Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is considered one of the most troublesome weeds in the southern and central United States, but results of previous research to determine the mode of inheritance of this trait have been conflicting and inconclusive. In this study, we examined segregation patterns of EPSPS gene-copy numbers in F1 and F2 generations of A. palmeri and found no evidence of a Mendelian single-gene pattern of inheritance. Transgressive segregation for copy number was exhibited by several F1 and all of the F2 families, most likely the product of EPSPS copy-number variation within each plant. This variation was confirmed by assaying gene-copy number across clonal generations and among individual shoots on the same plant, demonstrating that EPSPS amplification levels vary significantly within a single plant. Increases and decreases in copy number occurred in a controlled, stress-free environment in the absence of glyphosate, indicating that EPSPS gene amplification is a random and variable process within the plant. The ability of A. palmeri to gain or lose EPSPS gene copies is a valuable adaptive trait, allowing this species to respond rapidly to selection pressures and changing environments.

Effectiveness of cyclical in vitro selection for imazethapyr tolerance in tomato was studied. Cotyledons and leaf explants from shoots regenerated on media containing imazethapyr were used to initiate three successive cycles of selection. Increases in tissue viability, callus initiation, and callus proliferation were observed following three cycles of selection on 10−7 M imazethapyr. Dry matter accumulation of unselected tomato seedlings was reduced by 50% when imazethapyr was applied postemergence at a rate of 28 g ae ha−1. Progeny of eight tomato lines selected after one cycle of selection had greater tolerance to imazethapyr than control plants (60 to 70% dry matter accumulation versus 50%, respectively). Variability was also generated for imazethapyr tolerance among progeny of the same tomato line.

Dallisgrass (Paspalum dilatatum Poir.) is an important pasture grass on moist fertile sites in warm climates, however, it is limited in use by periodic low seed quality and resulting erratic availability and establishment difficulties. Due to apomictic reproduction, common pentaploid dallisgrass has not been responsive to traditional plant improvement approaches. Recent research has shown potential for some improvement in forage production and plant persistence with apomictic hexaploid varieties of dallisgrass. Tissue-culture regeneration has been reported to produce somaclonal variation in seed germination of common pentaploid dallisgrass and has recently resulted in availability of potentially useful regenerants of hexaploid dallisgrass. Seed of selected plants from both field and greenhouse environments was evaluated in replicated germination trials to determine whether hexaploid tissue-culture regenerants responded similarly to regenerants from common dallisgrass and to determine whether regenerants expressing potential agronomic usefulness as indicated by preliminary observations differed in germination. The eight germination trials included 299 regenerants with 57 of these being duplicate entries among trials. Germination did not differ among sources of regenerants. Within sources germination differed among regenerants with a few individuals exceeding germination percentage of parent lines in one evaluation of greenhouse-produced seed. Individual regenerants with agronomic potential and high percentage seed germination were identified. The results demonstrate that tissue-culture regeneration can be used to produce improved germination in agronomically useful genotypes of apomictic dallisgrass and suggest that such potential may exist for other recalcitrant plant species.