Selfing and Crossing Techniques
Sugarcane is normally crosspollinated.
If selfpollination
is required in the breeding program, the arrow is covered, usually with a paper bag as in corn, or isolated
from arrows of other clones. Crosspollinations
may be made either by bringing arrows from the male and female clones together in isolation and permitting natural
crosspollination
or by dusting pollen from the male clone over a flowering arrow of the female clone. Generally, emasculations are not made due to the large number
and small size of flowers on the arrow and to the 10to
14day
period it takes for the entire sugarcane arrow to complete flowering. In temperate climates, parent
clones are potted, placed on rail carts, and moved into a crossing house (Fig. 22.6). Within the crossing house, temperature and humidity are controlled for optimum
pollen production and stigma receptivity, and photoperiod is controlled to promote synchronization of flowering.
Male and female clones for specific crosses are placed together in isolated cubicles. Because the cubicle in the crossing house is free of wind currents that would
disseminate the pollen, the arrow of the male clone is placed above the arrow of the female clone so that pollen falls directly on the female flowers (Fig. 22.7). Tapping
the male plants lightly each day during
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Fig. 22.6.
Sugarcane clones on rail carts preparatory to being moved into a tall
crossing house. The airlayers
are visible as black bulges on the lower
stems just above the cans.
the pollination period helps to scatter the pollen over the female inflorescence. Natural selfsterility
of the female parent is depended on to prevent selfpollination.
The
modem crossing house has sufficient height to accommodate tall canes growing in a container, with tie bars to which the plants are fastened to hold them erect.
Crossing is accomplished also by dusting pollen from the male plant over the flowers on an arrow of the female plant, a process called pollenloading.
Arrows
collected from male plants in early morning and exposed to light at normal temperature will shed pollen. The pollen is collected on a clean paper and dusted over
isolated female parent arrows daily during the flowering period. Sugarcane pollen normally remains viable only for a few hours, but viability will be retained for several
weeks if the pollen is dried and stored at a temperature of –20ºC, or for periods up to one year if stored at –80ºC. The seed or fuzz ripens about three weeks
following pollination. Sugarcane seeds retain their viability only for a short period of time under normal storage temperatures, but the life of the seed is extended if dried
and stored at temperatures of 0 to 5 ºC.
Maintenance of Breeding Stocks
Clones to be used as parents in sugarcane crosses are normally selected while still growing in the field. Prior to flowering, the parent clones are potted and transported
to the crossing area or crossing house. Preservation of the flowering stalks of parent clones is prolonged by rooting while the stalk is still attached to the mother plant,
a process called airlayering or marcotting. An alternative procedure for preservation of flowering stalks and maintaining freshness is to keep detached stem sections
alive in a weak sulfurous acid solution.
AIRLAYERING OR MARCOTTING. About three weeks before flowering, a polyethylene strip containing a mixture of moist potting soil, or sphagnum moss, is
wrapped around a bud of the sugarcane stalk about two nodes above the ground level. If kept moist, roots will develop from the airlayered bud (Fig. 22.8). The
airlayered stalk is severed below the rooted node,
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Fig. 22.7.
Sugarcane parent clones isolated in cubicles in the crossing house.
Because there is no air circulation within the crossing cubicles, the
male clone is suspended above the female clone so that the pollen
as it matures will fall directly over the female flowers.
potted, and transferred to the crossing house. Airlayering keeps the tassels fresher and increases seed production compared to nonairlayered stalks.
SYNCHRONIZATION OF FLOWERING Arrows severed from the base of the cane and transported to a central crossing area or crossing house may be kept
alive in a fresh condition for several weeks by immersing the cut end in a weak sulphur dioxide solution containing 150 ppm SO2, 75 ppm H3PO4, and 37.5 ppm of
each H2SO4 and HNO4. The solution is recharged daily and changed twice weekly. The arrows from hybrid clones and most clones from S. officinarum and S.
robustum continue to flower in a normal manner and remain alive for three to four weeks permitting the seeds to mature. Survival rate for clones of S. spontaneum is
generally lower than for other species.
Synchronization of Flowering
Synchronization of flowering in parent clones is necessary to make crosspollinations.
Flower initiation in sugarcane is sensitive to photoperiod and temperature. It is
often possible to induce flowering in nonflowering clones or to hasten flowering in late flowering clones by reducing the length of the daylight period. Flowering may be
delayed and vegetative growth continued in early flowering clones by increasing the daylight period. Increasing the temperature also hastens flowering, and flowering is
delayed and pollen fertility reduced by lowering temperature. In temperate climates, crossing is done in crossing houses where both photoperiod and temperature are
controlled. The flowering response of clones to photoperiod and temperature may vary with the clone.
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Fig. 22.8.
Airlayered or marcotted stalks of sugarcane. A
polyethylene strip containing a soil mixture, or
sphagnum moss, is wrapped around the node of
the sugarcane stalk and kept moist. The stalk will
root within a threeweek
period and may then
be severed from its natural root system. Note the
root development within the clear plastic marcot.
Cytogenetics and Genetics
The Saccharum species are extremely complex allopolyploids, with small chromosomes that are difficult to study, and high chromosome numbers that vary with the
particular clone examined. The irregular number is due to presence of aneuploids and to meiotic irregularities that result in gains or losses of single chromosomes. The
basic chromosome numbers in Saccharum are 6, 8, and 10. S. officinarum is an octoploid with a basic chromosome number of 10 and 2n number of 80. The wild
species, S. robustum, has a basic chromosome number of 10, with 2n numbers of 60 and 80 being most common. Clones of S. barberi have been divided into four
groups based on chromosome number. The wild species, S. spontaneum, contains one polyploid group with a basic chromosome number of 8, and 2n numbers of
40, 48, 56, 64, 72, 80, 96, 104, 112, and 120; and a second polyploid group with a basic chromosome number of 10 and 2n numbers of 40, 50, 60, 70, 80, 100,
and 120. The number of chromosomes in commercial clones generally varies between 2n= 100 and 2n130.
Interspecific crosses can usually be made among clones of the five species within the genus Saccharum, although peculiar chromosome numbers are observed in the
progenies of certain crosses. Due to abnormalities in fertilization and embryo formation, the somatic chromosome number is transmitted to the progeny instead of the
gametic number of the pistillate parent, when S. officinarum is used as the maternal parent in crosses with S. spontaneum, S. barberi, or S. sinense. This
phenomenon does not occur when S. officinarum is used as the pollen parent as illustrated here:
Cross Chromosomes in F1
S. officinarum × S. spontaneum 2n 4n
S. officinarum × S. barberi 2n + n
S. officinarum × S. sinense 2n + n
S. officinarum × S. robustum n + n
S. spontaneum × S. officinarum n + n
S. barberi × S. officinarum n 4n
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S. sinense × S. officinarum n + n
S. robustum × S. officinarum n + n
Exceptions will be found among particular clones within each species. Various explanations have been proposed to explain the phenomenon, but none are universally
accepted.
In the breeding of sugarcane, it has been a general practice to cross the different species with the noble cane, S. officinarum, to combine the high sugar yield of the
officinarum clones with hardiness and disease resistance of the other species, a procedure called nobilization. Usually, two to three backcrosses to the noble parent
are necessary to recover satisfactory sucrose content.
Simple Mendelian genetic studies are virtually impossible in sugarcane owing to the high polyploid number and irregular transmission of individual chromosomes, to
meiotic irregularities arising with crossfertilization,
and to sterility problems that make crossing and selfing difficult. Each clone is a different heterozygous genotype
which is not reproducible through floral reproductive processes. Greater attention is given to quantitative genetic inheritance in sugarcane than to the inheritance of
qualitative characters. Inbreeding, where possible, leads to the rapid loss of vigor. Inbreeding is restricted in particular clones due to presence of selfsterility
or selfincompatibility.
Quantitative inheritance studies suggest that additive genetic variance is important for many agronomic characters and disease resistance and
nonadditive variance is important for cane and sugar yield.
Biotechnology
Plant cell and tissue culture techniques may be utilized to supplement conventional breeding programs in sugarcane. Micropropagation has been proposed as a means
for obtaining pathogenfree
seed canes for planting or longterm
storage, to facilitate shipping and movement of breeding canes through quarantine restrictions, and to
provide a source of genetic variants in a breeding program. The latter is a questionable use, considering the genetic complexity in present clones, and, in a clonal
propagated crop, the importance of striving for stability in maintenance of clones rather than increasing diversity. Research on genetic transformation was slow to
develop due to problems associated with utilization of the Agrobacterium technique in monocots. However, utilization of the particle gun technique for insertion of the
DNA has now permitted transformation in the sugarcane to be accomplished.
Methods of Breeding
The methods of breeding sugarcane include:
• germplasm collection, which plays an important role in supplying sources of breeding materials,
• clonal selection from populations of wild clones to isolate genotypes with desirable genes that can be transferred to commercial clones, and
• hybridization among commercial clones to obtain genetic recombinations.
Hybridization is the breeding procedure by which new cultivars are developed in sugarcane.
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Germplasm Collections
Germplasm collections containing cultivated clones with a wide range of diversity and representative clones from the different Saccharum species are normally
maintained by sugarcane breeding stations for use in their breeding programs. The germplasm collections of sugarcane differ from germplasm collections of wheat, or
maize, as they are maintained as living clones rather than by seeds (Fig. 22.9). The largest germplasm collection is maintained at the Indian Sugarcane Breeding
Institute, Coimbatore, India, with about 3500 clones. The United States Department of Agriculture maintains about 3000 clones at the Sugarcane Field Station, Canal
Point, Florida, and about 2000 clones at the National Clonal Germplasm Depository, Miami, Florida. About 3000 clones are maintained at the Copersucar
Technology Center, Piracicabu, Brazil. The germplasm collections contain both commercial clones and clones from wild species and related genera. The commercial
clones contain useful genes for sugar yield and purity. The clones from wild sugarcane species and related genera constitute a depository for genes contributing to
hardiness, drought and cold resistance, salt tolerance, and disease and insect resistance.
Clonal Selection
ClonaI selection refers to the isolation of a specific genotype that is maintained by vegetative propagation. Sugarcane breeding began by selection of clones from wild
populations. Clonal selection is currently utilized to isolate desirable genotypes from a genetically mixed population, such as a wild population, or a segregating
population obtained by hybridization, or from populations originating through recurrent selection.
Fig. 22.9.
Collection of clones of the wild sugarcane species, S. spontaneum,
maintained at the Sugarcane Research Institute, Coimbatore, India.
S. spontaneum is used as a parent in breeding for hardiness and
disease resistance.
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Hybridization
Hybridization between clones, followed by selection within the hybrid population, is the procedure by which sugarcane cultivars are currently developed. Because the
sugarcane plant is heterozygous, segregation will occur and selections are made in the F1 generation. Crosses are made freely, with several thousand Fl seedlings
grown from a single cross. If a particular cross is found to have seedlings with desirable characteristics in the progeny, it may be repeated. If a clone is found to
contribute desirable characteristics to a series of progenies (exhibits good general combining ability), it may be used as a parent in a large number of crosses.
Several types of crosses are made in the breeding of sugarcane, which are referred to as:
BIPARENTAL CROSSES. Biparental crosses are crosses between two specific clones. This is the most common type of cross. Arrows of two parent clones are
brought together in isolation to permit openpollination.
If the female parent is selfsterile,
hybrid seeds will be harvested. If the female parent is selffertile,
both selfed
and crossed seeds will be harvested. An alternative procedure is to isolate a selfsterile
parent as female and pollinate by hand.
AREA CROSSES. Crosses in which several malesterile
clones are brought together in isolation and pollinated by a single male parent. Unless the female parent
clones are selfsterile,
they will cross among themselves.
MELTINGPOT
CROSSES OR POLYCROSSES. Arrows of selected clones are brought together in isolation, permitting natural crosspollination
to occur.
Seeds from meltingpots
are harvested and kept separate by clones, in which case only the maternal parent is known. If the clones are selected for a common
outstanding character, such as yield, sugar content, or frost resistance, the harvested seeds may be used as the first selection cycle in a recurrent selection procedure.
Selection Procedures Following Hybridization
A selection procedure in which seedlings are planted in field plots as single plants is illustrated in Fig. 22.10. In the first season, sugarcane seeds are germinated in
greenhouse flats or seed beds soon after the seed is harvested (Fig. 22.11). Several thousand F1 seedlings may be grown from each cross, with a total of 100,000
seedlings being grown each season in a normal breeding program. The seedlings are transplanted to a field nursery when 6 to 12 weeks old. An alternative planting
method is to plant the seedlings in bunches of 3 to 15 seedlings. The bunch planting system permits growing a larger number of seedling plants on available land area
and reduces labor costs of transplanting compared to transplanting single seedlings. Evaluation of individual seedling plants growing in a bunch is more difficult than
when seedlings are grown singly. Preliminary screening occurs in the second and third seasons, with about 10% of the plants grown from each crop being selected.
Plants are selected in early stages on the basis of vigor, stalk size, erectness, freedom of disease, and sugar yield and quality. Yield trials are conducted from the fourth
through the eighth season, in replicated plots after which a superior clone, if identified, is increased and released as a cultivar for commercial production.
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Fig. 22.10.
A typical selection procedure following a biparental cross in sugarcane. Seeds are germinated
and seedling plants grown in the first season, followed by preliminary evaluation
trials in the 2nd and 3rd seasons and yield trials in the 4th to 8th seasons.
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Cultivars
Descriptions of new sugarcane cultivars are published and the published descriptions used as the basis for future identification of the clone. In the United States, new
cultivars are registered by the Crop Science Society of America, and the descriptions are published in Crop Science. It has become the practice in sugarcane
breeding to name new sugarcane cultivars by letters that identify the sugarcane breeding station where the clone was selected, followed by a number to identify the
specific clone. Letters identifying clones from a representative group of sugarcane breeding stations throughout the world are as follows:
Symbol Breeding Station
B Central Sugar Cane Breeding Station, Barbados, British West Indies
CO Sugarcane Breeding Institute, Coimbatore, India
CP USDA Sugarcane Field Station, Canal Point, Florida
H Hawaiian Sugar Planters' Association, Honolulu, Hawaii
L Louisiana State University, Baton Rouge, Louisiana
M Mauritius Sugar Industry Research Institute, Reduit, Mauritius
A South African Sugar Association, Natal, South Africa
POJ Java Sugar Experiment Station, Pasuruan, Java
T Texas A&M Research and Extension Center, Weslaco, Texas
Q Bureau of Sugar Experiment Stations, Brisbane, Queensland, Australia
A sugarcane cultivar tends to decline in yield after being grown for a few years in a particular area, making it necessary to replace the cultivar with a new clone in order
to maintain high yield. While the exact cause of the deterioration in yield is not always determined with certainty, the yield decline is often attributed to changes in
disease patterns.
Breeding Objectives
Yield, sugar content, and disease resistance have received major attention as breeding objectives in sugarcane breeding. Other objectives that are important in
particular breeding programs are maturity; resistance to lodging; resistance to environmental stress from frost, drought, flooding, high temperature, or salinity; disease
resistance; insect resistance; and ability to produce ratoon crops.
Potential for Cane and Sugar Yield
The sugarcane plant stores large quantities of juice containing sucrose in the stalk. Thus, tonnage of canes per unit of land area is one criterion of sugar yield. Because
thick and tall stalks can store more juice than thin and short stems, primary selection in tropical countries is for vigor of growth and for tall, largebarrelled
canes with
high tillering capacity. In subtropical areas with short growing seasons, as in Louisiana, selection is made for shorter stalks and high tillering. Juiciness of the stem, and
sugar content and recovery, are also important in yield of sugar per unit of land area. More progress has been made in increasing total sugar yield by breeding for
increased tonnage than has been made by breeding for
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Fig. 22.11.
Sugar seedlings growing in flats at the Sugarcane
Breeding Institute, Coimbatore, India.
increased sucrose content. Yield of cane harvested is also influenced by cultivar response to fertilization, resistance to climatic adversities, and resistance to disease
and insect pests.
Maturity
Maturity in sugarcane breeding refers to the stage of plant growth when maximum sugar accumulation has been achieved. The sugarcane plant is mature when sugar
accumulation is uniform from the bottom to the top of the stalk. Sugarcane cultivars may be classified as early, intermediate, or late. Cultivars that mature early are
desired for milling in the beginning of the season, with progressively later maturing cultivars being milled as the season progresses.
Lodging Resistance
Sugarcane plants grown with high fertility and optimum moisture need strong stalks so they will stand without lodging. Lodged canes fail to develop full normal growth,
provide favorable environments for the development of disease, and deteriorate in sugar content and quality. Resistance to lodging from strong winds and rainstorms
require a thick, stout stalk, a healthy and well developed root system, and freedom from disease or insect injury that weakens the stalk. While tall plants are necessary
and desirable for maintaining high yield, they are more susceptible to damage by strong wind and rain. A balance between excessive height and reduced yield from
shorter plants may need to be achieved.
Resistance to Adverse Environments: Frost, Drought, Waterlogging, Salinity
Resistance to cold and occasional frost is required for sugarcane cultivars in north India and southern United States. Tolerance to frost and drought is found in canes of
S. barberi and S. sinense, the indigenous sugarcanes of north India, and in canes of the wild species S. spontaneum. Combining genes for frost and drought hardiness
of these species with genes for high sugar yield from S. officinarum has been a major objective in the breeding of sugarcane
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for marginal environments. Certain clones of S. spontaneum are able to withstand waterlogged conditions for long periods. Resistance to waterlogging is characterized
by the production of a large matrix of fibrous roots extending from the base of the stem to the surface of the water. Breeding for tolerance to salinity generally involves
selection for tolerance to higher levels of sodium ions.
Disease Resistance
The breeding of sugarcane has been closely related to outbreaks of serious diseases in the crop. The sereh disease forced the abandonment of susceptible clones of
the noble sugarcanes in Java. Mosaic virus combined with red rot almost forced abandonment of the sugarcane industry in Louisiana. The diseases were later
controlled by breeding resistant clones. Due to the diverse genetic background in sugarcane, resistance appears to be available to most any disease provided the
breeder can devise efficient and effective screening techniques. Decline in yields of new cultivars after a few seasons of cultivation is usually believed to result from
changes in pathogen populations.
Breeding for resistance to sugarcane disease pathogens is generally based on increasing tolerance controlled by quantitative inheritance rather than utilization of a single
genespecific
type of resistance. Breeding procedures to increase disease resistance levels involve:
• the selection of parent clones with a high degree of resistance,
• crossing resistant clones to generate transgressive segregates with improved resistance, and
• screening seedling populations to identify the superior segregates.
Screening procedures are required that will identify resistant parent clones and Fl seedling plants.
S. spontaneum has been used widely in crosses as a source of disease resistance. S. officinarum and S. robustum have also served as sources of disease resistant
genes. Genes for resistance to rust, caused by Puccinia melanocephala H. and P. Sydow, and for resistance to sugarcane smut, caused by Ustilago scitaminea,
have been found in clones of S. officinarum and S. spontaneum; genes for resistance to red rot, caused by Physalospora tucumanesis Speg., and to sugarcane
mosaic, a virus disease, have been found in S. spontaneum; genes for tolerance to eyespot
disease, caused by Helminthosporium sacchari (B. de haan) Butl., were
found in clones of S. robustum. Genes for resistance are transferred from wild species to cultivated clones by the nobilization procedure.
Insect Resistance
The most destructive insect pests attacking sugarcane are the sugarcane stem or stalkborers. Diatraea saccharalis F. is a common sugarcane borer in the United
States, but other species are prevalent in other countries. Resistance to the sugarcane borer has been identified in clones of S. spontaneum. Resistance may result
from unattractiveness of the leaf for egg deposition, inability of young borers to become established, high fiber which hinders feeding of borers, or tolerance and ability
to produce good yields in spite of borer attack.
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Production of Ratoon Crops
Production of a ratoon crop refers to the production of a second crop after the first crop of sugarcane has been harvested. Growing a ratoon crop saves the cost of
replanting a new crop. Clones vary in their ability to survive and produce a profitable ratoon crop. The number of crops harvested from a single planting may vary
from one to as many as six to eight in tropical countries.
Quality
Factors considered in quality of sugarcane include millability, yield of juice, sugar content of juice, and quality of juice. Millability refers to characteristics of the cane
that make it possible to recover the sucrose from the stalk by normal methods of extraction. Characteristics desirable for good millability are moderate hardness of
rind, good length of fibre, long internodes, and low fibresucrose
ratio. Yield of juice and sucrose content of the juice are important as they determine the sugar yield.
The most important factor in quality of the juice is the percent sucrose. Other factors of importance are total solids, brix (the percent of solids in the juice), and the
nonsugar fraction of the juice. Breeding for sugar quality has been facilitated by development of handheld
instruments for measuring brix which can be taken to the
field so that measurements may be taken on individual canes. Little progress has been made in increasing sucrose content in new hybrid canes over the content of the
best of the older clones of S. officinarum.