Effect of light and temperature on germination of dimorphic achenes of Centautea solstitialis in California

Abstract

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.