Seven corn inbred lines previously shown to differ in response to soil residues of chlorsulfuron were characterized as to the target-enzyme acetolactate synthase (ALS) specific activity and to its susceptibility to the herbicide. ALS from plantlets at the five-leaf stage of growth was similarly susceptible to chlorsulfuron in all lines and its specific activity in the shoots was not significantly correlated with in vivo tolerance to the herbicide. By contrast, differences in ALS specific activity in roots of plants both at the five- and three-leaf stages of growth were significantly correlated (r = 0.96∗∗ and r = 0.93∗∗, respectively) with in vivo tolerance. Correlation was also noted in extracts from cultured excised root tips (r = 0.94∗∗). Callus tissue of a chlorsulfuron-tolerant line was less affected by the herbicide and had a significantly higher ALS specific activity than callus from a chlorsulfuron susceptible line, whereas inhibition of ALS by the herbicide was similar in both lines. These results indicate that the naturally occurring differences in ALS levels in the roots of the investigated inbred lines contribute largely to the differential in vivo response observed to chlorsulfuron.

Knowledge of how plants will partition their new biomass will aid in understanding competition between crops and weeds. This study determined if the amount of biomass partitioned to the root versus the shoot can be predicted from tissue carbon [C] and nitrogen [N] concentrations and the daily gain in C (GC) and N (GN) for each unit shoot and root biomass, respectively. Pots measuring 28 cm diameter and 60 cm deep were embedded in the ground, and each contained one plant of either corn or velvetleaf. Each plant received one of three nitrogen treatments: 0, 1, or 3 g of nitrogen applied as ammonium nitrate in 2001 and 0, 2, or 6 g of nitrogen in 2002. Measurements of total above- and belowground biomass and tissue [C] and [N] were made at 10 different sample dates during the growing season. Fraction of biomass partitioned to roots (Pr) was predicted from [C], [N], GC, and GN. Accurate prediction of the fraction of biomass partitioned to roots versus shoots was evaluated by comparing observed and predicted Pr across all treatments. The coordination model has potential as a reliable tool for predicting plant biomass partitioning. Normalized error values were close to zero for corn in 2001 and 2002 and for velvetleaf in 2001, indicating that biomass partitioning was correctly predicted.

Two herbicides, the alkanolamine salts of 2-sec-butyl-4,6-dinitrophenol (dinoseb) and 3-[p-(p-chlorophenoxy) phenyl]-1,1-dimethylurea (chloroxuron), were compared for their effects on soybean [Glycine max (L.) Merr., ‘Lee’] when applied topically. In initial studies, chloroxuron was damaging when applied twice to soybean and reduced yields in one of the two year's studies. In a third year's work, direct comparisons of topical applications of dinoseb and chloroxuron showed that soybean injury increased with increased rate of material at each soybean growth stage with the most severe initial damage in the two-trifoliate stage. Recovery was rapid with few significant yield decreases occurring. Chloroxuron was more effective in controlling the broadleaf weeds present. Both herbicides were most effective when weeds were in the seedling stage of growth.

The effect of glyphosate [N-(phosphonomethyl)glycine] on carbon dioxide. (CO2) levels, ethylene production, and cellulase activity was investigated. Production of ethylene increased within 12 h and CO2 increased within 24 h when 12-day-old bean plants (Phaseolus vulgaris L. ‘Red Kidney’) were treated with 20 mM isopropylamine salt of glyphosate. The CO2 cycled for 3 days and then increased around treated plants. Specific activity of cellulase was increased in debladed bean seedlings that had been pretreated with 20 mM isopropylamine salt of glyphosate. Cellulase enhancement was detected 2 days after the pretreated plants were debladed. Glyphosate-enhanced ethylene production may have increased the cellulase activity.

Competition between wet-seeded rice and barnyardgrass under two distinct environments was analyzed using a two-parameter response–surface model at the International Rice Research Institute in the Philippines. The findings showed that this model could predict crop yield loss due to weed competition over a wide range of crop and weed densities. The low-tillering, new plant–type cultivar was a weaker competitor and had a higher yield loss than high-tillering cultivar ‘IR72’ and a hybrid. Increasing the crop density could reduce yield loss due to weed competition. This effect was greater for the new plant type than for IR72 and the hybrid when barnyardgrass density was low. In contrast, this effect was less for the new plant type than for IR72 and the hybrid when the weed density was high. Competitiveness of the three rice cultivars was also affected by season. Crop yield loss was higher in the wet season than in the dry season.

The objective of this field research was to compare relative effectiveness and profitability of alternative herbicides applied to the same plots for four consecutive years for controlling and reducing dense Canada thistle infestations in no-till spring wheat. Chlorsulfuron at 30 g ai ha−1 plus nonionic surfactant or clopyralid plus 2,4-D at 70 plus 280 g ae ha−1, respectively, applied annually for 4 yr controlled Canada thistle and was more effective for gradually reducing Canada thistle stands than 2,4-D at 560 g ae ha−1, MCPA plus bromoxynil at 280 plus 280 g ae ha−1, or tribenuron at 20 g ai ha−1. Chlorsulfuron and clopyralid plus 2,4-D also controlled Canada thistle ≥ 90% earlier (by 2 yr) than other treatments. Stochastic dominance analysis, a form of economic analysis, predicted that either chlorsulfuron or clopyralid plus 2,4-D would be preferred by farmers to the untreated check, MCPA plus bromoxynil, or 2,4-D treatments. Chlorsulfuron also would be preferred to clopyralid plus 2,4-D by risk-neutral farmers, whereas clopyralid plus 2,4-D would be preferred to chlorsulfuron by highly risk-averse farmers, those who are most likely to pick only consistently effective herbicides.

Resistance to fluazifop-P and quizalofop-P, (aryloxyphenoxypropionates) and sethoxydim (cyclohexanedione) was identified in 2 populations of johnsongrass in both field and greenhouse studies. The cropping history (1983–1991) of the sites indicated 1 or more annual applications of a graminicide (primarily fluazifop-P) since the early 1980s. Under field conditions, control of resistant seedling and rhizome johnsongrass (R91F) with fluazifop-P, quizalofop-P, fenoxaprop-ethyl, and sethoxydim was less than 35%. Clethodim provided up to 80% control of R91F. Under greenhouse conditions, ratios (R/S) of the I50 values (amount of herbicide required to inhibit plant growth by 50%) of resistant (2 sites: R91F and R91S) to susceptible (S91H) seedling (20–30-cm height) plants were > 388 (fluazifop-P), > 15 (quizalofop-P), and from 2.3 (R91S) to 3.4 (R91F) (both sethoxydim). For rhizome (30–45 cm height) plants, the R/S ratios were > 388 (fluazifop-P), > 16 (quizalofop-P), and 2.8 (R91S) to 8.5 (R91F) (both sethoxydim). Labeled rates (in kg ai ha−1) of fluazifop-P (0.10 and 0.21), quizalofop-P (0.039 and 0.08), and sethoxydim (0.21 and 0.21) were applied on seedling and rhizome plants, respectively, and resulted in little or no control of resistant johnsongrass. Greenhouse studies indicated registered rates of clethodim (0.10 and 0.14 kg ai ha−1 for seedling and rhizome plants, respectively) effectively controlled the resistant populations, but tolerance was measured for both seedling and rhizome plants at sublethal doses (down to 0.007 and 0.009 kg ai ha−1, respectively), with I50 ratios ranging from 1.5 (R91S) to 2.1 (R91F) for seedling plants and 4.5 (R91S) to 4.8 (R91F) for rhizome plants. Control of resistant seedling and rhizome johnsongrass under field conditions was adequate with glyphosate at 0.84, glufosinate at 0.84, and sulfosate at 0.84 kg ai ha−1, indicating no cross-resistance.

Field experiments were conducted to evaluate postemergence combinations of imazethapyr, imazquin, or chlorimuron with low rates of imazapyr for Johnsongrass and pitted morningglory control. Imazapyr applied alone at rates up to 4 g ai/ha gave little or no control of either weed species. However, the addition of imazapyr to various rates of imazethapyr or imazaquin synergistically increased both johnsongrass and pitted morningglory control 8 weeks after treatment. The rates of imazethapyr or imazaquin required for significant enhancement of johnsongrass control were higher than those required for pitted morningglory control. No synergistic increases in control of either weed species were noted with mixtures of imazapyr and chlorimuron. Although not synergistic in every case, the mixtures of imazapyr at 4 g/ha with imazethapyr, imazaquin, or chlorimuron gave johnsongrass and pitted morningglory control equal to or better than the next higher rate of these herbicides applied alone. Imazapyr did not increase soybean injury or decrease yield provided by chlorimuron, imazaquin, or imazethapyr.

Weed seed rain was monitored in field plots under three fallow types and four land-use intensities in Ibadan, Nigeria, in 1994 and 1995. The fallow types were natural bush, planted Leucaena leucocephala, and Pueraria phaseoloides. The land-use intensities consisted of continuous cropping, involving Zea mays/Manihot esculenta and fallowing for 1, 2, and 3 yr, with each fallow period followed by 1 yr of Z. mays/M. esculenta cultivation. In 1994, seed rain in plots cropped after P. phaseoloides fallow was significantly lower than in plots cropped after bush or L. leucocephala fallow. Pueraria phaseoloides plots had similar seed rain as bush fallow plots in 1995, and the seed rain in these plots was significantly lower than in L. leucocephala plots. Weed seed rain was significantly higher in continuously cultivated plots across all fallow types than in plots that were cultivated after one or more years of fallow. The lowest seed rain was in plots that were cropped once after a 3-yr fallow. The largest quantity of weed seed input in the plots occurred in either August or September, reflecting the life cycle of the annual weeds that dominated the vegetation. Individual species differed in pattern and duration of shedding seeds within the fallow systems and land-use intensities. Annual weeds dominated the seed rain in continuously cropped plots, and seeds of perennial weeds were dominant in plots fallowed for more than 1 yr before cultivation. Weeds flowered earlier in continuously cropped plots than in plots that were cropped after 2 or 3 yr of fallow. Increased land-use intensity caused an increase in seed rain and consequently increased the soil seed bank. Pueraria phaseoloides fallow was more effective in shading weeds and probably reducing the quantity of light reaching them than the natural bush and planted L. leucocephala fallow systems, and this may have been the basis of the significantly lower seed rain in P. phaseoloides plots.

Disulfoton and phorate applied in the seed furrow greatly reduce clomazone phytotoxicity to cotton in the field, whereas aldicarb does not. An experiment was conducted to determine the effect of aldicarb, disulfoton, and phorate on 14C-clomazone absorption, translocation, and metabolism by cotton grown in a sandy loam soil. Clomazone at 0.87 μg g−1 of soil alone or in combination with aldicarb at 0.6 μg g−1 of soil reduced cotton root and shoot growth 26 to 33%. Root and shoot growth were not reduced by clomazone plus disulfoton or phorate at 0.6 μg g−1 of soil. Protection of cotton against injury by clomazone was not explained by reduced absorption or translocation of clomazone or a metabolite to the shoot. Clomazone metabolism was reduced by disulfoton and phorate, thus indicating a clomazone metabolite may be more toxic to cotton.

Due to known variation in canopy properties among sweet corn hybrids, weed suppressive ability (WSA), the crop's ability to reduce weed fitness, may not be uniform among hybrids. This hypothesis was tested using a range of wild proso millet densities subjected to four canopy treatments (three hybrids + weedy monoculture) under irrigated conditions in Washington and primarily rainfed conditions in Illinois. Parameter estimates for responses of weed growth and seed rain to wild proso millet density were used to quantify variation in WSA among hybrids. The same parameter estimates were used in a correlation analysis to identify associations between weed response and sweet corn canopy properties. Weed suppressive ability, as measured by wild proso millet shoot biomass and seed rain, varied among canopy treatments. Hybrid GH2547 was 25 to 31% more suppressive of wild proso millet than hybrid Spirit in Washington and 70 to 91% more suppressive in Illinois. Weed fitness was negatively correlated with leaf area index (LAI) after crop anthesis (−0.48 to −0.63), intercepted photosynthetically active radiation (PAR) at one of two harvest times (−0.51 to −0.56), and LAI at the 120- to 150-cm height (−0.51 to −0.55). Information on WSA may be useful in breeding programs; however, even near-term use of this knowledge offers modest but cumulative improvements to weed management systems in sweet corn.

Ten herbicides were evaluated for broomrape (Orobanche ramosa L. # ORARA) control and their effect on growth, yield, and chemical composition of four tobacco (Nicotiana tabaccum L.) cultivars. Of the preplant-incorporated (PPI) herbicides, only pebulate (S-propyl butylethylcarbamothioate) at 7.2 kg ai/ha and metham (methylcarbamodithioic acid) at 66 kg ai/ha provided, although inconsistently, 30 to 50% broomrape control in oriental but not in burley tobacco. Single postemergence (POE) applications of glyphosate [N-(phosphonomethyl)glycine] at 0.2 kg ai/ha or MH (1,2-dihydro-3,6-pyridazinedione) at 0.7 kg ai/ha at 40 days after transplanting resulted in 60 to 80% broomrape control. Glyphosate or MH applications at 40 days after transplanting and again at 60 days resulted in 100% and 80 to 90% control, respectively. Very good (80 to 90%) control also occurred with pebulate (PPI) at 7.2 kg ai/ha followed by glyphosate (POE) at 0.2 kg ai/ha at 40 days after transplanting. Tobacco yield increased significantly, compared to that of untreated plots, where glyphosate or MH was used, but nicotine, total nitrogen, and reducing sugar contents were not significantly affected.

The effects of short exposures to low temperature on cotton (Gossypium hirsutum L. ‘Stoneville 213′), velvetleaf (Abutilon theophrasti Medic), and spurred anoda [Anoda cristata (L.) Schlecht.] were studied in controlled environment chambers. A 3-day exposure to 17/13 C day/night temperatures 24 days after planting significantly reduced dry weight, leaf area and height in all three species when compared to control plants maintained at 26/21 C day/night. After subsequent periods of 7 days at 26/21 C and 8 days at 29/23 C, the two weed species recovered in growth more completely than did cotton. Therefore, the weed/crop ratios for dry weight and leaf area were increased by the imposition of the cold treatment. Mathematical analysis of the growth of the three species indicated that the growth reductions during the cold treatment were caused by decreases in both net assimilation rate and leaf area duration. The net photosynthetic rates and stomatal conductances of individual leaves of cotton and velvetleaf were significantly reduced during the cold treatment but recovered to control levels during a subsequent 4-day exposure to 26/21 C. We conclude that the adverse effects of low temperatures on the growth of cotton, velvetleaf, and spurred anoda are caused mainly by reductions in leaf area production. Because of its slower and less complete recovery, cotton is more severely affected by low temperature than either of the two weeds. Our results help explain the observation that velvetleaf and spurred anoda are more competitive with cotton following abnormally cool periods early in the growing season.

Triazine-resistant (TR) kochia control with herbicides was evaluated in the greenhouse and in the field on ridge-till and ecofallow corn. In the greenhouse, bromoxynil, dicamba, linuron, and paraquat applied POST controlled TR kochia > 86%. Control of TR kochia with 2,4-D ester at 0.3 kg ae ha−1 varied with kochia biotype. In three ridge-till fields in 1985, atrazine plus fluorochloridone at 2.2 plus 0.6 kg ai ha−1 applied before planting plus a layby cultivation was the only treatment that reduced TR kochia biomass to < 750 kg ha−1. In 1986, atrazine plus dicamba at 2.2 plus 0.3 kg ha−1, atrazine plus fluorochloridone at 2.2 plus 0.3 or 0.6 kg ha−1 applied EPOST; atrazine at 2.2 kg ha−1 applied EPOST followed by an MPOST application of bromoxynil at 0.3 or 0.4 kg ha−1, or dicamba plus 2,4-D ester at 0.3 plus 0.3 kg ha−1; and an MPOST application of atrazine plus dicamba at 1.3 plus 0.5 kg ha−1, atrazine plus pyridate at 1.1 plus 1.0 kg ai ha−1, or pyridate plus cyanazine at 1.0 plus 1.7 kg ai ha−1 followed by a layby cultivation reduced kochia biomass to < 400 kg ha−1. In ecofallow corn, the only herbicide treatment that consistently controlled TR kochia > 95% 21 to 30 d after application was pyridate plus fluorochloridone at 1.0 plus 0.6 kg ha−1. Other treatments were less effective because kochia was too large or additional kochia emerged after application. When added to atrazine plus cyanazine, paraquat at 0.3 kg ai ha−1 plus dicamba was more effective than paraquat plus 2,4-D ester or paraquat alone in controlling TR kochia.

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.

The emergence of annual species depends on the number of seeds present and the biotic and abiotic conditions directly surrounding those seeds (the microsite). A field experiment was conducted to study the relative importance of seed presence vs. microsite conditions in determining the emergence of four annual species. Green foxtail, wild mustard, wild oat, and canola were seeded at 200, 400, and 1,200 seeds m−2 in separate plots in a coarse, loamy, mixed Typic Haplocryoll and a fine, mixed Typic Haplocryoll soil. Five microsite modification treatments (control, irrigation, soil compaction, soil compaction plus irrigation, and no crop) were applied to all weed seed density treatments for each weed species. All plots were seeded to spring wheat. Irrigation or soil compaction increased percent emergence of wild oat. Green foxtail emergence tended to increase with soil compaction in 2001 but not in 2002. Wild mustard and canola emergence were largely unaffected by microsite modification treatments. Weed emergence increased with increasing seed density for all species, but the proportion of the total number of seeds emerging decreased with increasing seed density for all species. We suggest that the emergence of the four weed species in this experiment was both seed and microsite limited. Increasing the number of seeds in the soil increased the probability of seeds landing within an appropriate microsite. For these four species, therefore, weed spread and weed patch formation may be determined both by seed production and dispersal and by variability in soil microsite conditions. Results suggest that weed management practices should limit seed dispersal of all species and discourage weed emergence of hard-to-control species during critical establishment periods.

Laboratory and field research was conducted to determine the influence of winter wheat (Triticum aestivum L.) stubble on weed control and the dissipation of emulsifiable concentrate formulations (EC) of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], and acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide]; and microencapsulated formulations (ME) of alachlor and acetochlor. EC metolachlor provided better weed control than EC alachlor when applied 28 days before planting sorghum (Sorghum bicolor L.) and similar weed control when applied at 1 or 14 days before planting. This difference was attributed to more rapid EC alachlor dissipation. The order of relative herbicide persistence at 24 C and 33% (w/w) soil moisture in a silty clay loam was ME acetochlor = ME alachlor > EC metolachlor > EC acetochlor = EC alachlor. Acetanilide degradation was affected more by increasing temperature from 15 to 24 C than by increasing soil moisture from 15 to 33%. More herbicide was washed from overwintered than fresh straw in the first 1 cm of simulated rainfall. In the first simulated rainfall event, more herbicide was removed from straw that had been dry than wet at application, but the amount of herbicide remaining on dry and wet straw was similar after four events. Formulation generally did not affect herbicide wash-off.

The response of several spring wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and oat (Avena sativa L.) cultivars to preemergence and postemergence applications of chlorsulfuron (tested under code number DPX-4189) {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbonyl] benzenesulfonamide} was evaluated in the field and greenhouse in 1979. In greenhouse experiments, ‘Butte’ spring wheat was less susceptible than six other spring wheat cultivars to chlorsulfuron at 1.0 kg/ha applied preemergence or postemergence. ‘Morex’, ‘Conquest’, and ‘Bonanza’ barley cultivars were less susceptible than six other barley cultivars to preemergence chlorsulfuron, but barley cultivars were similar in response to postemergence chlorsulfuron. The eight oat cultivars responded similarly to preemergence and postemergence chlorsulfuron at 1.0 kg/ha. Data from two field experiments, which included evaluation of durum wheat [Triticum durum (Desf.)] cultivars, indicated that the four small grains had adequate tolerance to postemergence chlorsulfuron at 0.125 and 0.25 kg/ha, except that durum wheat grain yield was reduced by 0.25 kg/ha at the Rosemount location. No differential cultivar responses to chlorsulfuron occurred among the small grains in the field.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is a noxious grass weed that infests rice fields and causes huge crop yield losses. In this study, we collected 12 E. crus-galli populations from rice fields of Ningxia Province in China and investigated the resistance levels to the acetolactate synthase (ALS) inhibitor penoxsulam and the acetyl-CoA carboxylase (ACCase) inhibitor cyhalofop-butyl. The results showed that eight populations exhibited resistance to penoxsulam and four populations evolved resistance to cyhalofop-butyl. Moreover, all four cyhalofop-butyl–resistant populations (NX3, NX4, NX6, and NX7) displayed multiple herbicide resistance to both penoxsulam and cyhalofop-butyl. The alternative herbicides bispyribac-sodium, metamifop, and fenoxaprop-p-ethyl cannot effectively control the multiple herbicide–resistant (MHR) plants. To characterize the molecular mechanisms of resistance, we amplified and sequenced the target site–encoding genes in resistant and susceptible populations. Partial sequences of three ALS genes and six ACCase genes were examined. A Trp-574-Leu mutation was detected in EcALS1 and EcALS3 in two high-level (65.84- and 59.30-fold) penoxsulam-resistant populations, NX2 and NX10, respectively. In addition, one copy (EcACC4) of ACCase genes encodes a truncated aberrant protein due to a frameshift mutation in E. crus-galli populations. None of the amino acid substitutions that are known to confer herbicide resistance were detected in ALS and ACCase genes of MHR populations. Our study reveals the wide spread of MHR E. crus-galli populations in Ningxia Province that exhibit resistance to several ALS and ACCase inhibitors. Non–target site based mechanisms are likely to be involved in E. crus-galli resistance to the herbicides, at least in four MHR populations.

The comparative morphological development of six johnsongrass [Sorghum halepense (L.) Pers.] ecotypes was studied over a 10-week period. Three of the ecotypes were previously shown to be susceptible to dalapon (2,2-dichloropropionic acid) while others were resistant to the herbicide. The production of primary culms varied from 5.0 to 9.3 per plant system, and secondary culms ranged from 7 to 24 per plant system. Plant heights varied nearly two-fold, and the number of individual rhizomes produced by different ecotypes varied three-fold. Susceptible ecotypes from Georgia, Washington, and Illinois produced more rhizome nodes and higher dry and wet weights of culms and rhizomes than resistant ecotypes from Mississippi, Louisiana, and California. Ecotypes most resistant to dalapon had longer rhizome internodes and higher water content in culms and rhizomes than susceptible strains. Differences in anatomy and growth support the hypothesis of the existence of johnsongrass ecotypes, but the results were not consistent with the susceptibility of ecotypes to dalapon.