In this review, we examine current techniques and recent advances directed toward understanding cellular mechanisms involved in controlling dormancy in vegetative propagules. Vegetative propagules (including stems, rhizomes, tubers, bulbs, stolons, creeping roots, etc.) contain axillary and adventitious buds capable of producing new stems/branches under permissive environments. Axillary and adventitious buds are distinct in that axillary buds are formed in the axil of leaves and are responsible for production of lateral shoots (branches). Adventitious buds refer to buds that arise on the plant at places (stems, roots, or leaves) other than leaf axils. Both axillary and adventitious buds generally undergo periods of dormancy. Dormancy has been described as a temporary suspension of visible growth of any plant structure containing a meristem (Lang et al. 1987). Dormancy can be subdivided into three categories: (1) ecodormancy-arrest is under the control of external environmental factors; (2) paradormancy-arrest is under the control of external physiological factors within the plant; and (3) endodormancy-arrest is under the control of internal physiological factors. One common feature in all of these processes is prevention of growth under conditions where growth should otherwise continue. There is growing evidence that lack of growth is due to blockage of cell division resulting from interactions between the signaling pathways controlling dormancy and those controlling the cell cycle.

Nomenclature: Abscisic acid, ABA; CDK-activating kinase, CAK; cyclin-dependent protein kinase, CDK; extracellular-signal-regulated kinase, ERK; gibberellic acid, GA; growth factor receptor, GFR; mitogen-activated protein kinase, MAPK; underground adventitious buds, UABs; nuclear export signal, NES; nuclear localization signal, NLS; retinoblastoma, RB; virus-induced gene silencing, VIGS.

As a first step toward developing a genomics-based research program to study growth and development of underground adventitious shoot buds of leafy spurge, we initiated a leafy spurge expressed sequence tag (EST) database. From the approximately 2,000 clones randomly isolated from a cDNA library made from a population containing growth-induced underground adventitious shoot buds, we have obtained ESTs for 1,105 cDNAs. Approximately 29% of the leafy spurge EST database consists of expressed genes of unknown identity (hypothetical proteins), and 10% represents ribosomal proteins. The remaining 60% of the database is composed of expressed genes that show BLASTX sequence identity scores of ≥ 80 with known GenBank accessions. Clones showing sequence identity to a Histone H3, a gibberellic acid-responsive gene, Tubulin, and a light-harvesting chlorophyll a/b-binding protein were shown to be differentially expressed in underground adventitious shoot buds of leafy spurge after breaking of dormancy. RNA encoding a putative cyclin-dependent protein kinase (CDK)-activating kinase, a gene associated with cell division, and Scarecrow-like 7, a gene involved in GA signaling, were present at similar levels in dormant and growth-induced underground adventitious shoot buds. These data show how even a small EST database can be used to develop a genomics-based research program that will help us identify genes responsive to or involved in the mechanisms controlling underground adventitious shoot bud growth and development.

Nomenclature: Amp, ampicillin; CAK, CDK-activating protein kinase; CDK, cyclin-dependent protein kinase; EST, expressed sequence tag; LB, Luria–Bertani broth; Lhcb, light-harvesting chlorophyll a/b-binding protein; NCBI, National Center for Biotechnology Information; SSC, sodium chloride sodium citrate solution.

Several inbred lines of acetolactate synthase (ALS)-inhibiting herbicide-resistant (ALS-R) Palmer amaranth and ALS-susceptible (ALS-S) common waterhemp were developed in the greenhouse. Interspecific hybrids were obtained by allowing several ALS-S common waterhemp females to be pollinated only by ALS-R Palmer amaranth in a growth chamber. Putative hybrid progeny were treated with an ALS-inhibiting herbicide, and the hybrid nature verified using a polymorphism found in the parental ALS gene. Polymerase chain reaction (PCR) was used to amplify a region of the ALS gene in both parental plants and putative hybrids. Restriction enzyme digestion of the ALS-R Palmer amaranth PCR fragment resulted in two smaller fragments, whereas the PCR fragment in the ALS-S common waterhemp was not cut. Restriction digestion of the putative hybrid PCR fragment showed a combination of ALS-R Palmer amaranth double fragments and an ALS-S common waterhemp single fragment. Approximately 4 million flowers were present on 11 common waterhemp females and produced about 44,000 seeds that appeared viable. From the approximately 3,500 putative hybrid seedlings that were screened, 35 were confirmed as hybrids using herbicide resistance as a phenotypic and molecular marker. The data collected here verify that interspecific hybridization does occur between these two species, and even at a low rate, it could contribute to the rapid spread of ALS resistance in these species.

Nomenclature: Imazethapyr; common waterhemp, Amaranthus rudis Sauer AMATA; Palmer amaranth, Amaranthus palmeri S. Wats AMAPA.

Wheat cultivars resistant to imazamox will facilitate selective chemical control of many winter annual grass weeds, including jointed goatgrass, downy brome, and feral rye. These three weed species respond differently to imazamox postemergence treatments with feral rye, demonstrating more tolerance than jointed goatgrass or downy brome; therefore, growth chamber studies were conducted to evaluate imazamox absorption in all three weed species and translocation and metabolism in jointed goatgrass and feral rye. Adding nonionic surfactant (NIS) or methylated seed oil increased absorption in jointed goatgrass and feral rye but not in downy brome, compared to imazamox applied alone. Imazamox applied with NIS and urea ammonium nitrate resulted in the highest absorption in each species: 97, 91, and 92% of applied ¹⁴C for jointed goatgrass, downy brome, and feral rye, respectively, 48 h after treatment (HAT). Imazamox translocation from the treated leaf was similar for jointed goatgrass and feral rye across seven harvest intervals between 0 and 96 HAT. Shoot tissues of jointed goatgrass and feral rye accumulated 17 and 14% of applied ¹⁴C, respectively, by 96 HAT. Differential translocation of imazamox into root tissue was observed within 12 HAT; by 96 HAT, 20% of applied ¹⁴C translocated to jointed goatgrass roots compared to 27% for feral rye. Imazamox was readily metabolized in both weed species. At 96 HAT, 73 and 98% of the applied ¹⁴C was metabolized in the treated leaves of jointed goatgrass and feral rye, respectively. Metabolism was consistently higher in feral rye than in jointed goatgrass in all plant parts 96 HAT. On a whole-plant basis, metabolism was 25% greater in feral rye than in jointed goatgrass. The differential response of jointed goatgrass and feral rye to foliar applications of imazamox appears to be related to differences in translocation and metabolism but not in absorption.

Nomenclature: Imazamox; downy brome, Bromus tectorum L. BROTE; feral rye, Secale cereale L. SECCE; jointed goatgrass, Aegilops cylindrica L. AEGCY; wheat, Triticum aestivum L.

Disulfoton and phorate applied in the seed furrow greatly reduce clomazone phytotoxicity to cotton in the field, whereas aldicarb does not. An experiment was conducted to determine the effect of aldicarb, disulfoton, and phorate on ¹⁴C-clomazone absorption, translocation, and metabolism by cotton grown in a sandy loam soil. Clomazone at 0.87 μg g⁻¹ of soil alone or in combination with aldicarb at 0.6 μg g⁻¹ of soil reduced cotton root and shoot growth 26 to 33%. Root and shoot growth were not reduced by clomazone plus disulfoton or phorate at 0.6 μg g⁻¹ of soil. Protection of cotton against injury by clomazone was not explained by reduced absorption or translocation of clomazone or a metabolite to the shoot. Clomazone metabolism was reduced by disulfoton and phorate, thus indicating a clomazone metabolite may be more toxic to cotton.

Nomenclature: Aldicarb; clomazone; disulfoton; phorate; cotton, Gossypium hirsutum L. ‘McNair 235’.

The partitioning coefficient is defined as the proportion of new dry matter partitioned among different plant parts. Partitioning coefficients can be used to model plant dry matter accumulation. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of density and relative time of emergence of redroot pigweed on dry matter partitioning to stem, leaves, and reproductive parts throughout the season. Redroot pigweed was grown with sorghum and in monoculture at densities of 2, 4, and 12 plants m⁻¹ of row each year at each location. Dry matter partitioning during vegetative growth was not influenced by plant density. However, partition coefficients during the reproductive growth stage changed as a linear function of the time of pigweed emergence relative to the sorghum leaf stage. The later the emergence time relative to sorghum leaf stage, the higher the partitioning coefficient values for leaf (PC_leaf) and stem (PC_stem) and the lower the partitioning coefficient values for reproductive parts (PC_rp). The observed differences in partitioning coefficients due to relative emergence time are valuable information to those interested in simulating growth of competing plant species, especially with reference to their seed production.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) ECHCG; common lambsquarters, Chenopodium album L. CHEAL; redroot pigweed, Amaranthus retroflexus L. AMARE; sorghum, Sorghum bicolor (L.) Moench.

Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO₂]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO₂ reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO₂. In monoculture, elevated CO₂ significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO₂ did not adequately predict the CO₂ response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cocklebur and decreased significantly for sorghum in competitive mixtures at elevated CO₂. Data from this study indicate that vegetative growth, competition, and potential yield of economically important C₄ crops could be reduced by co-occurring C₃ weeds as atmospheric carbon dioxide increases.

Nomenclature: Common cocklebur, Xanthium strumarium L. XANST; sorghum, Sorghum bicolor L. Moench.

Greenhouse and laboratory studies were conducted to evaluate responses of ivyleaf morningglory, pitted morningglory, palmleaf morningglory, and smallflower morningglory to several herbicides in relation to leaf structure, epicuticular wax, and spray droplet behavior. Two- to four-leaf stage plants of each species were highly susceptible to acifluorfen, bentazon, bromoxynil, glufosinate, and glyphosate. However, at the five- to eight-leaf stage, these species were less susceptible, and control was herbicide specific. Spray droplets of these five herbicides had a higher contact angle on ivyleaf morningglory than the other three species with a few exceptions. Stomata and glands were present on both adaxial and abaxial leaf surfaces of all species, and palmleaf morningglory and smallflower morningglory had more of these than did the other two species. Trichomes were present on all species except palmleaf morningglory. Epicuticular wax mass was highest in ivyleaf morningglory (57 μg cm⁻²) and lowest in smallflower morningglory (14 μg cm⁻²). Wax consisted of homologous short-chain (< C₁₈) or long-chain (> C₂₀) hydrocarbons, alcohols, acids, and triterpenes. Smallflower morningglory waxes lacked short-chain length components. Triterpenes were absent in palmleaf morningglory and smallflower morningglory epicuticular waxes. Untriacontane (C₃₁ hydrocarbon) and tridecanol (C₃₀ alcohol) were common major long-chain components in waxes of all four species. Heptadecane (C₁₇ hydrocarbon) and octanoic acid (C₁₈) were common major short-chain length wax components in pitted, ivyleaf, and palmleaf morningglory. In spite of some differences in leaf surface structures, wax mass, and wax components among the four species, there was no clear relationship between these parameters and herbicide efficacy.

Nomenclature: Acifluorfen; bentazon; bromoxynil; glufosinate; glyphosate; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; palmleaf morningglory, Ipomoea wrightii Gray IPOWR; pitted morningglory, Ipomoea lacunosa L. IPOLA; smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb. IAQTA.

Giant foxtail putatively resistant to acetolactate synthase (ALS) inhibitors has been reported widely in the upper Midwest, typically in fields with a history of ALS inhibitor use in continuous corn or corn–soybean rotation. However, it is not known whether these giant foxtail populations vary in their response to ALS inhibitors. Therefore, our objectives were to confirm and quantify resistance of giant foxtail accessions from Wisconsin, Minnesota, and Illinois to imidazolinone and sulfonylurea herbicides; to determine the mechanism of resistance; and to determine the mechanism of resistance inheritance. Dose–response experiments using three- to four-leaf stage giant foxtail plants in the greenhouse confirmed cross-resistance of the Wisconsin, Minnesota, and Illinois accessions to imazethapyr and nicosulfuron. Based on ED₅₀ values (the effective dose that reduced shoot dry biomass by 50% compared to the nontreated plants), the Wisconsin, Minnesota, and Illinois accessions were 16-, 17-, and 15-fold resistant to imazethapyr, respectively, and 21-, 19-, and 9-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. In contrast, all accessions were susceptible and responded similarly to fluazifop-P. Based on an in vivo ALS assay, the Wisconsin, Minnesota, and Illinois accessions were > 750-, > 320-, and > 670-fold resistant to imazethapyr, respectively, and 1,900-, > 1,900-, and 80-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. To determine the inheritance of resistance traits, hybrid F₁ families were generated from crosses between ALS inhibitor–susceptible and -resistant plants from Minnesota. Three distinct plant phenotypes—resistant (R), intermediate (I), and susceptible (S)—were identified in the F₂ generation following exposure to imazethapyr. In repeated experiments, these phenotypes segregated in a 1:2:1 (R:I:S) ratio, indicative of a trait associated with a single, nuclear, semidominant allele.

Nomenclature: Imazethapyr; nicosulfuron; corn, Zea mays L.; giant foxtail, Setaria faberi Herrm. SETFA; soybean, Glycine max (L.) Merr.

Field experiments were conducted in 1996 and 1997 to evaluate the efficacy of combined cultural and biological weed control for management of Canada thistle in conservation tillage soybean production. For cultural control, we used a highly weed-competitive soybean variety (cv. ‘Kato’). The biological control agent was the phytopathogenic bacterium Pseudomonas syringae pv. tagetis (PST). The application of PST reduced Canada thistle survivorship, height growth, and seed production, although these reductions were usually less than those resulting from bentazon application. Only bentazon application resulted in significant reduction of biomass of Canada thistle plants that survived all season. These results suggest the value of PST for management of Canada thistle in conservation tillage systems due to its negative effects on survival, growth, and reproduction. However, the weed-competitive soybean variety did not affect Canada thistle performance differently than a less competitive variety used for comparison, and there was no indication of synergy between the effects of the two control methods.

Nomenclature: Bentazon; imazethapyr; Canada thistle, Cirsium arvense (L.) Scop. CIRAR; soybean, Glycine max (L.) Merr. ‘Kato’, ‘Evans’; Pseudomonas syringae pv. tagetis.

Field studies were conducted in 1996, 1997, and 1998 to determine the effectiveness of several grasses as filter strips for reducing sediment and herbicide losses in runoff. Big bluestem, eastern gamagrass, switchgrass, and tall fescue reduced total runoff volume by at least 55, 76, 49, and 46%, respectively. Within the 127-d sampling period, each perennial grass filter strip investigated reduced total sediment loss in surface runoff by at least 66%. All four species reduced total fluometuron loss in runoff at least 59%. Big bluestem and eastern gamagrass reduced norflurazon loss in runoff 63 and 86%, respectively. When a filter strip was present, fluometuron and norflurazon losses did not exceed 5 and 3% of the total applied, respectively, compared to 12 and 5%, respectively, when a filter strip was not present.

Nomenclature: Fluometuron; norflurazon; big bluestem, Andropogon gerardii Vitman; cotton, Gossypium hirsutum L. ‘DES-119’; eastern gamagrass, Tripsacum dactyloides L.; switchgrass, Panicum virgatum L., PANVI; tall fescue, Festuca arundinacea Schreb., FESAR.

The cross-tolerance of imidazolinone-tolerant (IMI-tolerant) rice to various acetolactate synthase (ALS)-inhibiting herbicides at one and two times labeled rates was studied. The IMI-tolerant rice is cross-tolerant to imazaquin, imazapyr, nicosulfuron, pyrithiobac, thifensulfuron plus tribenuron, and triasulfuron; is partially tolerant to imazamethabenz and metsulfuron; and is susceptible to chlorimuron, flumetsulam, imazamox, imazapic, primisulfuron, and rimsulfuron. In the greenhouse, IMI-tolerant rice injury with 70 and 140 g ai ha⁻¹ imazethapyr was 17 and 34%, respectively, 28 DAT. Both rates of imazapyr, imazaquin, rimsulfuron, nicosulfuron, thifensulfuron plus tribenuron, and pyrithiobac, and 25 g ai ha⁻¹ triasulfuron, injured rice the same as imazethapyr. Red rice control with 70 and 140 g ha⁻¹ imazethapyr was 97 and 98%, respectively, 28 DAT. At label and two times the label rate, all imidazolinones, nicosulfuron, and primisulfuron controlled red rice equivalent to imazethapyr. Red rice control with 28 g ai ha⁻¹ rimsulfuron was similar to control with 70 and 140 g ha⁻¹ imazethapyr 28 DAT. In the field, barnyardgrass control with two times the labeled rate of imazamox, imazapic, imazapyr, imazaquin, imazamethabenz, rimsulfuron, and nicosulfuron was equal or greater than control with imazethapyr 30 DAT; however, at two times the labeled rate of imazamox, imazapic, and rimsulfuron, injury was greater than imazethapyr. Of all the herbicides tested, only nicosulfuron, imazaquin, and imazapyr offer a combination of low rice injury and high red rice control compared with imazethapyr.

Nomenclature: Chlorimuron; flumetsulam; imazamethabenz; imazamox; imazapic; imazapyr; imazaquin; imazethapyr; metsulfuron; nicosulfuron; primisulfuron; pyrithiobac; rimsulfuron; thifensulfuron; triasulfuron; tribenuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; red rice, Oryza sativa L. ORYSA; rice, Oryza sativa L. ‘IMI-tolerant 93AS-3510’.

Cogongrass is a difficult weed to control in small-scale farming systems and often causes significant crop yield reduction. Field experiments were conducted from 1996 to 1999 at three sites located in the forest/savanna transition zone of Nigeria to determine the influence of intercropping cover crops on cogongrass, corn, and cassava growth. Total cogongrass biomass (shoots and rhizomes) at the onset of the study was highest at Ijaiye (889 g m⁻²), followed by Umumba (445 g m⁻²), and least in Ezillo (138 g m⁻²). Velvetbean had the highest percent ground cover at Umumba and Ijaiye (67 to 89%) 10 wk after planting and shaded the ground longer at all locations. Twelve months after planting, plots with cover crops had 66, 71, and 52% lower cogongrass biomass than the weedy control without cover crops at Ijaiye, Umumba, and Ezillo, respectively. Velvetbean at all locations, L. purpureus at Ijaiye, and tropical kudzu at Umumba and Ezillo were the cover crops most effective in reducing rhizome biomass of cogongrass. Annual weeds dominated the plots sown to cover crops after 2 to 3 yr. At Ijaiye and Umumba, cogongrass competition affected the yield of cassava more than the yield of corn. At all locations, cover crops and weeded control treatments had 27 to 52% more corn grain yield than the weedy control. At Ijaiye, corn grain yields from velvetbean and L. purpureus plots were similar to that from the weeded control plot. At Umumba, all plots with cover crops had corn grain yields similar to that of the weeded control. At all locations, almost all cover crop treatments had cassava root yields higher than the weedy control. Except at Ijaiye, root yields from weeded control plots were 17 to 88% higher than in cover-cropped treatments, suggesting competition between cover crops and cassava.

Nomenclature: Cassava, Manihot esculenta Crantz ‘TMS 30572’; cogongrass, Imperata cylindrica (L.) Beauv. IMPCY; corn, Zea mays L. ‘SUWAN 1 SR’; Lablab purpureus (L.); tropical kudzu, Pueraria phaseoloides (Roxb.) Benth.; velvetbean, Mucuna cochinchinensis (Lour.) A. Chev.

The effect temperature, light intensity, time to initial light exposure, relative humidity, and the presence of dew have on CGA-248757 and flumiclorac efficacy was evaluated in laboratory trials. Increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on common lambsquarters by 79 and 87%, respectively. Similarly, increasing temperature from 10 to 40 C increased CGA-248757 and flumiclorac activity on redroot pigweed by 68 and 60%, respectively. Increasing light intensity from 0 to 1,000 μmol m⁻² s⁻¹ increased CGA-248757 activity on common lambsquarters and redroot pigweed by 92 and 93%, while flumiclorac activity increased 91 and 99%. Time to initial light exposure and relative humidity did not affect CGA-248757 or flumiclorac activity on common lambsquarters and redroot pigweed. The presence of dew reduced herbicidal activity of both compounds on common lambsquarters by 5% and redroot pigweed control with CGA-248757 and flumiclorac by 21 and 20%, respectively. Field applications of CGA-248757 or flumiclorac at 6:00 a.m., 2:00 p.m., and 10:00 p.m. indicate environmental conditions at application strongly influence soybean tolerance and weed control with CGA-248757 and flumiclorac. The greatest soybean injury occurred from CGA-248757 or flumiclorac applications at 6:00 a.m. compared with applications at 2:00 p.m. or 10:00 p.m. Common lambsquarters control was greatest when CGA-248757 or flumiclorac was applied at 6:00 a.m. or 2:00 p.m. compared with 10:00 p.m. However, redroot pigweed control was greatest when CGA-248757 or flumiclorac was applied at 2:00 p.m. Application time of day did not affect velvetleaf control with either herbicide.

Nomenclature: CGA-248757, [[2-chloro-4-fluoro-5-[(tetrahydro-3-oxo-1H,3H-[1,3,4]thiadiazolo[3,4-a]pyridazin-1-ylidene)amino]phenyl]thio]aceate; flumiclorac; common lambsquarters, Chenopodium album L. CHEAL; redroot pigweed, Amaranthus retroflexus L. AMARE; soybean, Glycine max (L.) Merr. ‘Conrad’ GLYMA; velvetleaf, Abutilon theophrasti Medik. ABUTH.

The objectives of this study were to use a computer simulation model to predict the influence of herbicides and mechanical treatments on giant foxtail population dynamics, annualized net return (ANR), and the giant foxtail economic optimum threshold (EOT) in a corn–soybean rotation over 20 yr. Mechanical treatments were interrow cultivation in corn and rotary hoe in soybean. Herbicides at full (1 ×) and half (½ ×) rates applied alone reduced giant foxtail seedbank 95% within 4 and 8 yr, respectively. Predicted seedbank dynamics had more variability when managed with herbicides at ½ × than at 1 × rates applied alone. Mechanical treatments integrated with herbicide at ½ × rates resulted in giant foxtail seedbank and variability similar to herbicides at 1 × rates applied alone. ANR was maximized when herbicides were applied between ⅜ × and 9/16 × rates applied alone. As initial giant foxtail density increased from 100 to 10,000 seeds m⁻², the herbicide rate that maximized ANR increased. Economic optimum thresholds (EOTs) did not vary when herbicides were applied at different rates, but integrating mechanical treatment with herbicides increased the EOT from 0.1 to 0.7 seedlings m⁻². Sensitivity analysis determined that giant foxtail seedbank demographics, seedling survival, and seed production per plant had the most influence on model predictions. Model sensitivity varied little between 1 × and ½ × rates. Integrating herbicides and mechanical treatment decreased the sensitivity of the model to perturbations in parameter estimates. Herbicides at reduced rates were more profitable over the long term than 1 × rates, but risk of herbicide failure increased as rate decreased. Integration of herbicides applied at reduced rates with mechanical treatments increased ANR and minimized the risk of herbicide failure compared to herbicides applied at 1 × rates alone.

Nomenclature: Corn, Zea mays L.; giant foxtail, Setaria faberi Herrm. SETFA; soybean, Glycine max (L.) Merr.

Field and laboratory studies were conducted at Stoneville, MS, from 1996 to 1998 to determine the influence of subsoiling (SS) and conventional tillage (CT) of a Sharkey clay soil on microbial characteristics and herbicide degradation. Soil samples obtained from imazaquin-treated and nontreated plots from the soybean row and interrow position were analyzed. Because only the row position is actually disturbed by SS, a comparison of row and interrow position on the parameter was conducted. Imazaquin (preemergence, 140 g ai ha⁻¹) had no effect on microbial populations, microbial enzyme activity (fluorescein diacetate [FDA] hydrolysis and triphenyl-tetrazolium chloride [TTC] dehydrogenase), and organic carbon content. Estimates of microbial activity based on FDA hydrolysis and TTC dehydrogenase activity indicated greater activity under CT; however, microbial biomass and organic carbon were not affected by tillage or row position. A laboratory study assessed the degradation of carboxyl- and ring-labeled 2,4-D as influenced by tillage and row position. Soils from CT plots had an initially higher mineralization rate of ¹⁴C carboxyl-labeled 2,4-D compared to soils from SS plots; however, no effect of tillage or row position was observed on the cumulative amount of ¹⁴CO₂ evolved 14 d after treatment (DAT) in 1996 and 18 DAT in 1998. In studies with ring-labeled 2,4-D, a higher ¹⁴CO₂ evolution was detected in soils obtained from SS plots, regardless of row position, whereas a greater amount of radioactivity was observed in the unextractable fraction from CT soils. Because differences in 2,4-D mineralization between tillage regimes were minimal, adoption of SS as a tillage practice for heavy clay soils in the Mississippi Delta may have a limited effect on microbial characteristics and biodegradation of soil-applied herbicides.

Nomenclature: 2,4-D; 2,4-DCP, 2,4-dichlorophenol; imazaquin; FDA, fluorescein diacetate; TTC, triphenyl-tetrazolium chloride; soybean, Glycine max (L.) Merr.

Seed from six Australian near-isogenic lines of wild oat were germinated and grown in controlled-environment growth chambers under either ambient CO₂ (357 parts per million by volume [ppmv]) or elevated CO₂ (480 ppmv) at 20/16 C or 23/19 C. Three soil moisture treatments—−0.01 MPa (field capacity), −0.10 MPa, or −1.00 MPa—were imposed. Wild oat lines grown under elevated CO₂ had higher seed production and greater plant dry weights, although the response of these variates involved a complex of interactions with temperature, soil moisture, and line. Plant height varied with wild oat line, and plants grown at 20/16 C were taller than those grown at 23/19 C. At 23/19 C, time taken to mature was reduced for some wild oat lines, and elevated CO₂ reduced the time taken to maturity for some lines at 20/16 C. There was no significant difference in the level of dormancy developed in freshly harvested caryopses between the two CO₂ treatments, but an effect was present in seed that had been after-ripened for 193 d. These results indicate that the main climate change variables ([CO₂], soil moisture, and increased temperature) directly influence the growth and development of wild oat and are likely to affect the population dynamics of this species.

Nomenclature: Wild oat, Avena fatua AVEFA.