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.

Differences in Temperature Responses of Achene Types in Centaurea melitensis

“…The iCL achenes were more restricted to cooler temperatures. Germination over a variety of temperatures is consistent in Asteraceae as seen by the studies conducted on its closely related congener Centaurea soltitialis (Joley et al 1997;Young et al 2005).My data also showed that achene of Centaurea melitensis had little to no germination at temperature of 5ºC and 30ºC. These were indicative of a very cold winter day or a very hot summer day.…”

“…They also observed a plateau of 100% germination in the middle three temperatures which ranged between 10ºC and 25ºC. They also tested fresh and stored achenes and found achenes freshly collected in July germinated best at 25ºC:15ºC and germination for stored achenes decreased at 25ºC:15ºC and 30ºC:20ºC (Joley et al, 1997).…”

“…They also remain in the flower head until winter, until they are forced out through constant drying and wetting action that occurs with fall rains (Roche, 1992). The center achenes have a pappus and are located in the middle of the flower head are released first when the flower head senescence, usually during the summer (Di Tomaso, 2001;Di Tomaso 1996;Roche, 1992;Joley et al, 1997). In Centaurea melitensis the peripheral and center achenes both have a pappus.…”

“…A more extensive study was conducted by Joley et al (1997) comparing light and temperature responses on germination of Centaurea soltitialis achenes. For the purpose of this paper only the temperature aspects will be addressed.…”

“…Each flower head also contains a peripheral and center achene that vary morphologically. Furthermore, not only do they vary morphologically but, they develop at different positions within the same flower head and disperse at different times throughout the growing season (Joley et al, 1997;Porras and Munoz, 200ab;Robbins and Bellue et al, 1951;Di Tomaso, 2001). …”