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Introduction The agricultural use of saline water or soils can benefit many developing countries. Salt-tolerant plants can utilize land and water unsuitable for salt-sensitive crops (glycophytes) for the economic pro- duction of food, fodder, fuel, and other products. Halophytes (plants that grow in soils or waters containing significant amounts of inor- ganic salts) can harness saline resources that are generally neglected and are usually considered impediments rather than opportunities for development. Salts occur naturally in all soils. Rain dissolves these salts, which are then swept through streams and rivers to the sea. Where rainfall is sparse or there is no quick route to the sea, some of this water evaporates and the dissolved salts become more concentrated. In arid areas, this can result In the formation of salt lakes or in brackish groundwater, salinized soil, or salt deposits. There are three possible domains for the use of salt-tolerant plants in developing countries. These are: 1. Farmlands salinized by poor irrigation practices; 2. Arid areas that overlie reservoirs of brackish water; and 3. Coastal deserts. In some developing regions, there are millions of hectares of saTinized farmland resulting from poor irrigation practices. These lands would require large (and generally unavailable) amounts of 1
2 water to leach away the salts before conventional crops could be grown. However, there may be useful salt-tolerant plants that can be grown on them without this intervention. Although the introduction of salt-tolerant plants will not necessarily restore the soil to the point that conventional crops can be grown, soil character is often improved and erosion reduced. Moreover, many arid areas overlie saline aquifers groundwater containing salt levels too high for the irrigation of conventional, salt- sensitive crops. Many of these barren lands can become productive by growing selected salt-tolerant crops and employing special cultural techniques using this store of brackish water for irrigation. Throughout the developing world, there are extensive coastal deserts where seawater is the only water available. Although growing crops in sand and salty water is not a benign prospect for most farm- ers, for saline agriculture they can complement each other. The dis- advantages of sand for conventional crops become advantages when saline water and salt-tolerant plants are used. Sand is inherently low in the nutrients required for plant growth, has a high rate of water infiltration, and has low water-holding capacity. Therefore, agriculture on sand requires both irrigation and fertilizer. Surprisingly, 11 of the 13 mineral nutrients needed by plants are present in seawater in adequate concentrations for growing crops. In addition, the rapid infiltration of water through sand reduces salt buildup in the root zone when seawater is used for irrigation. The high aeration quality of sand is also valuable. This characteristic allows oxygen to reach the plant roots and facilitates growth. Although careful application of seawater and supplementary nutrients are necessary, the combination of sand, saltwater, sun, and salt-tolerant plants presents a valuable opportunity for many developing countries. Of these three possibilities for the introduction of salt-tolerant plants (sal~nized farmland, undeveloped barren land, and coastal deserts), the reclamation of degraded farmland has several advan- tages: people, equipment, buildings, roads, and services are usually present and a social structure and market system already exist. The potential use of saline aquifers beneath barren lands depends on both the concentration and nature of the salts. The direct use of seawater for agriculture is probably the most challenging potential application.
3 Most contemporary crops have been developed through the do- mestication of plants from nonsaTine environments. This is unfortu- nate since most of of the earth's water resources are too salty to grout them. From experience in irrigated agriculture, Miyamoto (personal communication) suggests the following classification of potential crop damage from increasing salt levels: Irrigation Water Salts, ppm Crop Problems Fresh <125 None Slightly saline 125-250 Rare Moderately saline 25~500 Occasional Saline 50~2,500 Conunon Highly saline 2,500-5,000 Severe Colorado River water, used for irrigation in the western United States, contains about 850 ppm of salts; seawater typically contains 32,00~36,000 ppm of salts. Salinity levels are usually expressed in terms of the electrical conductivity (EC) of the irrigation water or an aqueous extract of the soil; the higher the salt level, the greater the conductivity. The salinity of some typical water sources is shown in Table 1. TABLE 1 Water Salinity. . . megaton Salinity Water Quality Colorado Alamo Negev Pacific Measurement (Good) (Marginal) River River Groundwater Ocean Electrical conductivity (dS/m)* 0-1 1-3 1.3 4.0 4.0 - 7.0 46 Dissolved solids, ppm 0-500 500-1,500 850 3,000 3,000-4,500 35,000 *1 dS/m = 1 mmho/cm = (approx.) 0.06%NaC1 = (approve.) 0.01 mole/1 NaCl. 10,000 ppm = 10 o/oo (parts per thousand) = 10 grams per liter = 1.0% In the International System of Units (SI), the unit of conductivity is the Siemens symbol, S. per meter. The equivalent unit commonly appearing in the literature is the mho (reciprocal ohm); 1 mho equals 1 Siemen. SOURCE: Adapted from Epstein, 1983; Pastemak and De Malach, 1987; and Rhoades et al., 1988.
4 There are three broad approaches to utilizing saline water, de- pending on the salt levels present. These include the use of marginal to poor irrigation water with electrical conductivities (ECs) up to about 4 dS/m, the use of saline groundwaters such as those in Israel's Negev Desert with ECs up to about 8 dS/m, and the use of even more saline waters with salt concentrations up to that of seawater. At low, but potentially damaging, salt levels, Rhoades and coworkers (1988) have grown commercial crops without the yield losses that would normally be anticipated. Through knowledge of crop sensitivity to salt at various growth stages, they used combina- tions of Colorado River water and Alamo River water to minimize the use of the higher quality water. For example, wheat seedlings were established with Colorado River water; Alamo River water was then used for irrigation through harvest with no loss in yield. At higher salt levels, Pasternak and coworkers (1985) have devel- oped approaches that involve special breeding and selection of crops and meticulous water control. The agriculture of Negev settlements in Israel is based on the production of cotton with higher yields, quality tomatoes for the canning industry, and quality melons for export all grown with EC 4-7 dS/m groundwater. Experimental yields of a wide variety of traditional crops grown in Israel with water with ECs up to 15 dS/m, are shown in Table 6 (p. 35~. In west Texas (USA), Miya~noto and coworkers (1984) report commer- cial production of alfalfa, melons, and tomatoes with EC 3-5 dS/m irrigation water, and cotton with 8 dS/m irrigation water. The use of water with still higher salt levels up to, including, and even exceeding that of seawater for irrigation of various food, fuel, and fodder crops has been reported by many researchers including Aronson (1985; 1989), Boyko (1966), Epstein (1983; 1985), Gallagher (1985), Glenn and O'Leary (1985), Tyengar (1982), Pasternak (1987), Somers (1975), Yensen (1988), and others. These scientists have produced grains and oilseeds; grass, tree, and shrub fodder; tree and shrub fue~wood; and a variety of fiber, pharmaceutical, and other products using highly saline water. Thus, depending on the soil or water salinity levels, salt-tolerant plants can be identified that will perform well in many environments in developing countries. The salt tolerance of some of these plants enables them to produce yields under saline conditions that are comparable to those obtained from salt-sensitive crops grown under nonsaline conditions. The maximum amount and kind of salt that can be tolerated
s 120 Oh 100 IL cL ~ 80 O a, > Q 60 40 20 \ Halophytes Salt-Sensitive Crops - \ Salt-Tolerant Crops ~ 0 5 10 15 20 25 30 35 SALINITY, dS/m FIGURE 1 Growth response to salinity. Many halophytes, such as Suacda maritime, have increased yields at low salinity levels. Salt-tolerant crops, such as barley, maintain yields at low salinity levels but decrease as salt levels exceed a certain limit. Yields of salt-sensitive crops, such as beans, decrease sharply even in the presence of low levels of salt. SOURCE: Adapted from Greenway and Munns, 1980; Maas 1986; and Yensen, et al., 1985. by halophytes and other salt-tolerant plants varies among species and even varieties of species. Many halophytes have a special and distinguishing feature-their growth is improved by low levels of salt. Other salt-tolerant plants grow well at low salt levels but beyond a certain level growth is reduced. With salt-sensitive plants, each increment of salt decreases their yield (Figure 1~. Such data provide only relative guidelines for predicting yields of crops grown under saline conditions. Absolute yields are subject to numerous agricultural and environmental effects. Interactions between salinity and various soil, water, and climatic conditions all affect the plant's ability to tolerate salt. Some halophytes require fresh water for germination and early growth but can tolerate higher salt levels during later vegetative and reproductive stages. Some can
6 germinate at high salinities but require lower salinity for maximal growth. Traditional farming efforts usually focus on modifying the en- vironment to suit the crop. In saline agriculture, an alternative is to allow the environment to select the crops, to match salt-tolerant plants with desirable characteristics to the available saline resources. In many developing countries extensive areas of degraded and arid land are publicly owned and readily accessible for government- sponsored projects. These lands are often located in areas of high nutritional and economic need as well. If saline water is available, the introduction of salt-tolerant plants in these regions can unprove food or fuel supplies, increase employment, help stem desertification, and contribute to soil reclamation. LIMITATIONS Undomesticated salt-tolerant plants usually have poor agro- nom~c qualities such as wide variations in germination and matura- tion. Salt-tolerant grasses and grains are subject to seed shattering and lodging. The foliage of sal~tolerant plants may not be suitable for fodder because of its high salt content. Nutritional characteristics or even potential toxicities have not been established for many edible salt-tolerant plants. When saline irrigation water is used for crop production, careful control is necessary to avoid salt buildup in the soil and to prevent possible contamination of freshwater aquifers. Most importantly, salt-tolerant plants should not be cultivated as a substitute for good agricultural practice nor should they be used as a palliative for improper irrigation. They should be introduced only when and where conventional crops cannot be grown. Also, currently productive coastal areas (such as mangrove forests) should be managed and restored, not converted to other uses. All of these limitations are impediments to the use of conven- tional methods for culture and harvest of salt-tolerant plants and the est~rnation of their production economics. RESEARCH NEEDS - Increased research on the development of salt-tolerant cultivars of crop species could, with appropriate management, result in the broader use of saline soils. In the early selection and breeding pro- gra~ns of crop species for use in nonsaline environments, performance
7 was improved through the considerable genetic variability present in the unimproved crops and in their wild relatives. Since few crops have been subjected to selection for salinity tolerance, it is possible that variation In this characteristic may also exist. Conversely, few undomesticated salt-tolerant plants have been examined for variabil- ity in their agronorruc qualities, and it Is even more likely that such characteristics can be improved through breeding programs. In addition, germplasm collection and classification, breeding and selection, and development of cultural, harvest, and postharvest techniques are all needed. Basic information on the way in which plants adapt to salinity would significantly assist their economic development. Exploration for new species should continue to identify candi- dates for economic development. Research can then begin on ways to improve the agronomic qualities of these plants and to utilize their genetic traits. For example, seed from a wild tomato found on the seashore of the Galapagos Islands produced tomatoes that were small and bitter. When this species was crossed with a commercial tomato cultivar, flavorful} fruit the size and color of cherry tomatoes were obtained in 70 percent seawater. Recent advances in plant biotechnology include work on salinity tolerance and productivity. New techniques for in vitro selection of genotypes tolerant to high salinity leveLs have been found to improve the adaptability of conventional crops as well as assist in the selection of desired genotypes from a wide range of natural variability in individual salt-tolerant plants. Genotypes with increased tolerance to water and salinity stress have been identified and followed in genetic crosses with conventional genotypes using new techniques in gene mapping and cell physiology. Stress genes are now the target of research in genetic engineering. The transfer of these genes from sources in salt-tolerant species to more productive crops will require modifications in cultural practices as well as treatment of the plant products. Interdisciplinary communication is particularly important in re- search on salt- tolerant plants. Cooperation among plant ecologists, plant physiologists, plant breeders, soil scientists, and agricultural engineers could accelerate development of economic crops. Further, universities could introduce special programs to allow broad study of the special characteristics of saline agriculture to serve growing needs in this field.
8 REFERENCES AND SELECTED ~AD~GS Abrol, I. P., J. S. P. Yadav and F. I. Massoud. 1988. Salt-affceted Soils and Their Management. Soils Bulletin 39, FAO, Rome, Italy. Ahmad, R. 1987. Saline Agriculturc at Coastal Sandy Belt. University of Karachi, Karachi, Pakistan. Ahm ad, R. and A. San Pietro (eds.~. 1986. Prospects for Biosalinc Rcacarch University of Karachi, Karachi, Pakistan. Aronson, J. A. 1989. Haloph Salt tolerant Plants of the World University of Arizona, Tucson, Arizona, US. Aronson, J. A. 1985. Economic halophytes a global view. Pp. 177-188 in: G. E. Wickens, J. R. Goodin and D. V. Field (eds.) Plank for Arid Land,. George Allen and Unwin, London, UK. Bahri, A. 1987. Utilization of saline waters and soils in Tunisia. Results and research prospects. FcrNlizere and AgricuBurc 96:17-34. Barrett-Lennard, E. G., C. V. Malcolm, W. R. Stern and S. M. Wilkins (eds.~. 1986. Forage and Fuel Production. from Salt Affceted Waetclar~ Elsevier Publishers, Amsterdam, Netherlands. Bernstein, L. 1964. Salt Toleranec of Plank. USDA Bulletin No. 283, Washington, DC, US. Boyko, H. 1966. Salinity and Aridity. New Approachce to Old Problc~ru. Dr. W. Junk, Publisher, The Hague, Netherlands. Epstein, E. 1985. Salt tolerant crops: origins, development, and prospects of the concept. Plard and Soil 89:187-198. Epstein, E. 1983. Crops tolerant of salinity and other stresses. Pp. 61-82 in: Better Crops for Food. Pitman Books, London, UK. Epstein, E., J. D. Norlyn, D. W. Rush, R. W. Kingsbury, D. B. Kelley, G. A. Cunningham and A. F. Wrona. 1980. Saline culture of crops: a genetic approach. Science 210:399-404. Flowers, T. J., M. A. Haiibagheri and N. J. W. Clipson. 1986. Halophytes. The Quarterly Rctnew of Biology 61~3~:313-337. Gallagher, J. L. 1985. Halophytic crops for cultivation at seawater salinity. Plant and Soil 89:323-336. Glenn, E. P. and J. W. O'Leary. 1985. Productivity and irrigation requirements of halophytes grown with seawater in the Sonoran Desert. Journal of Arid Environmer~ 9~1~:81-91. Goodin, J. R. and D. K. Northington. 1979. Arid Land Plant Rceourecs. Texas Tech University, Lubbock, Texas, US. Greenway, H. and R. Munns. 1980. Mechanisms of salt tolerance in nonhalo- phytes. Anrwal Review of Plant Physiology 31:149-190. Iyengar, E. E. R. 1982. Research on seawater irriculture in India. Pp. 165-175 in: A. San Pietro (ed.) Biosalinc Research: A Look to the Inure. Plenum Press, New York, New York, US. Mans, E. V. 1986. Crop tolerance to saline soil and water. Pp. 205-219 in: R. Ahmad and A. San Pietro (eds.) Prospects for Bio~alinc Rcecarch. University of Karachi, Karachi, Pakistan. Miyamoto, S., J. Moore and C. Stichler. 1984. Overview of saline water irrigation in far west Texas. Pp. 222-230. in: Procecdir~ge of Irrigation. and Drainage Special*y Confcrcnec, ASCE, Flagstaff, Arizona, July 24-26, 1984.
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