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Dennis T. Ray and Youngkoo Cho

Progress toward the domestication of guayule (Parrhenium argentatum Gray) as a domestic source of latex, has been slowed because of its complicated reproductive biology. This work was performed to help elucidate the facultative nature of apomictic reproduction in guayule. Ploidy level and six morphological characters were measured in one-hundred and eighty open-pollinated, half-sib progeny derived from twelve different apomictic tetraploid (4n=4x=72) female parents. The number of chromosomes observed among the half-sib progeny varied from 36 to 81, with the majority (77.3%) of the progeny with 72 chromosomes. All of the observed chromosome numbers could be explained either by meiotic reduction in the megaspore mother cells or fertilization of egg cells or both. The frequencies of meiotic reduction and fertilization resulting in these progeny were estimated to be 27.2 and 22.7%, respectively. Among the six measured morphological characters, significant correlations were found only between chromosome numbers and plant height (r=0.37) and leaf area (r=0.31).

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Marietta Loehrlein and Dennis T. Ray

Triploid watermelon seed are produced by crosses between tetraploid female and diploid male plants. When open-pollinated, the resultant seed can be either tetraploid due to self-pollination or triploid from pollination by a diploid. This work was done to test if triploid and tetraploid seed can be separated on the basis of seed thickness and weight. Open-pollinated seed from a 4n × 2n cross were separated by either thickness (grouped into 0.1-mm increments) or weight (5-mg increments). Seed were germinated in a greenhouse and transplanted into the field. Plants were scored as either triploid or tetraploid by use of a genetic marker system. When separated by either thickness or weight, triploid and tetraploid seed were found in essentially each size category. There were no significant differences between populations for thickness, and the mean weights were essentially equal between triploid and tetraploid seed.

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Marietta Loehrlein and Dennis T. Ray

Triploid watermelon seed does not germinate in cold, wet soils as well as diploids; germination is slower due to reduced embryo size and thicker seed coat; fissures on the seed coat provide safe harbour for fungal spores; and triploid fruit set is later than most diploid cultivars. Because of these problems producers often transplant rather than direct-seed seedless watermelons. Seed priming has been shown to improve germination in other crops and would be an attractive method allowing for direct seeding of seedless watermelons. Seed from open-pollinated 4n × 2n crosses were primed in solutions of H2O, polyethylene glycol 8000, KNO3, or left untreated. Treatment times were 1, 3, or 6 days, and treated seed were subsequently dried for either 1 or 7 d. Seed were scored for germination in the laboratory and emergence under field conditions. Germination was better using H2O than KNO3 and PEG but not always better than the untreated control. Treatment time of 1 day was superior to 3 or 6 days, but length of drying time was insignificant. In the field trial, treatments did not differ in emergence.

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Maren E. Veatch* and Dennis T. Ray

Rubber production in guayule (Parthenium argentatum, Gray) is highest in the winter months. It has been suggested that lower nighttime temperatures stimulate rubber production. Higher rubber production could be the result of up regulation of the enzymes involved in rubber synthesis, or acclimation of the photosynthetic apparatus to cold temperatures, making photosynthates available for rubber production when the plants are not actively growing, or a combination of the two factors. The effect of low night temperatures on photosynthesis and rubber production was examined in greenhouse grown plants. The experiment was set up as a split plot with different sub plots enclosed in heated or non-heated containers during the night. Those placed in non-heated containers were exposed to the ambient night temperatures of the greenhouse. During the growing season, growth was measured as change in height, and midday and diurnal photosynthetic measurements were collected. At harvest fresh and dry weight of both roots and shoots were taken. Resin and rubber were then extracted with acetone and cyclohexane respectively. Midday photosynthesis was as high or higher in cold night plants than in the warm night plants. In addition, the cold night treatment had lower growth than the warm night treatment. Percent resin and rubber were significantly higher in both shoots and roots of cold night plants, as was the overall yield of rubber. An increase in photosynthetic rate in the cold night treatment, without the concomitant increase in growth seen in the warm night treatment, could account for the increase in rubber production during the winter months.

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Amed N. Mamood and Dennis T. Ray

Studies were conducted to evaluate container size and pretreatment on transplant survival and growth of guayule (Parthenium argentatum Gray). Seeds of cv. 11605 were planted in a greenhouse in two different container sizes. After 60 days half of the seedlings in both treatments were clipped, and then hand transplanted into the field. The height and width of 10 plants in each treatment were measured biweekly. The percent survival, date of flowering, seed germination and weight of 1000 seeds were determined. Plants produced in large containers had a higher survival rate, plant size and flowering rate. In addition non-clipped seedling had significantly higher survival rates. There were no significant differences in seed germination or in seed weights among treatments.

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Perry E. Nugent and Dennis T. Ray

Since 1968, three spontaneous 4× melons (Cucumis melo L.) plants were discovered in our field or greenhouse plantings. Two were found in the cultivar Planters Jumbo and one in the virescent marker C879-52. Each of these 4× plants had rounded cotyledons, shorter internodes, thicker stems and leaves, more hairs, and smaller fruits, with larger stem and blossom scars, than their 2× counterparts. Also, their flowers, pollen grains, stomates, and seeds were larger. The discovery of a 4× virescent plant in 1987 allows easier germplasm transfer between ploidy levels. Morphological characteristics of 2× and 4× melons will allow identification without need for chromosome counts.

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William J. Liesenbein and Dennis T. Ray

Guayule is a desert shrub that has been researched extensively for the purpose of developing it into a commercially viable source of high quality, hypoallergenic latex and rubber. Traditional breeding techniques have dramatically increased rubber yields, but genetic engineering techniques have the potential to increase rubber concentration in each plant while accelerating the development time of high-yielding guayule lines. Development of techniques to measure the flow of pollen from transgenic to non-transgenic plants is necessary to learn how to limit potential contamination of the environment due to transgene escape. Thus, the purpose of this work was to develop the methodology to quickly screen for transgenic seedlings. Guayule lines AZ 101, AZ-2, and N6-5, which were transformed with plasmids containing the NPTII gene that confers kanamycin resistance, were planted in a field surrounded by border rows and satellite plots of non-transgenic guayule. Seeds of known transgenic and non-transgenic guayule (controls) and seed collected from the non-transgenic plants were germinated and treated with an aerosol solution containing kanamycin at concentrations of 200, 300, and 400 mg·L-1. Kanamycin caused seedlings to develop blotchy yellow leaves at all concentrations. Kanamycin at 300 mg·L-1 was determined to be ideal for its time-to-symptom development and its lack of symptom severity found when treated at 400 mg·L-1. Plants showing kanamycin resistance or kanamycin damage were confirmed using PCR.

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Maren E. Veatch-Blohm and Dennis T. Ray

As a native of the Chihuahuan desert, guayule (Partheniumargentatum Gray) has a history of dealing with low water availability. Agronomic studies have shown that increasing irrigation increases overall rubber yields, but decreases rubber concentration per plant. As water availability is an important factor in agricultural production, this study was conducted to examine how drought affects plant growth and secondary compound distribution throughout the plant. One-year-old guayule plants were subjected to water stress from June through August, in 2003 and 2004. The well-watered treatment was irrigated daily, and the drought-stressed plants were irrigated when the soil water potential reached 6 (0.6) or 3 (0.3) bars (megapascals) in 2003 and 2004, respectively. Plant growth was monitored by measuring height, width, and stem diameter. Fresh weight of shoots and roots was recorded at harvest, and a subset of plants were defoliated and used to determine leaf weight and area. Resin and rubber were extracted from dried and ground plant samples. Growth, leaf weight, and leaf to stem ratio were decreased in the drought-stressed plants compared to the well-watered plants. Rubber concentration, but not resin concentration, was higher in the drought-stressed plants. There were no significant differences in resin and rubber concentration in the leaves and roots of the different treatments; however, they were both higher in the stems of the drought-stressed plants. In guayule, rubber is deposited mainly in the bark parenchyma of the stems. The drought-stressed plants had a greater contribution of stem biomass to overall biomass and a reduced stem diameter with higher bark to wood ratio, which could account for the higher rubber concentration in the drought-stressed plants.

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David A. Dierig, Pernell M. Tomasi, and Dennis T. Ray

Lesquerella fendleri (Gray) Wats. (lesquerella, Brassicaceae), native to the southwestern United States, is a potentially useful industrial oilseed crop. The seed oil contains hydroxy fatty acids, similar to castor (Ricinus communis L.) seed oil. The unique properties of the oil, along with coproducts, allow additional applications that would not compete with castor oil. Plants with vestigial anthers (male-sterile) were discovered in a greenhouse-grown, nonselected population in 1993. The inheritance of the trait was investigated through four crop seasons. Crosses were made among male-sterile and male-fertile plants from an open pollinated population, thus, they were heterozygous for many traits. Statistical analysis indicated that male sterility is expressed as a result of two nonlinked nuclear genes with epistatic relations and different cytoplasms, which cause partial or total fertility restoration. These ratios fit a 13:3 epistatic ratio, indicating that male sterility is controlled by homozygous recessive alleles at one locus in combination with at least one dominant allele at the second locus, i.e., ms1ms1 Ms2_. Some cross results were skewed in favor of fertile phenotypes presumably due to cytoplasmic effects causing partial fertility restoration. Male-sterile lines could be used for hybrid development and this information will be helpful in implementing a strategy for hybrid development. Hybrid plants and higher yields will enhance the potential for commercialization of this new alternative crop.

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Roy N. Keys, Dennis T. Ray, and David A. Dierig

Guayule (Parthenium argentatum Gray), a latex-producing perennial desert shrub and potential industrial crop for semiarid regions, exhibits reproductive modes ranging from sexual, self-sterile diploids to predominantly apomictic, self-compatible polyploids. The objectives of this study were to develop and evaluate a rapid, simple technique for characterizing apomictic potential (percentage of ovules that produce apomictic embryos) in guayule breeding lines. Initial in vivo experiments were based on an auxin test that permitted quantification of apomictic frequency in grasses. In our trials, floral application of NAA or IBA resulted in embryo production similar to that of open-pollinated controls, but 2,4-D inhibited embryo production. Breeding lines could be separated based on embryo production using an in vivo auxin test; however, accuracy of the results was questionable because 1) pollen release and insect activity within isolation bags prevented distinguishing between sexual and apomictic embryos, and 2) high temperatures and large humidity fluctuations could have affected results. Thus, in vitro flower culture was investigated using liquid medium, because it would provide better control of these factors. Flowers developed normally in vitro, except that pollen was not released from the anthers; therefore, any embryos produced in vitro were considered to be apomictic. Embryo production was similar on both Nitsch and Nitsch and Woody Plant Media. Addition of growth regulators inhibited embryo production. Embryo production was tested on Nitsch and Nitsch medium without growth regulators for seven breeding lines. Based on statistical analyses, four classes of apomictic potential were identified, ranging from none (sexual) to high. Chemical names used: 2,4-dichlorophenoxy acetic acid (2,4-D); indole-3-butyric acid (IBA); α-naphthaleneacetic acid (NAA).