More info for the terms: adventitious, apomixis, breeding system, competition, density, diploid, formation, fresh, genotype, hypocotyl, pappus, presence, radicle, root crown
Rush skeletonweed reproduces by seed, by root fragments, and by vegetative spread from vertical and lateral roots.
Breeding system: Rush skeletonweed is triploid and exhibits obligate apomixis ([24,58] and references therein), although sexual diploid populations can occur in the western part of Turkey ([16] and references therein). This breeding system may be beneficial to an invasive species in that it promotes genetic stability and also enables plants to reproduce in areas where environmental factors or pollinators may be limiting (Duke 1985, as cited by [54]).
Pollination: Experimental evidence presented by McVean [58] and Cuthbertson [24] indicates that viable seed production in rush skeletonweed is completely independent of pollination. Flowers are visited by a wide variety of insects, and bees are known to collect the pollen; however, these insects serve no function for the plant [40,58].
Seed production: Rush skeletonweed seed production is extremely variable [19]. Rush skeletonweed plants can produce large amounts of seed over a relatively long period. Potential seed production has been estimated in the order of 70,000seeds per m² in a dense infestation in Australia (Cuthbertson 1967, as cited by [67]). Seed production and viability are determined primarily by the vigor of the plant up to the point of seed maturity [19], and may depend on plant age, plant genotype, moisture availability, summer air temperatures, infection with biological control insects, interactions with other control methods, and other site management factors.
Rush skeletonweed flowerheads develop singly or in clusters of 2 to 5. Each head produces 9 to 12 florets, each floret forms a seed, and viability is generally high [24,54]. Most Australian first-year rush skeletonweed plants grown in a greenhouse produced between 59 and 150 flowerheads, with 10 to 12 flowers per head, and total seed production between 500 and 1,500 seeds per plant [58]. McVean [58] predicted that older, multiple-stemmed plants growing under field conditions could produce 10 times as many seeds.
In western Australia, rush skeletonweed plants (types A and C) grown in a greenhouse produced between 3,000 and 13,070 seeds per plant, with 82% viability on average. Total number of seeds in rush skeletonweed plants grown in agricultural fields ranged from 13,730 to 27,550 and averaged 19,926 seeds per plant, with 75% viability. Some plants less than 1 year old produced more than 10,000 seeds in the field [67].
Seed production characteristics also differed between the 2 types tested. At one site, type C plants produced significantly (p<0.001) more seed (14,120 vs. 8,430 seeds per plant), of greater viability (79.4% vs. 73.9%), and with lower primary seed dormancy (3.5% vs. 11%), with less variation between harvest dates than type A plants [28].
According to Cuthbertson [24] and McVean [58], moisture availability during flowering and seed maturation is a major factor in determining reproductive capacity in rush skeletonweed. McVean [58] found that well-watered plants produced seeds containing greater than 90% fully developed embryos, whereas droughted plants produced fewer than 10% viable seed. Under the most severe drought, flowering failed altogether. Similarly, Dodd and Panetta [28] found that simulated drought reduced seed numbers, viability, primary dormancy and seed weight in greenhouse-grown rush skeletonweed plants, although the 2 types, A and C, responded differently. Type A control plants averaged 7,570 seeds per plant one year and 8,290 seeds per plant the following year when type C control plants produced 2,750 seeds per plant. Drought reduced total seed numbers by 75% in type A plants, and by 46% in type C plants [28].
Conversely, in field experiments in western Australia, most field-grown plants (types A and C) produced abundant viable seed, even without substantial summer rainfall. Summer rain did not increase seed output or affect viability in established plants, and there were no relationships in either type between the various seed characteristics and cumulative rainfall for different 2-week periods prior to seed harvest. According to Panetta and Dodd [67], rush skeletonweed's extensive root system allows it to "become largely independent of" summer rainfall for flower and seed production. However, long-term drought retards growth and development of rush skeletonweed, and limits production of viable seeds [67,100].
Rush skeletonweed seed production and viability appear to be sensitive to high summer temperatures (daytime maxima exceeding 91 °F (33 °C)) with low air humidity and low soil moisture [58]. Exceptionally high air temperatures appeared to depress seed numbers and/or viability at several sites [28].
Activity of biological control agents can also affect viable seed production in rush skeletonweed (see Biological control). A large difference in rush skeletonweed seed production was observed between 2 sites near Canberra, Australia, with seed output at one site at 4 seeds per m² compared with 1,642 seeds per m² at another site. The only difference between the 2 areas was the presence of 3 biological control organisms (the rust, the mite, and the midge) at the former site [19]. Stem colonization by gall mites (Eriophyes chondrillae) reduces the number of flowers and seeds produced. In greenhouse experiments, rush skeletonweed plants without gall mites averaged 57 floral stems per plant, and the number of floral stems per plant decreased with increasing mite density. Plants averaging 180 mites per plant had 27 flowering stems; plants averaging 890 mites per plant had 18 flowering stems; and plants averaging 1600 mites per plant had 15 flowering stems [73]. In Washington, McLellan [57] observed reduced seed production in gall mite-infested rush skeletonweed plants compared with uninfested plants; and Cullen and others [20] demonstrated up to 96% reduction in flower production in rush skeletonweed plants infected by gall mites.
Mowing rush skeletonweed before flowering, once or twice in a season, resulted in significantly (p<0.05) lower seed production [57].
Seed dispersal: Rush skeletonweed seeds are readily dispersed. Seeds are small and possess both a pappus, which facilitates wind dispersal, and a rough seed coat with small teeth, which increases the chances of attachment to animals and other vectors. Rush skeletonweed seed may also be dispersed by a variety of human vectors, including rush skeletonweed contamination of agricultural products, especially hay. Transportation corridors are a common location for rush skeletonweed seed dispersal [67].
Seed banking: According to several researchers, rush skeletonweed seeds remain viable for several years in the laboratory. Cuthbertson [22] found that germination rates remain high (93.6% to 98.7%) for rush skeletonweed seed stored in open containers in the laboratory for 1 to 2 years. Germination rates decreased rapidly after that, with some germination after 3 years, 0% to 2.6% germination after 4 years, and no germination after 5 years of storage. Storage temperature and other conditions are not given [22]. According to Old [62] rush skeletonweed seeds refrigerated at 37 °F (3 °C) maintained a high degree of viability for up to 8 years and showed a low degree of viability after as long as 16 years. In most cases length of time to germinate increased with length of storage. Liao and others [53,54] found that up to 60% of rush skeletonweed seeds collected in Idaho remained viable after 1 year of storage, with viability decreasing over time. Stored seeds generally exhibited higher germination rates (90% average) than fresh seeds (67% average), indicating possible dormancy and afterripening effects [53].
Several authors suggest that most rush skeletonweed seeds exhibit little or no dormancy (estimates between 0 and 40% dormancy) [22,58,60,65,82], although this attribute appears to vary between rush skeletonweed types [67]. Because mature rush skeletonweed seeds typically germinate readily, regardless of collection date, over a wide range of temperatures, and independent of light availability [53,58,64], they may not form a long-lived seed bank. Further evidence to support this comes from studies indicating that rush skeletonweed seeds are short-lived under field conditions and generally survive for less than 6 to 18 months [64,82]. Liao and others [53] found up to 60% of non-germinating rush skeletonweed seeds were viable, suggesting that nongerminating seeds may persist in the seedbank under some conditions.
Panetta [64] found that buried rush skeletonweed seeds germinated readily following summer rainfall events of less than 0.4 inch (10 mm) in Australia. Seedlings did not survive due to inadequate moisture availability following germination. Seeds lying on the surface were less likely to germinate in response to small rainfall events, but were prone to predation by seed-harvesting ants. These conditions lead to considerable losses of rush skeletonweed seed. Panetta [64] also found that only "a few" rush skeletonweed seeds survived after 4 months burial at 2 inches (4 cm). According to Cullen and Groves [19], fire and breakdown by pathogens may also lead to rush skeletonweed seed losses, although no details are given.
There is little to no rush skeletonweed seed accumulation between years under field conditions [19,58,65]. Panetta [65] measured a maximum of 3.1% of a rush skeletonweed seed crop surviving between years, during a year with a particularly dry summer. In Washington, attempts to recover rush skeletonweed seeds from soil under dense infestations have been unsuccessful, a factor which gives some support to the Australian conclusion that few rush skeletonweed seeds remain viable in the field from one year to the next [82].
Germination: In general, rush skeletonweed seeds have high viability and high germination rates. Viability is not dependent on pollinators [24] and does not appear to be affected by moisture availability during the growing season [54], although it does appear to decrease during storage [22,54,60,62]. Germination of rush skeletonweed seeds does not require light [22,58] and occurs over a wide range of temperatures [58,60]. Germination is sensitive to moisture availability and depth of seed burial [22].
Cuthbertson [24] found 95.8% seed viability from unstressed rush skeletonweed plants, while McVean [58] found that, even under ideal germination conditions, up to 20% of ripe embryos may "remain dormant or die." Normally dispersed rush skeletonweed seeds collected in Washington gave no indication of innate dormancy. Immediately after collection, samples gave 95% germination on blotters [82]. Storage tests indicate viability may be lost over time (see Seed banking).
Rush skeletonweed seeds from 2 populations of rush skeletonweed in southwestern Idaho were capable of immediate germination without scarification or wet prechilling. Total germination generally ranged from 60 to 100% throughout the entire seed production period (July through October). Germination percentages were lower for rush skeletonweed seeds harvested late in the season. Germination was rapid, reaching 50% of total in less than 12 days. Germination differed among harvest dates, rearing sites, and between years of collection. Rush skeletonweed seed from the lower elevation site tended to have higher viability, especially early in the season. Seeds formed in the dry year (4%-33% below average precipitation) were as viable and had higher and faster germination than those from the wet year (12%-43% above average precipitation) [53,54].
Rush skeletonweed seed germination occurs over a wide range of temperatures between 45 and 104 °F (7-40°C) [58,60]. Germination is extremely rapid at the optimum temperature (around 77 °F (25 °C)), beginning within 6 to 8 hours and reaching 60% to 90% of its maximum within 24 hours [58]. Germination of rush skeletonweed seed was higher at an incubation temperature regime of 68/50 °F (20/10 °C) (50% germination in 6 days) as compared with 68/86 °F (20/30 °C) (50% germination in 9.5 days) (dark/light, respectively) [53].
Germination of rush skeletonweed seed is sensitive to moisture availability. Cuthbertson [22] found that rates and final percentages of germination were reduced progressively at osmotic tensions below -0.2 MPa, until germination ceased at -1.6 MPa. According to Schirman and Robocker [82], water held by the pappus of a group of rush skeletonweed seeds following a summer shower in Washington is sufficient to support radicle elongation. They found complete germination inhibition only at tensions exceeding -1.5 MPa [82]. Similarly, buried rush skeletonweed seeds germinated readily following summer rainfall events of less than 0.4 inch (10 mm) in Australia [64].
Moisture loss may be rapid when fully or partially imbibed rush skeletonweed seeds are exposed to drying influences, so germination may be promoted by slight burial [58,67]. In Australian studies, rush skeletonweed seeds lying on the surface were much less likely to germinate in response to small rainfall events [64]. Seedlings emerged successfully from rush skeletonweed seeds buried up to about 2 inches (5 cm) in sandy soil, but did not emerge from seeds at this depth in soils of finer texture [58]. Maximum depth of seed burial resulting in rush skeletonweed seedling emergence was 1 inch (2.5 cm) in a medium-textured soil, and no emergence was observed from seeds buried below 0.75 inch (2 cm) in clay soils ([60] and references therein). Rush skeletonweed seeds are sensitive to reduced oxygen and fail to germinate below the surface of waterlogged soil [58].
Seedling establishment and growth: Rush skeletonweed seedling establishment is highly variable from year to year and from site to site [19], and depends on moisture supply, soil texture and nutrient status, and depth of seed burial.
Rush skeletonweed seedlings require a continuous supply of moisture for 3 to 6 weeks in order to establish a viable root system and to avoid death by desiccation [19]. Depletion of seed pools may follow isolated rainfall during summer months, when there is enough moisture to allow germination, but not enough to support seedling establishment [58,64]. During experiments in Australia, only 0.03% of viable rush skeletonweed seeds established seedlings following 1.5 inches (38 mm) of simulated rainfall during January (summer), whereas 50% establishment occurred under similar conditions in autumn [58]. Panetta [64] observed virtually no establishment from summer germination.
Rush skeletonweed seedling establishment is also dependent on soil conditions. Experimental evidence presented by McVean [58] indicates that soil disturbance is extremely favorable to seedling establishment and must be a factor of considerable importance in the field. Cuthbertson [22] presents data on rush skeletonweed seed germination and seedling establishment in different soil types at varying burial depths. Emergence was reduced and delayed with increased burial depth in all soil types, with little emergence from rush skeletonweed seeds buried deeper than 1 inch (2.5 cm), and no emergence from seeds buried 2 inches (5 cm) deep in any soil type. Optimum emergence was from a depth of 0.2 inch (6 mm) in all soils except sand. Hypocotyl extension by rush skeletonweed was restricted in clays relative to other soils. Cracking and heaving in clay soils may also impede rush skeletonweed establishment [22].
Survivorship of 1st-year rush skeletonweed plants was higher on a deep sand than on a loamy sand overlaying a sandy clay (duplex soil) in Australia. Mortality on the latter was associated with drying of the A horizon during early summer. No rush skeletonweed plants produced seeds in their 1st year of growth on the duplex soil. Both rush skeletonweed plant types tested produced viable seeds when growing on the deep sand in both years of the study. The author suggests that while large areas of the Australian wheatbelt may be climatically suitable for rush skeletonweed, its invasive potential may be limited by edaphic characteristics over large areas [66].
Experimental evidence presented by McVean [58] indicates fairly high levels of readily available calcium and phosphorus are both important for initial establishment of rush skeletonweed seedlings. Low levels of nitrogen (0.062%) do not preclude rush skeletonweed establishment, although seedlings do better with higher levels. Rush skeletonweed seedlings are also sensitive to competition and shading from other plants [58,82].
Asexual regeneration: The primary means of local population increase of rush skeletonweed is through vegetative regeneration rather than by seedling establishment [19,67], although this may vary among years. Rush skeletonweed has an extensive and persistent root system and can reproduce vegetatively by adventitious buds on vertical and lateral roots in undisturbed plants, and in response to injury [23].
The taproot of rush skeletonweed may reach several meters into soil and branch at depth. Lateral root production in rush skeletonweed may vary with soil type, plant density, and plant biotype. Lateral roots occur most commonly on rush skeletonweed plants in sandy soils [19], and in sparse stands of rush skeletonweed rather than in dense stands. In Washington, the early-flowering type had more lateral roots than the late-flowering type [80]. Many laterals are short (<3 inches (8 cm) long) and ephemeral, lasting 1 season at most, while some laterals near the soil surface show taproot characteristics and grow horizontally for 6 to 20 inches (15-50 cm) before turning downward [67].
In established, undisturbed rush skeletonweed plants, daughter rosettes can develop from adventitious buds near the top of the tap root, giving rise to a root crown that bears several rosettes with a common root system [23]. These stem buds usually occur in the upper 2 to 4 inches (5-10 cm) of the main taproot [23,80]. Rosettes may also form from buds along the main lateral roots, 3 to 6 inches (8-15 cm) below the soil surface, and about 8 to 18 inches (20-45 cm) away from the parent plant. These rosettes may form their own roots to become satellite plants when the original lateral root connection with the parent plant breaks down [19,23,40,58,62,67,80]. Rush skeletonweed infestations may spread by as much as 2 feet (0.6 m) per year by formation of satellite plants along lateral roots [62].
It appears that the asexual regenerative capacity of rush skeletonweed plants during bolting, flowering and seed production is lower than that of vegetative plants. Rush skeletonweed plants observed in Washington ceased bud production after flower stem initiation in mid-April, and started again after seeds were dispersed in mid-September to early October [80].
When established rush skeletonweed plants are subjected to cutting, grazing, herbicide or insect damage, or any severe injury to rosettes or taproots, adventitious root buds may be produced almost anywhere on the remaining root system and give rise new rosettes [23,37,58,67,73]. New stems can reputedly reach the surface from buds over 3 feet (1 m) deep [58,67], although most regeneration occurs from depths of less than 18 inches (45cm) ([67] and references therein).
A critical factor in rush skeletonweed regeneration after injury is the energy balance of the root [19]. The relatively thick and fleshy taproot and main branch roots are usually rich in carbohydrate reserves ([23,67] and references therein). The level of carbohydrate reserves is relative to the age of the plant, time between successive injuries, and the rate of recovery as influenced by vigor of the resulting top growth [19]. Regeneration was observed from 5-week-old plants after removal of the rosette at ground level (Groves and Hull 1966, as cited by [23]). Under field conditions, however, the small roots of such young plants would be extremely susceptible to desiccation and therefore unlikely to regenerate [23].
Rush skeletonweed roots are extremely fragile [60] and when severed, new rush skeletonweed shoots can form from detached root fragments [23,37,58,62,63,80]. Cultivation or other soil disturbance can greatly increase rush skeletonweed rosette density since the roots become fragmented and each tiny fragment may give rise to a new plant [37,58,62,63,98]. The regenerative capacity of rush skeletonweed by root fragments is influenced by moisture availability, fragment size, depth of burial, soil type, and time of year; as well as plant characteristics such as plant size, depth of origin of the root fragment, and plant age, reproductive status, biotype and vigor [67].
In general the capacity of rush skeletonweed to regenerate from root fragments increases with the size of the fragment [23]. Rush skeletonweed root segments as small as 1 inch (2.5 cm) produced 3 buds (Kefford 1964, as cited by [60]). In a laboratory experiment, Cuthbertson [23] observed little to no shoot production from 0.4 inch (1 cm) rush skeletonweed root cuttings at all burial depths and soil types, while shoots from fragments 0.8 to 1.5 inches (2-4 cm) long emerged from up to 16 inches (40 cm) deep in sand. Fragments this small would likely die of desiccation in the field.
In general, the capacity of rush skeletonweed to regenerate from root fragments decreases as the depth of burial increases, but this also varies with soil type. Shoot growth and emergence from taproot cuttings are generally greater in sandy soils than in clays. Fragments 1.5 inches (4 cm) long were unable to emerge from 4 inches (10 cm) deep in heavy clay soil, and were able to emerge from all burial depths up to 16 inches (40 cm) deep in sand [23].
Root fragments had different sprouting capacities at different times of year in eastern Washington. Root sections formed buds from October to March, while bud formation was nearly zero during June, and increased in August and thereafter, decreasing in November. Decline in shoot production coincides with increased day length and flower stem initiation [80].
Rush skeletonweed plants attain the capacity to reproduce vegetatively at a very early age. Under laboratory conditions, shoots were produced by about 40% of cuttings taken from taproots of 2- to 4-week-old seedlings, and by 95% to 100% of cuttings from 5- to 7-week-old plants [23]. Excised roots averaging 7 inches (18 cm) long, from 36-day-old seedlings, produced an average of 5.8 buds per root after 21 days at 77 °F (25 °C) in darkness. Under the same conditions, 17-day-old rootlets averaging 6 inches (16 cm) in length failed to produce stem buds (Kefford 1964, as cited by [60]).
All parts of the taproot appear to be capable of producing new shoot initials, although the number of shoots produced per fragment decreased as depth of origin increased [23,80]. Distance from the root apex had no effect on the regeneration of 1.5-inch (4 cm) sections excised from 50 day old roots. All sections regenerated and produced approximately 3 buds per section (Kefford 1964, as cited by [60]).
In Australia, different types of rush skeletonweed differ in their regenerative capacity. Root cuttings from type C plants produced greater numbers of shoot buds than those of types A and B, regardless of the root diameter, plant age or depth of origin of the cutting. Bud production increased with age in types A and C, but decreased in Type B plants (Hull and Groves 1973, as cited by [67]). Similarly, Rosenthal and others [80] observed differences in root production between the early- and late-flowering biotypes in Washington. After 6 months growth in buried cans, the early-flowering type averaged 6.7 major roots per plant and the late-flowering type averaged 10.7 major roots per plant. After 18 months, the early-flowering plant showed no further increase in root numbers, while the late-flowering plants averaged 16 major roots per plant [80].
Regeneration from root fragments may also depend on the reproductive status of the plant from which the fragment originates. Regeneration occurred on 7% of cuttings from the top 16 inches (40 cm) of taproot from reproductive plants, compared with 15% for vegetative plants [23].