This study provides a comparative analysis of the dormancy and germination mechanisms of the indehiscent fruits of hoary cress (Lepidium draba L.) and hairy whitetop (Lepidium appelianum Al-Shehbaz), two invasive weeds of the Brassicaceae. Germination assays comparing isolated seeds (manually removed from the fruits) and intact indehiscent fruits showed that the isolated seeds are nondormant and provided full germination for both species. In contrast to this, the species differed in the germination properties of their indehiscent fruits, in that L. appelianum fruits were nondormant, while the L. draba fruit coat (pericarp) conferred a coat-imposed dormancy. The pericarp of L. draba fresh fruit was water permeable, and neither mechanical scarification nor surface sterilization affected germination, supporting the concept that pericarp-mediated dormancy was not due to water impermeability or mechanical constraint. Washing of L. draba fruits with water, afterripening (dry storage), and treatment with gibberellin (GA) stimulated the germination of this species, all of which are indicative of physiological dormancy. Analyses of endogenous abscisic acid (ABA) and GA levels combined with treatment experiments with wash water from fresh and afterripened L. draba pericarps and with ABA dose–response quantification of germination revealed that ABA is a key component of a pericarp-mediated chemical dormancy in this species. Consistent with this, pericarp ABA levels decreased during afterripening and upon fruit washing, and isolated fresh or afterripened seeds did not differ in their ABA sensitivities. The possible roles of the ABA-mediated pericarp dormancy for the germination ecophysiology and weed management of these species are discussed.

The allelopathic potentials of Hope white lupine (Lupinus albus L.) herbage and herbage extracts were evaluated. The emergence and growth of cotton (Gossypium hirsutum L. ‘Deltapine 45A’), soybean [Glycine max (L.) Merr. ‘Lee 68′], and six weed species under greenhouse conditions were measured after seeding directly into a moist, silt loam soil into which dry Hope lupine herbage, collected at second bloom, had been incorporated at rates from 500 to 8000 ppm (dry herbage/dry soil, w/w). At the lower rates, incorporated Hope white lupine stimulated the emergence of large crabgrass [Digitaria sanguinalis (L.) Scop.]; at the higher rates, cotton emergence was inhibited. Cotton and soybean fresh weights were stimulated at incorporation rates up to 2000 ppm, but were inhibited at higher concentrations. The growth of johnsongrass [Sorghum halepense (L.) Pers.] and large crabgrass was stimulated at most incorporation rates. Stimulated plants were chlorotic and had longer internodes. Emergence and growth of other species tested were not significantly affected by incorporated Hope white lupine. Equivalent concentrations of Hope white lupine herbage extracts were about 17 times more inhibitory to germination of cress (Lepidium sativum L. ‘Curlycress’) seed than to growth of sorghum [Sorghum bicolor (L.) Moench ‘AKS 663′] coleoptiles or radicles. The inhibitory principle extracted from Hope white lupine herbage was non-volatile, heat insensitive, and significantly less active in soil. Fractionation of a herbage extract suggested that neither the alkaloid nor phenolic acid content could account for herbage phytotoxicity in soil.

A large-scale continuous system for recovery of root exudates from millet and cowpea was developed and tested extensively. Large quantities of germination stimulants were exuded into the medium flowing through the plant trays, which could be demonstrated by germination of seeds of three species of angiospermous parasitic plants, Alectra vogelii, Striga gesneriodes, and witchweed. Recovery of root exudates was affected by adsorption on the macroreticular polymer resin XAD-4 followed by desorption with methanol. Furthermore, indications were found that germination inhibitors are released into the medium. Subsequent testing confirmed the quantitative release of the germination stimulants as well as a large number of other compounds.

Phenolic content, germination percentage, and host preference were compared among 11 seed populations of witchweed (Striga hermonthica (Del.) Benth. # STRHE). Significant differences in germination percentage occurred among the seed populations after preconditioning by their exposure to different durations of warm-moist conditions. Greatest percentage of seed germinated after 8 days of preconditioning. Preconditioning for more than 16 days reduced percentage germination. The host specificity range of witchweed seed populations that parasitized pearl millet [Pennisetum americanum (L.) K. Schum] was broader than were those seed populations that attacked sorghum [Sorghum bicolor (L.) Moench.]. Results of thin-layer chromatography indicated large variations in phenolic content among the witchweed seed populations.

Field and laboratory experiments were conducted with unicorn-plant to determine conditions required for germination and to measure seed production. Field-grown plants produced an average of 122 pods/plant with an average of 71 seed/pod. The highest percent germination from seed harvested in 1980 occurred following a 2-week prechill treatment of 4 C. Germination increase was greater by removing the seed coat and the membrane enclosing the embryo than by removing the seed coat alone. Germination of seed from all pod compartments was similar. Aqueous extracts of unicorn-plant testa, leaf, stem, root, and exocarp were inhibitory in petri dish bioassays to cotton radicle growth. Extracts of stem, root, and exocarp were inhibitory to wheat radicle growth, and extracts of endocarp, leaf, and exocarp were inhibitory to unicorn-plant radicle growth. Seed buried in the field 10 cm deep for 1 to 8 months showed increased germination over time. Germination was lower when seed were stored at 4 C for 1 to 8 months in a soil having 25% (v/v) water.

Benzyl isothiocyanate (BITC) was formulated in dry granules with tung oil and calcareous loess to facilitate safe handling of the water-sensitive lachrymator. Crop and weed seeds were exposed to granular BITC at 40 mg ai/kg (w/w) substrate on moist sand for 2 days. When the pretreated seeds were washed with water and placed on BITC-free sand in a germination chamber for 15 days, germination in comparison to untreated seeds was 0 to 8% for pitted morningglory (Ipomoea lacunosa L. # IPOLA), sicklepod (Cassia obtusifolia L. # CASOB), milo [Sorghum bicolor (L.) Moench. ‘Funks G-522DR’], and red rice (Oryza sativa L. # ORYSA) seeds; 15 or 16% for velvetleaf (Abutilon theophrasti Medic. # ABUTH) and johnsongrass [Sorghum halepense (L.) Pers. # SORHA]; and 39% for barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG]. Subjecting milo seeds to moist sand medium that contained BITC granules at 10, 20, and 40 mg ai/kg substrate reduced germination of milo by 78, 97, and 100%, respectively. However, BITC granules applied to the growth medium 2 to 4 days prior to sowing seeds had little effect on germination and growth of milo or sicklepod. Thus, BITC-treated soil soon became safe for newly sown seeds, but seeds present in soil when the toxin granules were applied failed to recover from exposure to BITC and deteriorated within 15 days.