Xanthium strumarium

XANTHIUM STRUMARIUM is distinguished from spiny clotbur (X. SPINOSUM) by its broader cockleburs, more ovoid leaves on long petioles, and lack of spines.

Cocklebur is often associated with open, disturbed areas, particularly flood-prone areas with good soil moisture (Martin and Carnahan 1982), but it is found in a wide variety of habitats. It frequents roadsides, railway banks, small streams, and riverbanks, as well as the edges of ponds and freshwater marshes and overgrazed pastures. It does not tolerate shading (Sen 1981).

Cocklebur grows on a wide range of soils (sands to heavy clays) and available moisture. On rich soils with abundant moisture and little competition from other plants, it grows tall and luxuriant, forming pure stands. In dry, poor soils, plants may grow to only a few centimeters high, persist through drought, and set seed. The ability to grow under a variety of conditions results in a continuous seed supply, if plants are not controlled (Holm et al. 1977).

Cocklebur withstands partial submergence for six to eight weeks by forming adventitious roots from the submerged portion of the stem. These roots float in water and often get infested with oxygen- producing green algae (DEDOGONIUM) which solves the problem of aeration (Ambasht 1977).

POPULATION DYNAMICS

Weaver and Lechowicz (1983) describe two types of cocklebur populations. Populations located along shores or water courses tend to be small, ephemeral, and homogeneous with seed dispersal by wind and water. Populations in ruderal (weedy) habitats, agricultural fields, or waste areas tend to be large, dense, and heterogeneous with tall, vigorous plants producing an abundance of seed. Seed dispersal here is primarily the result of human activities. Both types of populations, however, occupy unstable habitats and are continually shifting to newly disturbed areas.

Cocklebur is an extremely competitive weed in corn, cotton, and soybeans fields, particularly in the southeastern and midwestern U.S. (Miller 1970, Charudattan and Walker 1982). Though not as abundant in California, it is still a serious problem in agricul- tural areas (Vargas 1984), as well as in recreation areas and along reservoirs (Wright and Schweers 1984). Some plants appears to have allelopathic properties (Cutler 1983).

The burs cause an allergic reaction in some people (Parsons 1973) and are toxic to domestic animals (and perhaps to some wildlife). Poisoning threats are greatest in areas where other, more palatable plants have already been consumed (Holm et al. 1977). Ingesting an amount of seeds equal to only 0.3 percent of an animal's body weight will cause toxicity. Still, this rarely occurs as the spiny burs are not palatable to animals.

However, the cotyledons are palatable and also have the highest toxicity. Poisoning generally results when these are eaten. This situation occurs most at the edges of ponds, lakes, flood plains, or other bodies of water where shallow flooding followed by recession of the waterline occurs. Under such conditions seeds germinate readily, constantly supplying new generations of potentially poisonous seedlings as the water source dries out. Animals are attracted to such areas because of their need for drinking water. The problem is accentuated because XANTHIUM seeds do have natural dormancy and germinate over long periods of time. Ingestion of an amount of cotyledons equal to 0.75 to 1.5 percent of the animal's body weight will cause toxicity.

Toxicity decreases rapidly as true leaves are formed. Evidence of poisoning appears in about 12 to 48 hours, the symptoms being nausea, vomiting, lassitude, depression, weakened muscles, and prostration. Severe poisoning may result in convulsions and spasmodic running movements. Ruminants may not vomit. Death may occur within a few hours or days. Fatty substances such as milk, lard, or linseed oil have been recommended as antidotes (Kingsbury 1964).

Kaul (1971) includes the following reasons for cocklebur's ability to inhabit such a range of habitats: an effective dispersal mechanism, wide ecological amplitude, heavy output of seeds and high viability and germination under varied environments, high reproductive capacity, large seed size and weight, rapid seedling growth, and a well-developed root system.

FLORAL BIOLOGY

XANTHIUM STRUMARIUM is wind-pollinated, self-compatible, and predominately self-pollinated (Love and Dansereau 1959). The staminate heads of X. STRUMARIUM are located above the pistillate heads on the main axis and side shoots, an arrangement favoring inbreeding (Weaver and Lechowicz 1983). Moran and Marshal (1978) found the outcrossing rate in natural populations to be 0 to 12%.

In a Quebec experimental garden, individual plants produced from 611 to 1,488 male inflorescences (Weaver and Lechowicz 1983). The 100-150 male florets in each staminate head begin to shed their pollen from a few days before the stigmata are receptive until all female flowers are ripe. The slightest movement of the plant or a gust of wind causes the pollen to rain down over the exposed stigmata of the female flowers immediately below. The pollen of the plant itself is therefore most likely to ensure the fertiliza- tion of its female flowers, and only an accident, a strong wind, or crowded growth, accomplishes cross-fertilization. In XANTHIUM, inbreeding is thus the rule and outbreeding only an occasional occurrence (Love and Dansereau 1959).

Cocklebur has been widely used as an experimental plant in studies of photoperiod. Love and Dansereau (1959) list 34 articles on XANTHIUM photoperiod, and many more have been written since that time (Cleland and Ajami 1974). According to Salisbury (1969), X. STRUMARIUM is a short-day plant and usually does not flower when day length exceeds 14 hours. However, there is evidence of differences in light response among the complexes, as some plants flower with day lengths as long as 16 hours.

At high latitudes, day length is greater than 14 hours during summer, and therefore, X. STRUMARIUM does not flower until late summer, once day length is short enough to stimulate flowering. Seeds mature late under these conditions, usually in early autumn. These differences are considered to represent genetic adaptations of the reproductive system to environmental variables as a result of natural selection (Ray and Alexander 1966). The cotyledons do not play a role in flower induction (Holm et al. 1977).

SEED PRODUCTION AND DISPERSAL

Open grown X. STRUMARIUM plants produce 500 to 5,400 burs per plant. The number of fruits produced is dependent upon the amount of vegetative growth at the time of floral initiation. On crowded plants, production is reduced to 71 to 586 burs per plant (Weaver and Lechowicz 1983). Burs are buoyant and will float for up to 30 days (Kaul 1961), thus being easily dispersed to beaches and pastures subject to flooding. The burs also become entangled in animal hair or human clothing. The burs are a serious problem in sheep production areas where they become entangled in the wool, reducing its value (Wapshere 1974a). X. STRUMARIUM burs contain a highly toxic substance, carboxyatractyloside, capable of killing hogs, cattle, goats, horses, sheep, and poultry.

SEED VIABILITY AND GERMINATION

Germination of cocklebur seeds has been extensively researched (Crocker 1906, Davis 1930, Katoh and Esashi 1975, Zimmerman and Weis 1983). More than 80% of cocklebur seeds are viable in most populations (Weaver and Lechowicz 1983). Light is not required for germination, but seedlings seldom emerge from seeds lying on the surface or buried more than 15 cm in the soil (Kaul 1965a, Stoller and Wax 1973).

Seeds of XANTHIUM STRUMARIUM have a high moisture requirement for germination and show little germination in soils at less than 75% of field capacity, but they are able to absorb moisture at high osmotic concentrations (Kaul 1968). Cocklebur seed viability decreases over time, and seeds do not survive more than a few years (Wapshere 1974b). Seedlings are unusually large with foliar-type cotyledons that, through early photosynthetic function, enable the young seedling to become quickly established (Polunin 1966). Seedlings may be identified in the cotyledon stage by the presence (below ground) of the persistent bur, which usually remains attached to the seedling (Kingsbury 1964). The species does not reproduce vegetatively (Weaver and Lechowicz 1983).

XANTHIUM STRUMARIUM plants produce seeds of two types (termed somatic polymorphism). Each bur contains two seeds, with the smaller one often pushed upwards toward the beaked end of the fruit. The lower seed has a shorter dormant period and germinates first. Dormancy in XANTHIUM involves the presence of a different water-soluble germination inhibitor in each seed type, to which the testa are impermeable. The presence of oxygen causes degradation of these two inhibitors and subsequent rupture of the seed coat, but apparently at very different rates in the two types. Thus at least two batches of seeds are present in each generation to assure germination in the event the immediate environment happens to be unsuitable (Redosevich and Holt 1984).