Rising demand for organic soybeans (Glycine max L.), coupled with high price premiums for organic products, has producers interested in making the transition to organic soybean production. However, organic soybean producers and those making the transition cite weed management as their biggest limitation. Current organic weed management relies heavily on cultivation. Cultivation has negative consequences on soil health and producers are interested in organic reduced and no-till production. Rye (Secale cereal L. cv. Rymin) cover crop was evaluated for weed suppression abilities and effects on soybean yield. Experiments were planted in 2008 and 2009 at three site locations. Rye was planted in the fall of each year and killed at soybean planting with a roller-crimper or flail-mower, creating thick weed-suppressing mulch with potential allelopathic properties. The mulch was augmented with one of three additional weed control tactics: pre-emergence corn gluten meal (CGM), post-emergence clove oil, or post-emergence high-residue cultivation. Rolled-crimped and flail-mowed treatments had similar weed suppression abilities. There were no differences between CGM, clove oil, or cultivation at most sites. Rye biomass level was the only independent variable that affected weed control. Rye biomass levels higher than 9,000 kg ha-1 were sufficient in controlling weeds. Organic soybean yields were similar to weed-free soybean yields at rye biomass levels higher than 9,000 kg ha-1. Goldsboro 2008, where rye biomass was 10,854 kg ha-1, the organic rye-only treatment yielded at 2,190 kg ha-1 and the weed-free treatment yielded at 2,143 kg ha-1. Plymouth 2008, where rye biomass was 9,526 kg ha-1, the organic rye-only treatment yielded 2,694 kg ha-1 and the weed-free treatment yielded at 2,809 kg ha-1. On the contrary, at low rye biomass levels (4,450- 6,606 kg ha-1), the organic rye-only treatment yielded 628- 822 kg ha-1 less than the weed-free treatment. High rye biomass levels are critical to the success of this production system. However, at rye biomass levels greater than 10,000 kg ha-1, severe soybean lodging was induced and potentially limited yield potential.

Rhizoctonia solani causes stem rot and target spot of greenhouse-produced tobacco seedlings. No fungicides are registered for control of these diseases, so sanitation is the primary disease management strategy. Seedling resistance to R. solani has not been characterized in current tobacco germplasm. The objective of this study was to screen seedlings of a diverse array of accessions, including several classes of tobacco cultivars and related Nicotiana species for resistance to a stem rot (AG-4) and a target spot isolate (AG-3) of R. solani. Further studies were conducted to determine if the resistance identified is heritable. Experiments were conducted in environmentally controlled growth chambers at the NCSU phytotron. Tobacco seedlings were grown in polystyrene trays floating on a nutrient solution to replicate greenhouse growth conditions. Approximately two weeks after germination, rice grains colonized by R. solani were placed on the surface of the growth medium to infest the medium. Symptoms, including death, stem lesions, and target spot lesions, were observed for 42 days after infesting the soil for stem rot and 56 days for target spot. Data were analyzed using a GLM procedure in SAS (SAS Institute, Cary, NC). Significant differences were observed among the accessions in level of resistance to stem rot and target spot. Disease incidence ranged from 12.5 to 100% for stem rot and 6.2 to 97.9% for target spot. This wide range of disease incidence observed among accessions for both diseases indicates that useful levels of resistance may exist to both diseases and may be useful in future breeding efforts.

The literature has shown the possibility to enhance many of the soil properties that improve production with additions of plant biomass to the soil in conjunction with long-term conservation tillage. Since it has been proven that reduced tillage is extremely beneficial to a soilâs health, the key question is if the incorporation of deep rooted cover crops and/or large quantities of above ground biomass over a two-year period would result in extensive improvements on a soilâs natural properties throughout the effective rooting depth, or is this too short of a time period for any beneficial effects to occur to the chemical, physical and biological properties of the soil? While SOC additions to the soil surface through decaying biomass can make measurable improvements to a soil in the humid, tropical southeastern U. S., the dynamics of total carbon (total SOC), total nitrogen (TKN), particulate organic matter (POM), potentially mineralizable nitrogen (PMN), CEC and bulk density (Db) in the 0-5, 5-10, and 10-18 cm depths in a production system that incorporates deep rooted cover crops and/or additions of organic matter from cover crops at rates >6 Mg/ha/yr (3 T/ac) are unknown. The objectives of this research were to determine what effect rye (Scale cereale), barley (Hordeum vulgare), alfalfa (Medicago sativa), wheat (Triticum aestivum), triticale (Triticale hexaploide Lart.), annual white sweetclover (Melilotus officinalis), blue lupine (Lupinus angustifolius), rye/hairy vetch (Scale cereale/Vicia villiosa) and alfalfa/rye (Medicago sativa/Scale cereale) would have on the following parameters: total SOC, POM, TKN, PMN, CEC, and Db after two (2) years of seeding. Significant spatial/temporal interactions and main effects were found in Db, PMN, POM and CEC while significant main effects were found in total C and total N. Significant treatment interactions were found in Db, total C, PMN, POM and CEC. Relative to treatment biomass, significant effects were seen between treatments and treatment*year interaction. Results on Db found spatial variability with depth but not with season and a minimal treatment effect depending on surface texture. Total C and total N decreased with depth but were independent spatially and temporally; however, treatment effect on total C was <5 months. Between Db and total C, the two parameters were inversely correlated. PMN either fluctuated between sampling times or declined with time. Notably, rye and rye/hairy vetch effected PMN in the 0-5 cm depth where other treatments showed no effect. POM declined with time regardless of depth implying a priming effect was occurring; however, rye/hairy vetch appeared to show an early reversal trend. As for CEC, there was no consistent trend. Additionally, CEC exhibited a moderate correlation to POM but not total C. Biomass measurements indicated no treatment consistently exceeded >6 Mg/ha/yr. Rye and rye/hairy vetch more often produced the most biomass and lupine achieved the overall maximum yield. Relationships between biomass and N parameters found a moderate, positive effect from the biomass inputs that appeared to compound with time. Overall, rye and rye/hairy vetch were the best cover crop treatments but two years of biomass inputs were not enough to prevent declines in POM. Conversely, PMN fluxed from applied N, and biomass with low C:N ratios oxidized quickly resulting in less carbon. Total C, total N and Db were not responsive to short term management indicating more effort is needed to define and/or develop a cover crop that will consistently reach the >6 Mg/ha/yr.

Adequate stability of cotton (Gossypium hirsutum L.) lint yield is an integral criterion for cultivar release; however, the magnitude of lint yield variation today is close to six times greater than variation observed in the 1920?s. Yield stability has often been associated with genetic diversity. Observing cotton lint yield in diverse population types containing various levels and kinds of genetic diversity over many environments could reveal information about stability and how it relates to diversity. An 18-environment field study was undertaken to observe lint yield stability in four population types of cotton. These populations were pure lines grown in pure stands (homozygous/homogeneous), pure lines grown in blended stands (homozygous/heterogeneous), hybrids grown in pure stands (heterozygous/homogeneous), and hybrids grown in blended stands (heterozygous/heterogeneous). Lint yield components were also observed to determine the contribution each had towards lint yield stability. Differences were determined by observing the coefficient of variation (CV) for mean yield and yield components of population types and over environments. We found the heterozygous populations to be more stable than the homozygous populations. This was attributed to the hybrids and blends of hybrids out-yielding the parents and blends of parents in the low-yielding environments. This advantage was not observed in the high-yielding environments and, in effect, reduced the amount of variation observed over all environments. The number of bolls/hectare was the only yield component that showed definitive differences for stability between population types with the heterozygous populations having significantly higher stability than the homozygous populations. The superior stability of the heterozygous populations was attributed to an increased lint production in the lower yielding environments stemming from an increased number of bolls/hectare.

The purpose of the research was to gain further understanding of the responses of plants to stress in acid environments and the role of mycorrhizal fungi in stress tolerance. Acidic soils of the southeastern coastal plain typically have low fertility, but weeds remain problematic even in soybean and peanut fields where no fertilizer nitrogen is applied. Field experiments using ¹⁵N natural abundance examined whether nitrogen might be transferred between the N₂-fixing crop species and neighboring weeds. A five year field study demonstrated that substantial nitrogen transfer did occur. Because nitrogen transfer was largely dependent on the presence of arbuscular mycorrhizae, the results strongly suggested that N moved from plant to plant through mycorrhizal hyphae that connected plant root systems. Another characteristic of acid soils is the presence of high levels of aluminum. When soil pH is below 5.0, the presence of Al⁺³ in soil solution can cause aluminum toxicity to occur in many plants. Aluminum toxicity inhibits root growth and predisposes crop plants to drought and nutrient deficiencies. Loblolly pine was determined to be extremely tolerant to aluminum. In a series of experiments, we examined the ability of loblolly pine to exclude aluminum from root tip meristems which are known to be the main sites of aluminum toxicity. The primary and secondary roots of pine exhibited high degrees of Al tolerance. Tolerance was associated with Al exclusion from the root tips and, of the Al accumulating in the root, exclusion from the root meristem. Ectomycorrhizal colonization was found to contribute to aluminum tolerance, evidently by providing an extra barrier to Al entry into the root. Additional experiments examined Al relations of arbuscular mycorrhizal fungi. The root systems of more than 80% of all plant species are colonized by arbuscular mycorrhizae. In the presence of aluminum, colonization by arbuscular mycorrhizal fungal species was inhibited. The results suggested alterations in root function caused lower colonization, as mycorrhizal infection potential appeared unaffected. Aluminum also inhibited fungal spore germination, but only at very high Al levels. Much of the function of the mycorrhizal fungi was unaffected even as aluminum accumulated in fungal structures.

The bioprocessing of tobacco (Nicotiana tabacum L.) is attracting attention as a means for the production of transgenic proteins, as well as native proteins and secondary metabolites, and is doing so at a time when recent reductions in demand for the conventionally produced tobacco have created a need for alternative crops. Of the potentially valuable natural products produced by tobacco, many are sequestered on the leaf surface as cuticular wax or trichome exudate. Because of the potential high value of many of these leaf surface components as pharmaceuticals, fragrance elements, and pesticides, as well as the known ease of their removal from the leaf via a simple solvent wash, procedures have been developed to extract these compounds in a manner that could be readily integrated into a tobacco bioprocessing operation. To this end, well-established methods for the analytical extraction of the leaf surface chemistry have been modified to improve their suitability for such integration. A battery of solvents including methanol, ethanol, isopropanol, n-propanol, and acetone were selected for their theorized ability to extract the components of interest as well as the improvements in washed tissue condition and process cost and safety they would likely confer relative to methylene chloride, the analytical solvent of choice. The amounts of some major components (cis-abienol, á- & â-cembratrienediols, docosanol, and sucrose esters) extracted from tissue by sequential, timed washes with each solvent were quantified by gas chromatography and compared to amounts extracted by methylene chloride. Most of the solvents tested extracted high levels of the components analyzed with varying degrees of efficiency. Methanol, ethanol, and n-propanol were found to quickly extract levels of components generally comparable to those extracted by methylene chloride. Regardless of any other considerations, methylene chloride and similar solvents are generally not suitable for extraction of components from the surface of the tobacco leaf prior to extraction of proteins because they cause denaturation and precipitation of proteins subsequently extracted from tissue, as well as tissue wilting due to disruption of cellular membranes. All the solvents studied were found to have less deleterious effects on the integrity of the leaf than methylene chloride. In addition, solvent effects on the extraction and crystallization of ribulose bisphosphate carboxylase/oxygenase (Rubisco; ?Fraction 1 protein?) from washed tissue were studied. Fortuitously, methanol and ethanol washed tissue were found to result in higher levels of Rubisco crystallization than unwashed tissue. All other solvents, however, including n-propanol, were found to reduce crystallizable Rubisco. Notably, crystallizable Rubisco levels in all cases were increased relative to the amounts of other soluble (?Fraction 2?) proteins. To generally demonstrate the effect of surface washing on transgenic proteins produced in tobacco, tissue from plants modified to produce bovine stomach lysozyme (BSL) were washed with methylene chloride and the alternative solvents. Of the solvents tested, only methanol and isopropanol resulted in significant decreases in lysozyme activity in a total protein extract of washed tissue. Moreover, immunoblotting of extracts from washed tissue indicated that the observed activity reductions may have been caused by inactivation of the enzyme, rather than decrease in extractability. On the bases of surface extraction efficiency, Rubisco crystallization, and lysozyme activity, ethanol would appear to be the best choice for leaf surface extraction in an integrated bioprocess. However, solvent selection would also be dependent upon the relative importance of the products affected; if Rubisco is of much lower value than the other products, then it may be that n-propanol would be a better choice. Furthermore, it is readily apparent that solvent wash effects would need to be studied on a case by case basis for any other native or engineered proteins of interest.

In all agricultural systems using few or no herbicides, cultural and mechanical techniques are crucial to economic viability. Further advances in non-chemical weed control will require in-depth knowledge of weed biology. The objective of new strategies likely will be to alter competitive interactions between crop and weed species. An important part of weed biology, and often the basis for competitiveness, is species response to the environment. Environmental conditions differ greatly in different parts of the world. In agricultural systems, the low nitrogen status of some soils requires most crops to be amended. Weed populations can experience a wide range of fertilization regimes from year to year due to different crop rotations. Fertilization patterns in a rotation can lead to differential growth and reproductive performance of weeds and competitiveness with crops. In research described in the first two chapters, we attempt to determine the extent that lowering parental nutrition would impact weed growth, reproduction, and offspring vigor. The purpose was to begin evaluating benefits of controlling nitrogen fertility in agronomic systems on low fertility soils. Another environmental factor that could influence weed competitiveness is temperature. The significance of temperature responses will become increasingly important in the future. Global climate change models are predicting increases in the earth?s average surface temperature between 1.4- 5.8 ºC during the 21st century. As temperatures increase it is conceivable that warmer temperatures will alter competitive balance between crop and weed species, intensifying weed pressures. The expression of temperature response also will be a function of other interacting factors such as the ability to acquire water and nutrients. Resource acquisition is controlled to a large extent by root growth and interactions with soil microorganisms. The third chapter of this thesis is focused on temperature responses of selected plant species from an agroecological system. The purpose was to begin assessment of possible climate change effects on weed competitiveness. All of our experiments made extensive use of the weed Senna obtusifolia due to its importance in crops in the Southeastern U.S.

Greenhouse dose-response experiments were conducted on a biotype of johnsongrass from Washington County, Mississippi to determine the level of purported resistance to the aryloxyphenoxy propionate herbicide fluazifop-P and cyclohexanedione herbicides clethodim and sethoxydim. Both seedling and rhizome plants were evaluated. Resistant/susceptible ratios (R/S) were 11.0, 5.7, and 5.5 for clethodim, fluazifop-P, and sethoxydim, respectively, for seedling plants. R/S ratios were 15.6, 22.7, and 22.3 for clethodim, fluazifop-P, and sethoxydim, respectively, for rhizome plants. There was no difference between the resistant and susceptible biotypes in the absorption, translocation, or metabolism of 14C-clethodim in the resistant and susceptible biotypes. Specific activity of acetyl Co-A carboxylase (ACCase) from the susceptible and resistant johnsongrass biotypes (means of 0.221 and 0.223 nmol/mg protein/min, respectively) were similar. ACCase from the susceptible biotype was sensitive to clethodim, with an I50 value of 0.29 mM clethodim. ACCase from the resistant biotype was less sensitive, with an I50 value of 1.32 mM clethodim. The resultant R/S ratio for clethodim was 4.5. These results indicate that resistance to clethodim in this johnsongrass biotype resulted from an altered ACCase enzyme. The relative competitiveness and non-competitive productivity of R and S johnsongrass were assessed in greenhouse and growth chamber experiments. When grown in noncompetitive conditions in growth chamber experiments, photosynthetic rate, net assimilation rate, leaf number, leaf area, specific leaf area, leaf dry biomass, and shoot dry biomass were similar for R and S biotypes 21, 27, and 35 days after planting. The biotypes were similar in terms of plant height and leaf number. Relative crowding coefficients for above ground dry biomass similar, and a combined t-test indicated that the resistant and susceptible biotype did not differ for above ground dry biomass (tlof=0.54, 1.3; P=0.38, 0.23, respectively). There does not appear to be a fitness penalty associated with the resistance. A seedling bioassay was developed for the determination of resistance to clethodim and fluazifop-P in johnsongrass. The assay was based on differences in the coleoptile length of R and S seedlings exposed to clethodim and fluazifop-P in petri dishes for 5 d. A bioassay concentration of 0.09 mg/L clethodim and 0.18 mg/L fluazifiop-P where chosen as discriminant based on rate responses of each biotype to increasing herbicide dose. At 5 DAT, the R:S ratio for clethodim was 18.7, and the R:S ratio for fluazifop was 35.4. A study was conducted to determine the nuclear state and develop a suitable medium and culture method for in vitro germination of johnsongrass pollen. Johnsongrass pollen was trinucleate, and in vitro tests for pollen viability using Alexander?s stain and a fluorochromatic reaction method indicated johnsongrass pollen was viable (92.6-98.4%). A factorial treatment of four concentrations of sucrose, two concentrations of boric acid, and two concentrations of calcium nitrate was used to determine the optimum pollen germination media. The factorial study was conducted using three different cultural methods: suspension culture, agar culture, and cellophane membrane culture. Germination was highest in a suspension culture with media containing 0.3 M sucrose, 2.43 mM boric acid, and 3 mM calcium nitrate. In a second study, pollen germination using the above media was 78.9% when harvested from flowers just before anthesis. Three studies were conducted to develop pollen tests for the screening of ACCase target-site resistance in a biotype of johnsongrass using the developed germination media. Pollen from the susceptible biotype of johnsongrass was strongly inhibited by increasing concentrations of clethodim, with a GR50 of 25.8 (standard error of ±0.6) mM and GR50 of 16.4 (standard error of ±1.7) mM clethodim by visual assessment and spectrophotometric assessment, respectively. Minimum R/S values were >3.9 by visual assessment and >6.1 by spectrophotometric assessment. Both assessment methods differentiated the susceptible and resistant biotypes, and ACCase target-site resistance is expressed in johnsongrass pollen.

Antioxidants in the leaf apoplast have the potential to detoxify ozone (O3) and active oxygen species (AOS), and thus may play a role in preventing plant injury. To investigate this possibility, two experiments were conducted to compare antioxidant metabolism in the leaf extracellular spaces of O3-tolerant (Essex) and O3-sensitive (Forrest) cultivars of soybean [Glycine max (L.) Merr.]. In the first experiment, a canopy profile consisting of the 2nd, 4th and 6th main stem trifoliates was sampled from plants grown for four weeks in a greenhouse supplied with charcoal-filtered air (CF) to assess genetic and leaf age effects under low stress conditions. In general, apoplastic ascorbate (AA) levels were low (< 30 nmol AA g-1 FW) in both cultivars. Apoplastic dehydroascorbate (DHA) levels were higher (> 50 nmol DHA g-1 FW) relative to AA, particularly in younger leaves (250-450 nmol DHA g-1 FW), resulting in a low ascorbate redox state. Ascorbate oxidase (AO) and ascorbate peroxidase (APX) activity were found in leaf apoplast samples, suggesting that the enzymes played a role in maintaining ascorbate primarily in the oxidized state. In the second experiment, the second main stem trifoliates of four week-old plants were compared in the two genotypes following exposure to CF or elevated O3 conditions. Following a six-day treatment period with 77 ppb O3 for 7 h d-1, foliar injury was greater in O3-treated Forrest than in Essex, and total leaf guaiacol peroxidase activity was correlated with the greater O3 sensitivity of Forrest. Under both CF and elevated O3 conditions, there was a significantly higher apoplastic total antioxidant capacity in Essex than in Forrest correlating with greater O3 tolerance. Although reduced AA concentration was greater in apoplasts sample from O3-treated Essex than Forrest, the difference (6.5 nmol g-1 FW) was unlikely to play a major role in the differential O3 sensitivity of these two cultivars. Apoplast AA was generally less than 30% of the total antioxidant capacity found in apoplast samples, suggesting that other antioxidants might be involved in the detoxification of O3 in the extracellular space in these soybean cultivars.

Cotton (Gossypium hirsutum L.) is a perennial plant managed as an annual crop to optimize yield and fiber quality while managing inputs to maximize profit. Transgenic improvements have simplified cotton production. Resistance to the non-selective herbicide glyphosate and in-plant production of Bacillus thuringiensis (Bt) endotoxin are two such advances. Cultural practices including use of mepiquat chloride (MC), and optimizing planting date contribute to crop uniformity and decrease risk involved with environmental stresses. Comparisons between conventional and transgenic weed and insect management systems, optimal and late planting dates, overhead sprinkle irrigation and drip irrigation, and use of MC were evaluated. Optimal-planted cotton had better yield than cotton planted late. Mepiquat chloride did not always provide an advantage, however never caused an undesirable response. At times, cotton plants treated with MC showed improved micronaire, compensation for boll loss, and earlier maturity. Broadcast glyphosate at the eight-leaf stage reduced yield of optimal-planted cotton in 1 of 3 years and 2 of 3 years in late-planted cotton. Glyphosate contact after the four-leaf stage in 2 of 3 years shifted the majority of bolls above node 10. Lint yield results were variable between overhead sprinkle and drip irrigation systems. Mepiquat chloride did not affect yield in irrigated cotton, however did control plant height, and improve fruit retention and cotton maturity. Non-labeled glyphosate applications reduced maturity in each irrigation system in 1 of 3 years. Cotton injury caused by conventional herbicides resulted in yield loss and poor returns compared to glyphosate systems. Early-season weed competition from low input herbicide programs caused cotton biomass reduction. High costs of conventional herbicide programs offset the available profit margin compared to glyphosate systems when yields were similar. Glyphosate systems provided excellent control of all weed species, while conventional herbicides gave acceptable control in most cases. Glyphosate resistant cotton cultivars with had better yield and returned more profit than the same cultivars treated with conventional herbicides.