Laboratory studies were conducted to evaluate possible interactions of atrazine and glyphosate on soybeans at the molecular level. Because these compounds alter the extractable activities of nitrate reductase (NR)3 and phenylalanine ammonia-lyase (PAL), these enzymes were chosen as biochemical markers. Levels of anthocyanin, hydroxyphenolic compounds, and chlorophyll were also determined. Three-day-old dark-grown soybean seedlings, susceptible to both herbicides, were transferred to 2 mM CaSO4 or CaSO4 containing high-purity 10–4 M atrazine, 10–4M glyphosate, or a combination of both herbicides at 10–4 M each. Plants were placed in a growth chamber under continuous light (200 μE·m–2·s–1) at 25 C and harvested at 24-h intervals over a 96-h time course. Interactions of atrazine and glyphosate on hypocotyl elongation were detected after 96 h when the compounds were supplied simultaneously. Intermediate PAL levels were found in the atrazine + glyphosate treatment compared to the levels for each herbicide alone. At 96 h, anthocyanin content in hypocotyls in the atrazine + glyphosate treatment was twice that of the atrazine treatment, and hydroxyphenolic content was increased by 25% (per axis). Interactions of these herbicides were also apparent on NR activity; i.e., on a per organ basis the combination treatment resulted in levels 20% higher than glyphosate alone in roots and 16 times greater than glyphosate alone in leaves after 96 h. Total chlorophyll content in hypocotyls (μg/organ) was decreased by atrazine but not significantly by glyphosate after 96 h. Atrazine + glyphosate resulted in chlorophyll content equal to that of atrazine alone. Mathematical analyses showed that interactions on NR, PAL, anthocyanin levels, and hypocotyl elongation were antagonistic at 96 h. These data demonstrate interaction between atrazine and glyphosate in plant tissues that is related to some biochemical parameters associated with the secondary molecular mode of action of these herbicides.

The absorption, translocation, and metabolism of 14C-picloram and 14C-2,4-D applied alone and together to leafy spurge was evaluated. Leafy spurge absorbed 34 and 24% of the 14C-2,4-D applied alone and with picloram, respectively, and 14 and 10% of the 14C-picloram applied alone and with 2,4-D, respectively. More 14C-2,4-D was translocated in leafy spurge than 14C-picloram, and adding picloram to 14C-2,4-D decreased 14C translocation to the roots. Adding 2,4-D to 14C-picloram increased the percentage of absorbed 14C that translocated in leafy spurge from 28 to 48%. Generally, 14C-picloram and 14C-2,4-D remained as the parent acid in leafy spurge whether applied alone or together. Of 14C-picloram recovered from the roots, 83% was unmetabolized picloram when applied alone compared to 95% when applied with 2,4-D, which probably is the reason for increased leafy spurge control when these herbicides are applied together.

The effects of haloxyfop on elongation, mitotic index, and morphology of sorghum and unicorn-plant primary roots were examined. Elongation of sorghum roots was completely inhibited by haloxyfop concentrations of 10–6 M or greater 24 h after treatment, whereas unicorn-plant root elongation was unaffected by the same concentrations 72 h after treatment. Mitotic indices of sorghum roots were reduced by both 10–6 and 10–8 M haloxyfop, with the higher concentration reducing the index to near zero by 24 h of exposure. The mitotic indices of unicorn-plant roots were unaffected by the same levels of the herbicide after both 24- and 48-h treatment. Histological analyses showed that after 24 h exposure to 10–6 M haloxyfop, large vacuoles were present in cells at the root apex that normally did not exhibit these organelles. After 48 h of exposure many cells in the apical region appeared to lack visible cytoplasm and/or nuclei, and by 72 h only cell walls remained visibly evident, and many cells had collapsed. These changes are similar to those that occur in tissues undergoing senescence. Treatment of sorghum roots with 10–8 M haloxyfop did not cause discernible changes after 72 h. Unicorn-plant roots treated with 10–6 M haloxyfop appeared unaffected after 72 h.

In greenhouse studies, the calculated I50 (herbicide application resulting in 50% inhibition of shoot regrowth) in tall fescue was approximately 0.004 kg/ha for both sethoxydim and haloxyfop. In red fescue, the I50 for sethoxydim and haloxyfop was 9.4 kg/ha and 0.04 kg/ha, respectively. As measured in crude cell extracts of tall fescue, incorporation of 14C-acetyl-coenzyme A into fatty acids was inhibited 62 and 71% by 10 μM sethoxydim and 10 μM haloxyfop, respectively. In red fescue, 10 μM haloxyfop inhibited 14C-acetyl-CoA incorporation into fatty acids by 29%, whereas 10 μM sethoxydim had no effect. The I50 for inhibition of acetyl-coenzyme A carboxylase activity in tall fescue with sethoxydim and haloxyfop was 6.9 and 5.8 μM, respectively. In red fescue the I50 for haloxyfop was 118 μM. Sethoxydim concentrations as high as 1 mM had little effect on acetyl-coenzyme A carboxylase activity in red fescue. These results suggest that acetyl-coenzyme A carboxylase is a sensitive site of action for sethoxydim and haloxyfop in tall fescue, and that tolerance to these herbicides in red fescue is due to the presence of a more tolerant form of the enzyme.

Experiments were conducted in order to establish the mechanism by which the synergist MZH 2091 increases metribuzin activity on ivyleaf morningglory. Herbicide absorption and metabolism were compared for plants treated either with 14C-metribuzin alone or with 14C-metribuzin plus nonlabeled MZH 2091. The compounds were applied via the roots for 3 h and the absorption was determined. After the pulse treatment, the plants were grown for additional 0, 3, 6, 9, 12, 21, 45, or 69 h in water, and the metribuzin metabolism was studied in the cotyledons. The synergist did not interfere with herbicide uptake but did strongly inhibit metribuzin metabolism. The pathway of metribuzin metabolism in ivyleaf morningglory is discussed as well as the way it is affected by the synergist MZH 2091.

Prediction of the developmental stages of wheat and wild oats would be useful in order to: 1) correctly time the application of herbicides, and 2) accurately schedule research and cultural operations. The Haun developmental scale which numbers leaf development and describes floral development on the main stem of grasses was found to be suitable for describing the development of semidwarf wheat and wild oats in California. Haun developmental rates of wheat and wild oats were similar. Interference by wheat or wild oats in mixed cultures did not change the developmental rate of either species when grown with added nutrients and water. Degree days gave better correlations with development than calendar days when different planting dates, years, and locations were compared. A degree day model with a 5 C base temperature and a second-order polynomial expression gave accurate predictions of developmental stage, which correlated well with field data.

Downy brome seed production was measured in the field following the establishment of different moisture levels using a line source sprinkler irrigation system. Results indicated that seed production was strongly affected by different moisture levels, with a curvilinear decrease in seed number from higher to lower moisture. In a hydroponic study with water deficits induced by PEG-8000, no seeds were produced when plants were severely stressed (-1.1 MPa) for a 7-day period during culm elongation or anthesis. Number of seeds/panicle was reduced under severe water stress during seed fill, under moderate stress (-0.5 MPa) during anthesis or seed fill, and under mild stress (-0.1 MPa) during anthesis. Water stress also reduced apparent photosynthesis and increased diffusive resistance of flag leaves, particularly under severe stress. In general, water deficits did not affect seed weights or germination percentages in either study.

Bermudagrass plugs were transplanted from the greenhouse to the field at monthly intervals from March through October at Shafter, CA. Emergence began from the March plantings when soil temperatures at a depth of 5 cm reached 17 C. Although the initial growth of March, April, and May plantings was very slow, these plantings eventually produced about 60% more dry matter (2100 g/m2) than June, July, and August plantings (1300 g/m2) when harvested in December. At 8 weeks after planting, June to August plantings accumulated 120% more dry matter (460 g/m2) than March to May plantings (210 g/m2). At 12 weeks, dry matter accumulation was greatly reduced by shorter photoperiods and cool temperatures for March and September plantings (160 g/m2) when compared to all intermediate plantings (700 g/m2). All plantings, except October, produced rhizomes (15 to 120/m2) and seeds (154 to 73400/m2) before killing frosts occurred in late November. Although rhizomes and seeds were not collected until the 12-week harvest for the March to May plantings, these plantings produced or tended to produce the greatest number of rhizomes (85/m2) and seeds (49300/m2) by December. Rhizomes and seeds from the June to September plantings were collected within 8 weeks, indicating that declining photoperiods hastened rhizome and seed production.

Competitive interactions of wheat and wild oats in relation to variations in plant density of both species were investigated in field studies under irrigation and added fertilizer. Competition studies included comparisons by both additive and replacement series. The additive series was used to study plant responses to competition under fixed density of wheat and increasing density of wild oats. The replacement series was used to study plant responses to competition under constant total plant density with differing proportions of wheat and wild oats. On a per plant basis, shoot dry weight and leaf area index of wild oats were less than those of wheat at anthesis. However, the replacement series experiment indicated that wheat and wild oats were equivalent in competitiveness. Relative density of wild oats gave a better regression fit for wheat yield than did absolute wild oats density (r2 values of 0.83 and 0.61, respectively). Yield of wheat grain was linearly proportional to relative density of wild oats.

Replacement studies were conducted under greenhouse conditions to determine if available soil water influences the competitive interaction between soybean and Florida beggarweed. Stomatal conductance and leaf water potential were determined for both species under different levels of available soil water to identify possible mechanisms involved in changes in the relative competitiveness induced by the water deficits. Soybean leaf area and aboveground biomass were greater than for Florida beggarweed under optimum water, but equal to or less than Florida beggarweed with water stress. Soybean was more competitive than Florida beggarweed when there was adequate soil water but less competitive than Florida beggarweed under water stress. Stomatal conductance was higher for soybean with optimum soil water (at high leaf water potentials) but equal to Florida beggarweed as soil water became limiting (low leaf water potentials). These data indicated that water stress differentially affected soybean and Florida beggarweed.

Growth and morphology of two U.S. hydrilla biotypes grown under 10- and 16-h photoperiods in outdoor aquaria were investigated. The dioecious hydrilla biotype elongated extensively under both photoperiods and reached the water surface within 2 to 3 weeks after planting. In contrast, the monoecious biotype grew more densely near the sediment, primarily by producing a high number of horizontal stems, root crowns, and higher shoot densities. Monoecious hydrilla grown from tubers produced new tubers after 4 weeks of exposure to the 10-h photoperiod, and the number of tubers produced was five- to sevenfold higher in the monoecious than in the dioecious biotype. Furthermore, monoecious hydrilla produced tubers under both 10- and 16-h photoperiods, with tuber production much higher under the shorter photoperiod. Dioecious hydrilla grown from tubers under similar conditions produced new tubers after 8 weeks only under the 10-h photoperiod. No tubers were produced by the dioecious biotype throughout a 10-week exposure to the 16-h photoperiod.

Barnyardgrass seeded in field plots at monthly intervals from March through October at Shatter, CA, began to emerge in March when soil temperatures reached 17 C at a depth of 5 cm. For all plantings, most of the seeds germinated and produced seedlings within 2 weeks. Increasing day lengths favored vegetative growth, with April, May, and June plantings producing the tallest plants, the most dry matter, and the largest number of tillers. July and August plantings, the first to flower, began flowering and producing seeds within 5 to 6 weeks after planting, but the greatest number of seeds (15 000 to 25 000/plant) was produced by the larger plants of the April, May, and June plantings. September plantings produced few seeds/plant (37), when compared to March (5900), July (9800), and August (9700) plantings. The earliest that seed production would be expected from seedlings emerging in March and April would be early June, and seedlings emerging in September could produce seeds before killing frosts.

Gas exchange of jointed goatgrass leaves was affected by temperature, irradiance level, and soil matric potential. Net photosynthesis of leaves under saturating irradiance (PPFD3 = 1850 (μE·m–2·s−1) was optimum at about 20 C. At 25 C, net photosynthesis was nearly 90% of maximum at a PPFD of 800 μE·m–2·−1. Transpiration, and presumably water use, increased steadily with temperature from 10 to 40 C. Dark respiration rate and compensation points for light and for CO2 increased exponentially, or nearly so, from 10 to 40 C. Soil moisture deficits of −130 kPa reduced net photosynthesis and transpiration by about 30 and 55%, respectively, compared to well-watered plants.

Experiments were conducted to determine adaptation characteristics to reduced irradiance of velvetleaf, common lambsquarters, eastern black nightshade, tumble pigweed, and soybean. Plants were grown to the 5- to 8-leaf stage in the greenhouse at ambient radiation (850 μE·m–2·s–1), and 26 and 13% of ambient radiation. Tumble pigweed, a C4 plant, had the highest light-saturated photosynthetic rates at all growth irradiances, while common lambsquarters had the highest rates of the four C3 species. All species adjusted to reduced irradiance by decreasing light-saturated photosynthesis, leaf respiration rates, root:shoot ratios, and leaf densities, while increasing leaf area ratios (LAR)3. As growth irradiance was reduced, support tissues (roots, stems, and petioles):leaf ratios did not change for common lambsquarters or velvetleaf, increased for soybean, and decreased for eastern black nightshade and tumble pigweed, indicating superior adaptation of the latter two species for efficient light harvesting at reduced growth irradiances. Of these five species, eastern black nightshade had the lowest respiration rate, the highest LAR, and the lowest support:leaf ratio for optimum adaptation to shaded environments.

Three grass selective herbicides, sethoxydim, fluazifop, and haloxyfop, applied to wild oat plants at the five-leaf stage inhibited growth and induced chlorosis in leaves. Young and actively growing tissues were affected first. Stem elongation in wild oat was inhibited within 2 days of treatment with sethoxydim and within 5 days of treatment with fluazifop or haloxyfop. At these same observation times, internodes that were elongating rapidly at the time of treatment were constricted at the base. These symptoms were followed by necrosis of the internode tissue. Histological examination of the affected internodes indicated that the herbicides inhibited cell division in very young internodes and inhibited both cell division and cell elongation in slightly older internodes. Initial injury occurred in the epidermal, cortical, and procambium cells of the peripheral regions of the stems located at the base of the affected internodes. Necrosis then progressed to the center of the stem tissue and all cells in the internodes were killed within 14 days of treatment. All three herbicides caused similar morphological and histological effects on developing wild oat stems.

In field studies, sprinkler irrigation application of the butyl ester of fluazifop-P, the methyl ester of haloxyfop, and the ethyl ester of quizalofop controlled large crabgrass as well as conventional spray applications. In greenhouse investigations, root uptake of the herbicides from sprinkler irrigation applications injured large crabgrass more than root uptake from conventional applications, but large crabgrass injury from shoot uptake was equal with sprinkler irrigation and conventional applications. Droplets with dilute concentrations of herbicide and crop oil, simulating sprinkler irrigation, were more active when applied to the whorl or second leaf than to the first leaf of large crabgrass. An increase in concentration of nonemulsified oil in the treatment solution increased herbicide deposition and retention.

Triazine-resistant giant foxtail was identified in 1984 near Delta, PA. In field studies conducted from 1985 to 1987, preemergence applications of atrazine with cyanazine or simazine provided poor (≤60%) season-long control of this annual grass in no-tillage corn. Best season-long preemergence control was obtained with metolachlor or microencapsulated formulations of alachlor or EPTC. Postemergence applications of cyanazine or tridiphane + atrazine + crop oil provided poor giant foxtail control. Postdirected applications of paraquat resulted in fair (≥70%) control of giant foxtail through midseason. In greenhouse studies, triazineresistant (R) giant foxtail tolerated preemergence applications of atrazine or simazine at dosages to 9.0 kg ai/ha. Triazine-sensitive (S) giant foxtail was injured by 2.2 kg/ha and higher rates of atrazine and simazine. In laboratory studies, the I50 for inhibition by atrazine of photoinduced electron transport in thylakoids isolated from S and R biotypes was determined to be 0.24 and 205 μM, respectively. The differential sensitivity was paralleled by simazine. However, the limited solubility of simazine prevented determination of an I50 value with thylakoids from the R biotype.

Field experiments were conducted in 1986 and 1987 to evaluate the effect of herbicide incorporation to various depths on common cocklebur control with imazaquin, a tank mix of imazaquin and metribuzin, and a preformulated mixture of chlorimuron and metribuzin in soybeans. Herbicide rates included were two-thirds and the full registered rate. All treatments, except chlorimuron plus metribuzin (50 plus 320 g ai/ha, respectively), controlled more common cocklebur when incorporated to depths of 2.5 to 7.6 cm than when left unincorporated. Also, imazaquin at 90 g ai/ha and the tank mix of imazaquin plus metribuzin at 90 plus 280 g ai/ha, when incorporated, controlled more common cocklebur than higher rates of the same combination when not incorporated. Common cocklebur control was similar with 2.5-, 5.0-, and 7.6-cm soil incorporation depths.

Corn and giant foxtail response to foliar-applied sethoxydim at 67, 134, and 200 g ai/ha was evaluated in field studies. Sethoxydim applied over the top of corn (60 cm tall) caused greater whorl damage and reduced corn grain yield more than postdirected sethoxydim. Sethoxydim controlled giant foxtail best when used in conjunction with a preemergence application of metolachlor (2.2 kg/ha) and atrazine (1.7 kg/ha).

Degradation of trifluralin in four soils, each represented at four sites, under field conditions was determined quantitatively and described mathematically. A biexponential equation that resulted from integration of first-order and second-order differential rate equations described degradation data better than the first-order kinetic model for 15 of 25 soil-site combinations. Biexponential model regression coefficients indicated extent of degradation and that degradation is rapid at initially high trifluralin concentrations but slows as concentration decreases. The first-order kinetic model initially underestimated but ultimately overestimated degradation of trifluralin, thereby inferring that a first-order half-life is inadequate for predicting trifluralin persistence.