Volume 27 No27 February&March 2003
Miscellaneous
In Miscellaneous>The Long March ./. Most Peaceful Job ./. Arid Land Industrial crops ./. Exotic Tangier ./. The Allergy Of The Frog ./. Never Forget
Farming
AGRO-ECONOMY
 
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Arid Land Industrial Crops
By Dr. S.M Alam
PART I
Agricultural production in the arid and semiarid areas throughout the world is heavily dependent upon water from irrigation. The major crop plants of the world are neither xerophytic nor well adapted to arid environments, and are not productive without supplemental irrigation. The amount of water available for agriculture in the year 2000 in Arizona and other contiguous areas is projected to meet only about 50% of the needs for currently available, agriculturally productive land. It is clear that development of new crops with low water requirements is of the highest priority. Yields of irrigated, conventional crops are generally high in these areas, but production costs are also high and continuing to increase. Reduced water availability and increased production costs make conventional crops less attractive economically and provide an incentive for the development and adoption of new and alternative crops. Since plant productivity in arid areas is closely tied to water usages, an economically viable new crop should use significantly less water. However, developing a highly productive new crop that uses less water may be easier said than done. A commonly held misconception is that desert plants use water more efficiently than non-desert plants or conventional crops. However, most desert plants use water more efficiently when water is limited and can survive long periods of water stress. Xerophytic plants may not grow and produce much biomass during periods of extended water stress, but they have the capacity to survive and reproduced under condition that would cause death and complete crop failure to most mesophytic plants.
A new or alternative crop for future production in arid environment most likely will not be characterized as having high biomass yields. Crops produced and grown for high biomass yields for use as solid fuels or conversion to liquid fuels are more likely to be produced economically on non-arid land. Hay, grains, most oil seeds, sugar, pulp, and fiber crops that are consumed in relatively large quantities and generate relatively low prices are also not likely to be prime candidates as new for arid environments. In contrast, plants that produce significant yield of relatively high valued industrial feedstock’s and product such as rubber, resins, gums, waxes, pharmaceuticals, biologically active materials, essential oils, and other oils with unique fatty acids are likely new crop candidates for these lands.
The rationale should not be construed as an argument against research and development of new arid-adapted food and feed crops, which may have high social and economic impact in less developed countries. In fact, such research and developmental activity in less developed countries on arid-adapted food crops may pay much larger dividends for the resident population than concentrating efforts on industrial crops. There is good evidence from native plant population in arid areas suggesting that high productivity of native food crops can occur, and there are substantial opportunities for improving production in unfavorable environments. For example, more than 400 native species of noncultivated food plants have been identified in the Sonoran desert of Mexico and the southwestern United States. More than 40 of these have served as major local food resources for natïve inhabitants.
Notwithstanding, the framework of reference to which the preceding statements and the focus of this paper are addressed is toward development of new and alternative industrial crops. Preferably, new crops and provide a reliable domestic supply of essential industrial feed stocks. Undoubtedly, these new industrial crops would be of greater economic significance to the technically advanced countries then to those less developed. However, undr appropriate condition, the less developed countries should be able to capitalize on the benefits of producing new industrial crops for export arid as resources and stimuli for their own industrial development.
Many candidate species have been suggested for domestication and development as new crops for arid environments. Over a period of time, a rather large number of species have been tried, with varying degrees of successful development of any specific new crop adapted to arid environments is likely to be lower than the generally acknowledged low rate of adoption of new crops in temperate environments. In the past few years, only a limited number of species have received active attention. In total, only about six species including guayule, ojoba, lesquerella, buffalo gourd, and euphorbia have received what one might characterize as “major” attention. Currently, development of only guayule, jojoba, and lesquerella is being pursued actively.
Guayule (Parthenium argentatum Gray, Asteraceae): The United States is totally dependent upon importation of natural rubber for industrial and defense purposes at a high annual cost. The projected annual usage of natural rubber for 1990 in the United States was estimated at 0.9 million metric tons. Natural rubber is preferred to synthetic rubber where resiliency, high elasticity, and low heat buildup are essential in various products. Guayule, one of about 2000 plant species that produced rubber, has been long-recognized as a promising source of natural rubber which is essentially identical to that from the hevea rubber tree (Hevea brasiliensis). Guayule is a small woody perennial rubber –producing shrub native to the Chihuahuan desert of north Central Mexico and southwestern Texas. Unlike hevea, where latex flows from continuous ducts, obtaining rubber from guayule involves harvest of two to three years old plant tops by clipping or by digging whole plants, including the rubber producing roots. Extraction of rubber, which accumulates in the living parenchyma cells, is accomplished by grinding the stem and root tissues and using organic solvent extraction. Guayule plants also produce significant quantities of resins, which have potential uses as byproducts, and can be obtained as part of the rubber extraction process. Harvest of native stands and initial use of guayule as a source of natural rubber began in the late 1800s and became a major source for the USA & Mexico in the early 1900s. Native stands were rapidly depleted, and a minimal research effort to domesticate and develop guayule as a new crop started in 1907. Loss of rubber supplies from the Far East in 1942 led to the initiation of a crash guayule R& D program under the emergency Rubber Project of the U.S. Government. After a short 3 1/2 –year operational period, the development of synthetic rubber and the end of World War II precipitated the termination of the project, with only limited research continuing to 1959.
From a cultural standpoint, there appears to be no real constraints to full commercialization. However, full commercial production and utilization of guayule rubber are largely dependent upon development of higher-yielding cultivars through germplasm enhancement and plant breeding. A well coordinated, cooperative guayule breeding and genetic research program involving USDA/Agriculture Research Service at Phoenix, Arizona, the University of Arizona, Tucson, and the University of California, Riverside was initiated in 1986. The primary objective was to increase rubber yield to commercially acceptable levels. Rubber yield per unit of land area is an interrelated function of rubber concentration (%) and dry matter or biomass production. Other important secondary objectives include the development of genetically-enhanced germplasm and cultivars with improved seedling and mature plant vigor, plant architecture , fast regeneration following harvest by clipping increased cold tolerance and tolerance to diseases, pests, drought, and salinity, adaptation dry land as well as irrigated cultural system, and improved rubber quality and quality retention following harvest and processing..
Several germplasm collection of guayule and related Parthenium species have been made within their natural range. Most guayule germplasm consists of apomictically programs. Sexually reproducing, largely self incompatible diploids of guayule are found in limited numbers in a very restricted area in Mexico. Most related species are also diploids, although a polyploid series has been found in a few species. In the past, only limited use has been made of diploid guayule material in the breeding programs. However, a recent germplasm collection of new diploids and their use in a recurrent selection program along with use of interspecific hybridization is adding dimension to the total breeding effort. Much of the current germplasm being utilized originated from breeding material developed from two major collection made during the emergency Rubber Project. Most of this material traces back to a small number of accessions collected in a very limited area in the Mexican State of Durango. However, a surprisingly large amount of variability for rubber and resin quantity and quality and plant growth characteristics has been shown to exist within the apomictically reproducing polyploid germplasm. The facultative apomictic system found in the polyploidy material apparently serves as a mechanism for conservation and propagation of a wide array of both genic and chromosomal variation. New apomictic single plant and line selections have been made with desirable combination of rubber concentration (%) and yield (g/plant), biomass production, and vigorous top regrowth following harvest by clipping. Progeny of selected plants and lines reproduced by seeds and vegetative cuttings are currently undergoing further evaluation and reselection, and hold promise of producing cultivars with rubber concentration of 7 to 9% and rubber yield of over 1100 kg/ha/year. Under most conditions, such yields should make guayule production an economic success. The guayule research and commercialization program is a good example of the coordinated and cooperative involvement of federal, state, and Industry sectors. This multidisciplinary effort involving a full array of scientists, engineers, economists, and management specialists is developing a complete, viable system for producing guayule rubber from seed production to planting, harvesting, processing, and utilization. While good progress has been made, further gains are limited by underinvestment of resources in research and development. This is difficult to reconcile in light of the annual United States impordeficit of nearly $1 billion for natural rubber, back in year 2000. Such limited research support is most glaringly apparent in the guayule breeding and genetics program, which as with most crops is scale dependent.

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