(Compiled by F. A. Martin, Louisiana
Agricultural Experiment Station)
INTRODUCTION
Sugarcane is a large tropical grass with
an unusual ability to store sucrose in stem cell vacuoles. In
certain environments the plant can produce extremely high levels
of biomass that can be used for fiber or energy. The ability
to store sucrose, along with high biomass potential and ease of
cultivation, makes sugarcane one of the world's most productive
agricultural plants. Although genes for freeze tolerance exist
in the genome, most sugarcane cultivars are susceptible to freeze
damage and therefore the cultivation of sugarcane is restricted
to tropical and subtropical environments. In the U.S., four states,
Florida, Hawaii, Louisiana, and Texas, as well as Puerto Rico,
produce significant amounts of sucrose from sugarcane. In 1994,
936,800 acres were under cultivation with a total value of $900,827,000
(USDA National Agricultural Statistics Service 1996). The sucrose
produced from cane, along with that produced form sugar beets,
accounted for 64 percent of American domestic sugar consumption.
The remaining demand was satisfied by foreign producers. Although
sugarcane is a minor crop in the U.S., it is of major economic
importance to Florida, Hawaii, Louisiana, and Texas. With increased
emphasis on breeding for freeze tolerance and increased biomass,
the range of cultivation in the U.S. and the potential as an energy
and fiber crop could be enhanced.
PRESENT GERMPLASM ACTIVITIES
Historically, sugarcane germplasm has been vegetatively maintained. The genetic nature of Saccharum spp., along with the difficulty of producing true seed, made clonal populations the most manageable solution. After the discovery of true seed, sexual reproduction allowed new clones to be bred. The use of exotic germplasm for the improvement of sugarcane is an excellent example of the contributions wild relatives have made toward the genetic improvement of economically important crop species. The stimulus for the hybridization of S.officinarum, "noble canes," with wild relatives came largely from disease outbreaks. The noble canes had been the basis of the early sugar industries until the late 1880s when an epiphytotic of sereh disease was occurring. The initial hybridizations were with the wild species S.spontaneum and later with a naturally occurring hybrid, Kassoer. Interspecific hybrids not only solved the disease problem, they also provided increased yields, improved ratooning ability, and adaptability to stressful environments.
The need to preserve Saccharum germplasm as true seed and not as clones is obvious. Clonal material is difficult and relatively expensive to maintain in field planting and may be lost as a result of natural or manmade disaster, or as a consequence of routing movement and handling. For example, Berding and Koike (1980) reported that out of 1,260 clones collected on 14 expeditions from 1875 to 1957, only 338 or about 27 percent were still in collections in 1976. This high attrition rate argues well for the preservations of true seed.
There are two world collections -- one maintained by the USDA at Miami, Florida, USA and the other maintained by the Sugarcane Breeding Institute, Coimbatore, India. There are several sugarcane breeding stations in other nations that maintain large clonal populations, including Australia, Brazil, Indonesia, and Thailand. There are also several sugarcane experiment stations in the USA where clonal populations exist, including the Hawaiian Sugar Planters' Association, Hawaii, the USDA Breeding Station at Canal Point, Florida, and the USDA Houma Station at Houma, Louisiana. However, none of these collections by itself is complete, and there are no significant storage facilities to assure long-term germplasm preservation at any of these stations.
There is a unique collection of nearly 100
genotypes of North American Saccharum germplasm at the
Houma Station. The collection consists of seed, living plants
established in field plots, and local herbarium specimens. Official
herbarium specimens have been deposited at Texas A&M University.
Formerly placed in Erianthus, clones collected from southeastern
USA, were recently placed in Saccharum by Webster and Shaw
(1995): S. alopecuroideum (2n=30), S. baldwinii (2n=30),
S. brevibarbe var. brevibarbe (2n=60), S. brevibarbe
var. contortum (2n=60), S. coarctatum (2n=60), and
S. giganteum (2n=30, 60, and 90). Chromosome counts were
made by Burner and Webster (1994). Saccharum giganteum
is being studied at taxonomic (R.D. Webster), cytological (D.M.
Burner), and molecular (D.M. Burner and L. McIntyre) levels to
determine whether the cytotypes constitute unique species or subspecies.
The North American species have not been placed within the evolutionary
pathway of S. officinarum (Roach and Daniels, 1987), which
is logical given their geographic isolation. The breeding potential
of this germplasm has not been determined, but it possesses adaptability
to temperate climate and, therefore cold tolerance that may be
absent from cultivars. Successful crosses have been made between
elite sugarcane x brevibarbe var. contortum and
elite sugarcane x S. giganteum (Burner and Webster, 1994),
and additional crosses were made in 1995. The only known populations
of such hybrids are those being made at Houma. The North American
species appear to be cross-compatible with sugarcane, like their
Old World cousins, despite geographic isolation and genetic and
morphological diversity (North American and Old World taxa have
two and three stamens per floret, respectively).
STATUS OF CROP VULNERABILITY
The ability to produce fertile interspecific hybrids within the genus Saccharum initially broadened the genetic base of the crop. After the original interspecific crosses, sugarcane breeders concentrated their efforts on intercrossing the derivatives which increased yield and quality of commercial hybrids. A consequence of this approach was a reduction in genetic diversity. Most sugarcane cultivars in the world today were derived from crosses made with only a few clones that were selected in the late 19th Century and early 20th Century in India and Java. The original crosses include only 13 original clones, eight clones of which were S. officinarum, two were S. spontaneum, one was probably a natural hybrid of S. spontaneum and S. officinarum, and two clones of S. sinense. All commercial sugarcane cultivars were then developed by intercrossing these hybrids and backcrossing to S. officinarum. In India, which has one of the longest continuously running breeding programs, all commercial clones trace back to 25 S.officinarum, four S.barberi, and six S.spontaneum clones. The narrowness of the genetic base is very apparent, and it has become apparent that improvement of certain traits is increasingly more difficult. Thus, it is desirable to incorporate new germplasm from other sources of Saccharum and related species that are maintained by the USDA in the World Collection of Sugarcane and Related Grasses. Genes derived from these new sources of germplasm are necessary to improve such desirable characters as disease and insect resistance, cold tolerance and stubbling ability, and reduce vulnerability.
Because of its relatively narrow genetic base, several diseases, including rust, smut, Fiji, eyespot, and a host of other pests and pathogens, put the widespread cultivation of sugarcane at risk. The cytoplasm of all commercial canes worldwide has come from a few officinarums and, if these were to become susceptible to new races of Helminthosporium, an epidemic similar to the southern corn leaf blight could occur. The development of new races of these pathogens is inevitable and continued efforts must be made to retain current levels of resistance. The narrow genetic base has also resulted in a yield plateau for most breeding programs. Future yield gains under current conditions will be smaller than in previous cycles of selection. The genetic vulnerability can be reduced if current collections are more fully utilized.
The Beltsville Quarantine Station is an
intrigal part of domestic and foreign germplasm movement. Procedures
should be structured to minimize the risk of inadvertant movement
of disease causing organisms in germplasm. Accordingly, the Beltsville
Quarantine procedures should use the most up-to-date disease detection
methods. This will minimize the possibility of a new disease
or a new strain of an existing disease being introduced into domestic
or foreign sugarcane industries.
GERMPLASM NEEDS
Collection: In the germplasm area, the emphasis to date has been on collection. Between 1875 and 1957, 14 expeditions were conducted. The continued loss of clones from the world collection, along with agricultural, forestry, and mining development in the center of diversity of Saccharum, have made re-collection in these areas an urgent and essential task. Several recent collections were made from (1)New Guinea (Krishnamurthi and Koike, 1982) where 242 clones were collected; (2)the Indonesian Archipelago (Berding and Koike, 1980) where 571 clones were collected; (3)Sulawesi, Maluku, Sumatra and Irian Jaya (Tew et al., 1986) where 133 clones were collected; and (4)the Phillipines (Medina et al., 1986) where 319 clones were collected. In addition to these collecting expeditions, national collections have been made in India, Thailand, China and Indonesia. We feel that except for the need to sample a few other areas, the emphasis now should be redirected toward germplasm evaluation.
Maintenance: In the United States, vegetative collections of Saccharum and related genera are maintained at the USDA World Collection at Miami, Florida. This collection contains a great amount of genetic diversity and potential with over 700 clones of S.officinarum, the most important species within the group. Smaller collections are maintained at Hawaiian Sugar Planters' Association, Maunawili Station, and the USDA Breeding Station at Canal Point, Florida and the Houma Station at Houma, Louisiana.
Preservation. The need to preserve the
genes from the world collection is urgent. Seed from the S.
officinarum intercrosses and S. Spontaneum selfs have
been produced and sent to the National Germplasm Storage Bank
at Fort Collins, Colorado for storage. Similar seed production
and storage efforts should be undertaken with the other related
species.
EVALUATION AND ENHANCEMENT
Large genotype x environmental interactions in Saccharum have limited evaluation of collected material. Currently collections are maintained as clones and little evaluation has occurred. HSPA started a program to evaluate the S.officinarum group by estimating its breeding values from progeny testing. This type of evaluation also needs to be done with the clones at Miami, but the low flowering levels which occur there restrict the crosses which can be made.
In India, where the counterpart to the USDA
collection is maintained, evaluation of these materials has been
in progress since 1981. In their program multi-location testing
involving 10 locations is being used to evaluate the world germplasm
collection under Indian Agro-ecological zones. These evaluations
must also be made in other environments if we are to successfully
exploit these genetic resources. Germplasm enhancement for specific
regions of the world can proceed after these evaluations are completed.
There is tremedous opportunity for international cooperation
in this area.
RECOMMENDATIONS
The following recommendations are made in order of priority:
1. Continue to place seed of Saccharum species and related grasses in the National Seed Storage Laboratory long term storage facilities.
2. Utilize latest disease detection procedures at the Beltsville Quarantine Station.
3. Conduct research to fine tune molecular techniques for use in germplasm characterization and evaluation.
4. Increase emphasis on developing international cooperation in germplasm characterization and evaluation.
5. Expand the traditional evaluation program to include testing families in several environments in all sugarcane growing states.
6. Evaluate the World Collection to determine diseases that may be present.
7. Research the physiology of seed dormancy.
Berding and Koike, H. 1980. Germplasm conservation of the Saccharum complex: A collection from the Indonesian Archipelago. Hawaiian Planters' Records, 59:87-176.
Burner, D.M., Y.-B. Pan, and R.D. Webster. 1995. Random amplified polymorphic DNAs (RAPDs) in Saccharum native to North America. Agron. Abstr. 87:180.
Burner, D.M. and R.D. Webster. 1994. Cytological studies on North American species of Saccharum (Poaceae: Andropogoneae). SIDA 16: 233-244.
Krishnamurthi, M. and Koike, H., 1982. Sugarcane collecting expedition: Papua New Guinea, 1977. Hawaiian Planters' Records, 59:273-313.
Medina, R.E., Krishnamurthi, M., Lapastora, E., Dosayla, R., Divinagracia, H., Silverio,G., and Cano, I.B., 1986. Sugarcane germplasm collection in the Republic of the Phillipines. Proc. ISSCT 19:522-527.
Roach, B.T. and J. Daniels. 1987. A review of the origin and improvement of sugarcane. In: COPERSUCAR International Sugarcane Workshop, pp.1-31, COPERSUCAR, Sao Paulo, Brazil.
Tew, T.L., Purdy, L.H., Lamadji, S. and Irian, 1986. Indonesian sugarcane germplasm collecting expedition--1984. Sugar Cane, No.2, Mar-Apr., pp.15-17.
Webster, R.D. and R.B. Shaw. 1995. Taxonomy of the native North American species of Saccharum (Poaceae: Andropogoneae). Ann. Missouri Gard. (In Press).