Several factors, singly or synergistically, may influence the behavior of weedy species under conditions of intraspecific competition. The species studied were Bromus inermis, B. tectorum, Capsella bursa—pastoris, Conyza canadensis, Plantago lanceolata, P. major, Senecio sylvaticus, S. viscosus, and Silene anglica. Some or all of the species were subjected to conditions of intraspecific competition involving variable regimes of: (1) seed—sowing density, (2) soil surface, (3) fertilizer, (4) moisture, and (5) habitat, i.e., greenhouse and non—greenhouse conditions. Initially the vegetative dry weight production per unit area increased as density increased for every species. After a certain density (relative to the species) was reached, production was independent of density. All species displayed considerable plasticity in that similar genotypes showed a great increases in seed production between high and low densities (e.g., over a 100—fold increase for Capsella bursa—pastoris). Mean seed weight was constant under variable conditions of density, fertilizer, soil surface, and moisture in al species except Bromus tectorum. In mixed stands, the population of each species is smaller than its potential maximum. The size of the population of seedlings may be determined by the number of microenvironments suitable for germination (safe—sites) in an area. The populations of the respective species may be further regulated by: (1) self—controlled germination (Bromus inermis, Conyza canadensis, Plantago lanceolata, and Silene anglica); (2) mortality (Plantago major); and (3) individual plasticity (Bromus tectorum, Capsella bursa—pastoris, Senecio sylvaticus, and S. viscosus).

Heterotheca grandiflora Nutt. (Asteraceae, tribe Astereae) is one of the few native Californian plant species increasing its range as a weed. The production of dimorphic seed, together with flexible development in either the annual or biennial habit, may contribute to its range expansion. Well dispersed disc achenes germinate rapidly to high percentages while poorly dispersed ray achenes show considerable dormancy, germinating at a much lower rate to lower final percentages. Ray achenes appear more sensitive to environmental factors and have more specific germination requirements than do disc achenes. Thus, germination is distributed in space and time. Plants growing as annuals have one flowering period while those acting as biennials may flower up to three times, although seed production differs greatly among the stages. These factors aid in forming the "general purpose genotype" so frequently encountered in weedy species.

No abstract

Germination tests were conducted on seed from individual plants and from bulk samples representing 174 populations and 105 species to survey the prevalence of germination polymorphism in flowering plants in Utah and surrounding areas. Twenty—two populations representing 15 species (about 1/8 of the populations sampled) exhibited polymorphism, as measured by the germination of seed from individuals of percentages significantly different from those of bulk samples. Several of the 15 species, including Bromus tectorum, Rumex crispus, and Amaranthus retroflexus, were represented by populations with germination polymorphism and by populations without it.

Moist-chilled and freshly harvested seeds of redroot pigweed (Amaranthus retroflexus L.) and yellow foxtail [Setaria glauca (L.) Beauv.] were tested for germination responses to soil ammonium and nitrate. A controlled environment was used to simulate overwintering and germination conditions in the field. The results suggest that no alteration of weed germination behavior should be expected in the crop ecosystems studied if ammonium plus nitrification inhibitor replaces other fertilizer systems which result in substantial levels of nitrate in the soil solution. The only germination effect was one of nitrate on unchilled foxtail. Chilling lowered catalase activity in both species.