore the physiological basis and genetics of the two mechanisms of shoot drought tolerance previously reported in cowpea and to develop and evaluate a method of high-throughput phenotyping of drought tolerance in winter wheat using thermal imaging. In cowpea, a legume well known for its tight stomatal control, no differences in gas exchange between drought tolerant and susceptible genotypes were observed. A unifoliate stay-green trait was discovered that segregates as a single recessive gene. However, it did not correlate with trifoliate necrosis or overall drought tolerance. In winter wheat, CTD did not always correlate with yield under rainfed conditions. One drought-tolerant cultivar, in particular, had the hottest canopy temperature, possibly because it was able to conserve moisture by closing its stomata whereas another closely related drought-tolerant cultivar had the coolest canopy temperature. Therefore, it appears that no single method of phenotyping for drought tolerance can be broadly applied across all genotypes of a given species due to possible contrasting mechanisms of drought-tolerance and environmental differences.Item Telomerase activator1: a zinc-finger protein that acts synergistically with auxin to control telomerase expression in Arabidopsis thaliana(Texas A&M University, 2006-04-12) Ren, ShuxinTelomerase is the key enzyme synthesizing telomeric DNA in most eukaryotic organisms. In mammals, telomerase expression is abundant in the germline cells but is undetectable in most other differentiated organs. Intensive studies of telomerase have focused on human cancerous cells, where over 90% of all cancerous tissues examined have telomerase activity. In wild-type Arabidopsis, telomerase expression is abundant in reproductive organs and dedifferentiated tissues such as flowers, siliques and calli but barely detectable in vegetative tissues (both rosette and cauline leaves). In this study, a biochemical screen strategy was developed for isolation of telomerase activating mutants in Arabidopsis thaliana. Through screening of Arabidopsis activation-tagged lines by a PCR-based TRAP assay, two tac (for telomerase activator) mutants were isolated. RT-PCR analysis of AtTERT expression revealed that different mechanisms are involved in alternating telomerase activity in tac1 and tac2. We cloned and characterized the TAC1 gene. TAC1 encodes a single zinc finger protein and acts synergistically with auxin to induce telomerase expression without altering cell cycles. Telomere length was unperturbed in the mutant, but other phenotypes, such as altered root development and the ability of cells to grow in culture without exogenous auxin, indicated that TAC1 not only is part of the previously reported link between auxin and telomerase expression, but also potentiates other classic responses to this phytohormone. DNA microarrays were used to analyze the expression profile of the tac1 mutant and revealed that several drought-induced genes were up-regulated 3 to 10 fold in the tac1-1D mutant. RT-PCR analysis further confirmed this up-regulation for five of these genes. Investigation of root growth also indicated that tac1-1D roots were ~20% longer relative to wild-type. Further experiments demonstrated that over-expression of TAC1 does confer drought tolerance, but not salt tolerance. In addition, our preliminary result showed that treatment with a low concentration of IAA could induce drought tolerance in wild-type Arabidopsis. Although plants with constitutive expression of telomerase have no practical utility, the ability of TAC1 to confer drought tolerance could have significant agricultural applications.Item The Responses of Selected Garden Roses (Rosa x Hybrida) to Drought and Salt Stresses(2014-07-16) Cai, XiaoyaWater shortage and poor water quality are critical problems for agriculture in many regions of the world. Roses (Rosa ? hybrida L.) are some of the most common garden plants in the world. Despite their popularity, however, they can present challenges to gardeners, particularly in relation to their responses to environmental stresses, such as those caused by arid and semiarid regions. The objectives of the present study were to investigate the tolerance of selected garden roses by evaluating their growth and physiological responses under drought and salt stress conditions. Four experiments were conducted to evaluate 25 garden rose cultivars, including 22 Earth-Kind? rose cultivars. In Expt. 1, four garden roses were subjected to two watering treatments: well-irrigated and cyclic drought stress. ?RADrazz? was the most drought tolerant among the cultivars investigated. With lower gas exchange and greater reduction in flower numbers at low substrate moisture content (SMC), ?Marie Pavie? was the least drought tolerant. In Expt. 2, two Earth-Kind? rose cultivars were subjected to four constant SMC of 10, 20, 30, and 40 % by utilizing an automatic irrigation system. Plants at 30 and 40 % SMC maintained the highest shoot and root dry weight (DW), flower number, midday leaf water potential, and photosynthetic rate. Plants had excellent performance at 30 % SMC and acceptable growth and quality at 20 % SMC. The 10 % SMC led to significant growth reduction, poor quality, and 25 % mortality. In Expt. 3, six garden rose cultivars were subjected to three salinity levels at electrical conductivity (EC) of 1.5 (control), 4.0 or 8.0 dS?m^(-1). ?New Dawn? was considered to be the most salt tolerant, while ?Caldwell Pink?, ?Marie Pavie?, and ?The Fairy? were the least salt tolerant with their greater reductions in flower number and shoot DW. In Expt.4, 18 Earth-Kind? rose cultivars were subjected to two salinity levels at EC of 1.2 (control) and 10.0 dS?m^(-1). By comparing the growth and physiological responses at high saline solution among the 18 cultivars, ?Belinda?s Dream?, ?Climbing Pinkie?, ?Mrs.Dudley Cross?, ?Reve d?Or?, and ?Sea Foam? were the most salt tolerant, while Cecile Brunner?, ?Else Poulsen?, ?Madame Antoine Mari, ?Perle d?Or, ?Spice?, and ?Souvenir de St. Anne?s? were the least salt tolerant among the cultivars investigated.Item Two Approaches to Evaluate Drought Tolerance in Maize: Seedling Stress Response and Epicuticular Wax Accumulation(2010-12-02) Meeks, MeghynWe wanted to develop rapid and cost-effective drought tolerance screening methods for mass amounts of germplasm. In 2009 and 2010, we evaluated sixty-two maize inbred lines and their hybrid testcross progeny using seedling stress response and epicuticular wax accumulation as predictors of drought tolerance. The seedling screening method measured germination, survival and recovery percentages after a series of drought cycles in a greenhouse environment. Eight inbred lines had significantly (P < 0.05) lower germination than the mean estimate, but hybrid testcrosses were not significantly different. The second-to-last day of survival cycle and the second day of recovery cycle best explained genotypic differences for inbred lines and hybrid testcrosses respectively. One inbred line performed well as both an inbred line and hybrid testcross, but poor correlation over the sample set (R2 = 0.0097) was observed. Flag leaves taken at flowering from plants under full and limited irrigation regimes were sampled for epicuticular wax. Extracted wax weight for genotypes was compared as a percentage of leaf weight (percent wxlfwt) and leaf area (percent wxwta). Eleven genotypes had above average percent wxlfwt as both inbred lines and hybrid testcrosses. Thirteen genotypes had above average percent wxwta as either inbred lines or hybrid testcrosses. Irrigation treatment was not significant (P > 0.05). Heritability of percent wxlfwt was 0.17 (inbred lines) and 0.58 (hybrid testcrosses). Heritability of percent wxwta was 0.41 (inbred lines) and 0.59 (hybrid testcrosses). Correlations (R2) for percent wxlfwt and percent wxwta were 0.19 and 0.03, respectively. Heritability of grams of grain per ear and total grain yield was highest in hybrid testcrosses, with no correlation between inbreds and hybrids. The developed seedling screening method easily allowed visible drought tolerance observations in inbred lines and hybrid testcrosses but does not seem heritable in our germplasm. Epicuticular wax weight is not an ideal primary trait to evaluate for drought tolerance, but may be a good candidate to observe as a secondary trait in relation to grain yield production in hybrids. Results from this research best supports breeder selection of hybrid germplasm using seedling drought tolerance in conjunction with epicuticular wax.