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.

Ceutorhynchus litura dispersed 9 km in 15 yr from its point of release in 1972 with incident of infestation of Canada thistle plants at one study site reaching 92% in 1987. Plants generally survived larval feeding in the spring, but holes created by departing larvae provided entrances for arthropods, nematodes, and diseases. Subsequent mortality rates were high for belowground Canada thistle shoots. However, the number of new shoots from underground roots the following spring usually offset the mortality that had been indirectly caused by C. litura.

The interaction of reduced rates of bentazon, chlorimuron, imazaquin, and imazethapyr with cultivation for broadleaf weed control in soybeans was investigated in field experiments conducted at three sites in Missouri in 1987 and 1988. Single reduced-rate herbicide applications provided soybean yields equal to full rates although visual weed control was slightly lower. Sequential applications of all four herbicides at reduced rates provided weed control and soybean yields equal to full-rate applications. The number of velvetleaf plants m−2 and seeds plant−1 were not influenced by herbicide, herbicide rate, or application timing. Cultivation improved weed control and soybean yield and decreased late-season weed populations and seed production.

Seeds of eight weed species were heated for up to 7 days at 40, 50, 60, and 70 C in dry (2% moisture) and moist (19% moisture) Bosket very fine sandy loam to determine temperature-time treatments lethal to weed seeds. Seeds in dry soil were very tolerant to 60 C or less for up to 7 days but most seeds were killed at 70 C after 7 days. In moist soil, a few (1 to 12%) seeds of common purslane, redroot pigweed, johnsongrass, and spurred anoda survived for up to 3 days at 70 C. Some (4 to 30%) seeds of velvetleaf, pitted morningglory, and the above species survived up to 7 days at 60 C. No seeds of prickly sida and common cocklebur survived more than one-half day at 60 C. Three days at 50 C were lethal to all cocklebur seeds. Many of the sublethal treatments promoted germination of prickly sida, velvetleaf, spurred anoda, and pitted morningglory, presumably because high temperature broke dormancy of some hard seeds. The enhanced effect of heat treatments upon seeds in moist soil compared to seeds in dry soil was due to the higher moisture content of seeds, or in fruit tissues of dispersal units in moist soil. Because some seeds survived several days in moist soil at 60 and 70 C, it is unlikely that soil solarization or other natural methods of raising soil temperature will eliminate weed seeds from the field. However, high soil temperatures may reduce weed seed populations by killing heat-susceptible seeds and by breaking dormancy of hard seeds followed by thermal kill of seedlings.

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.

Chlorsulfuron was preemergence applied in field trials at La Coruña, Spain, at rates from 5 to 30 g ai ha−1 (1987–88) and from 5 to 100 g ai ha−1 (1988–89) for weed control in winter and spring wheat. Excellent control of broadleaf weeds was obtained in both; however, narrowleaf control at the last evaluation (heading) was poor. Chlorsulfuron at 30 g ha−1 did not cause a residual problem in 1987–88, with only 0.24 μg kg−1 of dry soil recovered after both a winter and spring wheat cropping season. However, chlorsulfuron persistence at the same rate was very high in the dry year 1988–89, with 0.43 μg kg−1 and 0.53 μg kg−1 recovered in winter and spring wheat, respectively. ‘Cardeno’ spring wheat showed no phytotoxic symptom at any rate of chlorsulfuron from 5 to 100 g ha−1. Its yield and yield components (spikes m−2, grain number spike−1, grain weight) were greater with all chlorsulfuron rates, but not in ‘Fiel’ winter wheat for rates of 30 g ha−1 or higher.

Bioeconomic sugarbeet weed management models for preplant, postemergence, and layby herbicides, and late-season handweeding decisions are presented. The personal computer/spreadsheet models are based on number of weed seed in soil, field survey of weed populations, growth stages of weeds and sugarbeets, expected yield loss from weeds, herbicide weed control, weed control cost, and sugarbeet price. The models incorporate two producer risk levels. Several weed scenarios were used to verify the models for reasonable recommendations.

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.

The influence of various factors on the tolerance of corn and selected weed species to thifensulfuron was studied in greenhouse experiments. Corn fresh weight was reduced by postemergence application of thifensulfuron when applied at 180 g ai ha−1 without surfactant or at 18 g ha−1 with a nonionic surfactant. Corn was more susceptible to root exposure whereas velvetleaf was more susceptible to foliar exposure. Velvetleaf was most susceptible when plants were young, when a nonionic surfactant was added, and at 20 rather than 30 C. Simulated rainfall 8 h after application reduced velvetleaf injury by thifensulfuron at 18 g ha−1 without surfactant; however, with addition of a nonionic surfactant, velvetleaf injury was reduced only if rainfall occurred less than 2 h after application. Soil moisture level did not affect velvetleaf susceptibility.

The performance of four sequential weed management models that assumed either low or high risk was compared to the performance of two sugarbeet consultants, one who assumed low risk and the other high risk. Weed management recommendations were performed over one growing season at two locations, each with several levels of weed populations. Recommendations for preplant, postemergence, and layby herbicide treatments or late-season handweeding differed among the four weed management levels. The high-risk management level was labor intensive and the low-risk management level was herbicide intensive. Weed populations at harvest, recoverable sucrose, and net return above weed control costs were not different among the four weed management levels. Weeds can be controlled in sugarbeets by employing weed management practices based on bioeconomic modeling.

Common ragweed was the most difficult to control of the species present and its control appeared to have the greatest effect on peanut yield. Preplant-incorporated (PPI) treatments provided no control of common ragweed but ethalfluralin plus vernolate PPI provided better than 90% control of yellow nutsedge and nearly 50% control of morningglory species. Single applications of paraquat at 0.14 kg ai ha−1 following PPI applications of ethalfluralin or ethalfluralin plus vernolate provided less than 75% common ragweed control. Sequential applications of paraquat applied 1 and 3 weeks after peanut emergence (1 + 3 WAE) provided at least 81% common ragweed control. Peanut yield with ethalfluralin plus vernolate PPI followed by paraquat 1 WAE (4400 kg ha−1) was equivalent to the handweeded yield (4470 kg ha−1). Yields were not significantly less with the same PPI application followed by paraquat 1 + 3 WAE (3730 kg ha−1) or by acifluorfen plus bentazon 3 WAE (3730 kg ha−1), and ethalfluralin PPI followed by paraquat 1 + 3 WAE (3740 kg ha−1). Ethalfluralin plus vernolate PPI and paraquat 1 WAE provided the highest net returns of $1370 ha−1.

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.

The arcsine and square root transformations were tested on 82 weed control data sets and 62 winter wheat winter survival data sets to determine effects on normality of the error terms, homogeneity of variance, and additivity of the model. Transformations appeared to correct deficiencies in these three parameters in the majority of data sets, but had adverse effects in certain other data sets. Performing the recommended transformation in conjunction with omitting treatments having identical replicate observations provided a high percentage of correction of non-normality, heterogeneity of variance, and nonadditivity. The arcsine transformation, not generally recommended for data sets having values from 0 to 20% or 80 to 100%, was as effective in correcting non-normality, heterogeneity of variance, and nonadditivity in these data sets as was the recommended square root transformation. A majority of data sets showed differences between transformed and nontransformed data in mean separations determined using LSD (0.05), although most of these differences were minor and had little effect on interpretation of results.

Greenhouse studies were conducted to determine the response of 200 corn hybrids and 29 inbreds to metolachlor applied at 4.5 kg ai ha−1. Both hybrids and inbreds varied in their response to the herbicide. The distribution of injury revealed a normal distribution curve with most of the hybrids having a midlevel of tolerance. Some hybrids were very tolerant, while others were quite sensitive. Laboratory studies were conducted to evaluate absorption and metabolism of 14C-metolachlor for a subset of tolerant and sensitive hybrids. There was no observed difference in the product of metolachlor metabolism in the tolerant and sensitive hybrids. The observed variability in metolachlor tolerance among hybrids appeared due to differences in the amount of metolachlor absorption and metabolism and differences at the site of metolachlor action. The tolerant ‘Great Lakes 584’ hybrid absorbed significantly less 14C-metolachlor than did the sensitive ‘Pioneer 3744’, while the tolerant ‘Cargill 7567’ metabolized significantly faster more 14C-metolachlor than the other hybrids. The internal concentrations of available 14C-metolachlor were the same for the tolerant Cargill 7567 and the sensitive ‘Northrup King 9283’ after 8 h, indicating differences at the site of action of metolachlor for these two hybrids.