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Weber, D.J., W.M. Hess, R.B. Bhat, and J. Huang. 1993. Chrysothamnus: A rubber-producing semi-arid shrub. p. 355-357. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Chrysothamnus: A Rubber-producing Semi-arid Shrub

D.J. Weber, W.M. Hess, R.B. Bhat, and J. Huang

  1. BOTANY AND HORTICULTURE
  2. LEAF TRICHOMES
  3. REFERENCES
  4. Fig. 1
Chrysothamnus, a multi-use desert shrub which can grown in a wide range of environmental conditions, has potential uses for revegetation (Sankhla et al. 1987), as a forage for wildlife and livestock (Bhat et al. 1989), for production of natural rubber (Hall and Goodspeed 1919), for production of hydrocarbons from its biomass (Buchannon et al. 1978), for resin for polymer plastics (Thames 1988), as a landscape shrub (Weber et al. 1986), and as a potential source of natural chemical compounds (Hegerhorst et al. 1987a,c). Production of natural rubber can be obtained from Chrysothamnus grown on semi-arid lands where cold temperatures would exclude guayule and it could provide resin for polymer plastics and specific chemical compounds for the chemical industry.

BOTANY AND HORTICULTURE

The genera, Chrysothamnus (Asteraceae), is endemic to the western United States with 5 sections, 16 species, and 41 subspecies (Anderson 1986). Plants in this genera can grow from Mexico to Canada and from sea level to 3,000 m.

The species, Chrysothamnus nauseosus (Pallas) Britt. (rubber rabbitbrush) is a vigorous pioneer plant in disturbed sites. Plants can reach heights of 30 to 180 cm with a few reaching heights of 366 cm. Rubber rabbitbrush has a high rate of photosynthesis and does not become light saturated at full sun (Davis et al. 1985). The average dry weight biomass per plant is 29 kg and the average number of plants per ha in a normal population is 2,632 (McKell and Van Epps 1980). Chrysothamnus nauseosus blooms in the fall and the flowers are born in heads that in turn are raised into cymes, racemes, or panicles. The heads contain five yellow disc flowers and are subtended by involucral bracts. Each flower contains a pappus of abundant white slender capillary bristles. The plant is self-fertile and entirely diploid, although polyploidy does occur in one species.

Chrysothamnus nauseosus is a prolific producer of easily harvestable seeds. Seeds germinate on most soils including saline sites in a few days under cool nights (5°C) and warm days (15°C) (Khan et al. 1987). Propagation by cuttings is difficult though possible, and in vitro propagation techniques have been developed (Upadhyaya et al. 1985). The plants grow well under arid conditions but produce more biomass with increased moisture. Chrysothamnus nauseosus can tolerate both low freezing and hot arid temperatures.

A number of the species of Chrysothamnus particularly nauseosus contain rubber up to 7% dry weight. The average molecular weight of the rubber varies in the species, but some species contain rubber with an average molecular weight of 585,000 (Weber and Fernandes 1991). Rubber content was found to be highest under stress conditions (Hegerhorst et al. 1987b).

The resin content of Chrysothamnus may be as high as 35% in some species (Bhat et al. 1989) and the resin has potential use as a plastic extender (Thames 1988). The resin, located in the leaves and in the glandular trichomes, contain a range of terpenoid compounds including monoterpenes and pregnanes (Weber and Fernandes 1991). The resin chemicals can be extracted and have potential as a source of natural terpenes and insect inhibitors. The pregnanes are chemically related to animal hormones (Deepak et al. 1989).

LEAF TRICHOMES

Plants and seeds of species and subspecies of Chrysothamnus were collected from the western United States from a range of environmental conditions and planted in a uniform garden. Leaf samples were collected from the uniform garden when the leaves were fully mature. The leaf samples were fixed, dehydrated, gold coated, and then observed with a scanning electron microscope. There were two major types of trichomes observed, filiform and glandular. A total of six variants of the filiform or glandular trichomes could be distinguished (Fig. 1). In some cases, two or three types of trichomes were present on the same leaf. In other cases, only one type of trichome was present. The ratio of filiform to glandular trichomes on a single leaf ranged from 1 in C. nauseous ssp. iridis L.C. Anderson to 11.9 in C. nauseous ssp. salicifolius (Rydb.) Hall & Clem. The density varied greatly on the leaves of the different species. The number of total trichomes per mm2 ranged from 1 in C. nauseous ssp. graveolens (Nutt.) Piper to 301 in C. nauseous ssp. viridulus (Hall) Hall & Clements. The number of trichomes per mm2 did not correlate with resin content, amount of precipitation, or summer temperatures.

REFERENCES



 Fig. 1. Leaf surfaces of Chrysothamnus showing trichome types. (1) C. nauseosus ssp. consimilis (Greene) Hall & Clem. showing filiform tubular (arrow) and glandular biseriate (double arrows) types x225. (2) C. nauseosus ssp. viridulus (Hall) Hall & Clements showing ribbon types x225. (3) C. nauseosus ssp. salicifolius (Rydb.) Hall & Clem showing filiform ribbon (arrow), glandular uniseriate (double arrows), and glandular biseriate (triple arrows) types x225. (4) C. viscidiflorus ssp. lanceolatus (Nutt.) Hall & Clem showing prickle types x225. (5, 6) C. viscidiflorus ssp. viscidiflorus (Hook.) Nutt. showing prickle multicellular (arrow) and glandular (double arrows) types. x90; x450.
Last update September 12, 1997 aw