The effects of five preemergence herbicides on turfgrass quality and control of crabgrass (Digitaria spp.) were measured to determine the influence of thatch on preemergence herbicide activity in Kentucky bluegrass (Poa pratensis L.) turf. Field applications of benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine), oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-Δ2-1,3,4-oxadiazolin -5-one], and prosulfalin {N-[[4-(dipropylamino)-3,5-dinitrophenyl] sulfonyl]-S,S-dimethylsulfilimine} to thatchy Kentucky bluegrass turf in spring caused injury that was apparent during periods of summer stress, while DCPA (dimethyl tetrachloroterephthalate) and bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide] caused little or no injury. The same treatments on thatch-free sites revealed that only prosulfalin was injurious. Adsorption of benefin, bensulide, and DCPA from solution was greater by thatch than by soil; however, the mobility of these herbicides in undisturbed thatch profiles was greater than measured in undisturbed surface soil from thatch-free sites. Thus, turfgrass injury from preemergence herbicides on thatchy sites is due to at least two factors: the greater mobility of preemergence herbicides in thatch than in soil; and the inherent susceptibility of Kentucky bluegrass to injury from preemergence herbicides that come into direct contact with roots and other absorbing organs.

Optimum germination of Canada thistle [Cirsium arvense (L.) Scop.] occurred at alternating temperatures of 20 to 30 C and 30 to 40 C and at a constant temperature of 30 C. Osmotic pressures of 7 bars and greater reduced germination, but 2% of the seeds germinated at 15 bars. Canada thistle seeds (14%) were able to germinate in NaCl concentrations of 20,000 ppmw. Light was important in seed germination, however, its effect could be overcome by adding GA3 (gibberellic acid) to the germination medium. Optimum pH for germination was between 5.8 and 7.0. Depth of planting influenced emergence; the highest percentage of seedlings emerged from depths of 0.5 to 1.5 cm. Canada thistle seedlings tolerated average soil moisture tensions of 1.3 bars and continued to grow. Canada thistle seedlings 19 days old and with two true leaves were able to resprout after top-growth removal. Regrowth was evident on 8% of the clipped plants 4 days after initial top-growth removal.

The persistence of napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] applied at 2.24- and 4.48-kg/ha was studied for 2 yr at two locations in the field. Wheat (Triticum aestivum L. ‘Arthur 71′) was severely injured when sown in the fall following a spring application of napropamide. A wheat root bioassay also showed that napropamide was persistent for more than 180 days. The yield of sweet corn (Zea mays L. var. rugosa Bonaf. ‘Gold Cup’) was not significantly reduced when grown 1 yr after the initial application of napropamide.

The rates of degradation of 14C-benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) and 14C-DCPA (dimethyl tetrachloroterephthalate) were significantly faster in thatch than in soil from a Kentucky bluegrass (Poa pratensis L.) turf. These results suggest that herbicides applied preemergence in turf will persist for shorter periods of time because of the carbon-enriched medium (thatch) and that higher rates or more frequent applications may be required to maintain concentrations at effective levels.

The influence of soil organic matter on the initial and residual phytotoxicity of thirteen s-triazine herbicides was investigated in greenhouse experiments using three Horotiu sandy loam soils with organic matter levels of 9.8, 15.5, and 20.6%. The amount of herbicide required to reduce the growth of oats (Avena sativa L. ‘Mapua’) by 50% (GR50) when compared with the control was determined for each herbicide and each organic matter level. Results showed that the GR50 values for all herbicides were highly and positively correlated with the soil organic matter. In general, the phytotoxicity of compounds of high water solubility was less influenced by soil organic matter than those having low water solubility. The chloro-triazines persisted longer in soil than did the methoxy- or methylthio-triazines. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] were the most persistent of the chloro-triazines.

Burning and 2,4-D [(2,4-dichlorophenoxy)acetic acid] were used on the early stages of beaked hazel (Corylus cornuta Marsh.) understory to prevent development of a dense, continuous understory. At this early stage beaked hazel understories consisted of seedlings, small clones, and large clones. The application of 1.12 kg/ha of 2,4-D in July and August killed 84 to 100% of the seedlings and small clones, 86 to 98% percent of the large clones; while the application of 0.56 kg/ha of 2,4-D in June, July, or August caused 77 to 98% mortality of seedlings and small clones, but only 57 to 83% of large clones. Effects of burning were much more variable and generally mortality was less than with 2,4-D. One or two burns killed 23 to 100% of seedlings and small clones, and an average of only 13% (range 0 to 93%) of large clones.

Two Nicotiana tabacum L. cultivars, ‘North Carolina 88’ and ‘Coker 319′, were grown in a charcoal-filtered greenhouse and treated with three different herbicides according to the herbicide labels. Labeled rates (X) for diphenamid (N,N-dimethyl-2,2-diphenylacetamide), isopropalin (2,6-dinitro-N,N-dipropylcumidine), and pebulate (S-propyl butylethylthiocarbamate) were 9.2 kg/ha, 2.3 kg/ha, and 6.2 kg/ha, respectively. Three weeks after treatment with 1/2X, X, or 2X rates of herbicide, half of the plants were fumigated with 15 μl/liter ozone for 2.5 h for four consecutive days. North Carolina 88 and Coker 319 grew equally well and were equally sensitive to ozone injury. Ozone reduced plant height and levels of reducing sugars and total nonstructural carbohydrates of both cultivars. Isopropalin, at X and 2X, reduced the ozone sensitivity and growth of both cultivars, while the X rate of pebulate reduced the ozone sensitivity of North Carolina 88. The chemical composition of the plants was not correlated with the sensitivity of the tobacco plants to ozone-induced injury. The major factors governing fleck intensity, growth, and chemical compositions were ozone and herbicide treatments.

Regional pine seedling tolerance experiments were conducted on five pine (Pinus spp.) species at ten forest nurseries in 1974. Similar studies in 1975 included four pine species at nine locations. From these studies, herbicides appearing safe for preemergence use are bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate] at 1.7, 3.4, and 6.7 kg/ha; butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] at 2.2, 3.4, 4.5, and 6.7 kg/ha, napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] at 1.7 kg/ha; napropamide at 1.1 kg/ha plus bifenox at 3.4 kg/ha; dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) at 0.37 and 0.74 kg/ha; diphenamid (N,N-dimethyl-2,2-diphenylacetamide) at 4.5 kg/ha, trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 1.1 kg/ha; and profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] at 1.1 and 2.2 kg/ha. Southern pine seedling tolerance to oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) was unacceptable at 1.1 and 2.2 kg/ha. Napropamide severely stunted seedlings at 6.7 kg/ha but affected seedlings only slightly at 3.4 kg/ha.

Purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (C. esculentus L.) appeared to be equally acceptable for oviposition by caged Bactra verutana Zeller, but purple nutsedge was significantly more suitable as a host: 90% of the larvae survived to maturity on purple nutsedge compared with 65% on yellow nutsedge. Responses of the plant species to both larval feeding injury and plant density were similar but purple nutsedge tended to be injured more than yellow nutsedge. At a high shoot density (nine shoots per pot), production of tubers by purple nutsedge was more adversely affected by feeding of five larvae per shoot than was production by yellow nutsedge: tuber dry weights were reduced 93 and 80% and numbers of tubers per pot were reduced 77 and 62%, respectively. Production of inflorescences was greatly reduced in both species. The effect of B. verutana on inflorescences may be more important for yellow nutsedge, which is generally considered to reproduce freely by seeds. Both species of nutsedge probably would be about equally affected by augmentation of B. verutana populations as a method of biological control.

A 2-yr study was conducted to determine the influence of early and late soybean [Glycine max (L.) Merr.] planting dates on the competitiveness of velvetleaf (Abutilon theophrasti Medic). Velvetleaf was planted at densities of one plant per 61 and 30 cm of row, and competition ranged from 4 weeks to full season. Soybean planting dates were mid-May and late-June. Velvetleaf emerging with soybeans in mid-May were twice as competitive as those emerging with soybeans planted in late-June. A density of one velvetleaf per 30 cm of row competing full season reduced soybean yields 27% for the early and only 14% for the late planting date. The competitive difference was due to the short-day photoperiodic response of velvetleaf. Although the early growth stages of soybeans are more competitive than those of velvetleaf, 10 weeks after emergence velvetleaf competition reduced soybean growth and development. Velvetleaf does not appear to be a potential major problem in Arkansas for soybeans planted in June due to its photoperiodic response and late season competitiveness. However, the weed could present problems for soybeans planted early, especially when an early maturing variety is used.

Differential napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] tolerance by redroot pigweed (Amaranthus retroflexus L.) and prostrate pigweed (Amaranthus blitoides S. Wats.) was noted in field study. Redroot pigweed was readily controlled whereas prostrate pigweed was not. Germination studies in which both pigweed species were directly exposed to napropamide (0 to 25 ppm) indicated that prostrate pigweed was the most susceptible of the two species. Root growth rates of untreated prostrate pigweed were 30% greater than redroot pigweed. When seeds of both species were germinated in a 4-cm layer of napropamide in greenhouse study each species was controlled equally well. Exposure of germinating seedlings of the two pigweed species to napropamide 1 day before emergence resulted in differential control. Seedlings of redroot pigweed never developed beyond the cotyledon stage; whereas, prostrate pigweed seedlings were initially suppressed by the herbicide, but surviving plants continued to grow. An early preemergence application or mechanical incorporation of napropamide should enhance control of prostrate pigweed.

Soybean [Glycine max (L.) Merr. ‘Hark’] plants responded similarly to a sublethal application of metribuzin [2-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] or to low light intensities. Both treatments increased soluble amino acid, ATP, nitrate reductase, and nitrate levels (up to 67, 140, 190, and 10,000%, respectively) and decreased soluble reducing sugars (33%), malate levels (86%), and the chlorophyll a/b ratio (24%). The similar response of light-limited and of chemically photosynthesis-inhibited plants is thought to be induced by a carbohydrate stress or another parameter related to a decreased rate of photosynthetic electron flow. However, during the induction process in shaded plants, light quality might play an important role in some cases. The results obtained with metribuzin and soybean plants are compared to and discussed together with (a) the results obtained by other authors working with s-triazine herbicides and (b) results obtained with methabenzthiazuron [N-(benzothiazol-2-yl)-N,N′-dimethylurea] in earlier experiments.

Pea (Pisum sativum L. ‘Elf’) root rot suppression by dinitroaniline herbicides could not be explained by a direct effect on the host. Pre-incubation of pea roots with 0.1 ppmw of oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) or trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) in solution culture did not alter their resistance to root rot. Furthermore, 0.1 ppmw of oryzalin or trifluralin did not alter the exudation of electrolytes or α-amino compounds from pea roots. Most of the dinitroaniline herbicides significantly inhibited mycelial radial growth of pathogen, Aphanomyces euteiches Drechs., at 1.0 ppmw and inhibited asexual spore production at 0.1 to 1.0 ppmw. Dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine), fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], and oryzalin inhibited A. euteiches mycelial growth and asexual reproduction more effectively than the other dinitroaniline herbicides studied. The production of motile zoospores, the infecting propagule of the pathogen, was the most sensitive stage in the life cycle and was completely inhibited by 0.01 ppmw of all dinitroaniline herbicides tested. Since this concentration is below that estimated in the soil solution at registered rates of application, inhibition of motile zoospore production is sufficient to explain root rot suppression. Inhibition of pathogen motility resulted in a 2-week delay in the infection of pea roots. This delay allowed sufficient additional plant growth that the peas could better withstand the effects of subsequent disease development.

Diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate} loses part of its activity on wild oats (Avena fatua L.) when it is applied in combination with ester or amine formulations of MCPA {[4-chloro-o-tolyl)oxy] acetic acid.} Uptake of diclofop-methyl is reduced in the presence of commercial formulations of MCPA, particularly amine formulations, which are only partly compatible with the diclofop-methyl formulation. With added MCPA, regardless of formulation, hydrolysis of diclofop-methyl to the acid diclofop is slowed down, both in intact wild oat plants and in a crude leaf extract. At the same time, conversion of any acid formed to inactive conjugates is accelerated in the presence of MCPA. It is concluded that the loss of activity of diclofop-methyl by MCPA can be accounted for by the effects of MCPA on uptake and metabolic conversion of diclofop-methyl.

The competitive effects of common cocklebur (Xanthium pensylvanicum Wallr.) on soybean [Glycine max (L.) Merr. ‘Lee 74′] were studied on Peridge silt loam during 1975 and 1976. Interspecific competition reduced the dry matter (DM) and leaf area of the individual plants in comparison to the plants grown under intraspecific conditions. Twelve weeks after emergence the percentage reductions in DM and leaf area due to interspecific competition were similar, were greater in common cocklebur than soybeans, and were greater in the dry year (1976) than in the wet year (1975). The root systems of both species varied with plant age, soil depth, and seasonal moisture. In general, the density of roots of both species was greatest in the soil profile at a depth interval of 0 to 13 cm and in the dry year more roots were found deeper in the soil. Extraction of soil water began in mid-July at the 91-cm depth and in late July at the 137-cm depth. Roots of common cocklebur and of the interspecific treatment explored a greater volume of soil for water than did those of soybeans. The total amount of water used by a pure stand of soybeans and a mixed stand of soybeans and common cocklebur was greater than that used by a pure stand of common cocklebur. Approximately 25% less water was lost as evapotranspiration (ET) in the dry year than in the wet year. Values for water use efficiency (WUE) increased during the season. Little difference in WUE between species was found in the wet year and up to 10 weeks after emergence in the dry year.

Diclofop-methyl {methyl ester of 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} applied as postemergence treatment at rates of 0.84 to 1.68 kg/ha controlled 65 to 87% of the wild oat (Avena fatua L.) without injuring flax (Linum usitatissimum L.) or rapeseed (Brassica campestris L. and B. napus L.) Yields of both crops were significantly increased because wild oat competition was minimized. Wild oat control was further improved when adjuvants were added with the herbicide at a concentration of 0.5% (v/v) to the spray mixture. Oil content, iodine number, and linolenic acid concentration of flax, and oil content of rapeseed were increased with the application of the herbicide, while the protein content of meal from both crops was unaffected. When flax was grown in the greenhouse under weed-free conditions, the 0.84 and 1.12 kg/ha rates of the herbicide did not affect seed and oil composition, whereas the 1.68 kg/ha rate decreased protein content, and slightly increased oil content, but had no significant effect on iodine number when compared with the untreated check. The change in seed and oil quality of flax appears to be related to a reduction in weed competition by the herbicide.

Thylakoid membranes isolated from chloroplasts of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]-susceptible (S) and -resistant (R) biotypes of common groundsel (Senecio vulgaris L.) were assayed to determine the effect of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], bromacil (5-bromo-3-sec-butyl-6-methyluracil) and several s-triazine herbicides on photosystem II (PSII) activity. Each herbicide inhibited photosynthetic electron transport on the reducing side of PSII in thylakoid membranes from the susceptible biotype. Thylakoids isolated from the resistant biotype were 60 to 3200 times more tolerant to the inhibitors than thylakoids from the susceptible biotype. Electrophoretic analysis revealed no differences in molecular weight of membrane polypeptides of either common groundsel biotype. Three-hour dark incubation did not enhance inhibitor activity in the resistant-type thylakoid membranes. Specific structural or configurational changes associated with the reactive site may account for the differences in Hill reaction inhibition between biotypes. A relationship between herbicide structure and Hill reaction activity is discussed.

Seventy-three rice (Oryza sativa L.) cultivars and uniform advanced-generation dwarf lines were evaluated for their response to molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) at concentrations of 0, 1, 2, and 4 ppmw in a 7-day laboratory screening test. These rice entries varied significantly in their response to each molinate concentration when shoot height of each entry was compared to that at the 0 ppmw level. Entries were ranked according to their molinate response at the 2 ppmw concentration and categorized as being either tolerant, intermediate or susceptible. Entries categorized as susceptible at 2 ppmw were further subdivided into highly susceptible and moderately susceptible categories based on their response to the 1 ppmw concentration. Those categorized as tolerant at the 2 ppmw concentration were likewise divided into highly tolerant and moderately tolerant categories on the basis of their response to the 4 ppmw concentration. The results of this bioassay indicates the possible existence of a genetically inherited response to molinate which might be manipulated by plant breeders to develop cultivars which have a high tolerance to molinate.

Field and greenhouse studies were conducted to determine the degree to which annual bluegrass (Poa annua L.) could be controlled by inhibiting seedheads. The materials used were: MH (1,2-dihydro-3,6-pyridazinedione); chlorflurenol (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate), plus methyl 9-hydroxyfluorene-9-carboxylate, and methyl 2,7-dichloro-9-hydroxyfluorene-9-carboxylate; and endothall [7-oxabicyclo (2.2.1) heptane-2,3-dicarboxylic acid]. The effects of these materials on pollen quality and the viability of seed produced by treated plants were also determined. For all chemicals used, multiple applications at low rates resulted in better seedhead inhibition than single treatments at higher rates and their effects lasted longer. However, treatments that inhibited seedheads by an amount predicted to reduce annual bluegrass (more than 75%) often caused objectionable foliar discoloration. Endothall, particularly the granular formulation, caused excessive injury at all rates. All growth regulators reduced the number of seed produced, which affected the number of seeds that germinated from soil that was taken from treated plots. The number of seed found in the soil was sufficient to allow the stand to be self-perpetuating. All treatments reduced the percentage of fertile pollen, however, this reduction was not significant because the germination of seed harvested from treated plants was not reduced significantly. Even though these treatments reduced seedheads significantly, the population of annual bluegrass the following year was not reduced.

The experimental herbicide R-24191 [1-(m-t-butylacetamidophenyl)-3-methyl-3-methoxy urea] inhibited growth of Chlorella sorokiniana Shihira and Kraus, O2 evolution during CO2 fixation by Chlorella and isolated leaf cells of spinach (Spinacia oleracea L. ‘Winter Bloomsdale 769′), the photoreduction of NADP by isolated chloroplasts using water or hydroxylamine as the electron donor, the photoreduction of 2,6-dichloroindophenol by isolated chloroplasts, and O2 uptake by isolated chloroplasts using paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) as the electron acceptor. The phenylurea herbicide had no effect on the last reaction when ascorbate plus 2,6-dichloroindophenol were used as the electron donor. The results indicate that the site of the phenylurea inhibition was in photosystem II between water-splitting and photosystem I.