Effects of environmental factors on germination and emergence of hairy beggarticks were examined in laboratory and greenhouse studies. Optimum temperature range for germination of hairy beggarticks was 25/20 to 35/30 C (day/night, 12/12 h). Germination decreased above or below this range. Temperatures below 15/10 C and above 45/40 C were unfavorable for germination. Hairy beggarticks seed can germinate under both a 12-h photoperiod and a 24-h dark regime. Seed germinated 78 to 90% in buffer solutions of pH 4 to 9. Radicle growth was more sensitive to extreme pH than germination. Osmotic stress up to −0.1 MPa had little effect on germination, but less than 3% of the seed germinated at an osmotic stress of −0.75 MPa. Hairy beggarticks seed (13%) germinated at NaCl concentration of 100 mM but failed to germinate at 200 mM NaCl. Maximum emergence occurred when seed were planted less than 1 cm deep. No seedlings emerged when planted 10 cm deep. Flooding even for a day following planting decreased emergence to 25% compared to no flooding (56%). Seedling emergence decreased sharply with a further increase in duration of flooding, and no seedlings emerged when flooding was maintained up to 28 d after planting.

Greenhouse and laboratory studies were conducted to determine the extent of cross-resistance of chlorsulfuron-resistant sugarbeet (CR1-B) to other herbicides that inhibit acetolactate synthase (ALS) and to determine the physiological basis of resistance. Cross-resistance to metsulfuron, imazaquin, and imazethapyr was not evident, while only marginal cross-resistance was observed to triasulfuron, DPX-L5300, and nicosulfuron. CR1-B was moderately resistant to chlorsulfuron and chlorimuron and was highly cross-resistant to thifensulfuron and primisulfuron. Further greenhouse studies demonstrated that CR1-B was not significantly injured by thifensulfuron and primisulfuron applied at or exceeding the field use rate. Studies with 14C-primisulfuron showed that differential absorption or metabolism of primisulfuron could not account for the observed resistance. ALS enzyme assays showed that the CR1-B ALS enzyme activity was 66, 26, and 13 times less sensitive to chlorsulfuron, thifensulfuron, and primisulfuron inhibition, respectively, compared to ALS enzyme extracted from sensitive sugarbeets. An altered ALS enzyme, which is less sensitive to sulfonylurea herbicide inhibition, appears to be the physiological basis of resistance.

A laboratory bioassay showed that inhibitors of gibberellin synthesis (flurprimidol, paclobutrazol, and uniconazole) reduced plant height but did not affect physiological parameters such as photosynthesis, respiration, and chlorophyll content in two weedy submersed aquatic plants, hydrilla and Eurasian watermilfoil. Eurasian watermilfoil was sensitive to these compounds at concentrations as low as 0.75 μg L−1. Hydrilla sensitivity was in the range of 75 to 750 μg L−1. The three compounds reduced main and lateral stem lengths in hydrilla; however, at 75 μg L−1 the number of lateral stems and roots was greatly increased over untreated controls resulting in a stoloniferous growth habit. Dominant growth form of treated Eurasian watermilfoil was a single shortened stem with numerous compacted buds. Photosynthesis, respiration, and chlorophyll content were not affected in either plant at nontoxic dosages in which stem reduction was obtained. Both plants required only a 24-h exposure to maintain main stem length reduction for 6 wk after transfer to untreated medium. Our results indicate that these gibberellin synthesis inhibitors would be effective in reducing plant height in aquatic systems.

Experiments were conducted to study absorption, translocation, and activity of clopyralid and tribenuron in perennial sowthistle. Absorption and translocation were compared in perennial sowthistle at the rosette and bud stage over a period of 9 d. Both 14C-clopyralid and tribenuron were absorbed slowly by perennial sowthistle leaves with 60 and 30% absorption, respectively, 216 h after application. Limited movement of the absorbed 14C was observed for both herbicides with less than 28% of either herbicide exported from the treated leaf. Primary movement of 14C-herbicide within the plant was acropetal; however, no more than 18% of either herbicide was recovered in the upper shoot or buds. After 216 h, 4.4% of the applied 14C-clopyralid and 2.5% of the applied 14C-tribenuron were recovered in the roots and secondary shoots of plants at the rosette stage. Application of either herbicide to perennial sowthistle resulted in a decrease in net carbon assimilation and leaf conductance. Transpiration was reduced in rosette stage plants by both herbicides; however, transpiration in plants at the bud stage was not reduced until 216 h after treatment. Plants treated with tribenuron had lower net carbon assimilation, leaf conductance, and transpiration rates than plants treated with clopyralid. Application of clopyralid at 150 and 300 g ha-1 resulted in visual injury ratings of 65 and 95%, respectively. Tribenuron applied at 10 and 20 g ha-1 gave similar results. Dry weight accumulation was reduced at all application rates compared to untreated plants. Limited absorption and translocation along with high herbicidal activity suggests that small amounts of either herbicide are sufficient to inhibit perennial sowthistle growth.

Research was conducted during 1988 and 1989 in Georgia and Florida to determine the densitydependent effects and critical periods of wild poinsettia interference with peanut. Peanut yield loss ranged from 0 to approximately 50% over the wild poinsettia density range from 0 to 32 plants 9 m−1 peanut row. Predicted peanut yield losses across 2 yr in Georgia were 0, 4, 8, 12, 15, 26, 40, and 54% for season-long wild poinsettia interference at densities of 0, 1, 2, 3, 4, 8, 16, and 32 plants 9 m−1 of row, respectively. Across 2 yr in Florida, predicted peanut yield losses were 0, 9, 14, 22, 30, 37, and 41% for wild poinsettia densities of 0, 1, 2, 4, 8, 16, and 32 plants 9 m−1 row, respectively. Peanuts had to be maintained poinsettia free for 10 wk after peanut emergence to prevent yield loss. Wild poinsettia that interfered with peanut for more than 2 wk after peanut emergence reduced peanut yield.

Induction of germination in dormant barnyardgrass seeds by wounding was investigated. Previous research indicated that a volatile compound was emitted during imbibition of wounded caryopses. When wounded caryopses were submerged in agar, total germination and speed of germination were stimulated, and the stimulation was dependent upon the concentration of agar. A twofold increase in germination occurred in 1% agar versus water, and a fivefold increase with caryopses placed in 5% agar. When wounded caryopses were imbibed, there was a fourfold increase in respiration over that of intact caryopses after 1 h. This increased rate of respiration of wounded caryopses continued for 7 h, while there was a gradual increase in respiration of intact caryopses. Carbon dioxide induced intact dormant caryopses to germinate but was not effective in stimulating germination of the seed (spikelet). High levels of abscisic acid found in the hulls of dormant seeds may have prevented the action of carbon dioxide. These results suggest that increased respiration resulting from wounding provides elevated levels of carbon dioxide in the microenvironment of the seed, thus stimulating germination. Removal of the hulls is necessary for germination even in high levels of carbon dioxide.

This work was initiated to develop a method of sampling quackgrass populations. Shoot and bud distribution of 36 field populations was obtained. The results demonstrated that shoot count was inappropriate in assessing quackgrass bud populations and that bud sampling depth should be at least 20 cm if an accurate estimate is required. When shoot count is the only possibility, quadrat size should be as large as possible to minimize the error. Descriptive statistics of the field populations were used to generate a series of 24 simulated quackgrass bud populations. The field and simulated data were computer sampled with a program designed to compare the effects of varying sample size (number of quadrats) and sample shape (quadrat size and form). The intensity of sampling must be a function of resources and cost, since sampling precision increased linearly with sample size. Precision of sampling was generally higher when a large number of small quadrats were used compared to a small number of large quadrats. Sampling intensity per unit area must be increased when bud density increases to maintain a given degree of precision, but it can be decreased when plot size increases.

Sethoxydim reacts spontaneously with water resulting in immediate structural changes. Simulation of field conditions of light, moisture, oxygen, pH, and soil and evaporation on siliceous surfaces duplicated this lability. Sethoxydim degradation was enhanced by alkaline conditions, ultraviolet and incandescent light, and adsorption on solid surfaces. No sethoxydim was detected immediately after application to moist soil. Less than 2% extractable sethoxydim was present in dry soil after 24 h.

Different growth rates of young seedlings (genets) and plants grown from root pieces (ramets) of yellow toadflax could influence their respective competitive ability and their susceptibility to management techniques. Shoot production was similar for genets and ramets (approximately 10 shoots were produced 12 or 13 wk after transplanting or cotyledon appearance, respectively), but the rate of shoot biomass accumulation was higher for genets than for ramets. Genets consistently produced more underground shoots than ramets. Replanted underground shoots separated from their roots were able to produce new shoots and roots. Rate of elongation for roots 0.5 to 1.5 mm in diameter was higher for ramets than for genets, but their shoot production potential was the same. Root pieces from genets did not have the ability to produce daughter shoots until 3 wk after cotyledon appearance. This indicates that very young genets would be more susceptible than older genets or ramets to management control systems.

Sorghum cultivars were compared for their root development and production of the witchweed germination stimulant sorgoleone. In pot experiments, the resistant cultivar IS-7777 yielded a greater root biomass and root length, but a smaller root length/biomass ratio than the highly susceptible cultivars CK-60B and Bimbiri. Seedlings of these latter two cultivars exuded a high concentration of sorgoleone, while the low-susceptible cultivar IS-14825 exhibited a low production of this compound. In contrast, the low-susceptible cultivar Seguetana Niarabougou exhibited a high production of sorgoleone, while the resistant cultivars IS-7777 and Framida exuded a moderate amount of this compound. These results indicate that witchweed seed avoidance by means of reduced root growth is unlikely to be an important factor involved in the resistance of IS-7777. Low production of sorgoleone could partly explain low susceptibility of IS-14825, but other compounds may play a more significant role in the stimulation of witchweed germination.

A study was conducted under controlled environmental conditions to determine the effect of soil temperature, soil moisture, and depth of seed burial on the emergence of redstem filaree. Emergence occurred from 5 to 30 C but was optimal at 5 to 15 C. Redstem filaree emergence was optimal in moist soil with water contents of −0.03 to −0.28 MPa. Emergence progressively decreased as soil moisture was lowered below these levels, with less than 25% emergence attained at −1.53 MPa regardless of soil temperature. Almost no germination was observed in warm (30 C), dry (-1.03 to −1.53 MPa) soils. Rate of emergence was affected more by soil temperature than by moisture. A decrease in temperature from 20 to 5 C increased the time to reach 50% emergence by 6 to 8 d but a decrease in soil moisture from −0.03 to −1.53 MPa increased the time to reach 50% emergence by only 1 to 2 d. Emergence was greatest at soil depths of 1 cm or less. No emergence occurred at depths of 8 cm or below.

Foliar absorption of picloram by broom snakeweed, a rangeland shrub, was investigated. Picloram uptake into leaf, axillary bud, and stem tissues was similar. In addition, picloram uptake by leaf tissue from greenhouse- and field-grown broom snakeweed did not differ. Picloram accumulated rapidly and absorption saturated between 15 min and 1 h of application; no further absorption occurred through 72 h with maximum uptake ca. 15% of applied picloram. Picloram content increased linearly with increasing external picloram concentration, implying that movement of the herbicide across the cuticle is via diffusion. Absorption was dependent on relative humidity and temperature with the greatest uptake at 94% relative humidity and 35 C, respectively. Absorption was pH dependent; picloram absorption was greatest at pH 4 and least at pH 8. In addition, picloram absorption was less at pH 3 compared to pH 4. These results provide evidence that picloram is absorbed across the cuticle via simple diffusion and absorption is dependent on environmental conditions and solution pH at and following application.

Research was conducted in soybean and common cocklebur to determine if the synergistic interactions of clomazone plus metribuzin and clomazone plus linuron were due to the effect of one herbicide on root uptake, partitioning, or metabolism of the other. Treatments consisted of 14C-clomazone alone and combined with metribuzin or linuron, 14C-metribuzin alone and combined with clomazone, and 14C-linuron alone and combined with clomazone. Root uptake and partitioning of clomazone applied alone in soybean differed compared to clomazone plus metribuzin. Root uptake and partitioning of metribuzin or linuron did not differ when applied alone or combined with clomazone. Binding of clomazone or its metabolites in an unextractable form may be a method of deactivating clomazone by soybean, but not by common cocklebur. Levels of parent clomazone were higher in common cocklebur roots when clomazone was combined with metribuzin and linuron compared to clomazone done. Levels of parent metribuzin were higher in soybean roots, and in common cocklebur roots and shoots when clomazone was combined with metribuzin compared to metribuzin alone. Levels of parent linuron were greater in soybean shoots when linuron was applied with clomazone compared to linuron alone. These results indicate that the metabolism of metribuzin and linuron is altered in both species when clomazone is applied, leading to increased phytotoxicity.

Dormant line M73 wild oat caryopses were utilized to develop a system for the culture of excised embryos, to evaluate whether embryo dormancy exists, and to investigate the physiological basis for breaking dormancy. Dormant embryos cultured on N6 medium solidified with 0.25% Bacto agar displayed 70% germination in 2 d compared with approximately 20% for the other gelling agents. The non-plant-based gelling agent gellum at a concentration of 0.25% was selected for further experiments on breaking dormancy after it was determined that concentrations ≥ 0.5% decreased the rate of germination. Amending N6 medium with concentrations of 0.1 to 10 μM gibberellic acid (GA) increased the rate and extent of germination. Embryos treated with 0 to 0.01 μM GA required 6 d to attain 90% germination. Germination of dormant embryos on N6 medium without GA suggested that either true embryo dormancy did not exist in M73 or some constituent of the N6 medium promoted breaking of dormancy. Subsequent experiments indicated that the 88 mM sucrose was the constituent in the N6 medium responsible for breaking dormancy. Concentrations of sucrose from 40 to 200 mM were effective in breaking dormancy. Ten μM GA increased the rate and extent of germination of embryos cultured with 88 to 200 mM sucrose. At 88 mM, fructose, maltose, glucose, and sucrose all broke embryo dormancy. Fructose was the most active soluble sugar for breaking embryo dormancy, promoting nearly 100% germination in 4 d. As with sucrose, there was an interaction between GA and the soluble sugars in breaking dormancy. Ten μM GA with 88 mM fructose provided nearly 100% germination in 1 d. Amylose, but not amylopectin or pullulan, may substitute for soluble sugars. However, with 10 μM GA amylose, amylopectin and pullulan were equally effective in breaking dormancy. Breaking dormancy of embryos on N6 medium was independent of temperatures from 12 to 24 C in the presence of GA, but in its absence the optimum was 12 C. Application of GA to dormant caryopses significantly increased and decreased the level of glucose and sucrose, respectively, in the embryo. Gibberellic acid had a similar effect on glucose and sucrose in the endosperm tissue, except the differences were not significant at all times after treatment. The change in carbohydrate metabolism, especially in embryo tissue, may be important when considered in context with the observation that soluble sugars and GA act independently in breaking dormancy in excised M73 embryos. Breaking wild oat embryo dormancy with GA may be mainly a substitution for sugar requirement.

Autumn-planted sugarcane was seeded with itchgrass in early March to give 1 plant per 30.5 cm of row length (1.8 plants m−2). In three of four experiments, itchgrass was allowed to reseed and develop naturally in the second or third-yr ratoon crops. In the first-yr crop, itchgrass stands developed by May 15 and itchgrass was removed after 30, 60, or 180 d of interference. After 30 d interference, on about June 15, itchgrass biomass ranged from 200 to 2700 kg ha−1 in four experiments and reduction in sugar yield averaged 7%. After 60 d interference, on July 15, the biomass ranged from 1400 to 2900 kg ha−1 in two experiments and reduction in sugar yield averaged 17%. Itchgrass removed at harvest in November, after 180 d interference, reduced sugar yield by 19% in four experiments. Itchgrass infestations removed in the second-yr crop on May 1, June 1, and November 15 harvest reduced sugar yield in one experiment by 3, 11, and 72%, respectively, compared to a 2-yr weed-free control. Itchgrass interference reduced sugar yield primarily by reducing stalk population even though full-season interference increased sugar concentration of juice by 2 to 10%. These studies indicate that itchgrass must be removed from sugarcane well before June 15 (30 d of interference) under Louisiana growing conditions to prevent a rapid increase in itchgrass biomass and loss in sugar yield. However, sugarcane stands and yield recovered almost completely when maintained weed free in the second-yr crop following full-season itchgrass interference in the first-yr crop.

Broadleaf weeds apparently have patchy distributions within a field while POST control decisions are made assuming a regular spatial distribution. As a result, yield loss from weed competition may be overestimated, possibly leading to mistakes in choosing the optimal control treatment. Data on distribution of broadleaf weeds in 14 soybean fields were used in simulation experiments to investigate the potential for improving decision making with information about weed patchiness. The feasibility of modeling weed distribution in individual fields was also examined. Overall, the cost of assuming a regular distribution when making POST decisions was found to be low. Errors that occurred most often involved recommending more intensive control than was actually required, although in a few cases less intensive control was recommended. Error in the yield loss estimated for the uncontrolled population did not indicate the potential for a mistake in decision making for a field. Accurately modeling distribution of weeds within fields may be difficult as a result of correlations between distributions of individual species within a field and variation in distributions between fields.

Three wild oats phenotypes were grown in wheat stands sown at different dates in greenhouse and field trials. Wild oats growth and seed output, and their effects on wheat biomass were not different among phenotypes when wild oats emerged 2 wk after the wheat. In experiments in which wild oats were planted in germinated wheat, one phenotype was shorter, weighed less, and produced fewer seed than the other phenotypes. Another phenotype reduced wheat biomass more than the other phenotypes. Vernalization increased vegetative growth and reduced spikelet production of one phenotype, but had no effect on its competitiveness with wheat.

Spatial distribution of broadleaf weeds within 14 North Carolina soybean fields was characterized by fitting negative binomial distributions to frequency distributions of weed counts in each field. In most cases, the data could be represented by a negative binomial distribution. Estimated values of the parameter K of this distribution were small, often less than one, indicating a high degree of patchiness. The data also indicated that the population as a whole was patchy. Counts of individual species were positively correlated with each other in some fields and total weed count could be represented by a negative binomial for 12 of the 14 fields.

Fourteen tomato genotypes, eastern black nightshade, and 35 Lycopersicon accessions were screened for tolerance to acifluorfen. Tolerant and susceptible genotypes occurred in most species. Fifteen genotypes were chosen for further study depending on their fresh weight after acifluorfen treatment Over all 15 genotypes, there was no correlation between trichome density and acifluorfen tolerance; however, in L. esculentum, cultivars with the most trichomes were the most tolerant There was an inverse relationship between stomata density and tolerance. Amount and composition of epicuticular wax and cuticle thickness did not correlate to acifluorfen tolerance.

Sicyos deppei G. Don (Cucurbitaceae) is an annual weed that disperses dormant seeds. The influence of scarification and light on their germination was studied in the laboratory. Scarified seeds collected in 1987, 6, 7, 8, and 9 mo postharvest, gave high germination both in white light (89 to 100%) and darkness (64 to 96%), in comparison with nonscarified seeds which germinated 7 to 20% in both conditions. Scarified seeds collected in 1989, 3 mo postharvest, germinated higher (52 to 69%) than nonscarified seeds (0 to 5%). Exposure to light did not break dormancy of nonscarified seeds. The reversal of dormancy by scarification describes them as hard seeds. Scarified seeds, 3 mo postharvest, were light indifferent, germinating even in the usually inhibitory condition of far-red light. However, a low proportion of scarified seeds, 6 and 7 mo postharvest, presented a positive photoblastic response and germinated 23 and 36%, respectively.