Yellow Starthistle Information
Reproduction
-Flower pollination
-Flower & seed
dev
-Seed dispersal
Germination
& dormancy
-Seed prod & types
-Seed development
-Germination
-Stress & germ
-Seasonal germ
-Seed longevity
Growth & establishment
-Seedling establish
-Roots
--growth
& dev
--water use
---competition
-Shoots
--growth &
dev
--light
--temperature
--allelopathy
Biology and Ecology
Germination and dormancy
Seed production and types
Average seed production ranges from about 35 to over 80 achenes per seedhead (Benefield et al. 2001, Maddox 1981), depending upon the site. Large plants can produce over 100,000 seeds. The number of seedheads and achenes per seedhead can vary dramatically and are often determined by soil moisture and other soil properties (Maddox 1981, Pitcairn et al. 1997, Roché 1991a).
Yellow starthistle infestations can produce 50-100 million seeds per acre (Callihan et al. 1992, DiTomaso et al. 1999a, Maddox 1981). Unlike most other Centaurea species, yellow starthistle produces dimorphic achenes, one type with a distinct pappus, and the other with a pappus either poorly developed or absent (Callihan et al. 1992). The pappus-bearing achenes are light to dark brown with tan striations throughout. By comparison, the non-pappus-bearing achenes are dark brown to black and lack striations. Non-pappus-bearing achenes occur in a single ring around the periphery of the head, whereas pappus-bearing achenes occur in many rings in the center of the seedhead. Development of achenes occurs centripetally, from the outer non-pappus-bearing achenes to the inner pappus-bearing achenes (Maddox et al. 1996). Of the total achenes produced, between 75% and 90% are pappus-bearing and 10% to 25% are non-pappus-bearing (Benefield et al. 2001, Maddox 1981, Roché 1965).
Time requirement for development of viable seed
No germinable seed are found before the late senescence stage of flower development (Benefield et al. 2001). Seedheads at the middle senescence stage or earlier contain only partially filled or unfilled seeds. Germination of both achene types significantly increase from the late senescence to the achene dispersal stage. The mean number of germinating seeds increases with advancing phenological stage (Maddox et al. 1996).
Although germinable seeds do not typically develop until the late senescence stage, eight days after flower initiation, some seed are viable even at the initial senescence stage five days after flower initiation. Consequently, to prevent significant seed production, late season management options should be conducted at very early stages of flower development.
Germination
Over 90% of yellow starthistle achenes are germinable one week after seed dispersal (Benefield et al. 2001, Joley et al. 1997, Roché et al. 1997, Sheley et al. 1983, 1993). Maximum germination of yellow starthistle achenes (nearly 100%) occurs when seeds are exposed to moisture, light and constant temperatures of 10, 15, or 20 oC, or alternative temperatures of 15:5 or 20:10 oC (Joley et al. 1997, Roché et al. 1997). At temperatures above 30 oC germination is dramatically reduced (Joley et al. 1997, Roché et al. 1997). Germination occurs rapidly (typically by 24 h) with nearly all seed germinating within 96 hours (Sheley et al. 1983c, 1993). Although previous studies are inconclusive (Joley et al. 1997), yellow starthistle appears to have a light requirement for germination (Joley, unpublished data).
Germination of both achene types are high, but germination of the pappus-bearing achenes is slightly greater than non-pappus-bearing achenes. In viability experiments using tetrazolium, Benefield et al. (2001) showed that germination rates and viability were not statistically different with achenes collected one week after dispersal. Thus, nearly all viable seed are able to germinate at the dispersal stage, suggesting that yellow starthistle may not have an innate or induced dormancy mechanism. Interestingly, achenes will germinate within a narrow temperature range shortly after dispersal. However, with increasing exposure to higher temperatures and low moisture (within 1 month), as would occur in later summer, seeds experience a secondary dormancy (Enloe, unpublished data). This ensures that all seed do not germinate following an occasional late summer thunderstorm, where subsequent seedling mortality may occur when no additional moisture is received over an extended time period.
Affect of stress on germination
Germination of yellow starthistle achenes can also be influenced by water or salt stress. Studies (Kiemnec and McInnis 1994, Larson et al. 1992, Larson and Kiemnec 1997) indicate that a slightly greater percentage of pappus-bearing achenes germinate at higher moisture stress than non-pappus bearing achenes. In addition, yellow starthistle germination is more sensitive to moisture stress than salt stress. The two different types might be associated with success of yellow starthistle in exploiting highly variable soil moisture regimes in semi-arid environments.
