Control of rattail fescue, a winter annual grass, can be difficult in spring or winter wheat. Although rattail fescue is not a new weed species in the Pacific Northwest, occurrences have been increasing in circumstances where soil disturbances are minimized, such as in direct-seed cropping systems. To develop integrated management strategies for rattail fescue, information is needed on the longevity of seed viability in the soil, the presence of seed dormancy, vernalization requirements, and optimal environmental conditions for seed germination and establishment under field conditions. Controlled experiments on the biology of rattail fescue indicated that newly mature seed required an afterripening period of 1 to 12 mo to obtain high levels of seed germination, depending on germination temperature. Maximum seed germination was observed at constant day/night temperatures of approximately 20 C from thermogradient plate studies. Germination tests from seed burial studies indicated that a majority of buried seed was not viable after 2 to 3 yr. Field-grown rattail fescue plants required vernalization to produce panicles and germinable seed. A short afterripening period, cool germination temperature, and vernalization requirements support the classification of rattail fescue as a winter annual. This information will facilitate development of rattail fescue management systems, including crop rotations and various control tactics such as tillage or herbicide application timing during fallow periods.

Fifty corn poppy populations originating from three counties of northern Greece were evaluated for resistance to tribenuron. Twelve of the populations (six sampled from cereal fields and six from margins) were sampled from the county of Thessaloniki, 15 (13 sampled from wheat fields and two from margins) from Kilkis, and 23 (21 sampled from cereal fields and two from margins) from Serres. Fifty, 39, and 95% of the populations sampled from winter wheat fields originating from Thessaloniki, Kilkis, and Serres were resistant (R), respectively. However, all populations sampled from margins of the same areas were susceptible (S). All populations examined were susceptible to 2,4-D and bromoxynil. The level of resistance to tribenuron varied among populations with the herbicide dose required to reduce growth by 50% (GR50) ranging from 41 g ha−1 (R/S, resistance ratio 137) for the least resistant to over 720 g ha−1 (R/S greater than 2,400) for the most resistant populations. Fresh weight accumulation, seed production, and capsule number of eight R populations grown under field conditions were similar to those recorded for eight S populations originating from sites with high proximity of their respective R populations. However, the estimated mean growth rate (MGR) indicated significant differences due to the resistance trait and the population's origin. In particular, the R populations that originating from Thessaloniki and Serres had MGR 1.3 to 4.3 times lower than the respective S populations, whereas the R populations from Kilkis had similar or higher MGR values compared to the respective S populations. The populations with the highest R/S (greater than 2,400) had low MGR values, and the populations with R/S ranging from 1,437 to 2,227 had high MGR values.

Effective weed seedbank management requires mechanistic understanding of ecological determinants of seed persistence in the soil seedbank. Chemical and physical defense of common lambsquarters, field pennycress, giant foxtail, kochia, velvetleaf, and yellow foxtail seeds were quantified in relation to short- and long-term seedbank persistence. Seed content of ortho-dihydroxyphenols (o-DHP), a class of putative seed defense compounds, varied more than threefold between the least protected species (common lambsquarters, 9.2 µg g seed−1) and the most protected species (kochia, 34.1 µg g seed−1). Seed o-DHP was inversely related (r = −0.77, P < 0.001) to seed half-life in the soil and to short-term seed persistence in burial assays (r = −0.82, P < 0.05). The relative importance of chemical seed protection in comparison to physical seed protection, as represented by the ratio of seed o-DHP concentration to seed coat thickness, decreased linearly with increasing short-term seed persistence (r = −0.96, P < 0.01) and nonlinearly with increasing long-term seed persistence in the soil seedbank (y = 0.16 + 0.21/(0.0432 + x), R2 = 0.99, P < 0.001). Mechanical damage to the seed coat, via piercing, slicing, or grinding treatments, increased short-term mortality during burial for all six species. Mortality of pierced seeds was negatively associated (r = −0.35, P < 0.05) with seed phenol concentration and positively associated with seed half-life (r = 0.42, P < 0.01) and seed coat thickness (r = 0.36, P < 0.05). Seed phenolics, as a class, supported the results for o-DHPs. Overall, these findings suggest a potential weakness, with respect to seedbank management, in the way weed seed defenses are constructed. Weed species with transient seedbanks appear to invest more in chemical defense than those species with highly persistent seedbanks. As a result, seeds in the latter category are relatively more dependent upon physical seed protection for persistence in the soil seedbank, and more vulnerable to management tactics that reduce the physical integrity of the weed seed coat.

At the time of grain harvest, weed seeds can be classed into one of four pools on the basis of dispersal status and location: (1) undispersed, remaining on the mother plant; (2) dispersed in the current year, on the soil surface; (3) dispersed in the current year and collected by harvest machinery; and (4) dispersed in a previous year and persisting within the soil seed bank. Knowledge of the relative sizes of these seed pools for different weed species under different crop environments will be useful for determining the best way to reduce the size of inputs to the soil seed bank. In fall 2004 and fall 2005, four randomly selected commercially managed corn and soybean fields in east-central Illinois were sampled to quantify weed seed pools at time of crop harvest. Thirty randomly located 0.125-m2 quadrats were placed within each field, the four seed pools mentioned above were sampled for each quadrat, and the species composition and abundance of each seed pool was determined. The magnitude of the weed seed rain varied among species and between years and crops. Twenty-six weed species were found to contribute to at least one of the four seed pools. However, the weed seed pools were consistently dominated by six species: velvetleaf, Amaranthus complex (redroot pigweed and waterhemp), ivyleaf morningglory, giant foxtail, prickly sida, and common cocklebur. For each of these species, the ratio of undispersed seeds to seeds in the soil seed bank at harvest time was ≥ 1 in at least one crop during one of the two experimental years, indicating a potential for the soil seed bank to be completely replenished or augmented by that year's seed rain. This analysis demonstrates the urgent need for techniques to limit weed seed inputs to the soil seed bank at the end of the growing season.

The molecular basis for acetolactate synthase (ALS)–inhibitor resistance was determined for 6 susceptible (HS) and 24 resistant (HR) kochia populations from western Canada. The latter included 3 HR populations from Alberta (AB), 3 from Manitoba (MB), and 18 from Saskatchewan (SK). HR plants survived application of the ALS-inhibitor herbicide thifensulfuron–tribenuron mixture in the greenhouse. Most of the HR populations were heterogeneous and contained both HR and HS individuals. The molecular basis for resistance was determined in 273 HR individuals by sequencing the ALS gene (2,270 base pair [bp]) or by conducting a TaqMan genotyping assay developed in this study using real-time polymerase chain reaction (PCR) for single nucleotide polymorphism (SNP) 1709, where a G to T substitution resulted in a Leu for Trp substitution at amino acid position 574 (Trp574Leu mutation). A total of 16 SNPs were identified in the ALS gene sequences (0.7% polymorphism), 5 of which resulted in amino acid changes that confer resistance to ALS-inhibiting herbicides. The SNPs correspond to three target-site mutations: Pro197 (SNPs 565 and 566), Asp376 (SNP 1116), and Trp574 (SNPs 1708 and 1709). The Trp574Leu mutation was predominant (189 HR plants). The next most common mutation was the highly variable residue Pro197 (44 HR plants) with substitution by one of nine amino acids. The least-frequent were Asp376Glu (9 plants) and Trp574Arg (3 plants) substitutions. The presence of two ALS target-site mutations was found in 30 individual kochia plants, the first report from field-selected weed populations. These include combinations Pro197 + Trp574 (23 plants) and Pro197 + Asp376 (7 plants). The detection of Pro197, Asp376, and Trp574 mutations, as well as both combinations, from geographically separate regions suggests multiple origins of these mutations.

Venice mallow is an emerging weed problem in many regions of the world in various crops. Studies on its hard seed coat were conducted by scanning electron, light, and fluorescence microscopy. The effects of environmental factors on germination and seedling emergence were examined in laboratory experiments. Seeds possessed physical dormancy (90%) that required immersion for 0.5 h in concentrated sulfuric acid to break without damaging the seed. After scarification, 29% of seeds exhibited primary embryo dormancy. Exposure of seeds to even short periods of 15 d of prechilling induced secondary dormancy (10% germination). The surface of hard seeds had a velvety appearance from numerous papillate structures and deposits (hydrophilic material). The location of the water barrier was very superficial in the outer seed coat. Although, in hard seeds, the hilar area appeared to have vertical ruptures and the hilum fissure appeared to be open, there was no water entry. It was observed that microruptures could be the region of structural weakness of hard seeds in relation to water permeability during prolonged burial (8 mo).

The highest (60%) germination was recorded at a day/night temperature of 30/20 C with a 12-h photoperiod. No germination was measured at either 10 or 45 C constant temperature. Germination was recorded with a broad range of pH (3-11) and seeds were rather tolerant to low water potential (20% germination at –1.2 MPa). Seedling emergence was higher for seeds buried at 2 cm than for those placed on the surface (54 vs. 38%, respectively). These results showed that Venice mallow is a rather unique species that possesses a complex mechanism of dormancy (physical, primary, and secondary). Information gained in this study will be used in developing management strategies for this species.

Gene flow between jointed goatgrass and winter wheat is a concern because transfer of herbicide-resistance genes from imidazolinone-resistant (IR) winter wheat cultivars to jointed goatgrass could restrict weed-management options for this serious weed of winter wheat cropping systems. The objectives of this study were (1) to investigate the frequency of interspecific hybridization between IR wheat and jointed goatgrass in eastern Colorado, and (2) to determine the gene action of the IR acetolactate synthase (ALS) allele in IR wheat by jointed goatgrass and in IR wheat by imidazolinone-susceptible (IS) wheat backgrounds. Jointed goatgrass was sampled side-by-side with IR wheat and at distances up to 53 m away in both experimental plots and at commercial field study sites in 2003, 2004, and 2005. A greenhouse-screening method was used to identify IR hybrids in collected jointed goatgrass seed. The average percentage of hybridization across sites and years when IR wheat and jointed goatgrass were grown side-by-side was 0.1%, and the maximum was 1.6%. The greatest distance over which hybridization was documented was 16 m. The IR ALS allele contributed 25% of untreated ALS activity in jointed goatgrass by IR wheat F1 plants, as measured by an in vitro ALS assay. The hybridization rate between wheat and jointed goatgrass and the expression of the IR wheat ALS allele in hybrid plants will both influence trait introgression into jointed goatgrass.

Biotypes of common lambsquarters with tolerance to glyphosate have been identified in a number of states, but little is known about their fitness characteristics. Field and greenhouse studies were conducted to characterize the response of selected glyphosate-tolerant common lambsquarters biotypes to glyphosate, and also their biological and reproductive characteristics. In a greenhouse dose-response study, GR50 and GR90 values for four tolerant biotypes ranged from 1.48 to 3.22 and 8.73 to 18.7 kg ae ha−1, respectively, compared to 0.57 and 2.39 kg ae ha−1, respectively, for a glyphosate-sensitive biotype. In a field dose-response study, the GR50 and GR90 values were 0.06 and 0.48 kg ae ha−1, respectively, for a tolerant biotype, compared to 0.036 and 0.19 kg ae ha−1, respectively, for the sensitive biotype. The growth rate, time until flowering, and seed production of eight tolerant and two sensitive biotypes was evaluated in a field study. The tolerant biotypes grew taller, amassed more leaf area and dry weight, and advanced through growth stages more rapidly than sensitive biotypes during the early portion of the growing season. The tolerant biotypes were taller than sensitive biotypes at 6 and 10 wk after transplanting, but had lower dry weight at maturity. Tolerant biotypes initiated flower primordia approximately 6 to 8 wk after transplanting, whereas sensitive biotypes required 12 wk. However, no apparent fitness penalties were observed in glyphosate-tolerant biotypes based on seed-production estimates.

Resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides in populations of eastern black nightshade from Ontario has been linked to an Ala205Val substitution in the AHAS enzyme. The aim of this study was to determine the mechanism of inheritance of AHAS inhibitor resistance and the genetic relationships among resistant (R) and susceptible (S) eastern black nightshade populations from Ontario. Homozygous R and S parental populations were crossed and the inheritance was analyzed in F1 (S × R), reciprocal F1 (R × S), F2, and backcross (S × F1) progenies after application of imazethapyr at 150 g ai ha−1. Compared to parental lines, the progenies were rated as R, intermediate (I), and S phenotypes. All the F1 progenies were of the I phenotype. The backcross progenies segregated in a 1:1 (S:I) ratio, and the F2 families segregated in a 1:2:1 (R:I:S) ratio. These results indicate that a single nuclear gene, with incomplete dominance, controls resistance to AHAS-inhibiting herbicides in R population of eastern black nightshade. Random amplified polymorphic DNA (RAPD) markers were screened among 25 R and S populations. The genetic relationship of R and S populations based on RAPD profiles generated from six RAPD primers indicated four groups of populations in which resistance seems to have arisen independently. However, based on similarity coefficients, resistance within three of the groups could have arisen by gene flow. Both similar local selection pressure and gene flow could explain the spread of the Ala205Val substitution in R populations of eastern black nightshade in Ontario.

Sorghum genes responsive to purple witchweed parasitism were isolated, and their expression was analyzed. Using the suppression-subtractive hybridization strategy, 30 genes that were up-regulated in response to purple witchweed parasitism were isolated from the roots of a susceptible sorghum cultivar ‘Abu 70’. The changes in the expression of each gene were investigated in the roots and leaves of the sorghum cultivars ‘Wad Ahmed’ and ‘Tabat’ were parasitized by purple witchweed. Tabat is more susceptible and Wad Ahmed is less susceptible to purple witchweed than Abu 70. Further, the changes in the gene expression and host susceptibility to purple witchweed were studied in the roots of the three sorghum cultivars after treatment with salicylic acid (SA) or methyl jasmonate (MeJA). Purple witchweed parasitism induced jasmonic acid (JA)-responsive genes and suppressed SA-responsive genes in the roots of the cultivars Abu 70 and Tabat. In contrast, purple witchweed parasitism in the less-susceptible cultivar Wad Ahmed induced SA-responsive genes and induced JA-responsive genes to a small extent. SA, but not MeJA, decreased the susceptibility of all the sorghum cultivars to purple witchweed. Systemic expression of a few genes was observed in the leaves of the purple witchweed–parasitized sorghum cultivars.

Tembotrione inhibits 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD) and was recently registered for use in all types of corn. Some sweet corn hybrids are killed by tembotrione, yet a mechanistic understanding of sensitivity has not been reported. Sensitivity of mesotrione, another HPPD-inhibitor, is conditioned by a single allele. Two hypotheses were tested: (1) response to tembotrione and mesotrione are conditioned by alleles at the same or closely linked loci and (2) the extent of early-season injury from tembotrione and mesotrione is similar on hybrids. The first hypothesis was tested by comparing responses to tembotrione and mesotrione in 136 F3:5 families derived from a cross of mesotrione-sensitive and mesotrione-tolerant sweet corn inbreds. F3 families cosegregated for responses to tembotrione and mesotrione: 94% of the families had the same response to both herbicides. Thus, the same gene or very closely linked genes condition response to both herbicides. On the basis of chi-square goodness of fit tests, responses of families to tembotrione fit a 3 : 2 : 3 sensitive : segregating : tolerant ratio (P = 0.24), which would be expected if sensitivity to tembotrione was conditioned by a single recessive allele. The second hypothesis was tested in six field experiments by quantifying the extent of early-season injury to 249 sweet corn hybrids 1 wk after treatment (WAT) of tembotrione (184 g ha−1) or mesotrione (210 g ha−1). One hundred ninety-three hybrids were tolerant to both herbicides. Seven sensitive hybrids that were severely injured by both herbicides 1 WAT differed in their response 3 to 4 WAT; sensitive hybrids treated with mesotrione had apparently resumed normal growth, whereas those treated with tembotrione died. Conversely, hybrids with intermediate levels of injury (> 10 to 50%) 1 WAT with mesotrione had no visual symptoms of injury from applications of tembotrione. Despite the common genetic basis for response to mesotrione and tembotrione, hybrids with sensitive or intermediate responses to mesotrione did not have identical responses to tembotrione.

The transfer of the imazethapyr-resistant gene from Clearfield™ (CL) rice to red rice is an ecological risk. Flowering synchronization and genetic compatibility between cultivated rice and red rice could influence gene transfer. We examined the (1) variability in maximum outcrossing rate between 12 red rice biotypes and ‘CL161’ rice during their peak flowering overlap in the field and (2) genetic compatibility of red rice biotypes with CL161 rice. Experiments were conducted at Stuttgart, AR, and Fayetteville, AR, from 2005 to 2007. To evaluate the flowering synchrony of red rice and CL161 rice as well as its impact on outcrossing rate, field experiments were conducted at four planting times from early April to late May. The red rice biotypes were planted in the middle row of nine-row CL161 plots and flowering was monitored. Outcrosses were evaluated in subsequent years by herbicide response and simple-sequence-repeat marker assays. To determine compatibility, manual crosses were performed between 12 red rice biotypes and CL161 rice in the greenhouse. The flowering duration of all red rice types ranged from 5 to 16 d after the onset of flowering in contrast to 6 d in CL161 rice. Ten of the twelve types of red rice had ≥ 70% overlap in flowering time with CL161 rice in at least one planting date. The maximum field outcrossing rate between red rice biotypes and CL161 ranged from 0.03 to 0.25%. The field outcrossing rate between red rice biotypes differed (P < 0.01), but flowering synchronization was not directly related to outcrossing rate. Manual crosses resulted in seed sets of 49 to 94%. The majority of red rice biotypes had similar compatibility with CL161 rice. Thus, other factors must contribute to hybridization rates in the field. Follow-up experiments should investigate other plant factors and environmental influence on hybridization rate.

The predictability of the spring emergence period of kochia is a potentially exploitable attribute that may be applied to weed management. The emergence timing of kochia was monitored, during the spring and summer of 2005 and 2006, in 12 farmed fields in the northern region of the Northern Great Plains (NGP) of North America. Kochia emergence began at 50 cumulative growing-degree days (GDD Tbase 0 C, starting January 1) and continued into late summer. The onset of kochia emergence occurred well before the emergence onset of other weed species common to this region. Fall soil samples revealed few kochia seeds remained in the seedbank. In growth room studies, kochia seed placed at the soil surface had greater emergence compared with seed burial to a 10 mm or greater depth. Kochia did not emerge from seed planted 80 mm deep; however, radical emergence was evident indicating fatal germination. Kochia should be controlled early in the growing season, and burying seed will greatly reduce future kochia populations.

Spreading orach is an annual weed that colonizes roadsides, field edges, and increasingly, no-till agricultural fields. It produces dimorphic seeds with different levels of physiological dormancy, but little is known about the germination ecology of the two seed types. Field and controlled-environment studies were conducted to determine seed responses to light and stratification, the pattern of seedling emergence in the field, and the effect of soil water content on the length of cold stratification required to break dormancy for each seed type. The large, brown seeds have three times the mass of the smaller, black seeds, primarily because of a larger embryo, but have a thinner seed coat. Germination of brown and black seeds in petri dishes was 98 and 90%, respectively, after stratification for 3 mo at 5 C, whereas germination of unstratified seeds was 19 and 12%, respectively. Light stimulated germination of both stratified and unstratified black seeds but did not increase germination in stratified brown seeds. Up to 40% of brown seeds germinated in situ during stratification, compared with only 2% for black seeds. Germination in petri dishes and emergence in the field were more rapid for brown seeds than for black seeds. Maximum germination of black seeds occurred after stratification for 2 or 3 mo at 5 C on soil that was waterlogged (pore-water matric potential, ψ = 0 kPa), wet (ψ = −0.38 kPa), or at field capacity (ψ = −10 kPa). For shorter periods of stratification, total germination and germination rate of black seeds declined as soil water content decreased from waterlogged to dry (ψ = −500 kPa). Seed dimorphism in spreading orach may provide a mechanism to enhance survival in uncertain or variable habitats such as disturbed agricultural fields.

Morningglories are troublesome weeds in row crops and other agricultural areas throughout the United States. Plants of pitted morningglory, sharppod morningglory, and a fertile “hybrid” between pitted and sharppod morningglory (hybrid morningglory), were compared with cypressvine, ivyleaf, palmleaf, purple moonflower, red, and smallflower morningglories in greenhouse studies at Stoneville, MS. Plants from each of 76 accessions were studied for number of nodes to first internode elongation; stem color and pubescence; leaf area and dry weight of first four full expanded leaves; leaf blade pubescence on abaxial and adaxial surfaces and margins; leaf color, shape, and lobing; petiole length, color, and pubescence; sepal length, color, and pubescence; and corolla color, diameter, and length. Among these morningglories, the most diverse traits were pubescence and flower characteristics. Greatest morphological diversity was among hybrid morningglory accessions because characteristics were intermediate to pitted morningglory and sharppod morningglory accessions. Sharppod morningglory had five nodes to first internode elongation compared to three nodes in pitted and hybrid morningglory. Corolla color was white (90%) or white with faint pink veins (10%) in pitted morningglory, lavender (100%) in sharppod morningglory, and varied from pinkish lavender (45%), lavender (38%), white (12%), to white with pink veins (5%) in hybrid morningglory accessions. Pitted, red, and smallflower morningglory corolla diameters were not only smaller, but less variable in size than cypressvine, hybrid, ivyleaf, palmleaf, purple moonflower, and sharppod morningglories. Corolla diameter and lengths were most variable in sharppod morningglory accessions when compared to other morningglory accessions. The sepal tip shape was broader (broadly acute to obtuse) in palmleaf and sharppod than in hybrid, pitted, or other morningglories (acute to narrowly acute). In future studies, these morphological traits will be compared to determine if any are correlated with glyphosate sensitivity.

Baseline information on inducing germination of dormant leafy spurge seeds with growth regulators and chemicals is lacking. This study was conducted to survey the effect of various substances on germination of leafy spurge seeds. The nontreated control seeds in this population were nearly fully imbibed in 3 h and displayed approximately 35% germination in 21 d under the normal alternating temperature of 20/30 C (16/8 h). Gibberellic acid (GA3, 10 mM) induced 65% germination at constant temperatures of 20 and 30 C. The alternating temperature increased the effectiveness of 10 mM GA3 with 94% germination, a twofold increase over the control. Nontreated seeds did not germinate at the constant temperatures, suggesting that alternating temperature acts via a GA-independent pathway. Kinetin at 0.1 to 1 mM was no more effective than the control, but a saturated solution of kinetin induced 73% germination. Ethephon at 0.01 to 1 mM induced 58 to 66% germination, although there was little response to different concentrations. Ethylene gas at 1 ppm stimulated germination to 77%, a 1.8-fold increase over the control. Germination of seeds incubated continuously in 1 and 10 mM nitrate displayed 35 and 40% germination, respectively. Seeds pulsed for 24 h with 100 mM nitrate displayed 58% germination after 21 d. Potassium phosphate–citrate buffer (pH 3.4) and its individual components induced 60 to 70% germination. Fluridone (10 and 100 µM), 1-naphthaleneacetic acid (NAA, 0.1 to 10 mM), and ethanol (0.2 to 15%) had no effect on germination, but subsequent elongation in the presence of NAA was inhibited because of swelling of the radicle. This research reveals that GA3 is the most effective growth regulator for germination of dormant leafy spurge seeds, and its effect is independent of temperature.

Weed management is complicated by the presence of soil seed banks. The complexity of soil–seed interactions means that seed persistence in the field is often difficult to measure, let alone predict. Field trials, although accurate in their context, are time-consuming and expensive to conduct for individual species. Some ex situ techniques for estimating seed life expectancy have been proposed, but these fail to simulate the environmental complexity of the field. Also, it has been questioned whether techniques such as the controlled aging test (CAT) are useful indicators of field persistence. This study aimed to test the validity of the standard CAT (seed aging at 45 C and 60% relative humidity) in use at the Royal Botanic Gardens, Kew, U.K., for predicting field seed-persistence. Comparison of seed persistence and CAT data for 27 northwest European species suggested a significant positive correlation of 0.31. Subsequently, 13 species of emerging and common weeds of Queensland were assessed for their seed longevity using the CAT. The seed longevity data of these species in the CAT were linked with field seed-persistence data according to three broad seed-persistence categories: < 1 yr, 1 to 3 yr, and > 3 yr. We discuss the scope for using the CAT as a tool for rapid assignment of species to these categories. There is a need for further studies that compare predictions of seed persistence based on the CAT with seed persistence in the field for a larger range of species and environments.

Croftonweed has established itself as a pernicious weed in China. Experiments were conducted to test for anemophily, self-pollination, hand crossing, and apomixis. Additionally, pollen viability, seed set, and seed viability were investigated. Pollen cultured in vitro with the use of 14 culture media did not germinate, nor did self- or cross-pollen germinate on the stigma, and no pollen tubes were seen in the style. Young embryos developed into globular- or heart-shaped embryos before the opening of the captimulum, although neither meiosis to form an embryo sac nor fertilization occurred. Furthermore, seeds developed normally after closed flowers were emasculated and bagged. The plump seed ratio (plump seeds/total seeds) from apomixis (emasculation and bagged treatment) was high (41 ± 17.0%), and plump seed germination ratio exceeded 50%. These results provided additional evidence for the previous conclusion that croftonweed reproduces by apomixis. Apomixis, high seed production, and high seed germination may explain why this weed spreads so rapidly.

Nalta jute and redweed are widespread and significant broadleaf weed species of the tropics. Experiments were conducted to determine the effects of various environmental factors on seed germination and seedling emergence of these species. Light was not required for germination in either species. Germination was stimulated by seed scarification, suggesting that inhibition of germination in these species is mainly due to the hard seed coat. Scarified seeds of both species germinated over a wide range of alternating temperatures (25/15, 30/20, and 35/25 C). Both species were moderately tolerant of salt and osmotic stress, but nalta jute tolerated more stresses than redweed. Seedling emergence of nalta jute and redweed was greater than 87 and 93%, respectively, at soil depths of 0 to 2 cm but decreased as depth increased, with no emergence at 8 cm. Seedling emergence of both species was reduced by the addition of rice straw, though a high amount (4 to 6 t ha−1) of straw was required to suppress emergence significantly. The information gained from this study could facilitate the development of effective weed control programs.

Resistance to protoporphyrinogen oxidase (PPO)-inhibiting herbicides in waterhemp has been shown previously to be the result of a unique mechanism. Specifically, a three–base-pair (3-bp) deletion in the PPX2L gene, a gene encoding both plastid- and mitochondria-targeted PPO enzymes, confers herbicide resistance in this species. Furthermore, when this unique mechanism was initially characterized it was presumed that waterhemp contained three PPX genes, PPX1, PPX2S, and PPX2L, and that the resistant biotypes were missing PPX2S. Here, allele testing and examination of genetic sequence data demonstrate that there are likely only two PPX genes in waterhemp, PPX1 and PPX2L. Next, to determine the prevalence of this mechanism of resistance in Illinois waterhemp, we developed an allele-specific polymerase chain reaction (PCR) marker that amplifies only the 3-bp deletion allele, ΔG210, of PPX2L. By utilizing this marker, we show that the ΔG210 PPX2L allele correlated with whole-plant resistance to PPO inhibitors in each of four other waterhemp populations evaluated from Illinois.