Foliar-applied clopyralid was translocated much more readily than chlorsulfuron in the phloem of Tartary buckwheat plants. This result was not due to greater penetration of clopyralid into the treated leaf or to greater retention of chlorsulfuron in the cuticle. Experiments with excised leaf disks indicated that chlorsulfuron was taken up more readily by the leaf tissue and accumulated in the tissue to a higher concentration than clopyralid. Both herbicides effluxed readily from the tissue after transfer to herbicide-free medium, indicating that the accumulation was not due to irreversible binding within the tissue. Chlorsulfuron (2.8 nmol) applied with 14C-sucrose reduced 14C export from the treated leaf. Chlorsulfuron also reduced export of 14C following exposure of the treated leaf to 14CO2 at 6, 12, or 24 h after herbicide application. This effect of chlorsulfuron could be partially reversed by pretreating the plants with a combination of 1 mM valine, leucine, and isoleucine. In similar experiments clopyralid had no effect on assimilate transport. It is concluded that phloem translocation of chlorsulfuron in sensitive species is limited by a rapid, indirect effect on phloem transport that reduces both its own translocation and that of assimilate.

Growth chamber experiments were conducted to elucidate the morphological and physiological responses of rice to postemergence application of fenoxaprop. Two rice cultivars, ‘Newbonnet’ and ‘Mars’, and barnyardgrass were treated with 0.17 kg ai ha−1 fenoxaprop at the five-leaf stage. Within 2 days of fenoxaprop application, rice cultivars developed a white chlorotic band across leaves that were in direct contact with spray droplets. Leaf elongation rates for the two rice cultivars were inhibited by 40% after 4 days and by over 50% after 14 days. Inhibition of leaf elongation by fenoxaprop contributed to an overall decrease in leaf area and shoot dry weight. Net photosynthesis was reduced by 35% in fenoxaprop-treated plants 11 days after application, although stomatal conductance was not affected. Nitrogen fertilization prior to fenoxaprop application increased foliar injury of both rice cultivars. Visible injury symptoms showed the following order of susceptibility to foliar-applied fenoxaprop: barnyardgrass > Mars rice > Newbonnet rice.

The herbicide safener OTC (L-2-oxothiazolidine-4-carboxylic acid) increased the amount of reduced thiol compounds in sorghum [Sorghum bicolor (L.) Moench. ‘DK 427′] seedlings. When seedlings were grown in liquid culture medium containing 35S-OTC, the compound was metabolized to radiolabeled cysteine and glutathione. The addition of tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] increased conversion of 35S-OTC to cysteine and resulted in the formation of one additional 35S-labeled compound. When 35S-glutathione was injected into germinating seeds it was converted to 35S-cysteine and both thiols were subsequently found in roots and shoots. Seeds injected with 35S-OTC both translocated the compound to developing roots and shoots and metabolized 35S-OTC to cysteine and glutathione. Excised roots and shoots also metabolized 35S-OTC to the thiols. In an in vitro assay the enzyme 5-oxoprolinase converted OTC to cysteine.

Haloxyfop-methyl toxicity to johnsongrass was reduced when sprayed in combination with 2,4-D in greenhouse studies. The antagonism was not seen when 2,4-D application followed haloxyfop-methyl treatment by 72 h or preceded it by 24 h. The antagonism of haloxyfopmethyl activity was evident regardless of chemical form of 2,4-D used (DMA, BOE ester, IPA, or acid). Absorption of 14C-haloxyfop-methyl into johnsongrass leaves was slowed by the presence of 2,4-D in the treatment solution but total absorption after 72 h was not affected. Translocation of 14C was decreased when the 14C-haloxyfop-methyl was applied in combination with 2,4-D. More haloxyfop-methyl, less haloxyfop, and more ether-soluble haloxyfop-methyl metabolites were found in treated leaves when haloxyfop-methyl was applied with 2,4-D compared to haloxyfop-methyl applied alone. The changes in haloxyfop-methyl translocation and metabolism caused by 2,4-D can quantitatively account for the antagonism observed.

Glasshouse studies were undertaken to determine the effect of imposed moisture stress on the phytotoxicity of haloxyfop; the absorption, translocation, and metabolism of 14C-haloxyfop; and 14C-photoassimilate partitioning in johnsongrass and large crabgrass. Following foliar applications of haloxyfop at 30 and 25 g ai ha–1 to large crabgrass and johnsongrass, respectively, control 15 days after treatment was 92% for nonstressed plants and less than 12% for water-stressed plants. Foliar absorption of 14C-haloxyfop was reduced by moisture stress 1, 3, 5, and 24 h after treatment (HAT) in large crabgrass and 1, 3, 5, 48, and 72 HAT in johnsongrass. Regardless of stress treatment, absorption in both species reached a maximum by 24 HAT. Translocation of the radiolabel from the treated leaf to plant parts above and below the node of the treated leaf was inhibited by moisture stress in large crabgrass and johnsongrass at all harvest intervals beginning 5 and 24 HAT, respectively. Metabolism of 14C-haloxyfop was not altered by moisture stress. Fixation of 14CO2 and subsequent distribution of the 14C-photoassimilates were reduced by moisture stress. Decreases in photoassimilate translocation were similar to reductions in 14C-haloxyfop translocation. Moisture stress reduced the phytotoxicity of haloxyfop in the two grasses, and the reduced activity of haloxyfop appeared to be partially related to changes in herbicide absorption and translocation.

Glasshouse experiments were conducted to evaluate the effects of preemergence applications of metribuzin, pendimethalin, alachlor, and imazaquin on the competitive relationship between soybean and sicklepod, tall morningglory, or common cocklebur. An equal-ratio series design was employed where the proportion of species in a mixture remained constant and total plant density was varied. The ratio of soybean:sicklepod fresh weights was altered by alachlor and imazaquin while the ratio of soybean:tall morningglory fresh weights was altered only by imazaquin. The ratio of soybean:common cocklebur fresh weights was altered by metribuzin and imazaquin. Increasing herbicide rates resulted in higher soybean:weed fresh weight ratios and higher herbicide response coefficients.

The mechanism of glyphosate tolerance was investigated in nine birdsfoot trefoil selections that exhibited a threefold difference in glyphosate tolerance. Single-stemmed ramets (vegetative clones) established from the nine selections were used to evaluate tolerance to glyphosate, spray retention, and 14C-glyphosate absorption and translocation in growth chamber experiments. The tolerance of greenhouse-grown ramets correlated with the tolerance of field-grown plants, indicating that tolerance was not a function of plant size or affected by environment. The nine selections differed in spray retention and 14C-glyphosate translocation but not in glyphosate absorption. The differences in retention and translocation were not correlated with the level of glyphosate tolerance but could contribute to the tolerance of an individual plant. The specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) (EC 2.5.1.19) ranged from 1.3 to 3.5 nmol min−1 mg−1 protein among the nine selections assayed and was positively correlated with plant tolerance level. These results indicate that the primary mechanism of glyphosate tolerance in birdsfoot trefoil is based on the level of EPSPS activity.

The photodynamic herbicidal performance of δ-aminolevulinic acid in combination with four chlorophyll biosynthesis modulators was evaluated under greenhouse conditions, using corn, soybean, and ten weed species. Treatments resulted in accumulation of various amounts of protoporphyrin IX and of monovinyl and divinyl Mg-protoporphyrin IX and protochlorophyllide. Accumulation of these tetrapyrroles was accompanied by various degrees of photodynamic injury, depending on treatment, plant species, and somewhat the modulator. The lower photodynamic susceptibility of dark monovinyl/light monovinyl and dark divinyl/light divinyl plants toward the accumulation of monovinyl and divinyl protochlotophyllide, respectively, was attributed to their greater abilities to metabolize these protochlorophyllides in the light. On the other hand, the higher photodynamic susceptibility of the dark monovinyl/light divinyl weed species toward the accumulation of monovinyl protochlorophyllide was attributed to their lower ability to metabolize the accumulated monovinyl protochlorophyllide in the light.

Phenmedipham metabolism in leaf tissue of sugarbeet (tolerant) and rapeseed (sensitive) was compared. Sugarbeet leaf discs metabolized phenmedipham much more rapidly than rapeseed leaf discs, forming two metabolites of relatively low polarity. The less polar of these (metabolite 21) was a precursor to the other (metabolite 11), and its properties indicate derivation from phenmedipham by a single hydroxylation and monoglycosylation. Synthetic N-hydroxyphenmedipham was converted by both species into a compound that cochromatographs with metabolite 21. Purified metabolite 21 was much less inhibitory to light-driven oxygen evolution by isolated thylakoids of both species than was phenmedipham. Hydroxylation/glycosylation without prior carbamate hydrolysis appears to be a major factor in the tolerance of sugarbeet to phenmedipham.

In greenhouse replacement series experiments, ‘Atlantic’ and ‘Russet Burbank’ potatoes and barnyardgrass were more competitive than redroot pigweed as measured by relative competitive ability. In additive design field studies, four redroot pigweed or barnyardgrass per meter of row did not reduce Atlantic tuber yield when planted between the row following hilling (6 to 7 weeks after planting). However, as few as one barnyardgrass or redroot pigweed per meter of row reduced marketable tuber yield 19 to 33% when seeded in the row at the time of potato planting. Redroot pigweed reduced tuber yield more than barnyardgrass did in 1 yr of research. The correlation coefficients were similar for both weed density and weed biomass regressed on tuber yield. Aboveground potato biomass was not consistent in predicting total tuber yield. Furthermore, variability in aboveground potato biomass was not due to increasing weed weight or density. Neither specific gravity nor tuber quality was altered by the presence of any density of either weed species.

Multiple regression and response surface plots were used to analyze the effects of common lambsquarters population density and interference duration on weed growth and soybean seed yield. Under favorable growing conditions in 1986, weed biomass production at all population densities and interference durations was four to five times that produced in 1987, under less favorable conditions. However, there was no significant treatment by year interaction for soybean seed yield reduction by common lambsquarters, and production of each kg/ha weed biomass resulted in an average soybean yield reduction of 0.26 kg/ha. Utilizing 5% yield loss as an arbitrary threshold level, the regression equation predicted a common lambsquarters density threshold of 2 plants/m of row for 5 weeks of interference after crop emergence and 1 plant/m of row for 7 weeks. Seed production by individual common lambsquarters plants was highly correlated (r=0.92) with weed dry weight, and seed production ranged from 30 000 to 176 000 seeds/plant.

Glasshouse experiments were conducted to determine the effects of trifluralin on root growth and mineral relations of corn seedlings. Root weight to shoot weight ratios of corn seedlings were positively correlated to concentrations of trifluralin in soil. Root length to shoot weight ratios, however, were inversely related to trifluralin concentrations. Phosphorous and potassium concentrations in shoot tissue were reduced 60 and 35%, respectively, by 0.25 mg trifluralin kg−1 soil. Growth inhibition due to trifluralin was partially overcome by supplementing soil with nutrients.

Experiments were conducted in the greenhouse and growth chamber to evaluate competitive mechanisms of soybean and common cocklebur during seedling growth. Soybean was approximately 1.5 to 2 times taller and was more competitive aboveground than common cocklebur during the first few weeks of growth. Common cocklebur had an approximate 20 to 50% finer root system and was more competitive belowground than soybean during early growth.

Survival and growth of five collections of big sagebrush were compared in reciprocal transplant gardens. The study was designed to further the understanding of seedling ecology of this important rangeland plant. Survival of transplanted seedlings was not tied to their origin, but antagonism for collection site in survival was apparent for some collections. The site with the highest environmental potential as indexed by native plant community, soil development, and precipitation had the poorest seedling survival. The site with the lowest environmental potential produced seedlings that had the highest survival across all reciprocal gardens. The average mortality level after 3 yr was 11%, indicating that seedlings of big sagebrush can be transplanted successfully. In dry years, the collection from the driest location had the highest amount of juvenile biomass accumulation. However, during a season with much above-normal precipitation this was reversed and the amount of growth for this source was below levels observed for the dry years. The relation between subspecies of big sagebrush and survival or growth of the collections used in this study was not clear. Because collections were made on arid extremes of the subspecies they may reflect introgressive populations among subspecies.

Fall-planted longspine sandbur began to emerge in April, peaked in late April or May, and continued at low levels through October. During the first year after seeding, 98% of the total seedlings emerged prior to June. Fall-planted puncturevine began to emerge in late April or May and continued through October. Puncturevine emergence was multipeaked and 75% of each season's total emerged prior to mid-July. A similar number of puncturevine seedlings emerged the second and third years after planting as in the first. The emergence pattern for a species was similar each year regardless of how long the seed had lain in the soil. Longspine sandbur required 7 to 13 weeks from emergence to heading and produced approximately 2600, 1300, 180, and 3 spikes per plant when planted in May, June, July, and August, respectively. Puncturevine flowered within 3 to 4 weeks after emergence when temperatures were favorable and produced an average of 5600, 5200, 3600, and 200 burs per plant when planted in May, June, July, and August, respectively.

Field experiments were conducted in 1987 and 1988 at Frederick, MD, to determine established horsenettle (Solanum carolinense L.) interference with snapbean. Horsenettle grown for 3 yr prior to planting snapbeans reduced yield 65% in 1987 and 48% in 1988. Horsenettle established for 1 yr reduced snapbean yield 20 and 18% in 1987 and 1988, respectively. As the distance of the snapbean row from the horsenettle row increased, there was less effect on the yield of snapbean. In 1987, snapbean yield in rows 15, 30, or 60 cm from horsenettle was reduced 43, 29, and 15%, respectively, while during 1988 in the same plots snapbean yields were reduced 38, 26, and 11%. Eight horsenettle planted in a 4.6-m row the first year and 16 horsenettle per row for the second year reduced snapbean yield 36 and 55% respectively. Horsenettle fruit number was 16% lower the first year and 45% lower the second year when snapbeans were grown at a 15-cm row spacing instead of a 60-cm row spacing.

Johnsongrass leaves were covered with epicuticular wax that varied from 16 to 25 μg/cm2 on leaf blades and 56 to 206 μg/cm2 on leaf sheaths. At emergence, leaves were covered with a layer of smooth amorphous wax, but crystalline wax (wax plates) began to form on the amorphous wax within 1 or 2 days. This continued until all leaf surfaces were covered with wax plates. At 3 to 4 weeks of age, a smooth layer of coalescence wax was deposited over the wax plates. Formation of coalescence wax continued until nearly all leaf surfaces were covered with a smooth wax layer. Production of wax filaments began when plants were 3 to 4 weeks old and these tubular structures extended 100 to 200 μm above all other wax formations. Deposition of amorphous wax continued after stomata closed in the darkness, sealing over stomata, but the wax layer was broken when stomata opened again in the light. A capillary method was devised that was used to evaporate chloroform containing leaf waxes through 0.1- to 1.2-μm pores in inert filters to recrystallize amorphous wax and wax plates similar to that produced on johnsongrass leaves. Recrystallization of wax from wax filaments dissolved in chloroform produced the same structures of amorphous wax and wax plates as when only wax from leaves with amorphous wax and wax plates was used. Wax washed from leaves also produced wax plates and a variety of crystalline structures on the walls of glass vials after chloroform solutions were evaporated. This result indicated that the morphology of epicuticular waxes is influenced more by their inherent chemical and physical properties than by underlying cells or the cuticular membrane.

Temperature changes prior to picloram application affects its activity in leafy spurge. Absorption of 14C picloram was directly correlated with temperature changes; each 1 C increase in air temperature 24 h before treatment resulted in a 1% increase of 14C-picloram absorption in leafy spurge and vice versa. The greatest 14C-picloram absorption averaged 47% of applied 14C when the temperature increased from 18 C to 24 or 30 C 24 h before treatment compared to 33% when temperatures were constant. Translocation of 14C picloram was more sensitive than absorption to temperature changes with 4.3 and 1% of applied 14C-picloram translocated to the roots when the plants were maintained at 12 and 30 C, respectively. Even though absorption increased directly with temperature, 14C-picloram translocation to the root system declined as temperature increased.

Field surveys were conducted during 1978 and 1979 to determine the abundance and distribution of weeds in fields seeded to barley, oats, spring wheat, and mixtures of barley and oats in various proportions in the province of Prince Edward Island. Using a stratified random sampling procedure, weeds were counted in 536 fields during the 2-yr survey period. The weed flora had a large number of species that occurred at high densities, probably due to the limited herbicide use on Prince Edward Island. The average total number of species (64), number of species per field (20), and weed density (253 plants/m2) were similar among the four crop types and the five Extension Districts of the province. Only 49 of the 77 species encountered during the survey were found in 5% or more of the fields. Low cudweed, corn spurry, and common lambsquarters were the most abundant species, occurring in more than 80% of the fields at a mean density higher than 33.4 plants/m2. Red sorrel, smartweed, common hempnettle, broadleaf plantain, and quackgrass were also found in 80% or more of the fields, but at a mean density from 14.4 to 16.5 plants/m2. The perennials accounted for 51% of the commonly occurring species, annual and biennial broadleaf species accounted for 43%, and annual grasses were a minor group with only 6% of the species.

Spurred anoda and velvetleaf were grown for 40 days in controlled-environment chambers in monocultures at densities of 2, 4, 8, and 12 plants per 20-cm-diam pot and in mixed culture with all combinations of 2, 4, 8, and 12 plants of each species per pot. The day/night temperature was 29/23 C, and the photosynthetic photon flux density (PPFD) was 1000 μE m–2 s–1. Shoot dry weights and leaf areas of the two species were similar when they were grown in monoculture. However, in mixed culture spurred anoda exceeded velvetleaf in leaf area/plant and shoot weight/plant in 15 out of 16 treatments. Multiple linear regression equations relating shoot biomass/plant to the density of both species in mixed culture were calculated. Comparison of competition coefficients from these equations indicated that the competitive impact of a single spurred anoda plant was equivalent to the impact of 2.5 velvetleaf plants. In mixed culture, spurred anoda always contributed a greater proportion of the total shoot weight and total leaf area per pot than would be predicted from its proportion of the total plant population. These results indicate spurred anoda is competitively superior to velvetleaf.