Oldenlandia herbacea

Oldenlandia herbacea is an erect annual or perennial herb with much-branched wiry stems. It is a common weed found throughout the warmer parts of Asia and Africa and in some parts of tropical America. Although reported in cowpea, rice and oil palm fields, it is not known to cause problems. There is no evidence of it being invasive.

Oldenlandia and Hedyotis, two of the largest genera within the Rubiaceae, occur throughout tropical and subtropical regions worldwide. Both are grouped in the Hedyotis-Oldenlandia complex, comprising more than 500 species; these are very similar and share a herbaceous or shrubby habit, relatively small flowers with four petals, 2-locular ovaries and dehiscent or indehiscent capsules with many seeds (Neupane et al., 2009; Guo et al., 2013).

The generic delimitation in the Hedyotis-Oldenlandia complex has a taxonomically confused history due to the broad geographic distribution, species richness and morphological diversity of the members. Previous treatments included accepting a very broad-sensed Hedyotis, to partially or completely segregating both taxa into different numbers of smaller genera (Guo et al., 2013; Wikström et al., 2013; Neupane et al., 2015; Hsu and Chen, 2017). Many species have a scientific name in Hedyotis and also a synonym in Oldenlandia and vice versa (Wong et al., 2019).

Lewis (1965) noted that Africa has more Oldenlandia species than exist in all other tropical regions combined and is the centre of its morphological diversity. Several species are common agricultural weeds (Holm et al., 1979).

Oldenlandia herbacea is a common weed in tropical regions of Asia and Africa. The genus name Oldenlandia honours Henrik Bernard Oldenlan, a 17^th century Danish physician and botanist. The specific epithet refers to the habit of the plant.

The following description is from Wong et al. (2019):

Herbs, annual or perennial. Stems erect or suberect, weakly to sharply 4-ribbed, often narrowly winged at nodes, glabrous. Stipules reduced or truncate, fused to petiole bases, 2-4 × 0.2-0.3 mm, with few setae or bristles on the margin. Leaves: lamina linear or linear-lanceolate, 1-5.5 × 0.1-0.3 cm, apex acute, base acute to obtuse, margins weakly to strongly revolute, membranous to papery, glabrous to scaberulous on both sides, secondary veins not distinct; sessile or subsessile. Inflorescences axillary, solitary, rarely several-flowered and fasciculate to cymose, glabrous. Flowers 3-4 mm long, homostylous, rarely heterostylous, pedicels 6-22 mm long, as long as or longer than the leaves; hypanthium subglobose to ovoid, 0.8-1 mm long, usually glabrous, calyx lobes 4, narrowly triangular to linear, 0.5-1.5 mm long, scabridulous at the margin; corolla white to reddish or pale purple, infundibuliform, outside glabrous, tube 2-3 mm long, glabrous at throat, lobes ovate to oblong, 0.5-1 × 0.2-1 mm, acute; stamens four, inserted within tube or on throat, often visible, filaments very short in isostylous flowers, anthers c. 0.2 mm long, linear; ovary 0.5-1 mm long, ovules many; style slender, 3-3.5 mm long, stigma bilobed, lobes filiform, c. 0.8 mm long. Capsules globose or ovoid, 2-2.5 mm long, top protruding beyond the erect calyx segments, glabrous, loculicidally dehiscent through the beaked apex. Seeds many, angular, c. 0.2 mm diam., exotesta strongly reticulate, brownish, mucilaginous.

Oldenlandia herbacea is widespread in tropical Africa and Asia, Madagascar and other Indian Ocean islands (Hyde et al., 2016). Native to India, Sri Lanka, Bangladesh, sub-Saharan Africa, Madagascar, this species has been introduced to Indochina, Malesia, China, New Guinea, Central America and South America (northern) (EPPO, 2016).

Oldenlandia herbacea is a weed in cowpea (Vigna unguiculata) fields in Sierra Leone (Kanteh et al., 2013), yam (Dioscorea spp.) fields in Nigeria (Aliyu et al., 2021), rice (Oryza sativa) fields in Malaysia (Hakim et al., 2013), oil palm (Elaeis guineensis) plantations in Côte d'Ivoire (Traore et al., 2007) and Malaysia (Ismail et al., 1995), rubber plantations in Malaysia (Ismail et al., 1995) and in experimental plots of an intercrop of juvenile oil palm trees with maize (Zea mays), okra (Abelmoschus esculentus) and pepper (Capsicum annuum var. abbreviatum) fields in Nigeria (Oluwatobi and Olorunmaiye, 2014).

In Africa, O. herbacea can be confused with Oldenlandia corymbosa. It is possible to distinguish the two species by their habit; O. herbacea is upright, grows in compact tufts, is highly branched, while O. corymbosa is semi-upright, has a slender stem with relatively little branching. The flowers of O. corymbosa are grouped while those of O. herbacea are generally solitary (Salamero et al., 1997).

Many American collections of Oldenlandia lancifolia have been misidentified as O. herbacea: in the Caribbean, O. herbacea has been misapplied to O. lancifolia (Acevedo-Rodriguez and Strong, 2012).

Oldenlandia herbacea is found in a wide variety of habitats, in rocky or marshy areas, near edges of ponds, roadsides, in harvested rice fields, among grasses or sand dunes, grassland, dry woodland, dry waterholes and damp or swampy areas (Verdcourt, 1989; Wong et al., 2019). In Zimbabwe, it occurs in dry woodland to damp grassland and seepage zones and roadsides (Hyde et al., 2016). In Madagascar, this species is widespread in wooded vegetation on both lateritic and quartzite substrates and has been collected in the grasslands of the central plateau (Missouri Botanical Garden, 2016). O. herbacea is found from sea level to an altitude of about 2000 m (Missouri Botanical Garden, 2016).

In Nigeria, O. herbacea is recorded as a host of the plant viruses, Tomato yellow leaf curl virus (TYLCV) in field-grown tomatoes (Solanum lycopersicum) (Abraham et al., 2021) and Yam mosaic virus (YMV) in yam fields (Aliyu et al., 2021), indicating the possibility of this species acting as a reservoir for both viruses.

Bentham (1861) described O. herbacea as a common weed found throughout the warmer parts of Asia and Africa and in some parts of tropical America. It has been reported in cowpea, rice, yam, rubber and oil palm fields (Ismail et al., 1995; Traore et al., 2007; Hakim et al., 2013; Kanteh et al., 2013; Aliyu et al., 2021) and in experimental plots of an intercrop of juvenile oil palm trees with maize, okra and pepper (Oluwatobi and Olorunmaiye, 2014).

This species is also listed as one of the problem plants in southern Africa (Wells et al., 1986). Although buried seeds of O. herbacea in the soil of rubber and oil palm fields in Malaysia showed high viability, this weed has not been reported to cause problems in either rubber or oil palm growing areas (Ismail et al., 1995).