Oryza longistaminata (perennial wild rice)

Oryza longistaminata is a member of the ‘O. sativa’ complex of rice species. It is a wild rice native to Africa. It is an erect or spreading, robust perennial grass with extensive creeping, branched rhizomes. Its flowers have unusually long stamens and hence its name, long-stamen rice. The species occurs in swamp and floodplain grasslands, along edges of rivers and dams, on riverbanks and in irrigation canals. It generally grows in shallow water, but can also be found in water as deep as 4 m. It is propagated through rhizomes and seed, and is difficult to control. The species is restricted to sub Saharan Africa and Madagascar. Efforts should be made to prevent its inadvertent introduction to rice growing areas elsewhere in Africa and in the world. Long distance dispersal of this species is possible through shipments of contaminated grain or exchange of rice germplasm. O. longistaminata is listed as a quarantine pest in Mexico and the USA. For India, import regulations require that phytosanitary certificates for pigeon pea imported from Kenya, Malawi, Mozambique and Tanzania include declarations that the pigeon pea is free from O. longistaminata and other important weed species. The species was assessed for The IUCN Red List of Threatened Species in 2017 and listed as ‘Least Concern’.

Oryza longistaminata is a member of the 'O. sativa' complex of rice species, sharing both the AA genome and chromosome number (2n=24) with the cultivated species, O. sativa and O. glaberrima (Khush, 1997). The African cultigen O. glaberrima was domesticated from the annual wild rice O. barthii in the basin of the upper Niger river in West Africa. O. longistaminata is thought to have been the progenitor of O. barthii, having itself evolved from a common ancestor of O. rufipogon, the progenitor of O. sativa in Asia. A recent study on population structure and diversity of the species by Labroo et al. (2023) confirmed this belief. The names of the wild rices have been widely misapplied, and many publications have referred to the perennial form of wild rice in Africa as O. barthii. This confusion was discussed by Clayton (1968) whose nomenclature confirming O. longistaminata as the perennial and O. barthii as the annual species is now followed by African regional flora.

Common names for O. longistaminata include red rice and long-stamen rice (which has unusually long stamens) (Tong et al., 2023). The name ‘red rice’ is not exclusive to O. longistaminata but is also used to refer to some of the other rice species for example O. punctata and O. rufipogon. The rice grains are red due to presence of anthocyanins (Wangiyana et al., 2020).

Oryza longistaminata is an erect or spreading, robust perennial grass with extensive creeping, branched rhizomes. Glabrous, smooth culms are up to 250 cm tall, up to 2.5 cm diameter at the base, have up to ten nodes, are erect but sometimes floating, weak and spongy with adventitious roots developing from lower nodes. Leaf sheaths are pale-green to brownish, glabrous, nearly as long as the internodes, with 15 mm long auricles at the junction with the blade. The triangular ligule is often split down the middle, 0.8-5.5 cm long with an acute tip. Leaf blades are bright to dark green,10-75 cm long, 0.5-2.5 cm wide, are broadest below the middle, rough along the margins, otherwise smooth, glabrous with an indistinct mid-rib. The panicle is 16-40 cm long, erect or slightly drooping with a tuft of hairs at the base of branches. Narrowly oblong spikelets, on 0.5-4 mm long pedicels, are 7-15 mm long, scabrid to hispid and shed when mature. Glumes are reduced to a narrow membranous rim. Sterile lemmas are 2-3.8 mm long, glabrous and smooth. Fertile lemmas are slightly shorter than the spikelet, stiffly hispid, with six stamens, a blackish stigma and awns 2.6-7.5 cm, usually pink or purplish when fresh. The seeds are 7.5-8.5 mm long, oblong, glabrous, light-brown and glossy.

Both O. longistaminata and O. barthii evolved in West Africa (Khush, 1997) and are restricted to the African continent. O. longistaminata occurs between latitudes of 3°38´N and 26°17´S, and longitudes of 15°34´E and 48°58´E (Melaku et al., 2012).

As O. longistaminata and O. barthii are restricted to Africa, every effort should be made to prevent their inadvertent introduction to rice growing areas elsewhere in the world. This could happen due to contamination of grain shipments or exchange of rice germplasm. All consignments from infested areas should be carefully inspected. O. longistaminata is listed as a quarantine pest in Mexico and the USA (EPPO, 2024). Both O. barthii and O. punctata are listed as economically important species which are a potential problem to the USA agriculture, and should therefore be kept out of the country by plant quarantine procedures (Reed, 1977). Import regulations require that phytosanitary certificates for pigeon pea imported to India from Kenya, Malawi, Mozambique and Tanzania include declarations that the pigeon pea is free from O. longistaminata and other important weed species (Government of India, undated). The wheat imported to Egypt for consumption should have a phytosanitary certificate and a minimum of 25 seeds per kg for the combined total of seeds of O. longistaminata and eight other specified weed species (Australian Government, 2023).

The rhizomes can be dispersed by water, for example, during floods.

Seeds of O. longistaminata can be dispersed by birds and other animals (Wambugu, 2017). Migratory birds are suspected to have distributed some of the rice species, between continents, thousands of years ago (Vaughan et al., 2008).

Possible accidental introduction methods include planting of cultivated rice seeds that are contaminated with weedy rice species (weeds that belong to the genus Oryza and behave as rice (Ferrero, 2003; Goulart et al., 2014)), transportation of fragments of rhizomes by agricultural machinery from infested fields to clean fields (Gyasi et al., 2009) and of seed during harvesting (Ferrero, 2003).

As a weed of wetland habitats, O. longistaminata is only found as a problem in lowland rice crops.

Although all rice growing regions of the world have 'weedy' or wild rice problems, O. longistaminata is only likely to be confused with two other species within its range in Africa. Even so, it is the only rhizomatous form found on the continent. The annual species O. barthii has an extensive synonymy, having previously been referred to as O. silvestris var. barthii, O. breviligulata, O. mezii, O. stapfi and O. perennis subsp. barthii. The species has a similar distribution to O. longistaminata and has been recorded in Burkina Faso, Côte d'Ivoire, Gambia, Guinea, Mali, Liberia, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, Sudan, Tanzania, Uganda, Zambia and Zimbabwe (Clayton, 1970; Fernandes et al., 1971; Hepper, 1972). Another African wild rice, O. punctata, is also distinguished from O. longistaminata by the absence of rhizomes. It is synonymous with O. schweinfurthiana, O. sativa var. punctata and O. eichingeri var. longearistata, and is known from Angola, Côte d'Ivoire, Ghana, Kenya, Madagascar, Nigeria, Sudan, Eswatini, Tanzania, Uganda, Zanzibar and is also present in Thailand (Clayton, 1970; Fernandes et al., 1971; Hepper, 1972). The annual O. punctata is distinguished from O. barthii by its small seeds (<5 mm long). The spikelets of O. punctata are up to 6.2 mm long and they are approximately 2.5 times as long as they are wide. In O. barthii, the spikelets are somewhat larger (7-11 mm long). The seeds of both these wild rices are shed at maturity, unlike those of the cultivated forms O. glaberrima and O. sativa. Red rice, weedy, shattering forms of O. sativa with a red pericarp or occasionally with black seeds have on rare occasions been introduced with planting seed into Africa, for example in Eswatini (Parker and Dean, 1976). These are sometimes referred to as O. rufipogon, although this is strictly a perennial species. The 'red rices' cause serious weed problems in the Americas (Holm et al., 1997). They lack rhizomes, but in common with O. longistaminata have very long ligules (1.25-3.0 cm) on lower leaves. In comparison, those of O. barthii and O. punctata are less than 10 mm long.

Oryza longistaminata occurs in swamp, floodplain grasslands and openings in forests, generally growing in shallow water up to 1 m of less, but can even grow in water that is 4 m deep. It is also found along edges of rivers, dams or on riverbanks, in irrigation canals, in and around rice fields (Lu et al., undated). It can be found from sea-level to a height of 1400 m (Clayton, 1970) and even 1800 m (Brink, 2006). The species grows in black cotton soils, sandy soil and alluvial soils (Lu et al., undated).

Oryza longistaminata has high genetic variability (Melaku et al., 2019). The study by Labroo et al. (2023) identified three genetic subpopulations of O. longistaminata corresponding to Northwestern Africa, Pan-Africa and Southern Africa. The study also identified hybrids of O. longistaminata and O. sativa.

Little information has been published on the biology or ecology of the African wild rice species. The species does well at flow depths between 0.1 and 0.5 m and flooding is important for seed production (Marneweck, 2003 as cited by Kleynhans et al., 2007). The population of the species declines if drought occurs for three consecutive years. In South Africa, the species requires at least 25 days of continuous flooding to flower and set seed. O. longistaminata may be partially or completely sterile, but seed can be produced adding to weed problems (Parker and Dean, 1976). Seed will not germinate in the absence of adequate oxygen, such as under flooded conditions. O. longistaminata and O. barthii both display delayed germination over considerable periods when conditions for germination are otherwise favourable (Katayama, 1969), a trait that is presumably due to variable levels of dormancy in the samples tested. These species were dormant for at least 5 months after collection in a study conducted in Mali (WARDA, 1979). O. punctata may remain dormant for 1-5 years (Armstrong, 1968; Maijisu, 1970). There is some experimental evidence that O. barthii is allelopathic, reducing germination and establishment of O. sativa (Watanabe, 1989).

Pests and pathogens of O. longistaminata, such as rice yellow mottle sobemovirus (John et al., 1984), rice bacterial blight (Xanthomonas oryzae pv. oryzae) (Buddenhagen, 1982) and rice root nematode (Hirschmanniella spinicaudata) (Babatola, 1980), are also important economic pests of the rice crop.

Although there are numerous reports that wild rice species are widespread constraints to lowland rice production in many parts of Africa, there are few detailed studies of the actual yield losses caused. In Mali, populations of O. longistaminata have been reported to be as high as 150 plants per m² in fields in the Niger valley. A study of related species in Latin America indicated that 24 wild rice plants per m² caused yield losses of 75% (Fischer and Ramirez, 1993). In West Africa, it is not uncommon for heavily infested fields to be abandoned (Davies, 1983). Yield losses have been estimated to be in the order of 85% in fields with severe infestations of O. longistaminata in Mali (IER, 1989), whereas in Senegal, yield losses of over 90% have been associated with infestations of O. barthii (Davies, 1983). In Ghana, infestations of 100 shoots per square meter were estimated to cause losses of about 0.9 tonnes of rice per ha (Johnson, 2002). Although seed is shed at maturity, some wild rice seed may contaminate commercial rice grain (Parker and Dean, 1976), reducing its quality, as additional polishing may be required to remove the red pericarp of wild rice grains. As well as the competitive effect on the crop, both O. longistaminata and O. barthii are alternative hosts of some important rice pests and pathogens including rice yellow mottle sobemovirus (John et al., 1984) and rice bacterial blight (Xanthomonas oryzae pv. oryzae) (Buddenhagen, 1982). In addition, management of the weed is mostly long-term. For example, in the irrigated rice schemes at Tono and Vea in Ghana, O. longistaminata was reported to be a major problem, and canals needed to be cleared repeatedly (Johnson, 2002).

In some countries, the weed sometimes has economic benefits. Grains of O. longistaminata are sometimes used as food during famine e.g. in Ethiopia and Sudan, plants as grazing material for livestock, and straw for roofing of houses (Brink, 2006).

Populations of O. longistaminata in irrigated rice systems are likely to increase due to climate change (Rodenburg et al., 2011) and this will probably result in increased impacts.

Oryza longistaminata competes for nutrients, space and light with domesticated rice and other species. In a weed management study by Johnson et al. (2004), O. longistaminata affected growth of cultivated rice. Perennial weeds O. longistaminata and Bolboschoenus maritimus were more dominant than annual weeds in plots where weeds were not controlled compared to plots where weeds were controlled. The species can also suppress growth of some plant species. For example, it was reported to be allelopathic on growth of barnyard grass (Shen et al., 2016), a weed that can be found in rice fields.

Oryza longistaminata is sometimes used to supplement cultivated rice as a food grain in the Niger Valley of Mali (Nyoka, 1983). As the seed is shed at maturity, it has to be collected from the soil surface after flood waters have receded. A full day is needed for one person to gather 2-3 kg. As it is a member of the primary gene pool, sharing the AA genome with O. sativa, hybridization with cultivated rice has been detected in cultivated rice fields where the species occurs as a weed (Kilewa, 2014) and the transfer of useful traits, including resistance genes, from O. longistaminata to the crop can be accomplished through conventional hybridization and selection procedures (Khush, 1997). Accessions of the species with immunity to rice yellow mottle sobemovirus, resistance to bacterial blight (Xanthomonas oryzae pv. oryzae) and rice blast (Magnaporthe grisea) have been described and have potential for use as parents in breeding programmes (Vales, 1985; Thottapilly and Rossel, 1993; Zhang et al., 1994). Other traits that have been identified in O. longistaminata that could be transferred to cultivated rice include resistance to lodging (Long et al., 2020), heat tolerance (Fan et al., 2023), salt tolerance (Padmavathi et al., 2024) and perenniality (Zhang et al., 2023). Another strategy that has been proposed is domestication of the species via gene editing (Tong et al., 2023). Varieties of perennial cultivated rice that have been developed by crossing O. longistaminata with cultivated rice and released in China, according to information cited by Zhang et al. (2023), include PR23 and PR25.

Identification may be possible in the field, for example using existing identification keys. An identification manual for southern African grasses authored by Fish et al. (2015) has a key that distinguishes between O. longistaminata, O. punctata, O. barthii and O. sativa. Another document on taxonomy of wild species of rice by Lu et al. (undated) has a key that can be used to identify O. longistaminata and other wild rice species.

It may be difficult to visually distinguish the seed of O. longistaminata from seed of other rice species.

Weedy rice species are difficult to control, and prevention is the best way managing infestation (Ferrero, 2003). To reduce chances of introduction of weedy rice species to clean fields, use certified seeds and clean machinery (Ferrero, 2003).

As the wild rices do not germinate in the absence of adequate oxygen, they are only weed problems if they are allowed to germinate and establish in drained soil prior to flooding or in the case of O. longistaminata, when the weed regenerates from rhizomes. The annual species are therefore not serious weeds of transplanted rice, provided clean seed is used and adequate water control is employed (Parker and Dean, 1976). Seedlings of the rice crop and wild rices are morphologically similar. Planting the crop in rows can therefore help to identify any wild rice plants developing in the inter-rows. In India, purple-leaved rice cultivars were developed so that crop and weedy rices could be readily distinguished to facilitate hand weeding (Malik and Moorthy, 1996). Use of a short-term rice cultivar, which matures before the wild rice plants have begun to shed seed, results in more contamination of the harvested grain, but reduces the long-term problem by breaking the cycle of introduction of new weed seed into the soil (Parker and Dean, 1976). A traditional control measure for O. longistaminata, practised in Mali, has been the rotational 'underwater' mowing of plants on fallow areas of the Niger floodplain (Nyoka, 1983). This provides adequate suppression of the weed to allow production of a rice crop in the subsequent rainy season although the technique is very labour intensive. The rhizome system of O. longistaminata is relatively shallow, so only shallow tillage is needed to sever it from adventitious roots. Repeated dry-season tillage can therefore lead to the desiccation and death of the rhizome system. Timing in relation to soil moisture can be critical. Although a single ploughing prior to flooding failed to control the weed in trials conducted in Mali (WARDA, 1979), 'undercutting' with a blade set shallowly in the soil was effective if carried out twice prior to planting. Tillage has also been shown to influence O. barthii populations (Davies, 1983). A stale seedbed technique using rotary cultivations was shown to be particularly effective, leading to reduced levels of infestation and increased rice yields in Senegal.

Studies in West Africa demonstrated that growing sweet potatoes and vegetables during the dry season can reduce the population of O. longistaminata in rice growing fields (Johnson, 2002). Population of O. longistaminata in rice was four plants/metre² for fields that had sweet potatoes in the previous season, and 50 plants/metre² where sweet potatoes had not been grown.

The herbicide dalapon has given satisfactory control of both O. longistaminata and O. barthii on fallow land (Boeken, 1972). Pre-sowing applications of glyphosate or use of the herbicide on fallow land provide effective control of both species (Aubin et al., 1975; Diarra, 1978; Davies, 1983). Post-emergent applications herbicides for control of weedy rice species in cultivated rice crops is uncommon because herbicides that can control weedy rice species are also likely to cause injury to the rice crops (Ferrero, 2003). However post-emergent herbicides can be used to manage weedy rice for rice cultivars that are resistant to herbicides. For example, in an experiment at Long Ashton, post emergent applications of low rates of imazethapyr were successful in the control O. barthii and O. longistaminata in plots of imidazolinone resistant rice cultivars (Johnson, 2002).

Use of IPM can be a good strategy of controlling O. longistaminata. Since the species is a rhizomatous weed, hand pulling alone cannot easily get rid of the weed. In Senegal, farmers combine hand weeding with use of herbicides, pre-irrigation, planting of clean seed, transplanting of rice seedlings and crop rotations (Diallo, 1999 as cited by Rodenburg and Johnson, 2009)

Information quantifying the impacts of O. longistaminata is sparse.