22. Breeding Sugarcane
The breeding of sugarcane (Saccharum spp.) differs from that of other major field crops. A cultivar in cultivated sugarcane refers to a specific genotype, or clone,
that is propagated vegetatively through seedcanes or setts. The cultivated sugarcane clone is a complex, heterozygous polyploid that originated by chance segregation
in a hybrid population. Most commercial clones contain germplasm introgressed from three or more different Saccharum species. The agronomic performance of the
clone can be evaluated through field trials, but its genetic contribution as a parent in a breeding program is difficult to evaluate due to its heterozygous genotype.
Sugarcane is widely grown in the tropical and subtropical areas of the world. In tonnage of sugarcane produced, Brazil and India lead all countries. In the United
States, sugarcane is grown in Florida, Hawaii, Louisiana, and Texas.
Species of Sugarcane
Sugarcane is classified in the genus Saccharum, tribe Andropogoneae, family Gramineae. Within the genus Saccharum there are three species of cultivated sugarcane
(S. officinarum L., S. barberi Jesw., and S. sinense Roxb.) and two species of wild sugarcane (S. robustum Brandes and Jeswiet ex Grassl, and S. spontaneum L.)
(Fig. 22.1). Sugarcane clones presently in cultivation are complex hybrids among these species and cannot be classified as belonging to any particular species. Another
species of Saccharum, S. edule Hassk., has an edible inflorescence used for food but has little or no sugar and can scarcely be regarded as sugarcane.
SACCHARUM OFFICINARUM. This species includes the tropical, ''noble" canes indigenous to the New Guinea region of the South Pacific. Canes of S.
officinarum are found only in cultivation in native gardens and are no longer found growing wild. They are characterized by thick stems, soft rinds, high cane yield or
tonnage, low fiber, and high content and purity of sucrose. Originally, they were grown as chewing canes and for centuries were the only cultivated canes in the
tropical regions of the South Pacific. The term "noble" was applied to
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Fig. 22.1.
Canes of the cultivated and wild species of sugarcane.
(A) S. officinarum (noble cane), (B) S. barberi (north Indian
cane), (C) S. sinense (Chinese cane), (D) S. robustum
(wild cane), and (E) S. spontaneum (wild cane).
the tall, handsome, largebarrelled,
colorful canes of this species by early Dutch researchers in Java. The canes of S. officinarum do not withstand well the rigors of
drought, such as occurs in the subtropical climate of north India, or occasional frost, as occurs in the temperate climate of southern United States. Particular clones are
resistant to rust, smut, Fiji disease, or downy mildew, but outbreaks of red rot and mosaic at one time virtually eliminated the species from largescale
commercial
cultivation.
SACCHARUM BARBERI AND S. SINENSE. Saccharum barberi, the north India sugarcane, and S. sinense, the Chinese sugarcane, are indigenous to north
India, Bangladesh, and the BurmaChina
region. The species are reported to have originated by hybridization between S. officinarum and S. spontaneum. The canes
are characterized by thin stems, great vigor, early maturity, and wide adaptability. They can withstand light frost and drought, features which adapt them for cultivation
in north India. They are poor in cane yield but intermediate to good in sucrose content, resistant to red rot and sereh disease, susceptible to smut, and vary according
to the clone from susceptible to immune for mosaic disease. Clones of these species contribute genes for vigor, hardiness, and disease resistance in sugarcane crosses.
SACCHARUM ROBUSTUM. Saccharum robustum is a wild species that originated in New Guinea. The species has great vigor and wide adaptability. The
canes are tall with medium thickness, high in fiber, low in sugar content, susceptible to mosaic, but some clones are resistant to eyespot
disease. S. robustum has not
been utilized extensively in the breeding of commercial cultivars.
SACCHARUM SPONTANEUM. The clones of the wild species, S. spontaneum, form a complex group with great diversity and much natural hardiness. The
distribution of this species is widespread, being found in India, China, Taiwan, the Philippines, the South Pacific Islands, and Africa. In general, the clones are
perennial, rhizomatous, high tillering with slender stalks, high in fiber, and low in sucrose. Most clones flower abundantly with good seed set. Particular clones of S.
spontaneum are resistant to rust, smut, sereh, mosaic, red rot, or pythium root rot. Clones of S. spontaneum are widely used in sugarcane breeding to contribute
genes for vigor, hardiness, tillering capacity, and disease resistance.
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Related Genera
The genus Saccharum and four related genera, Erianthus, Miscanthus, Narenga, and Sclerostachya, constitute what is known as the Saccharum complex.
Intergeneric crosses have been successfully accomplished between sugarcane and genera within this group. These related genera possess a reservoir of genes for
agronomic characters and disease resistance that have not been used in the breeding of commercial clones of sugarcane due to meiotic abnormalities in the hybrid
progenies. Several backcrosses to sugarcane will undoubtedly be needed to transfer genes for desirable characteristics from these genera to sugarcane and recover
usable commercial clones.
Origin of Sugarcane
Cultivated sugarcane had two geographic centers of origin, New Guinea and the northern IndiaBurmaChina
region. The largebarrelled,
tropical species, S.
officinarum, probably originated from the wild species, S. robustum, in the New Guinea region. As S. officinarum migrated outward from its center of origin, it
became modified through natural hybridization with related genera. The north IndiaBurmaChina
sugarcanes, S. sinense and S. barberi, based on appearance,
probably have one and maybe two unidentified species involved in their origin.
Botany of Sugarcane
Sugarcane flowers sparsely except in tropical climates. The flowering response differs with the genotype of the clone, the temperature, the photoperiod, the soil
moisture, and the nutrition of the clone. Flowering is favored by warm nights and high humidity and inhibited by cool weather and high altitudes. Flowering is
undesirable in commercial canes as it leads to rapid maturity and reduces total sugar yield. In sugarcane breeding, flowering and production of fertile pollen and
true seeds are requirements for obtaining genetic recombination.
The sugarcane inflorescence consists of an open, branched panicle, known as an arrow, that bears thousands of flowers (Fig. 22.2). The flowers are borne in paired
spikelets, one sessile and one pedicellate (Fig. 22.3). The flowers open in early morning, usually between 5 and 6 a.m. Flowering starts at the top of the arrow and
proceeds downward, requiring 7 to 14 days for completion. Most flowFig.
22.2.
Flowering arrow of sugarcane plant. The
sugarcane arrow may bear thousands of flowers.
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Fig. 22.3.
Part of inflorescence of sugarcane plant showing
sessile and pedicellate spikelets. The sessile
spikelet flowers before the pedicellate spikelet.
ers are perfect; others are imperfect with either the pistil or, more frequently, the stamens aborting. Sugarcane clones vary widely in fertility and seed production; from
high pollen production to malesterility,
from complete selffertility
to selfsterility,
and even selfincompatibility.
The seeds produced are extremely small in size
averaging 200 to 300 seeds per gram. The seeds are often poorly developed and inviable. The seeds and accompanying floral structures, including the long silken hairs
at the base of the spikelets, are referred to as fuzz (Fig. 22.4). The fuzz breaks off easily and may be carried away by the wind. In breeding experiments care must be
taken to prevent loss of the seed in this manner. This is usually accomplished by bagging the tassel about 15 to 20 days after a cross has been made. As seeds mature
and fall, they are caught in the bag and are harvested about 30 days later.
Commercial sugarcane clones are propagated vegetatively by stem cuttings. The stem sections are about one meter in length and contain a lateral bud at each node
that germinates to form shoots and roots (see Fig. 2.10). The shoot develops into a primary stem from which secondary stems or tillers arise. The leaves may be loose
on the stem and break away easily, in which case they are said to be free trashing. Leaf sheaths that adhere tightly are undesirable as they hold water, permitting root
primordia to develop aerial roots. Breeding clones are propagated by singlebud
cuttings (Fig. 22.5). Several singlebud
cuttings may be made from each selected
clone. Stem nodal sections about 5 to 8 cm in length are prepared; each stem section contains a node and a bud or eye at the node. Upon germination of the bud,
shoots and roots develop from the nodal bud. Thousands of the nodal buds can be germinated in relatively small areas of greenhouse benches (Fig. 22.5).