About Rottboellia cochinchinensis (Lour.) Clayton
Rottboellia cochinchinensis (Lour.) Clayton is an annual grass. It has prop roots that support erect, laterally-branching culms, which typically measure 30–300 cm in length, though culms up to 400 cm high have been recorded. Cauline leaves grow from hispid leaf sheaths that bear irritating hairs, especially on lower sheaths. At the collar, leaf sheaths are the same width as the leaf blade, and have a truncated, membranous ligule 1mm long. Leaf blades are slightly rough (scabridulous), generally shaped linear to linear-lanceolate, and end in an acuminate apex. Leaves are usually 15–45 cm long and 5-20mm wide, though maximum upper limits of 80 cm long and 45mm wide have been recorded. The racemes of this species are 3–15 cm long, cylindrical, hairless (glabrous), and erect, growing on terminal and axillary culms. Each raceme is either embraced at its base by a leaf growing from an inflated leaf sheath, or projects beyond this leaf. The peduncle at the base of each raceme is rough and widens toward its apex, where the raceme is divided by fragile nodes between each rachis. Rachides are strongly inflated, around 2–3.5mm wide, with 5-7mm between each node, and bear pairs of spikelets abaxially: one is sterile, attached with a pedicel fused to the internode, while the other is fertile and attached directly (sessile). The raceme tapers at the apex with a number of reduced, sterile spikelets, forming a false panicle that appears heavily branched. In each spikelet pair, the sterile pedicelled spikelet is well developed, around 3-5mm long, egg-shaped and dorsally flattened. This spikelet is enclosed by hardened, smooth, obtuse, blunt, distinctly veined, glabrous bracts called glumes, which are winged on the margins. The sterile spikelet is deciduous along with the fertile spikelet; both break off together with a central peg, where the pair was attached transversely at each rachis node. Fertile sessile spikelets are very similar to sterile spikelets, measuring around 3.5-5mm long, ovate and dorsally flattened, but differ in how they attach to the rachis. Their glumes are not alike: the lower glume is wider than the upper glume and not winged, while the upper glume is V-shaped. The lower glume has 13 veins and is 2-keeled, while the upper glume has 11 veins and a single keel. Both glumes reach the apex of the florets, but the lower glume apex is notched (emarginate), while the upper glume apex is acute. Like most members of the Poaceae family, fertile spikelets hold florets that have two lodicules: fleshy scales that open the glumes during flowering. These lodicules are the same length as the 3 anthers in the florets, 2mm long. Inside the florets are the female floral structures, which consist of 2 stigmas. Also typical for Poaceae, the fruit this species produces is a caryopsis: a single seed fused to the pericarp. However, in R. cochinchinensis, the pericarp becomes free over time. The caryopsis is 3.5mm in length, with the embryo making up around half of its total length. The hilum, the scar where the caryopsis attached to the placenta, is point-like (punctiform), and the endosperm is covered in a mealy powder (farinose). The caryopsis is dispersed while still attached to the rachis internode and a structure called an eliastome (callus knob). This species has variable chromosome numbers: diploid cells may have 20, 36, 40 or 60 chromosomes, so common basic chromosome numbers are either x=10, or sometimes x=9. Rottboellia cochinchinensis uses C4 carbon fixation for photosynthesis, so it grows mostly in warm tropical climates with high sunlight, generally found in the tropics between the northern and southern 20 °C isotherms. This carbon fixation pathway lets the plant avoid the wasteful process of photorespiration, giving it a competitive advantage over C3 plants in conditions of high light intensity, high heat, and low humidity. It can grow in a diverse range of habitats including grassland and marginal land, and is a major weed of perennial and rotation crops across the tropics. Globally, it is considered an important weed in at least 18 crop species, including sugarcane (Saccharum), maize (Zea), upland rice (Oryza), cotton (Gossypium), soy (Glycine), Sorghum, and peanuts (Arachis). In Asian countries such as India, R. cochinchinensis is not a similarly serious weed, which suggests some biotypes of this species are more vigorously competitive with crops than others. This species is relatively shade tolerant, and can also grow rapidly under high light exposure. It is usually found at altitudes up to 2300m; low temperatures are typically the limiting factor for growth above this altitude, and it favors acidic soils. It flowers year-round in tropical climates, but flowers during July and August in the United States. It is a prodigious seed producer, sometimes producing over 2000 seeds per plant (though not all of these seeds are viable), and can produce over 650 kg of seeds per hectare. Seed germination patterns are unclear and vary across this species' pantropical distribution; recent research aims to better understand seedling emergence patterns to inform effective weed control regimes. Seeds are dispersed by floodwater, birds, small mammals, and more recently by humans and vehicles. The eliastome (callus knob) that is dispersed along with the caryopsis contains oils that may attract ants, which helps aid seed dispersal. The natural distribution of R. cochinchinensis is somewhat unclear, as the species has been very successful at expanding its range. Most sources assume the species is native to South-East Asia, since it was first described from specimens collected in this area, though some sources cite India as its native range. It is now pantropical in distribution, found across the Old World tropics from southern Asia, Sub-Saharan Africa, Madagascar, and Indonesia to New Guinea and the Solomon Islands, and Australia as far north as Queensland, with one georeferenced record from New South Wales. It has also been widely introduced across tropical the Americas, possibly first to Cuba or Brazil, before spreading rapidly across the Caribbean, southern United States, and all of tropical Central and South America. Introductions are speculated to have come from the transportation of crop products, agricultural and forestry machinery, or even intentional introduction for grazing in the Caribbean.
