Portulaca oleracea (purslane)

Portulaca oleracea (purslane) is an ancient, cosmopolitan species in which self fertilization is the rule. Hence, local populations exist which reflect variable morphological and physiological traits expressed as part of the genome of that population. However, purslane has not been split into a series of microspecies.The two most recent comprehensive monographs, Legrand (1962) and Geesink (1969) provide, respectively, only three and two names as synonyms. Gorske et al. (1979) conducted a numerical taxonomic analysis of 36 morphological characters on 44 ecotypes from 18 countries, which included the cultivated form commonly known as Portulaca oleracea var. sativa. They found three morphological groups, that form a clime: (1) cool temperate, (2) warm temperate to subtropic and (3) humid subtropic to tropic. They did not propose that these groups should receive nomenclatural recognition.Danin et al. (1978) proposed nine subspecies, on the basis of seed size, seed surface pattern and chromosome number. However, it may be that these subspecies could be expanded almost ad infinitum on the basis of the variety of seed surface patterns that can be found in the species (Matthews et al., 1993). Legrand (1962) stated that seed size, sepal wings and number of stamens are environmentally influenced and that seed surface pattern was not predictable or dependable as a taxonomic character because of the infinite transition patterns that are present.In a chemotaxonomic study comparing proteins and free amino acids, Prabhakar and Ramayya (1988) found that, within the complex P. oleracea, the var. ophemera is distinct from vars oleracea and sativa.Matthews et al. (1993) concluded that P. oleracea exists as a polymorphic species and is not readily divisible into subspecies on the basis of seed surface patterns, chromosome number or other morphological traits that are subject to environmental influences.

P. oleracea is mostly an annual, but it may be perennial in the tropics. Stems are glabrous, fleshy, purplish-red to green, arising from a taproot, often prostrate, forming mats. The leaves (also fleshy) are alternate, subalternate or opposite, obovate to spatulate with an obtuse or truncate-emarginate apex. The leaves may range from 40 mm x 15 mm up to 60 mm x 25 mm in fertile soils. Apical whorls have 2-5 leaves, usually 4. Axillary hairs are missing, inconspicuous or barely visible. Flowers are in a group at the end of the stem. The 2 sepals are fused at the base of the ovary and may form a wing-like carina 3-4 mm long that can cover the fruit. There are (4)5(6) yellow petals ranging from 3 to 10 mm long by 2 to 8 mm wide with 6-15 (3-20) stamens. The style branches are 3-6, the capsule ranges from 4 to 9 mm, opening at or just below the middle. Seeds are black when mature, but may be red or brown when immature. The seeds are 0.6-1 mm long, usually with granulate to flat-stellate surfaces. However, other patterns, with raised stellate and tuberculate surfaces can occur.

P. oleracea grows from sea level to 2600 m (Vengris et al., 1972) and is most common in the temperate and subtropical regions, although it extends into the tropics and higher latitudes. Common latitudes are between 45°N and 40°S, with extension to 58°N in North America and 54°N in Europe (Matthews et al., 1993).

The region of origin is uncertain, possibly an arid climate such as North Africa (Chapman et al., 1974). Although spred to the New World was thought to have been due to post-Columbian humans (Matthews et al., 1993), archaeological evidence (pollen analysis) suggests that P. oleracea arrived in the New World in pre-Columbian times (McAndrews, 1975).

P. oleracea competes for resources with many field crops, particularly herbaceous species that are germinating or growing in competition. Affected crops include: asparagus, red beets, celery, crucifers, cotton, maize, onions, potatoes, rice, soyabeans, sugarcane, tomatoes and wheat.

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P. oleracea is common in fields, gardens, vineyards, lawns, driveways, dunes, beaches, salt marshes, waste areas, eroded slopes, bluffs and riverbanks.

GeneticsCytologically, P. oleracea is characterized by a sequence of polyploids, from a base number of x=9. There are diploid races (2n=18) in Africa, Central and North America; tetraploid races (2n=36) in India, Central and North America and hexaploid races (2n=54) in India, Africa, Europe, North America and Hawaii. There are additional reports of 2n=45 in India and 2n=52 in Japan. For further information see Hagerup (1932), Cooper (1935), Sugiura (1936), Steiner (1944), Heiser and Whitaker (1948), Sharma and Bhattacharyya (1956), Mulligan (1961), Walters (1964), Bouharmont (1965), Khullar and Dutta (1973), Danin et al. (1978), Sanjappa (1978), Boquar (1986), Danin and Anderson (1986), Nyananyo and Okoli (1987), Kim and Carr (1990b), Matthews et al. (1993).Physiology, Phenology and Reproductive BiologyP. oleracea is mostly an annual, but it may be perennial in the tropics. From a typical, open, disturbed habitat purslane grows rapidly, producing flowers, fruits and seeds within 6 weeks of germination. It has a wide tolerance of photoperiod, light intensity, temperature, moisture and soil type. Seeds germinate under conditions that enhance the survival of seedlings. The species is self-compatible.Purslane reproduces primarily from seed. Over 6000 seeds can be produced after the first flush of flowers (5-6 weeks of growth). One plant can produce between 100,000 and 242,000 seeds over an entire season. A germination rate of over 90% has been recorded after 2.5 years and other germination studies have shown 39% germination after 0.2 years, 78% after 1 year, 59% after 7 years, and 59% after 14 years; 40-year-old seeds were viable. Over 60% of seed remains viable after passage through a house sparrow (Passer domesticus).Light is required for germination, but the temperature requirement is variable. Seeds can germinate at 10°C in the northern USA and in India, seeds germinate over the range 10-40°C, but not above 50°C. Germination response to light and temperature varies according to the site of origin and the time of seed maturation. In the dry season, seeds that developed on the upper 20% of the plant were less dormant than seeds from the lower 20%. El-Keblawy and Al-Ansari (2000) investigated the effects of site of origin, time of seed maturation and seed age on germination behaviour.The temperature below which development of P. oleracea ceases was determined by Steinmaus et al. (2000). Kruk and Benech Arnold (1998) modelled thermal responses in Argentina. Results allowed the determination of seed germination models that predict the occurrence of seedling emergence in the field and the dynamics of seed dormancy within those periods. P. oleracea seedlings can represent about 15% of the seed bank each year in the Corn Belt of the USA. Vegetative reproduction can occur by the development of adventitious roots from the base of cut shoots, but there is no evidence of adventitious rooting from unwounded shoots.After germination, purslane branches almost immediately. Flowers can be produced in day lengths from 4-24 hours. There is no flowering photoperiod. Capsule production and overall plant growth increase with day length. Capsules can mature under soil conditions of high or low moisture. Flowers will not open on cloudy days or days when the temperature is below 21°C. When opened, they remain open for four hours. The flowers are self-fertile and do not exhibit apomixis. No insect pollinators have been observed during a three-year study. Some investigators have said that the flowers are wind-pollinated, but the pollen is very sticky, a characteristic that is not present in windborne pollen.Environmental RequirementsC4 metabolism allows P. oleracea to optimize photosynthesis in conditions of high heat and bright sunlight while enduring periods of limited water availability (Koch and Kennedy, 1982). Lara et al. (2003) suggested that there is an induction of a Crassulacean acid-like metabolism (CAM) after 21-23 days of drought stress in P. oleracea.

The natural enemies of P. oleracea listed refer to recent investigations aimed at finding biological control agents; it is premature at this stage to indicate their importance. For further investigations into the potential for biocontrol of this weed see Zakharyan and Akopyan (1974); Gadoury and Watson (1987); Waterhouse (1993).The insects include Schizocerella pilicornis, Hypurus bertrandi, Nysius vinitor and Baris arctithorax. Their primary activity involves leaf mining, with some activity on the external parts of leaves, stems and fruits.For further information, see Norris (1985), Awadallah et al. (1976a, b), Elshafie (1976), Gorske and Hoppen (1976), Gorske et al. (1976), and Clement and Norris (1982).FungiFungal infections include Dichotomophthora portulacae, Drechslera indica, Helminthosporium portulacae [Drechslera portulacae] and several strains of Actinomycetes.For further information, see Norris (no date), Klisiewicz et al. (1983), Strider and Chi (1984), Vegh and LeBerre (1984), Klisiewicz (1985), Baudoia (1986), Mitchell (1986), Evans (1987), Kenfield et al. (1989) and Sugawara et al. (1992).

P. oleracea is an aggressive weed in most agricultural settings. Seeds on or near the surface of the soil germinate rapidly following ploughing (seeds require light for germination), so there is immediate competition with newly sown crops. This rapid growth is usually horizontal, covering the surface of the soil. Yields can be reduced by 20-40%, depending on the crop. Purslane grows best under warm conditions, so crops in subtropical areas are affected more than those in temperate areas.A field experiment was conducted in China to determine the relationship between the yield loss of summer maize and infestations of P. oleracea. Weed infestations did not significantly affect grain weight or ear number of the maize. The relationship between yield loss and the density of P. oleracea was S-shaped (Ni HanWen et al., 2000).Field experiments were conducted in Brazil to determine the effect of different periods of weed competetion on groundnuts. The presence of weeds including P. oleracea resulted in decreased pod and kernel yields and groundnut dry matter (Kasai et al., 1997). Reservoir for Other PestsPurslane can also act as a reservoir for other diseases, particularly those caused by nematodes and some viruses.For further information on nematodes, see Ferraz et al. (1978), Bendixen (1982), Kholod (1983), Zem and Lordello (1983), Khan and Khan (1985), Maqbool et al. (1986), Izquierdo et al. (1987), Tedford and Fortnum (1988), Inserra et al. (1989), Dabaj and Jenser (1990), Zehr et al. (1990), Salawu and Afolabi (1994).For further information on viruses, see Locatelli et al. (1976), Pochard (1977), Locatelli et al. (1978), Dodds and Taylor (1980), Allen et al. (1983), Nasser and Basky (1988), Dikova (1989), van Os et al. (1993), Stevens et al. (1994).

P. oleracea appears to be an excellent candidate for inclusion in saline drainage water reuse systems (Grieve and Suarez, 1997). It is highly tolerant of both chloride- and sulphate-dominated salinities, is a moderate selenium accumulator and a valuable vegetable crop for human consumption (Bianco et al., 1998) and for livestock forage. It is also a source of a gum with emulsification properties that can be used in the food industry (Garti et al., 1999). P. oleracea is a common weed in Australia and can be used as a demulcent, diuretic, antinflammatory and antibiotic (Cowper, 1996).

Introduction

The fleshiness of this weed makes it very resistant to desiccation, and hence often inadequately controlled by hoeing.

Physical control can be by mechanical means (Miyanishi and Cavers, 1981) or by the use of polyethylene film mulches to prevent germination (Inada et al., 1973; Kang et al., 1986; Ricotta and Masiunas, 1991; Zhang et al., 1992).

The natural enemies of P. oleracea listed refer to recent investigations aimed at finding biological control agents; it is premature at this stage to indicate their importance. For further investigations into the potential for biocontrol of this weed see Zakharyan and Akopyan (1974); Gadoury and Watson (1987); Waterhouse (1993); however, the primary method of control appears to be by chemical means.

Chemical Control

Atrazine, bentazone, bromoxynil, chloramben, chlorbufam, chlorpropham, chlorsulfuron, clomazone, clopyralid, dimethametryn, diuron, fluazifop-butyl, fluroxypyr, imazethapyr, linuron, methabenzthiazuron, metolachlor, metribuzin, napropamide, naptalam, oxadiazon, oxyfluorfen, pendimethalin, piperophos, pretilachlor, prodiamine, propanil, sethoxydim, simazine, thiazopyr and trifluralin have been used in attempts to control P. oleracea; this list is derived from hundreds of field trial reports, involving preplanting, pre-emergent and post-emergent treatments (concentrations and combinations of different chemicals are available in the literature).

The application of imazaquin, sulfentrazole and diclosulam pre-emergence; propaquizafop post-emergence followed by oxasulfuron + lactofen; haloxyfop followed by chlorimuron + lactofen gave good control of P. oleracea in soyabean (Laca Buendia et al., 1999).

Flumetsulam + trifluralin were more than 90% efficient in controlling P. oleracea with no apparent phytotoxicity (Jovanovic Radovanov et al., 1999).

Selected references for crops used in tests of herbicides to control P. oleracea:

Asparagus: Granier (1990)
Bananas: de Almeida and Texeira (1974)
Celery: Dusky (1983)
Maize: Kahurananga et al. (1973)
Cotton: Quinones (1987); Ramesh-Babu and Rao (1993)
Crucifers: Marion et al. (1985); Sieczka and Creighton (1985)
Onions: Blanco et al. (1982); Deuber et al. (1983)
Potatoes: Shehata et al. (1990)
Rice: dos Santos and Garcia (1983)
Soyabeans: William and Chiang (1976); Gazziero (1982); Gazziero et al., 1983)
Sugarcane: Kuntohartono and Tarmani (1980); Webb and Feez (1987)
Tomatoes: Rizzotto (1972); Acosta (1981)
Wheat: CIMMYT report on wheat improvement in 1979 (1981).

Herbicide Resistance

Greenhouse experiments were conducted to confirm and quantify linuron resistance in P. oleracea collected from a carrot field in Michigan, USA. A preliminary evaluation was made using a flotation test kit to identify resistance to linuron and atrazine. Subsequent greenhouse experiments indicated that P. oleracea was resistant to certains rates of linuron and atrazine. The resistant P. oleracea was also highly resistant to diuron, cyanazine and prometryn but had a low level of cross resistance bromoxynil. Both resistant and susceptible biotypes of P. oleracea were sensitive to hexazinone and bentazone (Masabni and Zanstra, 1999a, b).

Cross resistance to triazines, ureas and amides has been reported (Heap, 2000).

Allelopathy

An infusion of rue (Ruta graveolens) was tested for inhibitory effects on germination and growth of the radicle of P. oleracea. The rue infusion and its isolated allelochemicals (5-methoxysporalen, 8-methoxysporalen and quercetin) delayed the onset of germination and decreased germination. It also damaged the radicle of P. oleracea seedlings (Aliotta et al., 1996). These findings offer some promise in the search for natural herbicides.