Cucumber (Cucumis sativus L.) accessions which had demonstrated allelopathy under controlled environmental conditions were evaluated in the field. Plant introduction (PI) 169391 suppressed proso millet (Panicum miliaceum L.) fresh weight and population 58 and 84%, respectively. Total overseeded and volunteer weed population was reduced 54% when grown in association with PI 169391. Plant introduction 169391 was approximately twice as effective as PI 285605 in suppressing proso millet growth. Inhibition of weed species did not occur consistently in the field. The allelopathic effect of cucumbers was suppressed during periods of increased rainfall. However, these tests demonstrated that allelopathic activity could be obtained under certain edaphic and environmental conditions.

Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), dinitramine (N4,N4-diethyl-α,α,α-rifluoro-3,5-dinitrotoluene-2,4-diamine), profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro 2,6-dinitro-N-propyl-p-toluidine], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], and USB 3153 (N3,N3-di-N-propyl-2,4-dinitro-6-trifluoromethyl-m-phenylenediamine) were applied to evaluate weed control, cotton (Gossypium hirsutum L. ‘Dunn 119′) tolerance, and effect on rotational crops. Each herbicide was applied at 1-month intervals for 6 months prior to planting at 1, 1.25, 1.5, and 2X the recommended rates. Pigweed (Amaranthus hybridus L. and A. retroflexus L.) population counts and visual control ratings showed that weed control improved when treatments were applied closer to planting. Applications in November and December at 1.25X profluralin, 1.5X trifluralin, fluchloralin, and pendimethalin, 1X USB 3153, and 2X butralin were required to give effective weed control. Spring applications of 1X rates of profluralin, trifluralin, fluchloralin, and USB 3153, 1.25X rates of pendimethalin and dinitramine and 1.5X rate of butralin were required for adequate weed control. USB 3153 at all rates and dates of application severely limited growth and yields of wheat (Triticum aestivum L. ‘Funk 332′) planted after harvest and also the following May. Spring applications of profluralin at 1.5 and 2X rates and trifluralin at the 2X rate resulted in significant injury of rotational sorghum [Sorghum bicolor (L.) Moench ‘Dekalb E-59+’].

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 methyl ester of diclofop [2-(4-(2,4-dichlorophenoxy) phenoxy) propionic acid] was used in all of a series of experiments. When it was applied postemergence, it selectively controlled barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in sugarbeets (Beta vulgaris L. ‘USH 9′). Tank-mixing diclofop with desmedipham [ethyl m-hydroxycarbanilate carbanilate (ester)] resulted in decreased control of barnyardgrass under greenhouse and field conditions. Losses in activity ranged from 10 to 80%. Delaying application of desmedipham following application of diclofop by 4 days in the greenhouse or in the field resulted in no reduction in barnyardgrass control. Increased rates of diclofop offset the antagonistic effect of desmedipham. Similar losses in activity occurred when Hoe-29152 {methyl-2-[4-(4-trifluoromethylphenoxy)phenoxy] propanoate} was tank-mixed with desmedipham.

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.

‘Forrest’ soybean [Glycine max (L.) Merr.] infested with entireleaf morningglory [Ipomoea hederacea (L.) Jacq. var. integriusula Gray] were treated in the field with three postemergence herbicides, dinoseb (2-sec-butyl-4,6-dinitrophenol), sodium salt of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, and 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid]. Foliar nitrogen (N) loss, transpiration, and CO2 equilibrium concentrations were measured from 4 to 148 h after treatment. In morningglory foliar N loss was increased by dinoseb, decreased by acifluorfen, and first increased and then decreased by 2,4-DB treatments. Transpiration in this weed species was decreased by all three herbicides. In soybean, N loss was increased and transpiration decreased with dinoseb; acifluorfen did not affect either activity; and 2,4-DB decreased both processes during one sampling period. In both plant species dinoseb increased the CO2 equilibrium concentration within 4 h after treatment. Treatments with acifluorfen and 2,4-DB initially increased the CO2 concentration in morningglory with major increases noted at the onset of chlorosis 52 h after treatment. In soybean, some significant decreases in CO2 equilibrium concentrations occurred with the lowest rate of all three herbicides.

Detached leaves of hemp dogbane (Apocynum cannabinum L.) were treated with 14C-glyphosate [N-(phosphonomethyl)glycine] with and without adjuvants in the treatment solution. In the absence of adjuvants, 14C absorption was not significant after the first 30 min harvest. Adjuvants increased the amount of glyphosate initially absorbed but did not extend the period of absorption. The pattern of glyphosate absorption in isolated leaf cells was similar to the pattern found in detached leaves. Significant absorption was obtained at the first 15-min sampling with no additional absorption occurring during the remainder of the 2-h incubation period. The quantity of 14C absorbed by cells was proportional to the external concentration, but the pattern of cellular absorption was unaffected by the glyphosate concentration. After 2 h of incubation, an average of 0.1% of the glyphosate had been absorbed by the leaf cells. In similar studies using 14C-leucine, 1.0% of the 14C was absorbed after 2 h with absorption still increasing in a linear fashion. A uniform distribution of 14C was obtained in detached leaves that had absorbed 14C-glyphosate through the stem. In vitro and in vivo binding studies indicated this absorbed 14C was not tightly bound to cellular components and was probably free to be absorbed by and translocated out of the cells. Since the absorption pattern in detached leaves was similar to the pattern obtained in isolated cells, cellular absorption appears to represent the major barrier in the foliar absorption of glyphosate by hemp dogbane.

The competitive effects of hemp sesbania [Sesbania exaltata (Raf.) Cory] on soybeans [Glycine max (L.) Merr. ‘Forrest’] were studied on Sharkey clay for 2 yr. In full-season competition, hemp sesbania populations of 1,600, 3,200, 3,900, and 5,500 plants/ha did not reduce adjusted soybean yields, but populations of 8,100 to 129,200 plants/ha reduced yields 10 to 80%. Competition by hemp sesbania at 68,000 plants/ha for 1 to 4 weeks after soybean emergence reduced soybean yields 8% or less, whereas competition by the same population for 6, 8, and 10 weeks after soybean emergence reduced adjusted yields 18, 27, and 43%, respectively. Hemp sesbania populations of 3,200 plants/ha or more reduced the grade of harvested soybeans and populations of 5,500 plants/ha increased the level of foreign material found in seed samples taken at harvest. Hemp sesbania populations above 10,700 plants/ha increased the levels of damaged kernels and moisture in harvested soybean seed. Early-season control was required for highest soybean yields and total returns. The most critical period for control was 4 to 10 weeks after emergence of soybeans.

Mathematical growth analysis techniques were used to study the effects of simulated field temperatures and chilling events on the growth of the exotic noxious weed itchgrass (Rottboellia exaltata L. f.) and adapted varieties of corn (Zea mays L. ‘DeKalb XL43’ and ‘DeKalb XL395′), and soybean [Glycine max (L.) Merr. ‘Corsoy’, ‘Williams', and ‘Tracy’] in climate-controlled greenhouses and growth chambers. Itchgrass grew vigorously and produced seed in temperature regimes simulating the warmest 4 months of the growing season in the Gulf Coast states (30/25 C, day/night), the central Midwest (27/21 C), and the northern Midwest (23/18 C). At 30/25 C day/night, itchgrass produced 707 g of dry matter and 1907 seed per plant after 119 days. Itchgrass produced 621 g dry matter and 1429 seed at 27/21 C and 499 g dry matter and 1160 seed at 23/18 C. Itchgrass and adapted varieties of corn and/or soybeans were grown in four additional temperature regimes simulating the first 5 weeks of the growing seasons for corn and soybean at Madison, Wisconsin (19/11 C day/night); for soybean at Carbondale, Illinois (24/20 C); for corn at Waycross, Georgia (20/14 C); and for soybean at Baton Rouge, Louisiana (27/23 C). After 24 days of growth, half the plants in each regime were exposed to 3 days of chilling (11/4 C for Madison, 17/10 C for Carbondale, 15/6 C for Waycross, and 24/18 C for Baton Rouge) and returned to the original growth regimes for recovery. The chilling treatments reduced dry matter production, net assimilation rate, and leaf area duration more in itchgrass than in corn or soybean. After recovery, the previously chilled itchgrass plants had greater reductions in height, leaf area, and dry weight, compared to unchilled controls, than did corn or soybean. Weed/crop ratios in height, leaf area, and dry weight for itchgrass/corn were significantly reduced by chilling. In the two cooler regimes, weed/crop ratios in leaf area and dry weight for itchgrass/soybean were significantly reduced by chilling. Because of its sensitivity to cool temperatures, itchgrass, is unlikely to be an important early-season competitor with corn or with soybean outside the South.

The effects of shade on the growth and photosynthetic capacity of the exotic noxious weed itchgrass (Rottboellia exaltata L. f.) were determined under controlled environment conditions. The plants were grown at day/night temperatures of 29/23 C under 100, 60, 25, and 2% sunlight in a climate-controlled greenhouse. Mathematical growth analysis techniques were used to evaluate the effects of shading on dry matter production and leaf area production. Infrared gas analysis and diffusion porometry techniques were used to evaluate the effects of shading on photosynthesis and stomatal resistance. Shading markedly reduced dry matter production. At 40 days after planting, plants grown in 2, 25, and 60% sunlight had 0.3, 16, and 55%, respectively, of the dry weight of the plants grown at 100% sunlight. Leaf area production was less severely retarded by shading; the plants grown at 2, 25, and 60% sunlight had, respectively, 1.7, 42, and 99% of the leaf area of the plants grown at 100% sunlight. Ambient photosynthetic rates of recently expanded, single, fully exposed leaves were 22.5, 51.6, and 65.5 mg CO2 dm-2h-1 in the 25, 60, and 100% sunlight treatments, respectively. Photosynthetic rates at saturating irradiance did not differ significantly in plants grown at 25, 60, or 100% sunlight and ranged from 76.4 to 78.0 mg CO2 dm-2h-1. Stomatal resistances, ranging from 6.0 to 7.5 s cm-1, also did not differ significantly among these plants. In terms of dry matter production, itchgrass is a shade-intolerant plant. However, even when grown in shade, itchgrass maintains the capacity for high photosynthetic rates and high growth rates when subsequently exposed to high irradiance. These characteristics help explain its competitiveness with crop species.

Seed and tubers and dwarf spikerush [Eleocharis coloradoensis (Britt.) Gilly] survived extremely cold and hot temperatures if damp-dried before exposure. Damp-dried seed survived 14 days exposure to −196 C and 21 days exposure to 72 C, and then germinated 59 and 24%, respectively. Seed submersed in water germinated 20% when exposed to −15 C for 30 days and 4% when exposed to 60 C for 3 days. When damp-dried seed were warmed at 37 C for 21 days the germination was nearly twice that for wet seed treated similarly. Tubers that were damp-dried before exposure survived −196 C for 14 days and 49 C for 21 days and then germinated 20 and 49%, respectively. Tubers that were exposed wet to hot or cold temperatures were killed at less extreme temperatures than seed. None of the tubers exposed wet survivied −15 C for 30 days or 49 C for 3 days.

The double pot technique was used to study the effects of uptake site of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl—phenyl]-4(1H)-pyridinone} on early shoot and root development of soybean [Glycine max (L.) Merr. ‘Williams’] and cotton (Gossypium hirsutum L. ‘Acala SJ-1′). Soybean is susceptible and cotton is resistant to fluridone. Root uptake in soybean resulted in increased shoot length in the epicotyl and shoot apex, reduced shoot fresh weight, chlorosis of primary and trifoliolate leaves, as well as reduced root length, branching, and fresh weight. Shoot uptake resulted in reduced shoot length above the hypocotyl, reduced shoot weight, severe chlorosis of primary leaves, and inhibition of trifoliolate leaf formation. The reduction in the root length, branching, and weight was less by shoot uptake than by root uptake. In cotton, root uptake resulted in reduced shoot length in the epicotyl and shoot apex, slight reduction in shoot weight, no effect on chlorosis of cotyledon leaves, moderate to slight chlorosis of the true leaves, and no effect on root length, branching, or fresh weight. Shoot uptake resulted in reduced length of all shoot components and shoot weight, chlorosis of cotyledon leaves and the true leaves, and reduced root length, branching, and weight. Compared to soybean, cotton required much higher fluridone concentrations to show the above effects. It was generally concluded that in soybean, uptake by both the root and the shoot contributed significantly to the injury, whereas in cotton, shoot uptake was relatively more important.

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.

Fall applications of 0.14 kg/ha of picloram (4-amino-3,5,6-trichloropicolinic acid) to musk thistle (Carduus nutans L.) rosettes controlled musk thistle stems better and more consistently than fall applications of 2.2 kg/ha of 2,4-D amine [(2,4-dichlorophenoxy)acetic acid], 0.6 kg/ha of dicamba (3,6-dichloro-o-anisic acid), or a mixture of 0.3 kg/ha dicamba + 1.1 kg/ha of 2,4-D. Dry conditions in 1975 and cool conditions in November, 1976, hampered control with all herbicides except picloram. In spring applications when growing conditions were favorable, herbicide differences were usually not significant. For all herbicides, treatment of bolted stems did not control musk thistle satisfactorily. Picloram was superior for reducing stems 12 to 18 months after application in 2 of 3 yr. Residual control resulted principally because the herbicide killed many seedlings and first-year rosettes at the time of application.

Disk harrowing, spring tooth harrowing, and bed conditioning, alone or followed by bedding the rows, were evaluated in field studies to determine their efficacy to soil incorporate trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) plus metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5 (4H)-one] in soybeans [Glycine max (L.) Merr.]. Effects of the time interval between herbicide application and soybean planting were also evaluated. Effective weed control and adequate crop selectivity were obtained from all incorporation methods except those followed by bedding. Bedding occasionally reduced weed control and soybean stands as compared to incorporation without bedding. Excessive crop injury resulted from the application of a band of the herbicide mixture as a subsurface layer. The herbicide mixture was applied up to 6 weeks before soybean planting without adversely affecting crop or weed selectivity when incorporated with the spring tooth harrow without bedding.

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.

Sublethal doses of glyphosate [N-(phosphonomethyl)glycine] applied in a 50-μl drop in the partially furled third true leaf of 30-day-old sorghum [Sorghum bicolor (L.) Moench ‘Tophand’] seedlings induced basal stem swelling and bud release. Combining glyphosate with cycloheximide {3-[2-(3,5-dimethyl-2-oxycyclohexyl)-2-hydroxyethyl]glutarimide}, a cytokinin [N6 (2-isopentenyl)adenine], or L-phenylalanine significantly reduced the incidence of basal stem swelling. No such reductions was observed when indole-3-acetic acid or L-tyrosine was combined with glyphosate. The fresh weights of the seedlings given the combination treatments reflected the effects of glyphosate, except one combination that included tyrosine significantly reduced fresh weight and three combinations that included phenylalanine significantly increased fresh weight.

Definitions and properties of light, the effects of light on plant growth and physiological processes, and methodology and terminology for the measurement and reporting of light are reviewed. Alternatives to photometric methods now generally used in studies of the effects of light on plants in weed research are proposed. For studies of leaf energy budgets and plant water relations, the irradiance may be appropriately measured with radiometric methods and the data reported as W [watts] m-2. For studies of photosynthesis, the photosynthetic photon flux density may be measured and the data reported as μE [einsteins] m-2 s-1 or μmol [μmoles of photons] m-2 s-1. Different types of instruments are described and sources for these instruments are provided.

Seed of johnsongrass [Sorghum halepense (L.) Pers.] were planted during 1976 and 1977 at monthly intervals from March through October to study the growth and reproductive habits of this weed at Shafter, California. Plants were grown for 12 weeks and harvested at 3-week intervals. Emergence and development of plants increased with temperature. April through August plantings yielded 10-to 20-fold as much total fresh weight as March and September plantings. After planting, rhizome production normally began in 3 to 6 weeks and seed production in 6 to 9 weeks. From the April through August plantings, rhizomes accounted for averages of 13, 25, and 40% of the total fresh weight of plants 6, 9, and 12 weeks after planting, respectively. This compared, respectively, with 16, 9, and 6% for roots and 70, 66, and 54% for shoots. Seedlings emerging in March produced rhizomes in early June and viable seed in late June. Seedlings emerging as late as September produced rhizomes and some viable seed before killing frosts in November.