Comparison of WeatherMax^®and Engame^TMFormulations of Glyphosate on Cotyledon Surface Structure, Chlorophyll A Fluorescence and Shikimate Levels in Isogenic Cotton Cultivars Differing in Roundup Resistance

The effects of Roundup WeatherMax® and EngameTM formulations of glyphosate were investigated on the cotyledons of glyphosate resistant (GR) and glyphosate sensitive (GS) isogenic cotton cultivars. EngameTM is a mixture of glyphosate and 1-aminomethanamide dihydrogen tetraoxosulfate (AMADS). Fully expanded cotton cotyledons treated with EngameTM or AMADS developed surface lesions within 2 hours after treatment whereas surfactant-treated control or WeatherMax®-treated tissues did not develop lesions. The EngameTM and AMADS damage appeared as depressions which were confirmed by scanning electron microscopy. Light micrographs of cross sections through the depressions revealed collapsed and compressed epidermal and mesophyll cells with congealed cytoplasmic contents in the palisade and spongy mesophyll cells. Changes to photosynthetic electron transport were evident at 4 hours after treatment (HAT) in all treatments as revealed by chlorophyll A fluorescence. In GR cotton, the fluorescence perturbations decreased with time such that at 72 HAT EngameTM-treated cotyledons could not be distinguished from the surfactant-or WeatherMax®-treated plants. The GS cotton continued to show progressive decreases in the fluorescence parameters Fv/Fm and performance index (PI) to 72 HAT. Shikimate levels increased following glyphosate treatment in glyphosate sensitive cotton and EngameTM caused a two-to three-fold greater increase in shikimate compared to WeatherMax®. These results indicate that the EngameTM-based glyphosate formulation involved structural tissue damage which likely increased glyphosate uptake and subsequently increased inhibition of photosynthesis and the shikimate pathway.

Morphological Characterization of <i>Amaranthus palmeri</i>x <i>A. spinosus</i>Hybrids

The growth of clones of seven Amaranthus palmeri x A. spinosus hybrids was compared to type specimens of A. palmeri and A. spinosus. The hybrids came from the field where they were originally discovered and clones of the type specimens and hybrids were established under greenhouse conditions and used to compare growth rates. A. palmeri had the highest growth rate and A. spinosus the lowest growth rate based on height, node counts, and dry weight accumulation. A. palmeri also had the greatest number of days to flowering and A. spinosus the fewest. Hybrids had intermediary growth rates and days to flowering, but differed from each other with regard to sex identity. The hybrids were either dioecious like A. palmeri or, if monoecious, had patterns unlike A. spinosus. Spine length and texture also varied in hybrids and some were without spines. Hybrid 16Ci was short compared to all others and had succulent leaves and stems, which easily separated from the plant body. These hybridizations resulted in morphologically distinct types with acquisition of physical traits intermediate to the type specimens which may drive evolution of these species.

Influence of Water Quality, Formulation, Adjuvant, Rainfastness, and Nozzle Type on Efficacy of Fomesafen on Palmer Amaranth (<i>Amaranthus palmeri</i>) Control

Protoporphyrinogen oxidase (PPO) inhibitors are one of the few remaining postemergence herbicide options for controlling Palmer amaranth in soybean growing areas of Mississippi, USA. Most Palmer amaranth populations in Mississippi are resistant to both glyphosate and acetolactate synthase inhibitors. Resistance to PPO inhibiting herbicides in Palmer amaranth has very recently been reported in Arkansas, Tennessee, and isolated pockets of Mississippi. A significant proportion of reports of PPO inhibitor failures in Mississippi are not considered to be resistance-related at this time. Therefore, the objective of this research was to evaluate factors affecting the efficacy of fomesafen on Palmer amaranth including: quality of spray carrier (water), formulations, adjuvant, rainfastness, and nozzle type. All water samples and formulation combinations provided >95% control of Palmer amaranth 3 WAT. Some combinations of water samples and formulations did not result in complete control of the treated plants, with one or two individuals surviving 3 WAT. Formulation 1 provided 99% control compared to 95% from formulation 2. Irrespective of combinations of herbicide, adjuvant and height, control of Palmer amaranth was ≥91%. Formulation 1 provided 94% control compared to 88% from formulation 2. The adjuvant x height interaction was significant, owing to a 10% reduction in control of larger plants (86%) compared to smaller plants (96%) in presence of COC. COC provided better control (93%) than NIS (88%). Simulated rainfall applied ≥60 min after herbicide application did not adversely affect efficacy on Palmer amaranth when formulation 1 was applied in combination with NIS, with control ranging from 94% to 100%. Formulation 1 with COC provided ≥93% control at all rainfall application times, except 30 min after herbicide treatment, which resulted in 79% control. Formulation 2 provided better control with COC (79% to 100%) than NIS (71% to 90%), in general, across the rainfall treatments applied at various times following herbicide application. All nozzle and weed height combinations resulted in 89% or better control of Palmer amaranth. In summary, water quality, formulation, adjuvant, rainfastness, or nozzle type did not affect the activity of fomesafen under optimal application conditions in the greenhouse.

Glyphosate Resistance in Giant Ragweed (<i>Ambrosia trifida</i>L.) from Mississippi Is Partly Due to Reduced Translocation

A giant ragweed population from a glyphosate-resistant (GR) soybean field in Mississippi, USA was suspected to be resistant to glyphosate. Greenhouse and laboratory studies were conducted to confirm and quantify the magnitude of glyphosate resistance in a resistant biotype selected from this population and to elucidate possible physiological and molecular mechanisms of glyphosate resistance. Glyphosate dose response studies indicated that ED50 (effective dose required to reduce plant growth by 50%) values for glyphosate-resistant (GR-MS) and glyphosate-susceptible (GS-MS) biotypes, based on percent injury, were 0.52 and 0.34 kg ae/ha glyphosate, respectively, indicating a 1.5-fold level of resistance in GR-MS. The absorption pattern of 14C-glyphosate in the two giant ragweed biotypes was similar throughout the measured time course of 168 h after treatment (HAT). The amount of 14C-glyphosate that translocated out of treated leaves of the GR-MS and GS-MS plants was similar up to 24 HAT. However, the GS-MS biotype translocated more (71% and 76% of absorbed at 48 and 96 HAT, respectively) 14C-glyphosate than the GR-MS biotype (44% and 66% of absorbed at 48 and 96 HAT, respectively) out of the treated leaf. No target site mutation was identified at the Pro106 location of the EPSPS gene of the GR-MS biotype. The mechanism of resistance to glyphosate in giant ragweed from Mississippi, at least, is due to reduced glyphosate translocation.

Cottonseed Protein, Oil, and Mineral Nutrition in Near-Isogenic <i>Gossypium hirsutum</i>Cotton Lines Expressing Leaf Color Phenotypes under Field Conditions

Information about the effects of phenotype traits on cottonseed protein, oil, and nutrients is scarce. The objective of this research was to investigate the effects of leaf color trait on seed nutrition in near-isogenic Gossypium hirsutum cotton expressing green (G) and yellow (Y) leaf color phenotypes. Our hypothesis was that leaf color can influence the accumulation of nutrients in seeds. Sets of isogenic lines were: DES 119 (G) and DES 119 (Y);DP 5690 (G) and DP 5690 (Y);MD 51ne (G) and MD 51ne (Y);SG 747 (G) and SG 747 (Y). Each NIL set is 98.44 % identical. Parent line SA 30 (P) was used as the control. The experiment was repeated for two years (2014 and 2015). The results showed that, in 2014, seed oil in DES 119 (G) and SG 747 (G) were significantly higher than their equivalent yellow lines. Green lines showed higher content of phosphorus compared with yellow lines. Higher levels of Cu, Fe, Mn, Ni, and Zn were recorded in DES 119 (G) and MD 51ne (G). In 2015, seed protein, oil, C, N, P, B, Cu, and Fe were higher in green lines than in yellow lines. There was a significant correlation between protein and nutrients, and between oil and nutrients in 2015, but not in 2014 as the temperature was warmer in 2015 than in 2014. This research demonstrated that leaf color can alter seed composition and mineral nutrition under certain environmental growing conditions such as temperature.

Variation in 5-Enolpyruvylshikimate-3-Phosphate Synthase (EPSPS) Coding Sequences and Glyphosate Response among <i>Cyperus rotundus</i>L. Populations

The gene sequence encoding 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), the enzymatic target site of the herbicide glyphosate, was determined for several purple nutsedge (Cyperus rotundus L.) accessions from geographically distant locations and these were aligned to generate a consensus sequence. The EPSPS sequences each had single nucleotide polymorphisms (SNPs) only a few of which were predicted to cause an amino acid change in the EPSP synthase. None had the proline to serine substitution or other substitutions responsible for glyphosate resistance reported in other species. A dendrogram generated from the cluster analysis of the EPSPS gene sequences indicated similarities between accessions from Tanzania, Indonesia, California-2, Greece, Brazil, Argentina and Iran much like cluster analysis previously reported based on RAPD scores and morphological traits possibly indicating a common genetic background or origin. Considering the differences in EPSPS sequences, the response of these purple nutsedge accessions to 0.84 kg·ae·ha-1 of glyphosate was assessed to determine whether differential tolerance was present. At 7 days after the first application control ranged from 9% for the accession from Greece to 73% for the accession from Tanzania. Control of these accessions increased to 45% and 93% respectively by 14 days after the second application. The I50’s for glyphosate inhibition of growth for four accessions from geographically distant countries (Mississippi, Brazil, Indonesia and Tanzania) were 0.21, 0.10, 0.25 and 0.06 kg·ha-1, respectively, which represented a 4-fold difference. The difference in sensitivity to glyphosate may be a result of a non-target site mechanism such as differences in sequestration, translocation or cuticle thickness rather than alterations in EPSPS.

Target Site-Based Resistance to ALS Inhibitors, Glyphosate, and PPO Inhibitors in an <i>Amaranthus palmeri</i>Accession from Mississippi

Extensive acceptance of glyphosate-resistant (GR) row crops coupled with the simultaneous increase in glyphosate usage has sped the evolution of glyphosate resistance in economically important weeds. GR Amaranthus palmeri populations are widespread across the state with some exhibiting multiple resistance to acetolactate synthase (ALS) inhibiting herbicides such as pyrithiobac. A GR and ALS inhibitor-resistant accession was also resistant to the protoporphyrinogen oxidase (PPO) inhibiting herbicide fomesafen. The PPO inhibitor resistance profile and multiple herbicide resistance mechanisms in this accession were investigated. In addition to fomesafen, resistance to postemergence applications of acifluorfen, lactofen, carfentrazone, and sulfentrazone was confirmed. There was no resistance to preemergence application of fomesafen, flumioxazin, or oxyfluorfen. Molecular analysis of the ALS gene indicated the presence of point mutations leading to single nucleotide substitutions at codons 197, 377, 574, and 653, resulting in proline-to-serine, arginine-to-glutamine, tryptophan-to-leucine, and serine-to-asparagine replacements, respectively. The resistant accession contained up to 87-fold more copies of the EPSPS gene compared to a susceptible accession. A mutation leading to a deletion of glycine at codon 210 (ΔG210) of PPO2 gene was also detected. These results indicate that the mechanism of resistance in the Palmer amaranth accession is target-site based, i.e., altered target site for ALS and PPO inhibitor resistance and gene amplification for glyphosate resistance.

Soybean Seed Nutrition as Affected by Cotton, Wheat, and Fallow Rotation

Limited information is available on the effects of crop rotation on seed nutrition. Therefore, the objective of the current research was to determine whether crop rotations are beneficial to soybean seed nutrition for the first two complete rotation cycles in an experiment conducted from 2007 through 2012. The first complete rotation cycle (experiment one) was conducted in 2009, then repeated in 2010, and the second complete rotation cycle (experiment two) was conducted in 2011, and then repeated in 2012. The rotation sequences were: wheat-late cotton-fallow-soybean (WCFS), fallow-cotton-wheat-soybean (FCWS), and fallow-cotton-fallow-soybean (FCFS). The results showed that WCFS and FCFS resulted in higher seed oil, palmitic and stearic acids, glucose, sucrose, fructose, Fe, P, and B. No consistent effects on seed protein, oleic acid, linoleic acid, linolenic acid, raffinose, stachyose, and Mn contents were observed. These changes were accompanied by higher P, K, B, Fe in soil and N, K, and B in leaves, indicating that soil and leaf nutrients may result in continuous supply and mobility of nutrients from leaves to seed during seed fill. Our research demonstrated that crop rotation management can result in seed nutrient changes, affecting seed quality.

Genetic Diversity among Geographically Separated <i>Cyperus rotundus</i>Accessions Based on RAPD Markers and Morphological Characteristics

The diversity of globally distributed populations of purple nutsedge was assessed using molecular marker data and morphological traits. Cluster analysis of binary random amplified polymorphic DNA (RAPD) data and morphological traits indicated that the global population of purple nutsedge consisted of two clades. Cluster analysis of the RAPD data supported separation of the purple nutsedge accessions evaluated into two distinct clades of 11 and 33 accessions. Except for accessions identified as California* and Arizona, all USA accessions were clustered with accessions from Taiwan, Western Samoa, New Zealand, Malaysia, Japan, El Salvador, Columbia, Australia, Thailand and West Indies. A second cluster included accessions from Sudan, Greece, Iran, California*, Arizona, Brazil, Argentina, Mauritius, Philippines, Indonesia and Tanzania. The accessions from Sudan, Greece, Iran, Mauritius, and Tanzania were distributed along a similar longitudinal axis. Cluster analysis based on morphological traits though not identical to that based on RAPD data also supported separation into two clades and perhaps a third. The lack of genetic diversity among accessions supported the hypothesis that spread and propagation into new environments were largely by tubers which preserved genetic identity. The lack of diversity particularly among New World and USA accessions may also reflect a relatively recent introduction of the species into the Americas and a low level of outcrossing.

Corn Yield Response to Reduced Water Use at Different Growth Stages

To develop an efficient water use strategy for crop irrigation, we need to know how much water can be reduced without decreasing yield. A study was designed to determine corn growth stages at which water could be reduced without affecting grain yield, and at what soil moisture level water deficit stress begins in the plants in a silt loam soil. An experiment was conducted in a randomized complete block with a 3 × 4 factorial design in four replications, where treatments consisted of three soil moisture levels [100%, 75%, and 50% of field capacity (FC) of a silt loam soil by weight] and four growth stages [fourteen leaf stage (V14), silking (R1), milk (R3), and dent (R5) stages] in a greenhouse. Growth stages at the reproductive and grain fill stages of corn were selected because this study was intended for the Mississippi Delta, where there is frequent drought during these growth stages making irrigation necessary for corn production, whereas there is usually adequate rainfall during the vegetative growth stages. Results from this study showed that reducing soil moisture from 100% FC (fully irrigated) to 75% FC of a silt loam soil starting at the R1 growth stage in corn did not reduce yield significantly compared to yield from the 100% FC, while saving a significant amount of water. Physiological investigations at the three soil moisture treatments showed that a mild moisture deficit stress might have started at the 75% FC treatment. With further investigation, if savings in water at 75% FC result in a significant reduction in energy cost, it may be profitable to reduce soil moisture to 75% FC in a silt loam soil.

Early Growth and Development of Horseweed (<i>Conyza canadensis</i>(L.) Cronq.)

Horseweed (Conyza canadensis (L.) Cronq.) produces thousands of small elongated seeds which are botanically defined as achenes;yet, relative to the quantity of achenes produced, few seedlings survive to produce mature plants. The developmental progression from achene to 4 mm seedlings was documented, and seedling response to moisture deprivation was described. Radical protrusion through the pericarp occurred between 18 and 30 hours after onset of imbibition in water or when germinated on soil at or greater than field capacity. A ring of root hair initials formed immediately after radical emergence at the interface of what was to become the separation between the root and hypocotyl. By 48 hours post imbibition, radicals differentiated into a distinct root with root cap and a hypocotyl, and root hairs elongated. By 72 hours post imbibition, seedlings had emerged from the pericarp, and had: expanded photosynthetic cotyledons, a clearly defined hypocotyl, a ring of elongated root hairs exceeding 1 mm in length, and a root equal or longer than the hypocotyl. The epicotyl had not yet emerged, and the total seedling length was approximately 3 to 4 mm. Germination was delayed on soil at or below field capacity. More than 95% of two- and four-day-old seedlings that had been desiccated for more than 24 hours died after being rehydrated.