Rubber tree

The rubber tree (Hevea brasiliensis) is a large deciduous tree up to 43 m tall in the wild, though much smaller in tapping plantations (usually 15–25 m). The trunk is cylindrical, sometimes with a swollen, bottle-shaped base; the bark is brown and its inner layer holds latex vessels that spiral up in a right-handed helix at about 30° to the horizontal and can reach 15 m high. The leaves are trifoliate and spirally arranged, and the tree is deciduous for a short dry season. The flowers are unisexual (separate male and female), creamy yellow and petal-less, borne in pungent clusters. The fruit is a capsule with three large seeds that splits explosively when ripe. The chromosome number is 2n = 36.

According to Kew POWO the native range of Hevea brasiliensis is South Tropical America — Bolivia, Brazil North/South/West-Central, Colombia, French Guiana, Peru and Venezuela — where it is a forest tree of humid tropical habitat. Amazonian peoples used rubber latex long ago; after Goodyear's invention of vulcanisation in 1839 demand surged, triggering the Amazon rubber boom (1840–1913) that enriched Belem, Manaus and Iquitos. Attempts to grow the tree outside Brazil began in 1873; in 1875–1876 Henry Wickham brought some 70,000 seeds to Kew, from which about 2,000 seedlings were sent to Ceylon (Sri Lanka) and Singapore, and then to Buitenzorg (Java) in 1883. Because South American leaf blight ravaged its homeland, large plantations shifted to Asia; by 1898 Malaya had rubber estates, and today Thailand, Indonesia, Vietnam, India, China and West Africa are the main producing regions.

Fresh rubber latex is a white emulsion of about 60% water, 33–35% cis-1,4-polyisoprene (natural rubber) and about 5% non-rubber matter. The rubber particles are about 0.5 µm across, and the polymer has a molecular weight (Mw) of roughly 5×10^5–2×10^6 Da, giving its characteristic elasticity. The non-rubber fraction includes proteins (Hev b 1 — rubber elongation factor, REF; Hev b 3 — small rubber particle protein, SRPP; Hev b 2 — β-1,3-glucanase; Hev b 6 — hevein… making up the 15 allergens Hev b 1–15), lipids and phospholipids, carbohydrates with the cyclitol quebrachitol, resin, flavonoids and mineral salts. The seeds also contain cyanogenic glycosides (linamarin) as a defence, their carbon and nitrogen recyclable for growth and latex production. Commercial latex typically holds 30–35% rubber solids.

Rubber is a strategic industrial material. Vulcanisation (1839) turns polyisoprene into a durable, elastic elastomer used for tyres, inner tubes, shoe soles, belts, conveyor belts and anti-vibration mounts; concentrated latex, ribbed smoked sheet (RSS) and technically specified rubber (TSR) go into medical gloves, condoms, catheters, teats and adhesives. Rubberwood is made into furniture once plantations pass tapping age (25–30 years), and rubber seed oil is used in paints, soap and biodiesel. In research, Berthelot et al. (2012) clarified the roles of REF (Hev b 1) and SRPP (Hev b 3) in latex allergy; Guyot & Le Guen (2018) reviewed a century of work on South American leaf blight; and recent reviews explore rubber latex as a biocompatible biomedical and sensor material while seeking alternative rubber sources (guayule, Russian dandelion Taraxacum kok-saghyz).

On safety, natural rubber latex allergy is a significant occupational health issue: an estimated 150,000–1 million healthcare workers and over 15 million people worldwide are affected, through the Hev b proteins; powdered gloves disperse allergens into the air, so low-protein gloves are now preferred. Solvents, preservative ammonia and processing chemicals require safe handling. In the past arsenic trioxide was used on plantations (notably in Malaysia), with environmental risk. On conservation, the IUCN lists Hevea brasiliensis as Least Concern (LC) in its 2020 assessment (Fabriani & Hills, e.T62003521A62003529). However, plantation clones have a very narrow genetic base and are all susceptible to South American leaf blight caused by the fungus Pseudocercospora ulei (formerly Microcyclus ulei), so wild Amazonian populations and genebanks are vital breeding resources; deforestation and monoculture threaten this gene pool.