Various facets of the pollination biology affecting achene production of Centaurea solstitialis L. were examined. Production of viable achenes commenced in capitula bagged or excised in the late flowering stage (F-2 midstage). In all studies in which capitula were prevented from crossing, only one plant had a high level of achene production, all others producing either a few achenes or none at all. Achene production in plants allowed to cross was high and significantly greater than in plants prevented from crossing. Persistent low levels of self-fertilization in most plants indicated that there was selection against inbreeding. The pollen presentation mechanism for C. solstitialis was similar to that of other Centaurea spp. described previously. The most commonly observed insect visitors of yellow starthistle capitula were Hymenoptera spp., especially the European honeybee. Keywords: Centaurea, pollination, yellow starthistle.
Yellow starthistle (Centaurea solstitialis L.) is a major rangeland weed pest in California and other western states. Two complementary aspects of seed biology were examined: germination and seed bank dynamics. Achenes were tested for changes in germinability over time using various light, temperature, and moisture treatments. Soil cores were collected over time to monitor changes in achene density. Both plumed and nonplumed achenes exhibited photo reversible responses to red and far-red light. White and red light enhanced germination above that occurring in darkness. Achenes showed seasonal changes in germinability, which were characteristic of conditional dormancy. Exposure of achenes to field conditions during summer alleviated primary dormancy, allowing germination to occur at higher temperatures and lower substrate moisture. Nonplumed achenes collected in winter showed reduced germinability at or above moderate temperatures under both single and multiple light exposures. Imbibition and moderately low temperatures with subsequent drying induced secondary conditional dormancy in some achenes. Plumed achenes dominated the soil seed bank before the rainy season, then declined rapidly following rain. Nonplumed achenes peaked after rains began, then declined. Although nearly all achenes were depleted after 4 years, both types remained, showing annual cycles of high and low germinability in autumn and late spring, respectively.Key words: light, phytochrome, conditional dormancy, longevity.
Centciurea solstitialis L. is a winter annual weed of Eurasian origin that is widespread in the western United States. Capitula produce dimorphic achenes that are dispersed at different times. Factors investigated in the laboratory that affected germination were light, temperature, achene type (plumed and nonplumed), collection date, dormancy, and storage. The presence of large interactions among the factors affecting germination was constant throughout these studies. Although some germination occurred in the dark, a daily (8 h light: 16 h dark) photoperiod of white light produced higher germination percentages in both fresh and stored achenes. Even a single brief (10-min) exposure to light was almost comparable with a daily 8-h photoperiod. Maximum germination of nearly 100% was achieved in light under constant temperatures of 10, 15, and 20 °C and alternating temperatures of 15:5 and 20:10 °C. Broadening of temperature optima related to afterripening occurred for July achenes stored for 2 months and August-fresh achenes under alternating temperatures. Germination differences between achene types were more prevalent under dark than under light conditions. In general, germination of plumed achenes was higher than that of nonplumed achenes. In darkness, nonplumed achenes showed a shift in optimum germination temperature (from 15 to 20 °C) from July to September. Germination of nonplumed achenes in intact capitula was never greater than 21%, regardless of time of collection, because of the presence of the involucral bracts. Key words: yellow starthistle, germination, light, temperature, dimorphic achenes.
Studies were conducted to compare changes in germinability and/or viability of yellow starthistle achenes buried (in packets) in soil or stored dry in the laboratory, and to determine the rate of achene depletion from a soil seed bank. In one study, after 72 mo, 0 to 96% germinable achenes remained in packets buried 5 cm deep, and a mean above 99% after dry storage. In another study, total live (germinable plus viable) achenes declined from 100% at harvest to 77.6% after 24 mo of burial. Germinable pappus-bearing achenes remaining in packets increased with depth of burial. After 12 mo of burial, maximum survival of achenes occurred at depths of 5 cm or more. The density of yellow starthistle achenes and seedlings in a natural soil seed bank declined, when achene rain was prevented, to 3.9 and 1.1% of initial density, respectively, after 36 mo. Decline was attributed primarily to seedling emergence and achene death. Achene density in the upper 2.5 cm of soil appeared to be a good predictor of seedling emergence after autumn rains.
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