Developing New Crops for Arid Lands

Table of Contents

Introduction

Roughly one-third of the world's land mass is chaparral, scrub, or dry desert where conventional crops cannot be produced without irrigation. However, the world's burgeoning demands for food and fiber have placed increasing pressure on these marginal regions where attempts to cultivate crops are being initiated now more than ever before. Generally, the consequences have ranged from disappointing to disastrous.

When prolonged irrigation and other intensive agricultural practices become established in arid and semi-arid lands, soil salinity buildup and severe erosion inevitably follow. These problems are now major factors contributing to a worldwide epidemic of desertification and cropland loss. Approximately one-third of the world's 160 million hectares of irrigated lands are already affected by salinity problems, and cost-intensive methods are required to prevent more widespread damage. A further problem with irrigation is that it often relies on non-renewable underground water reserves. As the aquifer is depleted, the pumping costs and energy requirements increase—thus compounding the already high price of irrigation. Farm profits are routinely swallowed by these costs in the southwestern U.S., where thousands of acres of abandoned croplands stand as evidence of a single fact: conventional agriculture in arid lands is ultimately a self-limiting endeavor.

The most practical solution for the long term is likely to involve a change in current agricultural strategies, ultimately leading to the cultivation of crops that are better adapted to the desert. Diversified cropping schemes requiring little or no irrigation would bring with them corresponding reductions in the problems of erosion and soil salinization; such crops also would have greatly reduced growing costs. And a crop uniquely adapted to an arid habitat could be grown on land where agriculture is not impractical or impossible, thus conferring an additional cost advantage by avoiding competition with more traditional crops.

Conventional wisdom would imply that farming in the desert is a pipe dream In fact, many areas where rainfall cannot support conventional crops are still surprisingly productive of biomass. Uncultivated desert and semidesert regions generate about one-fourth as much of the world's primary productivity as do all the cultivated lands. Clearly, the potential exists for beneficial use of these low-rainfall areas.

The key to obtaining resources from marginal lands without causing environmental damage lies in understanding the unique adaptations, productivity, and limitations of plant life in the desert. These ecosystems are characterized by low standing biomass, although many desert-adapted species exhibit a very high photosynthetic conversion efficiency during favorable parts of the year and are able to make use of short and erratic periods of rainfall.

Traditionally, plants that have economic or ethnobotanical importance in the desert, other than food crops, are most likely to be species that produce compounds rich in carbon and hydrogen, such as rubber, paraffins, resins and lower-molecular-weight specialty chemicals. Although they have a slower overall growth rate than conventional crops, these plants show increased water economy by producing a high energy content per unit of dry weight biomass. When water is limited, any plant—regardless of its ability to tolerate heat and drought—will produce a relatively low amount of biomass. Thus, the most logical approach to bioresource utilization in the desert is to breed and cultivate plants that produce oils, resins, and other plant substances that have a high value per unit of volume.

Developing a Renewable Bioresource-based Industry

Introducing bioresources-based commodities and industries into the economy requires the development of a complex support system. Crop cultivation must be integrated with the transportation of raw materials, the exchange of feedstocks and intermediate products, and economical systems for processing and marketing the end products.

A strategy for bioresource development is one that stresses high-priced commodities along with the replacement of certain petroleum-derived chemicals. In some cases this may involve the reestablishment of biomass sources for chemicals that were originally plant-derived, such as the resin products used in the naval stores industry.

Biomass holds promise as an alternative source of chemical feedstocks, since these are specialized, lower-volume, and higher-priced commodities than fuel. The price of a reactive compound when marketed as a chemical is generally about three times its fuel value. These economic considerations are very important to the near-term success of a bioresources industry.

Ultimately, however, it will be important to devise wholly integrated systems for producing a variety of products from biomass, including fuels. A complementary relationship between raw material inputs, processes, byproducts, genetic selection, and the environment can significantly improve the new economics and engergetics of a production system. Careful planning and design can accommodate the use of process byproducts, waste-treatment functions, and the use of resources such as municipal wastes and saline agricultural effluents that are now considered liabilities. This can reduce the environmental impact and overall energy budget, and help to tailor a bioresource-based industry to suit geographic and social needs.

The research program of the Natural Products Center recognizes the importance of long term planning for development. New agricultural options for arid and semiarid lands are needed immediately in many parts of the industrialized world where the emphasis must be on industrial chemicals, pharmaceuticals, and other non-fuel options, with fuels as possible byproducts

On the other hand, fully integrated systems emphasizing fuel production may be appropriate for parts of the world where the petrochemical industry is less well entrenched, or where dependence on petroleum resources is problematic or impossible. These areas have a great likelihood of meeting their energy demands with biomass sources in the near future.

It is inevitable that biomass will someday contribute significantly to the world's energy needs. The advantage of a fuel and chemical industry based on indigenous raw materials, rather than imports, is obvious. Environmental considerations also are important. Photosynthesis takes carbon dioxide from the atmosphere and releases oxygen, and could ameliorate the dangerous greenhouse effect of CO2 buildup from burning fossil fuels. In contrast to the toxic byproducts of petroleum-based industries, the byproducts of biomass processing can actually benefit the environment. And the most persuasive argument of all, of course, is that the earth's fossil fuel reserves are finite, whereas well-managed, self-renewing biomass production systems are as unlimited as sunlight.

While biofuels are not yet competitive with petroleum, there are still many incentives for developing bioresource systems. New, locally-adapted crops will permit a continued reliance on an agricultural economy in areas threatened by declining water supplies. New crops also will provide new commercial products, with an especially significant impact expected in the areas of industrial polymers, pharmaceuticals, and environmentally safe pesticides.

Finally, biomass crops are a renewable resource that can be sustained in a wide variety of climates, and which can yield products to meet many—perhaps most—of the world's changing needs. This is a challenge the Natural Products Center continually seeks to address.

Economic Botany

The standard approach to modern agriculture in arid lands has been to seek ways of growing conventional crops in an environment for which these plants are not suited. Our strategy is the opposite. We seek ways to obtain valuable harvests from plants that have already undergone an extraordinarily effective breeding program: millions of years of natural selection resulting in excellent adaptation to desert conditions.

Certainly we are not the first to adopt this philosophy of expedience. Native populations of the Sonoran and other deserts have a long history of seeking and finding from desert plants virtually all the commodities they needed for survival.

The economic botany program at the Bioresources Research Facility is carrying out a modern version of this tradition. While native people of the desert valued plants primarily as sources of food or fiber, our emphasis is on specialty products that are not produced in the nation's breadbasket, and that have a high value in today's economy-crop returns that will keep a modern farmer in business. Food, animal feed, and fiber, when produced as byproducts of a crop whose primary product is a pharmaceutical or industrial chemical, can further improve the economic picture.

Plant Collection and Assessment

When a plant sample is collected from a wild population, analyzed in the laboratory, and found to have economically promising characteristics, its career as a "crop candidate" at the Natural Products Center has begun.

The first stage of development then involves an assessment of germplasm collected from the wild and grown under standardized field conditions, to determine the species' potential adaptability to an agricultural regimen. Trial specimens are planted and cultivated in observation plots where we can monitor the agronomic parameters that are critical to a crop's ultimate commercial success. Intrinsic characteristics of the plant are noted during this phase: whether it is annual or perennial, its habits of growth and flowering, the practicality of harvesting the important plant parts, and other information that will be of value in choosing plants for domestication.

Several acres of observation plots are located on site at the Natural Products Center. As the agronomic development of a selected species advances, more space is needed for larger-scale testing and bulk harvesting. Larger field for this purpose are available on the University of Arizona farms and on cooperative farm sites throughout the southwestern United States.

Quantitative Agronomic Studies

After an initial assessment has demonstrated a plant's sustainability for agricultural development, rigorous feasibility studies begin. These experiments quantitatively determine the agricultural strengths and weaknesses of a crop, including such things as water and nutrient needs, drought tolerance, and susceptibility to pathogens.

The most important single criterion determining a plant's feasibility for arid lands cropping is its water-use efficiency. This can be measured in field experiments by means of metered irrigation and gravimetric soil analysis.

Another important aspect of agricultural feasibility study is the design of appropriate cropping schemes. In order to do this, we do research to acquire a thorough knowledge of a plant's optimal planting and harvest dates, densities, harvesting requirements, potential for multiple harvests, and its performance as a mixed crop or monoculture. Potential yield is another essential piece of agronomic information we study in order to estimate the crop's agricultural and economic potential.

Plant Improvement

Seldom, if ever, can a plant species be taken directly from its native state into intensive cultivation without the need for some kind of improvement. Even in ancient times, newly domesticated grains were subjected to a gradual selection process for such features as upright habit and seed-retention (for more efficient harvesting), greater productivity, and improved aesthetic and nutritional properties. The crops that are grown and mechanically harvested by the modern farmer generally bear little resemblance to their native progenitors.

Similarly, the native species selected as candidate crops at the Natural Products Center are expected to require genetic improvement before they can be seriously considered as crop options for the farmer.

Plant breeding and genetic studies at the Facility are conducted within the greater context of a multidisciplinary crop development program. The analytical laboratory, for example, provides phytochemical information on a plant product's quantity and quality that gives necessary feedback to the breeding program. Crop growth and vigor are also important factors that must be evaluated separately.

Product Analysis

The ultimate value of a candidate plant, no matter how amenable it might be to domestication, is largely determined by its chemical makeup. Chemical analysis therefore provides critical insight into the crop development process at several stages of the research process.

Qualitative analysis of a plant's constituents are performed early during the evaluation and selection process. Modern phytochemical techniques and an array of quantitative analyses form the basis of the initial screening. If a commercially valuable product, specialty chemical or precursor is identified in a plant sample, the species may be chosen for further development.

At this point, quantitative analysis also becomes important. State-of-the-art techniques, including high pressure liquid chromatography with photo-diode array detectors and gas chromatography, permit sensitive quantification of key products. In this way plant yields can be monitored for their response to changes in agricultural parameters and genetic makeup, providing the information needed to maximize or minimize particular chemical components. Using these techniques, researchers can also delineate the important physico-chemical properties of a plant compound, and determine how much further product development may be needed to produce a marketable commodity.

Economic Analysis

Commercial production of bioresource crops will occur only when there is an adequate return on investment for all segments of the industry. Therefore, economic considerations influence even the early stages of a crop's development.

Scientists at the Natural Products Center have developed models for analyzing the economics and energetics of various projected production schemes. The important variables in these generalized schemes are feedstock costs, energy requirements and yields, product costs, and product marketability. The development of these models has given practical value to the research program, allowing the researchers to make early projections about the commercial prospects of a new crop.

For agriculture in an arid environment, the most important determinant of input costs and energy requirements is the amount of irrigation required to produce the crop. This finding emphasizes the value of an approach in which plants are sought first for their arid-adaptedness, and secondly for other attributes.

In the face of depleted aquifers, rising irrigation costs, erosion, and soil salinity problems, a crop that can be grown with little irrigation and good economic returns would be a welcome means of salvaging the flagging agricultural economy of these regions.

The economic advantages of bioresource development will become increasingly evident in the decades to come. A number of the projects that are being explored at the Facility already have been proven viable, in theory or in practice. The production of bulk specialty chemicals, in particular, has been shown to be highly cost-effective.

Process Development and Demonstration

One of the most challenging aspects of new crop development is the need for designing new systems for producing and refining the end products. Some steps of the process, such as plant harvesting, may be accomplished by traditional means, but frequently the technology required for a process does not exist and must be invented. Thus, the feasibility of the entire system depends not only on the vigor of the crop, but on the equipment needed to transform bulk plant material into refined, marketable end products. The design of this equipment, the integration of the complete production system, and the construction of demonstration plants are all part of our research plan.

Extraction technology is emphasized in our program because of its flexibility; similar equipment can be used for isolating a broad range of products from appropriate plant species. The same extraction facilities that are now used on a large scale for producing bulk specialty chemicals can be used later for extracting biocrude for used as a liquid fuel.

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Last updated: 17 September 2001