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
Growth and establishment (continued)
Shoots
Growth and development
Seedlings that typically germinate in late fall or early winter overwinter as basal rosettes. Rosettes develop slowly throughout the early spring. In the Central Valley and foothills of California, shoot bolting typically occurs in early summer and by mid-summer spines appear on developing seedheads. At the more mature stages of development, the pubescence and waxy grayish coating on the foliage of yellow starthistle reflects a considerable about of light. This reduces the heat load and the transpiration demand during the hot and dry summer months. The winged stems add surface area and also act to dissipate heat like a radiator (Prather 1994). These characteristics, as well as a deep root system, allow yellow starthistle to thrive under full sunlight in hot and dry conditions. Vigorous shoot growth coincides with increased light availability due to senescence and desiccation of neighboring annual species. Moreover, the presence of spines on the bracts surrounding the seedhead provides protection against herbivory. This is particularly important during the vulnerable flowering and seed development stages.
Senescence typically occurs in fall when moisture becomes limiting and plants are exposed to frost. Flowers can abort development before completion. Senesced stems can contain the non-pappus-bearing seeds for about a month until the spiny bracts and phyllaries fall off. Because the receptacle of the flowerheads contain an abundant amount of fine chaff, the seedheads will have a cotton-tip appearance. In contrast, tocalote and sulfur starthistle do not have cotton-tip seedheads after senescence. Stems of yellow starthistle degrade slowly and may remain erect for at least one year. In addition, both tocalote and sulfur starthistle have only pappus-bearing seeds that disperse once the seedhead has lost its senescenced flowers.
In grasslands where yellow starthistle rosettes are exposed to low light, the leaves are larger and develop a more vertical or erect growth form that may reach 10 inches in height. This upright form allows them to capture more light until the reproductive shoots bolt through the senescing canopy of the annual grasses (Roché et al. 1994). In contrast, when rosettes are exposed to full sunlight, they develop flatter and more compacted leaves horizontally oriented to the soil surface.
Dense yellow starthistle seedling cover can significantly suppress the establishment of annual grasses and forbs. However, yellow starthistle rosettes are also very susceptible to light suppression, and will produce shorter roots, larger leaves, more erect rosettes, and fewer flowers than plants in full sunlight (Roché and Roché 1991, Roché et al. 1994). Consequently, yellow starthistle does not survive well in shaded areas, and is less competitive in areas dominated by shrubs, trees, taller perennial forbs and grasses, or late season annuals. For this reason, infestations are nearly always restricted to open disturbed sites or grasslands dominated by winter annuals. Even in areas dominated by yellow starthistle, the level of competition for light can be so intense that seedlings will vigorously compete with each other, accounting for the low rate of seedling survival through self-thinning.
Yellow starthistle plants are insensitive to photoperiod and lack a vernalization requirement (Roché et al. 1997). This allows late germinating plants to continually reproduce provided moisture is adequate. Flowering continues until newly developing buds are killed by frost. In climates with milder winters, plants can act as biennials. However, in colder climates more typical to Northern California or other western states, mature plants rarely survive the winter. Whereas seedling can survive extended frost periods, mature plants are not considered to be frost tolerant. Cold tolerance (hardiness) appears to be lost during the transition from vegetative to reproductive phases.
A number of researchers have isolated a variety of chemical compounds from yellow starthistle (Binder et al. 1990, Bruno et al. 1991, Buttery et al. 1986, Merrill 1989, Merrill and Stevens 1985, Stevens and Merrill 1985, Stevens et al. 1990). One of the major groups of compounds identified in starthistle is the sesquiterpene lactones (Binder et al. 1990, Merrill and Stevens 1985, Stevens and Merrill 1985). The sesquiterpene lactone repin has been implicated in the toxicity of starthistle in horses.
It has been suggested that yellow starthistle might release chemical compounds that inhibit the growth of other plants, a process known as allelopathy (Kelsey and Bedunah 1989, Merrill and Stevens 1985, Stevens and Merrill 1985, Zamora et al. 1983). Allelopathy involves the release of chemical substances by one plant that inhibit the growth or development of other plants. The release of allelopathic compounds could be through glands or duct on the stem or foliage, leaching from degrading plant residue, or exudation from roots. In addition to its possible toxic effect on horses, repin has been postulated to be allelopathic to plants (Merrill and Stevens 1985, Stevens and Merrill 1985). Despite these implications, there is no direct evidence to substantiate the claim that yellow starthistle has an allelopathic effect on surrounding vegetation.
