GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS
R. T. Ramage Science & Education Administration, U.S.D.A. Department of Plant Sciences University of Arizona Tucson, Arizona 85719
Increasing demands for plant products for food, chemicals and energy will require that many of the arid and semi-arid regions of the world be used for crop production. Soil salinity is a perennial problem in arid and semi-arid areas. Crops grown in these areas are quite often irrigated and irrigation frequently compounds difficulties with soil salinity. Also, in many irrigated areas, substantial amounts of brackish water are available to augment irrigation supplies. Soil salinity and the potential use of brackish water for irrigation in the arid and semi-arid regions of the world have created a great need for salt tolerant crops. Consequently, much effort has been expended in recent years searching for crops and varieties that are salt tolerant. Much of this work has been done in countries that have extensive arid and semi-arid areas that are irrigated, such as the U.S.S.R., India, Pakistan, Israel and Egypt. There exists an enormous volume of literature pertaining to plant responses to salinity. The U. S. Department of Agriculture (USDA, 1978) has published an indexed bibliography that lists 2,357 references concerning plant responses to salinity~ Many additional references may be found in the various lists of titles and abstracts such as Field Crop Abstracts and Plant Breeding Abstracts. According to this volume of literature, over 1,500 species have been used in studying plant responses to salinity. Over 50 crop species have been evaluated for varieties that exhibit salt tolerance. These include cereal crops, fiber crops, forage grasses and legumes, both small- and tree-fruits, oil-seed crops, pulses, sugar crops, vegetable crops and trees (for reforestation and for streetside plantings where salt is used for deicing). 311
D. W. Rains et al. (eds.), Genetic Engineering of Osmoregulation © Plenum Press, New York 1980
312
R.T.RAMAGE
PLANT RESPONSE TO SALINITY Different salts have been used in determining plant responses to salinity. In addition to NaCI, CaCI2, KCI, K2S04, MgCI2, MgS04' Na2C03' NaHC0 3 and Na2S04 have been used, either singly, in combination with each other or in combinations with NaCI. Plant responses have also been studied by growing plants in saline and alkaline soils and by irrigating with water containing various salt solutions and concentrations. A number of studies report using seawater as irrigation water. Plants usually show greater sensitivity to single salts than to salt mixtures, probably because nutritional inbalances and specific ion toxicities are more likely to occur if one salt predominates under saline conditions. Plant responses to salinity have been reported from plants germinated and grown in many different media, containers and treated fields. Seed have been germinated in nutrient solutions, petri dishes, growth pouches, blotter sandwiches in plastic boxes, and pots and trays in growth chambers and germination cabinets as well as in naturally and artifically salinized soils. Plants have been grown to various stages of maturity in nutrient solutions, growth pouches and pots in greenhouses and growth chambers and in saline and alkaline fields. Growth stages where responses to salinity have been measured include imbibition, germination, emergence, seedling, tillering and mature plant stages. There is disagreement in the literature concerning the relationship of salt tolerance to the developmental stage of a plant. Some growth stages in some plants appear to be more tolerant to salinity than other growth stages. A number of experiments have shown that salinity may reduce the rate of germination but have little, if any, effect on total germination. Also, some plants have been reported to be tolerant at the seedling stage and susceptible at later growth stages while others are susceptible as seedlings but resistant at later stages. A number of authors insist that salt tolerance at germination gives the best measure of general salt tolerance on the assumption that, if the plants are to be grown in saline soils or with brackish water, a lack of salt tolerance for germination makes any potential tolerance of mature plants of little consequence. A large number of measurements, or indices, of salt tolerance have been used. These include rate of imbibition measured by weight gain or by seed size increase, percent germination, rate of germination, seedling survival, seedling growth, fresh weight of seedlings, root and shoot growth, plant height, tillering capacity, leaf area, spike or panicle weight, number of seed per spike or panicle, and, actual field yields. Also, salt tolerance has been estimated by uptake of potassium, tetrazolium staining and electric potential.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS
313
Salinity appears to affect rate of germination more than total germination and rate of development or growth more than total growth BREEDING FOR SALT TOLERANT PLANTS Some authors have reported that hybrids are more tolerant than their parents while others report that tolerance is intermediate between the parents, is similar to the more tolerant parent, or that tolerance is similar to the less tolerant parent. Wild relatives of some crops are reported to be more tolerant to salinity than cultivated varieties while wild relatives of other crops appear to be more susceptible than cultivated varieties. Apparently, in the domestication and breeding of certain crops, such as carrot and celery, salt tolerance has been inadvertently bred into the cultivated varieties. Salt tolerance has been correlated with both drought tolerance and winter survival. Usually, salt tolerant varieties have been found to be more drought and winter tolerant than salt susceptible varieties. Very little actual breeding for salt tolerance has been done. Few deliberate efforts have been made to test potential parents for tolerance or to select tolerant plants in the segregating progeny of crosses. The development of the existing salt tolerant varieties has been mainly a matter of chance as most of them have been selected from among varieties that had been bred for other reasons. A few salt tolerant varieties have been intentionally selected from local land races. The literature records several attempts to isolate induced mutations for salt tolerance. Tissue and cell culture techniques have been suggested as means of selecting salt tolerant varieties (cf. article in this volume). The purpose of plant breeding is to increase or stabilize yields of desirable plant parts or products. Yield and traits that tend to stabilize yield are characters that are under genetic controls. A fundamental tenet of genetics is that every characteristic of every living organism is the product of an interaction of a specific genetic constitution with a specific environment. In practicing plant breeding, we should keep in mind that the phenotype of a plant is the result of all of the genes of the plant interacting with each other and with the environment. We should appreciate that a successful variety is one that has a genotype that is balanced to accommodate the particular genetic characters of the variety. A successful variety is one that has a genotype that is balanced to accommodate the particular plant height, maturity, leaf width, chlorophyll content, salt tolerance, etc., of that variety. An optimum balance exists between the action of a particular gene and its background genotype. Each character or combination of
314
R. T. RAMAGE
characters has a background genotype that is most favorable for the expression of that character or combination of characters. Our premise is that there is a most favorable background genotype for every character or combination of characters and that success in plant breeding depends upon selecting the background genotype that is most favorable for the expression of the particular combination of characters that a variety must possess (Ramage, 1977). Any breeding program consists of two distinct phases: (1) generation of variation and (2) exploitation of the variation. In a conventional pedigree or backcross program, relatively little of the total effort goes into generation of variation -- most of it goes into exploitation, including yield tests. We believe that greater, and particularly more rapid, progress can be made if more of the total breeding effort is spent in generating variability and thereby reducing the amount of effort necessary for successful exploitation. Two of the more serious impediments of a plant breeding program are (1) the amount of time that is required between cycles of effective selection and (2) the difficulty in maintaining sufficient genetic diversity, to allow for selection of rare gene combinations in a background genotype that is most favorable for their expression. RECURRENT SELECTION FOR SALT TOLERANT PLANTS Salt tolerance is an extremely complex trait and no single criterion of measurement will be adequate. High levels of salt repress seed germination and plant growth by both non-specific, osmotic effects and by the toxic effects of specific ions. Also, salt affects growth stages differently. The nature of salt tolerance for imbibition is probably very different than for germination, for seedling survival or for plant growth. In breeding for such an intricate character as salt tolerance, selecting a most favorable background genotype will probably be much more important than selecting the character itself. A salt tolerant variety will be of little value if it does not produce an acceptable yield of desirable plant parts or products. Recurrent selection is the most feasible means of simultaneous selection for a character and its most favorable background genotype. Recurrent selection shortens the time that is required between cycles of effective selection and allows the use of the numbers of plants necessary to maintain the genetic diversity that is needed for the selection of rare gene combinations. Recurrent selection is concerned primarily with generation of variation and is used to develop populations and information for exploitation by other breeding schemes.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS
315
Recurrent selection is a breeding technique designed to accumulate or concentrate genes for a particular quantitative character in a population without a significant loss of genetic variability. The procedure was proposed and developed for corn breeding (Sprague and Brumhall, 1950). As the method depends upon obtaining rather large numbers of progenies from crosses, it has generally been considered to have the greatest usefulness for breeding cross pollinated crops. A typical recurrent selection program begins with the establishment of a base population that contains genetic variability for the character that is to be selected for. From this base population, plants that are superior for the particular character are selected and selfed seed are obtained from them. The selfed progenies of the selected plants are grown in a plant-to-row design. All possible intercrosses are made among the selfed progenies. Equal quantities of seed of all of the intercrosses are bulked and the progeny grown for one or more generations to allow genetic recombination to occur. This completes the first cycle of recurrent selection. The increase of the bulked intercrosses serves as the base population for the next cycle of recurrent selection. Again, superior plants are selected, selfed seed from them obtained, planted in a plant-to-row design and all possible intercrosses between progenies are made. The crosses are bulked and the progeny grown for one or more generations. The seed increase serves as the base population for the next cycle of recurrent selection. Cycles of recurrent selection are repeated for as long as there is improvement for the selected character. The population is then exploited by selecting plants that will be inbred to produce lines that will be used in hybrids, synthetic varieties, composites, etc. This 'basic program has been modified and adapted to breeding a numbe'r of crops such as sorghum, sugar beets, forage crops, and others. The typical recurrent selection program assumes that genetic variation for the selection character is limited to that contained in the beginning base population. Supposedly, a cycle of recurrent selection will be reached where there is no longer significant improvement for the selected character. The system also assumes that a population is to be developed, exploited and then discarded. Recurrent selection will be most effective in breeding for characters whose expression can be modified as a result of changing the background genotype, transgressive segregation or accumulation of minor genes. In many instances, the character expression of a gene or genes can be modified strikingly by changing the genetic background of the gene (Ramage, 1977). Transgressive segregants
316
R.T.RAMAGE
are individuals that show a more extreme development of a character than was present in the parental material. They are generally assumed to be due to cumulative and complementary effects of genes contributed by the original parents. The use of a kind of transgressive segregation for increasing and stabilizing disease resistance has been proposed as the accumulation of minor genes. Salt tolerance is a character whose expression can probably be modified both by changing the background genotype and by obtaining transgressive segregants or accumulating minor genes. The use of a typical recurrent selection breeding program is not feasible in breeding a self pollinated crop because of limitations imposed by the way self pollinated crops must be crossed. Genetic male sterility can be used to greatly facilitate crossing in self pollinated crops. The capacity for crossing permitted by the use of genetic male sterility allows the use of recurrent selection in breeding self pollinated crops. We have used modifications of the basic recurrent selection technique in breeding barley for such characters as salt tolerance, ability to cross pollinate, plant height, earliness, seed size, etc. We have called such breeding schemes "male sterile facilitated recurrent selection (Ramage, 1977b). A male sterile facilitated recurrent selection program begins with the establishment of a base population that is segregating for both male sterility and the character that is to be selected for. Relatively large numbers of both male sterile and male fertile plants that are superior for the particular character are selected. The selected plants are crossed using male sterile plants as females. The crossed seed are bulked and the Fl generation is grown. This completes the first cycle of recurrent selection. As we have been using it, a cycle of male sterile facilitated recurrent selection differs from a typical recurrent selection program primarily in two ways: (1) we make crosses between selected plants rather than between selfed progenies of selected plants, and (2) we usually use a selected plant in only one cross rather than make all possible intercrosses between selfed progenies of selected plants. As selection based on an individual plant is not nearly as effective as selection based on a progeny row, we select relatively large numbers of plants in order to maintain a high degree of variability in the population. By selecting large numbers of plants and making only single crosses between them, we may not be making the most rapid progress in selecting for the particular character, but we maintain a very high degree of variability for other characters. This increases the chances of selecting the character in a more favorable background genotype. The high degree of variability for other characters also makes the population much better suited for continuous exploitation.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS.
317
Just as in a typical recurrent selection program, the F2 of the bulked crosses between ~elected plants serves as the base population for the next cycle of selection. Cycles of recurrent selection are repeated for as long as there is improvement for the selected character. Plants may be selected from the F2 of any cycle and inbred for use as varieties, parents of hybrids, parents for conventional breeding programs, etc. Another modification of a typical recurrent selection program that we employ involves adding new sources of germplasm to the base population. In our schemes, we may introduce additional sources of germplasm into the population in any cycle. The new germplasm may be for the selected character or for any other desired character, such as disease resistance, local adaptation, large seed, etc. Usually, the new germplasm sources will be male fertile and are crossed onto male sterile plants selected from the population. The number of crosses made depends upon how much we want to dilute the base population. Also, we intercross populations that have been selected for different characters to start new populations. In most of our populations, simultaneous selection for several characters is practiced. As we use it, male sterile facilitated recurrent selection is an extremely flexible system. There is no limit to the amount of variation that can be carried in a population. The populations should continue to improve with each cycle of recurrent selection. Our populations are designed to be continued indefinitely and regularly exploited. This makes variety development from the population a continuous process. The basic features that make male sterile facilitated recurrent selection effective are: (1) the generation and maintenance of a wide genetic base from which to select, (2) the ability to make the large number of crosses necessary to obtain rare gene combinations, (3) the opportunity to apply extreme selection pressure to large populations generated by the system, and (4) the shortened time between cycles of effective selection. Male sterile facilitated recurrent selection produces populations that may be exploited in a number of ways. The simplest way is to select male fertile plants from the F2 of any cycle or recurrent selection and carry them on in a conventional pedigree program. For this type of exploitation, plants may be selected from any cycle of the recurrent selection program, thus making variety development a continuous process. Another way to effectively use recurrently selected populations is to select plants from different populations to be used as parents in conventional breeding programs. Techniques for breeding salt tolerant crops are relatively simple. Recurrent selection can be used to accumulate genes for
318
R. T. RAMAGE
salt tolerance in background genotypes that are most favorable for their expression. Populations developed by recurrent selection can be readily exploited by conventional breeding programs. The limiting component of a recurrent selection procedure is the selection of plants that are superior for the character being selected for. Rapid, accurate and inexpensive selection of a very few plants from a very large population is essential to a successful breeding program. Appropriate selection methods need to be developed for salt tolerance. Before better selection techniques can be developed, a better understanding of how non-specific osmotic effects and toxic effects of specific ions affect the different growth stages of plants must be gained. REFERENCES Ramage, R. T., 1977a, Male sterile facilitated recurrent selection and happy homes, Proc. Fourth Reg. Winter Cereals Workshop (Barley), Vol. II:92. Ramage, R. T., 1977b, Varietal improvement of wheat through male sterile facilitated recurrent selection, ASPAC Tech. Bull., No. 37. Sprague, G. F. and Brimhall, B., 1950, Relative effectiveness of two systems of sele~tion for oil content of the corn kernel, Agron. Jour., 42:83. U. S. Department of Agriculture, SEA, 1978, Plant responses to salinity: an indexed bibliography, ARM-W-6.
R. T. Ramage Science & Education Administration, U.S.D.A. Department of Plant Sciences University of Arizona Tucson, Arizona 85719
Increasing demands for plant products for food, chemicals and energy will require that many of the arid and semi-arid regions of the world be used for crop production. Soil salinity is a perennial problem in arid and semi-arid areas. Crops grown in these areas are quite often irrigated and irrigation frequently compounds difficulties with soil salinity. Also, in many irrigated areas, substantial amounts of brackish water are available to augment irrigation supplies. Soil salinity and the potential use of brackish water for irrigation in the arid and semi-arid regions of the world have created a great need for salt tolerant crops. Consequently, much effort has been expended in recent years searching for crops and varieties that are salt tolerant. Much of this work has been done in countries that have extensive arid and semi-arid areas that are irrigated, such as the U.S.S.R., India, Pakistan, Israel and Egypt. There exists an enormous volume of literature pertaining to plant responses to salinity. The U. S. Department of Agriculture (USDA, 1978) has published an indexed bibliography that lists 2,357 references concerning plant responses to salinity~ Many additional references may be found in the various lists of titles and abstracts such as Field Crop Abstracts and Plant Breeding Abstracts. According to this volume of literature, over 1,500 species have been used in studying plant responses to salinity. Over 50 crop species have been evaluated for varieties that exhibit salt tolerance. These include cereal crops, fiber crops, forage grasses and legumes, both small- and tree-fruits, oil-seed crops, pulses, sugar crops, vegetable crops and trees (for reforestation and for streetside plantings where salt is used for deicing). 311
D. W. Rains et al. (eds.), Genetic Engineering of Osmoregulation © Plenum Press, New York 1980
312
R.T.RAMAGE
PLANT RESPONSE TO SALINITY Different salts have been used in determining plant responses to salinity. In addition to NaCI, CaCI2, KCI, K2S04, MgCI2, MgS04' Na2C03' NaHC0 3 and Na2S04 have been used, either singly, in combination with each other or in combinations with NaCI. Plant responses have also been studied by growing plants in saline and alkaline soils and by irrigating with water containing various salt solutions and concentrations. A number of studies report using seawater as irrigation water. Plants usually show greater sensitivity to single salts than to salt mixtures, probably because nutritional inbalances and specific ion toxicities are more likely to occur if one salt predominates under saline conditions. Plant responses to salinity have been reported from plants germinated and grown in many different media, containers and treated fields. Seed have been germinated in nutrient solutions, petri dishes, growth pouches, blotter sandwiches in plastic boxes, and pots and trays in growth chambers and germination cabinets as well as in naturally and artifically salinized soils. Plants have been grown to various stages of maturity in nutrient solutions, growth pouches and pots in greenhouses and growth chambers and in saline and alkaline fields. Growth stages where responses to salinity have been measured include imbibition, germination, emergence, seedling, tillering and mature plant stages. There is disagreement in the literature concerning the relationship of salt tolerance to the developmental stage of a plant. Some growth stages in some plants appear to be more tolerant to salinity than other growth stages. A number of experiments have shown that salinity may reduce the rate of germination but have little, if any, effect on total germination. Also, some plants have been reported to be tolerant at the seedling stage and susceptible at later growth stages while others are susceptible as seedlings but resistant at later stages. A number of authors insist that salt tolerance at germination gives the best measure of general salt tolerance on the assumption that, if the plants are to be grown in saline soils or with brackish water, a lack of salt tolerance for germination makes any potential tolerance of mature plants of little consequence. A large number of measurements, or indices, of salt tolerance have been used. These include rate of imbibition measured by weight gain or by seed size increase, percent germination, rate of germination, seedling survival, seedling growth, fresh weight of seedlings, root and shoot growth, plant height, tillering capacity, leaf area, spike or panicle weight, number of seed per spike or panicle, and, actual field yields. Also, salt tolerance has been estimated by uptake of potassium, tetrazolium staining and electric potential.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS
313
Salinity appears to affect rate of germination more than total germination and rate of development or growth more than total growth BREEDING FOR SALT TOLERANT PLANTS Some authors have reported that hybrids are more tolerant than their parents while others report that tolerance is intermediate between the parents, is similar to the more tolerant parent, or that tolerance is similar to the less tolerant parent. Wild relatives of some crops are reported to be more tolerant to salinity than cultivated varieties while wild relatives of other crops appear to be more susceptible than cultivated varieties. Apparently, in the domestication and breeding of certain crops, such as carrot and celery, salt tolerance has been inadvertently bred into the cultivated varieties. Salt tolerance has been correlated with both drought tolerance and winter survival. Usually, salt tolerant varieties have been found to be more drought and winter tolerant than salt susceptible varieties. Very little actual breeding for salt tolerance has been done. Few deliberate efforts have been made to test potential parents for tolerance or to select tolerant plants in the segregating progeny of crosses. The development of the existing salt tolerant varieties has been mainly a matter of chance as most of them have been selected from among varieties that had been bred for other reasons. A few salt tolerant varieties have been intentionally selected from local land races. The literature records several attempts to isolate induced mutations for salt tolerance. Tissue and cell culture techniques have been suggested as means of selecting salt tolerant varieties (cf. article in this volume). The purpose of plant breeding is to increase or stabilize yields of desirable plant parts or products. Yield and traits that tend to stabilize yield are characters that are under genetic controls. A fundamental tenet of genetics is that every characteristic of every living organism is the product of an interaction of a specific genetic constitution with a specific environment. In practicing plant breeding, we should keep in mind that the phenotype of a plant is the result of all of the genes of the plant interacting with each other and with the environment. We should appreciate that a successful variety is one that has a genotype that is balanced to accommodate the particular genetic characters of the variety. A successful variety is one that has a genotype that is balanced to accommodate the particular plant height, maturity, leaf width, chlorophyll content, salt tolerance, etc., of that variety. An optimum balance exists between the action of a particular gene and its background genotype. Each character or combination of
314
R. T. RAMAGE
characters has a background genotype that is most favorable for the expression of that character or combination of characters. Our premise is that there is a most favorable background genotype for every character or combination of characters and that success in plant breeding depends upon selecting the background genotype that is most favorable for the expression of the particular combination of characters that a variety must possess (Ramage, 1977). Any breeding program consists of two distinct phases: (1) generation of variation and (2) exploitation of the variation. In a conventional pedigree or backcross program, relatively little of the total effort goes into generation of variation -- most of it goes into exploitation, including yield tests. We believe that greater, and particularly more rapid, progress can be made if more of the total breeding effort is spent in generating variability and thereby reducing the amount of effort necessary for successful exploitation. Two of the more serious impediments of a plant breeding program are (1) the amount of time that is required between cycles of effective selection and (2) the difficulty in maintaining sufficient genetic diversity, to allow for selection of rare gene combinations in a background genotype that is most favorable for their expression. RECURRENT SELECTION FOR SALT TOLERANT PLANTS Salt tolerance is an extremely complex trait and no single criterion of measurement will be adequate. High levels of salt repress seed germination and plant growth by both non-specific, osmotic effects and by the toxic effects of specific ions. Also, salt affects growth stages differently. The nature of salt tolerance for imbibition is probably very different than for germination, for seedling survival or for plant growth. In breeding for such an intricate character as salt tolerance, selecting a most favorable background genotype will probably be much more important than selecting the character itself. A salt tolerant variety will be of little value if it does not produce an acceptable yield of desirable plant parts or products. Recurrent selection is the most feasible means of simultaneous selection for a character and its most favorable background genotype. Recurrent selection shortens the time that is required between cycles of effective selection and allows the use of the numbers of plants necessary to maintain the genetic diversity that is needed for the selection of rare gene combinations. Recurrent selection is concerned primarily with generation of variation and is used to develop populations and information for exploitation by other breeding schemes.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS
315
Recurrent selection is a breeding technique designed to accumulate or concentrate genes for a particular quantitative character in a population without a significant loss of genetic variability. The procedure was proposed and developed for corn breeding (Sprague and Brumhall, 1950). As the method depends upon obtaining rather large numbers of progenies from crosses, it has generally been considered to have the greatest usefulness for breeding cross pollinated crops. A typical recurrent selection program begins with the establishment of a base population that contains genetic variability for the character that is to be selected for. From this base population, plants that are superior for the particular character are selected and selfed seed are obtained from them. The selfed progenies of the selected plants are grown in a plant-to-row design. All possible intercrosses are made among the selfed progenies. Equal quantities of seed of all of the intercrosses are bulked and the progeny grown for one or more generations to allow genetic recombination to occur. This completes the first cycle of recurrent selection. The increase of the bulked intercrosses serves as the base population for the next cycle of recurrent selection. Again, superior plants are selected, selfed seed from them obtained, planted in a plant-to-row design and all possible intercrosses between progenies are made. The crosses are bulked and the progeny grown for one or more generations. The seed increase serves as the base population for the next cycle of recurrent selection. Cycles of recurrent selection are repeated for as long as there is improvement for the selected character. The population is then exploited by selecting plants that will be inbred to produce lines that will be used in hybrids, synthetic varieties, composites, etc. This 'basic program has been modified and adapted to breeding a numbe'r of crops such as sorghum, sugar beets, forage crops, and others. The typical recurrent selection program assumes that genetic variation for the selection character is limited to that contained in the beginning base population. Supposedly, a cycle of recurrent selection will be reached where there is no longer significant improvement for the selected character. The system also assumes that a population is to be developed, exploited and then discarded. Recurrent selection will be most effective in breeding for characters whose expression can be modified as a result of changing the background genotype, transgressive segregation or accumulation of minor genes. In many instances, the character expression of a gene or genes can be modified strikingly by changing the genetic background of the gene (Ramage, 1977). Transgressive segregants
316
R.T.RAMAGE
are individuals that show a more extreme development of a character than was present in the parental material. They are generally assumed to be due to cumulative and complementary effects of genes contributed by the original parents. The use of a kind of transgressive segregation for increasing and stabilizing disease resistance has been proposed as the accumulation of minor genes. Salt tolerance is a character whose expression can probably be modified both by changing the background genotype and by obtaining transgressive segregants or accumulating minor genes. The use of a typical recurrent selection breeding program is not feasible in breeding a self pollinated crop because of limitations imposed by the way self pollinated crops must be crossed. Genetic male sterility can be used to greatly facilitate crossing in self pollinated crops. The capacity for crossing permitted by the use of genetic male sterility allows the use of recurrent selection in breeding self pollinated crops. We have used modifications of the basic recurrent selection technique in breeding barley for such characters as salt tolerance, ability to cross pollinate, plant height, earliness, seed size, etc. We have called such breeding schemes "male sterile facilitated recurrent selection (Ramage, 1977b). A male sterile facilitated recurrent selection program begins with the establishment of a base population that is segregating for both male sterility and the character that is to be selected for. Relatively large numbers of both male sterile and male fertile plants that are superior for the particular character are selected. The selected plants are crossed using male sterile plants as females. The crossed seed are bulked and the Fl generation is grown. This completes the first cycle of recurrent selection. As we have been using it, a cycle of male sterile facilitated recurrent selection differs from a typical recurrent selection program primarily in two ways: (1) we make crosses between selected plants rather than between selfed progenies of selected plants, and (2) we usually use a selected plant in only one cross rather than make all possible intercrosses between selfed progenies of selected plants. As selection based on an individual plant is not nearly as effective as selection based on a progeny row, we select relatively large numbers of plants in order to maintain a high degree of variability in the population. By selecting large numbers of plants and making only single crosses between them, we may not be making the most rapid progress in selecting for the particular character, but we maintain a very high degree of variability for other characters. This increases the chances of selecting the character in a more favorable background genotype. The high degree of variability for other characters also makes the population much better suited for continuous exploitation.
GENETIC METHODS TO BREED SALT TOLERANCE IN PLANTS.
317
Just as in a typical recurrent selection program, the F2 of the bulked crosses between ~elected plants serves as the base population for the next cycle of selection. Cycles of recurrent selection are repeated for as long as there is improvement for the selected character. Plants may be selected from the F2 of any cycle and inbred for use as varieties, parents of hybrids, parents for conventional breeding programs, etc. Another modification of a typical recurrent selection program that we employ involves adding new sources of germplasm to the base population. In our schemes, we may introduce additional sources of germplasm into the population in any cycle. The new germplasm may be for the selected character or for any other desired character, such as disease resistance, local adaptation, large seed, etc. Usually, the new germplasm sources will be male fertile and are crossed onto male sterile plants selected from the population. The number of crosses made depends upon how much we want to dilute the base population. Also, we intercross populations that have been selected for different characters to start new populations. In most of our populations, simultaneous selection for several characters is practiced. As we use it, male sterile facilitated recurrent selection is an extremely flexible system. There is no limit to the amount of variation that can be carried in a population. The populations should continue to improve with each cycle of recurrent selection. Our populations are designed to be continued indefinitely and regularly exploited. This makes variety development from the population a continuous process. The basic features that make male sterile facilitated recurrent selection effective are: (1) the generation and maintenance of a wide genetic base from which to select, (2) the ability to make the large number of crosses necessary to obtain rare gene combinations, (3) the opportunity to apply extreme selection pressure to large populations generated by the system, and (4) the shortened time between cycles of effective selection. Male sterile facilitated recurrent selection produces populations that may be exploited in a number of ways. The simplest way is to select male fertile plants from the F2 of any cycle or recurrent selection and carry them on in a conventional pedigree program. For this type of exploitation, plants may be selected from any cycle of the recurrent selection program, thus making variety development a continuous process. Another way to effectively use recurrently selected populations is to select plants from different populations to be used as parents in conventional breeding programs. Techniques for breeding salt tolerant crops are relatively simple. Recurrent selection can be used to accumulate genes for
318
R. T. RAMAGE
salt tolerance in background genotypes that are most favorable for their expression. Populations developed by recurrent selection can be readily exploited by conventional breeding programs. The limiting component of a recurrent selection procedure is the selection of plants that are superior for the character being selected for. Rapid, accurate and inexpensive selection of a very few plants from a very large population is essential to a successful breeding program. Appropriate selection methods need to be developed for salt tolerance. Before better selection techniques can be developed, a better understanding of how non-specific osmotic effects and toxic effects of specific ions affect the different growth stages of plants must be gained. REFERENCES Ramage, R. T., 1977a, Male sterile facilitated recurrent selection and happy homes, Proc. Fourth Reg. Winter Cereals Workshop (Barley), Vol. II:92. Ramage, R. T., 1977b, Varietal improvement of wheat through male sterile facilitated recurrent selection, ASPAC Tech. Bull., No. 37. Sprague, G. F. and Brimhall, B., 1950, Relative effectiveness of two systems of sele~tion for oil content of the corn kernel, Agron. Jour., 42:83. U. S. Department of Agriculture, SEA, 1978, Plant responses to salinity: an indexed bibliography, ARM-W-6.
