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.

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.

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.

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.

Densities up to 100 downy brome m2 were established in winter wheat in southeastern Wyoming and west-central Kansas to quantify wheat yield loss from downy brome interference and to approximate economic threshold levels. A quadratic equation best described wheat yield loss as a function of weed density when downy brome emerged within 14 days after wheat emergence. Densities of 24, 40, and 65 downy brome m2 reduced wheat yield by 10, 15, and 20%, respectively. Wheat yield was not reduced when downy brome emerged 21 or more days later than wheat. Economic thresholds varied with changes in downy brome density, cost of control, wheat price, and potential wheat yield. In a greenhouse experiment, dry weight of 72-day-old wheat plants grown in association with downy brome was not affected by the distance between the weeds and wheat, whereas downy brome plant dry weight increased with increasing distance between the weeds and wheat.

Two adjuvants, Triton 11 XE (Isoctyl phenyl polyethoxy ethanol) and urea were tank mixed with four herbicides, methabenzthiazuron [N-(benzothiazol-2-yl)N,N′-dimethyl urea] at 1.0 and 1.5 kg ai/ha, terbutryn [N-(1,1-dimethylethyl)-N′-ethyl-6-(methylthio)-1,3,5-triazine-2,4-diamine] at 0.50 and 0.75 kg/ha, metoxuron [N-3-chloro-4-methoxyphenyl)N,N-dimethylurea] at 1.0 and 1.75 kg/ha, and isoproturon [(N-(4-isopropylphenyl)-N,N-dimethylurea] at 0.50 and 0.75 kg/ha and applied to weeds in wheat. Metoxuron reduced weed dry matter by 90% over a 2-yr period. Isoproturon, terbutryn, and methabenzthiazuron reduced weed dry matter by 82%. Tank-mixing of Triton 11 XE at 0.5% (v/v) with metoxuron at 1.0 kg ai/ha, terbutryn at 0.5 kg/ha, isoproturon at 0.5 kg/ha, and methabenzthiazuron at 1.0 kg/ha reduced weed density and biomass by 52 to 67% and enhanced grain yield by 33% over herbicide alone. None of the herbicides had any significant interaction with adjuvants in terms of weed control and grain yield.

Laboratory studies were conducted to determine the ability of the extenders dietholate and SC-0058 to prevent enhanced biodegradation of EPTC and butylate applied to Wisconsin soils with different histories of carbamothioate herbicide use. Enhanced EPTC and butylate biodegradation occurred in soils previously treated with those herbicides. Enhanced biodegradation of dietholate occurred on soils previously treated with that extender plus either EPTC or butylate. Enhanced dietholate biodegradation was observed when applied alone or in combination with butylate or EPTC. Application with dietholate prevented enhanced biodegradation of butylate but not EPTC even though enhanced biodegradation of dietholate was occurring. Enhanced biodegradation of SC-0058 did not occur. SC-0058 prevented enhanced EPTC and butylate biodegradation even in soils previously treated for three consecutive years with the respective herbicide plus SC-0058. Application of either 1.1 or 2.2 kg/ha SC-0058 plus 6.7 kg/ha EPTC provided equal or better wild proso millet control and sweet corn yields than applications of EPTC alone or EPTC plus dietholate.

Field experiments were conducted from 1985 to 1987 to evaluate herbicide systems for minimum-tillage and conventional-tillage peanut production. While acceptable weed control could be achieved in both tillage systems, minimum-tillage systems generally had to be more herbicide intensive. Preemergence or preplant-incorporated within-the-row applications of either ethalfluralin or pendimethalin plus postemergence applications of paraquat and sethoxydim provided Texas panicum control equivalent to preplant-incorporated applications of ethalfluralin or pendimethalin. Early-postemergence paraquat applications improved Florida beggarweed and pitted morningglory control in conventional-tillage systems at least 15% compared to the same systems without paraquat Control of bristly starbur and sicklepod in conventional-tillage systems did not increase with paraquat application. Broadleaf weed control did not differ between tillage systems, except pitted morningglory control was lower in the minimum-tillage system. Conventional-tillage peanuts produced yields 800 to 1900 kg/ha higher, depending on herbicide system, and also provided greater net returns than minimum-tillage peanuts. The greater yield and net returns in conventional- versus minimum-tillage systems were not attributed to weed control or disease problems.

Chlorsulfuron and metsulfuron were effective on Texas whitebrush, killing 70 to 75% of the plants at 0.28 kg ai ha, marginally effective on Macartney rose, killing 15 to 43% of the plants at 1.12 kg ha, but ineffective on honey mesquite and huisache, killing no more than 10% of the plants at 1.12 kg ha. Sulfometuron was ineffective on all woody species at 1.12 kg ai ha. Clopyralid and picloram were effective, killing 70 to 92% of the huisache at 2.24 kg ae ha. Tebuthiuron killed 72% of the Texas whitebrush at 1.12 kg ai ha. Sulfometuron generally reduced herbaceous plant cover the year of application. Chlorsulfuron and metsulfuron often resulted in increased grass cover the year of application. Most herbicides reduced broadleaf weeds the year of application. Woolly croton often was the first prominent broadleaf species to return to areas treated with clopyralid and sulfometuron. Few differences occurred among herbicides in herbaceous weed cover the year following treatment.

The overall degradation of chlorimuron was very similar at −0.1 and −1.5 MPa and slightly less in air-dry soil. Degradation rates increased with increasing temperature. The primary 14C-labeled compounds observed in moist-soil extracts were desmethyl chlorimuron and saccharin, while the primary 14C-labeled compound observed in air-dry soil extracts was saccharin. Saccharin is formed quantitatively from ethyl 2-(aminosulfonyl)benzoate (phenylsulfonamide) during extraction and therefore represents phenylsulfonamide formed in the soil as a result of chemical hydrolysis of the sulfonylurea bridge. These degradation products suggest that chemical hydrolysis of the sulfonylurea bridge is the primary mode of degradation in air-dry soil, while microbial degradation and chemical hydrolysis both occur in moist soil. These laboratory results demonstrate that chlorimuron will degrade in air-dry soil at a temperature-dependent rate by chemical hydrolysis.

Adsorption and desorption isotherms for atrazine and two metabolites, hydroxyatrazine (HA) and S-glutathione atrazine (GSHA), were determined by batch equilibration on Plano and Waukegan silt loam soils at two soil pH levels (Plano, 6.1 and 4.5; Waukegan, 6.1 and 4.0). Freundlich adsorption isotherms were not affected by soil type except for GSHA at pH 4.0 to 4.5. When averaged over both soils, the order of adsorption at pH 6.1 was atrazine (Kf = 3.7) < GSHA (Kf = 7.3) << HA (Kf = 25) and at pH 4.0–4.5 was atrazine (Kf = 6.1) << HA (Kf = 58) ≤ GSHA (Kf: Plano = 35; Waukegan = 78). The average slope of the adsorption isotherms (1/nads) was 0.81. The slopes of all desorption isotherms (1/ndes) were less than their respective 1/nads, indicating hysteresis. Atrazine desorbed into soil solution (1/ndes > 0.0). With the exception of GSHA which desorbed from the pH 6.1 Plano silt loam (1/ndes = 0.15), desorption of HA and GSHA from other treatments was negligible (1/ndes = 0.0). Consequently, leaching of HA and GSHA in these and similar soils is not likely, due to high adsorption and low desorption.

Field trials were conducted in 1986 and 1987 to determine the effects of moisture on herbicidal activity of cinmethylin applied preemergence at 0.0, 0.3, 0.6, and 0.9 kg ai ha to both dry and moist sandy loam soil. Herbicide application was followed by varying amounts of irrigation. Weed species included velvetleaf, prickly sida, green foxtail, and barnyardgrass. When cinmethylin was applied to a moist soil or when 2.5 cm of irrigation was applied 5 days after cinmethylin application to a dry soil, overall weed control was reduced. Optimum weed control resulted from cinmethylin application to dry soil followed either by a 2.5-cm irrigation within 8 h or a 7.6-cm irrigation within 36 h.

Chinese tamarisk is an invader of riparian sites in the southwestern United States and northern Mexico. Plant canopy light reflectance measurements showed that Chinese tamarisk had higher visible (0.55- and 0.65-μm wavelengths and 0.63- to 0.69-μm waveband) reflectance than did associated woody and herbaceous plant species in the late fall-early winter period when its foliage turned a yellow-orange to orange-brown color prior to leaf drop. Chinese tamarisk had a yellow-orange color on conventional color (0.40- to 0.70-μm) aerial photographs during this phenological stage that made it distinguishable from other plant species. Computer analyses of conventional color film positive transparencies showed that Chinese tamarisk populations could be quantified from associated vegetation. This technique can permit area estimates of Chinese tamarisk infestations on wildland areas.

Leaf surface morphology and physical characteristics of herbicide deposits on leaf surfaces can influence herbicide performance. Leaf surface topography, the degree and type of epicuticular wax formation, and the presence, type, and distribution of trichomes all influence the distribution of a given herbicide formulation sprayed onto a leaf surface. Depressions above anticlinal cell walls accumulate herbicide, thus lessening uniform distribution. As the amount of particulate wax increases, the size of individual spray drop deposits on the leaf decreases, thus resulting in reduced coverage. In many instances the presence of trichomes reduces optimal epidermal coverage by intercepting spray drops before they reach the epidermal surface. Adjuvants reduce the adverse influence of leaf topography, epicuticular wax, and trichomes on herbicide distribution, but their use usually does not yield an even coating over the entire leaf surface. Many herbicides, in pure form, are solids (i.e., crystals) rather than liquids. For most applications, herbicides are dissolved, dispersed, or emulsified in a water-based spray solution. After spraying, water and any solvents evaporate from the leaf surface and herbicides often return to their solid crystalline form. In the few cases that have been studied, less herbicide is absorbed when present on the leaf surface as a solid rather than as a liquid. In many instances, greater effectiveness of a postemergence herbicide may be obtained if attention is given to optimizing the distribution and physical form on sprayed leaf surfaces.

Relevant data on the sorption of organic compounds by isolated plant cuticles are reviewed and discussed in relation to the foliar penetration process. The chemical properties and structure of plant cuticles favor sorption of lipophilic compounds and play an important role in the penetration of biologically active substances. With organic acid auxins 2,4-D and NAA, and methylene blue as molecular probes, concentration, pH, temperature, and surfactants were important factors affecting sorption. The constituent waxes of the cuticle markedly inhibit sorption of a wide range of organic compounds. Octoxynol surfactants that have 5 or 7.5 polyoxyethylene groups interact with the epicuticular wax to enhance the sorption of NAA. At sorption equilibrium, the cuticle has a lower affinity for methylene blue in the region usually rich in cuticular waxes and in a narrow band in the cuticular pegs.

Herbicide translocation in the phloem is dependent on the production of transport sugar(s) in source tissues and their loading into the phloem. The movement of sugars (primarily sucrose) from mesophyll cells to the sieve element-companion cell complex has been the subject of much investigation in recent years. The current evidence suggests that there are at least two sucrose loading mechanisms in higher plants: one involves sucrose transfer via the apoplasm and subsequent loading into the phloem, while a second mechanism involves movement through continuous symplasmic connections between the mesophyll cells and the phloem. Similarly, both symplasmic and apoplasmic routes may exist at the site of unloading in sink tissue. It is postulated that the mechanism of sucrose loading may influence herbicide entry into the phloem and the likelihood of herbicide transfer to the xylem at the site of loading or unloading.

A physicochemical approach to understanding phloem transport of xenobiotics requires that similarities in transport processes in different plant species far outweigh any differences. There is now evidence that this is so, though additional factors such as rate of cuticular penetration, metabolism, and phytotoxicity may differ among plant species and thus may influence distribution patterns. Most herbicides that are translocated in phloem are weak acids, and their transport behavior can now be explained, at least in part, in terms of accumulation and retention in phloem. These processes can, in turn, be explained reasonably well by considering the polarity and acid strength of each compound. Specific carrier processes do not appear to be involved in the transport of most phloem-mobile herbicides. Phloem transport of herbicides has been assessed using the castor bean plant. For acids of pKa <4, intermediate lipophilicity is required for good phloem transport, while weaker acids of pKa >5 and nonionized compounds need to be more polar in order to move well.

A refined model for the phloem translocation of xenobiotics in living plants is presented. The model accounts for the translocation of a xenobiotic that can exist in several ionic forms in terms of the permeabilities of these forms and the fractions of the forms that exist both within the sieve tubes and in the surrounding apoplast. Predictions of the model are quite sensitive to the description of the membranes used in the model, with optimum log Kow's and pKa's becoming more positive as the membranes become less permeable. The literature generally supports the predictions of the model. When applied to patented chemical hybridizing agents, the model predicts that most are phloem-mobile. This prediction strongly suggests that the chemical hybridizing agents exert their pollen-suppressing activity by direct effects on pollen and/or the surrounding maternal tissues. With low transpiration rates and the discontinuity of xylem in many reproductive tissues, phloem translocatability may be an essential requirement for the whole plant activity of many chemical hybridizing agents.

Translocation of phloem-mobile herbicides was inhibited by their phytotoxic action on processes that maintain assimilate translocation. Glyphosate lowered import into developing sink leaves soon after it was applied to exporting sugarbeet leaves. Later, photosynthesis slowed down and starch accumulation stopped, but export of both assimilate and glyphosate continued until it was limited by starch availability at night Experiments with field pennycress and Tartary buckwheat indicated that self-limitation of chlorsulfuron translocation probably occurred and that it resulted from lowered assimilate entry into phloem rather than from inhibition of photosynthesis or carbon allocation. Leakage of chlorsulfuron from the phloem when export was slowed down also may have contributed to its reduced translocation.