Some sugarcane facts…………

K K Vinod

Sugarcane (Saccharum officinarum) falls under the family Graminae, is known to be under cultivation in India from the Vedic times. India is considered to be one of the centres of diversity for Saccharum and allied genera. Sugarcane is one of the important agro-industrial crops in the country. Sugarcane is grown in India in 3.8 million hectares, producing 300 million tonnes of cane at approximately 71 tonne/ha of cane yield. The country produces about 20 million tonnes of sugar through 470 sugar mills spread all over the country giving employment to around 20 million people directly or indirectly. Of the total sugarcane production, about 60 per cent is utilised by the white sugar industry and the rest for gur, khandsari, seed, chewing etc. India is the highest producer of sugar in the world at present, however, the demand for sugar for internal consumption is growing due to the increased per capita consumption and the increase in population size. The increased requirement of sugar has to be met mostly through increased production per hectare since there is no possibility of further increase in the area under sugarcane due to competition from other crops, urbanisation etc.

The theoretical maximum yield of cane has been estimated to be 339.42 tonnes per hectare on the basis of the efficiency (3.6%) of use of total incident solar radiation. The world’s highest recorded yield is reported to be 255 tonnes per hectare. Recently, farmers in Gujarat are reported to have achieved yields as high as 340 tonnes per hectare. Compared to these yield levels, the national average is around 71 tonnes with the yields in various states ranging from 46 tonnes in Bihar to 113 tonnes in Tamil Nadu. Thus there is ample scope for improvement of cane and sugar productivity in the country.

The Genus Saccharum and the ‘Saccharum Complex’

Sugarcane belongs to the family Graminae in the genus Saccharum, a member of the tribe Andropogoneae abundant in tropical and subtropical regions. The term ‘Saccharum complex’ was used by Mukherjee (1957) to denote that the genera Saccharum, Erianthus (sect. Ripidium), Sclerostachya and Narenga constituted a closely related interbreeding group involved in the origin of sugarcane, to which Daniels et al. (1975) added the genus Miscanthus sect Diandra Keng (Table). The taxonomy, evolution, distribution and characteristics of genera in the Saccharum complex and the species of the genus Saccharum have been reviewed by Daniels and Roach (1987). Salient features of the Saccharum species and of the allied genera relevant to their use in pre-breeding are outlined.

Species of Saccharum complex

Species	Chromosome numbers (2n)
Saccharum spontaneum	40, 48, 56, 64, 80, 128
S. officinarum	80
S. barberi	81-124
S. sinense	111 - 120
S. robustum	60, 80
S. edule	60, 80
Narenga phorphyrocoma	30
Sclerostachya fusca	30
Erianthus spp.	20, 30, 40, 60
Miscanthus spp.	30, 40

S. officinarum L., an octaploid, 2n = 80, indigenous to New Guinea, has provided the genetic background and the sucrose genes of modern sugarcane hybrids. Clones of S. officinarum have thick stalks, high juice purity, low fibre and starch content, low vigour and adaptability to environmental stresses, and they are susceptible to diseases.

S. spontaneum L., 2n = 40-128, is distributed widely from New Guinea, the Mediterranean to Africa. Clones of S. spontaneum are highly polymorphic from bushy types to tall stalks with low sucrose and high fibre content. They played an important role in providing disease resistance and vigour to modern hybrids.

Saccharum robustum Brandes and Jesweit ex Grassl., indigenous to New Guinea, has stalks of up to 10 m high which are hard, woody and pithy with little juice. Five groups are recognized with two cytotypes 2n = 60 and 2n = 80 (Price, 1965). This species contributed towards the evolution of some Hawaiian varieties.

Saccharum barberi Jesw. and Saccharum sinense Roxb., 2n = 81-124, presumably evolved in Northern India and China, are characterized by thin to medium stalks, low sucrose, high fibre and tolerance to stress conditions. They are of limited fertility and their use in breeding is restricted. Molecular studies have confirmed that both S. barberi and S. sinense possess S. officinarum and S. spontaneum genomes.

The genus Erianthus Michx Sect. Ripidium Henrard, 2n = 20-60, is distributed in India, South­east Asia to Japan, Indonesia and New Guinea. Seven species are described. Clones of Erianthus are highly vigorous, tall with slender stalks of good diameter and display disease resistance, excellent ratooning ability and drought tolerance.

The genus Miscanthus Anderss., 2n = 38-114, is distributed from Tahiti through Eastern Indonesia, Indo­China to northern China, Siberia and Japan. The species vary from small wiry­leafed types to taller ones, occurring from sea level in Indonesia to 3300m in Taiwan. Four sections have been described with prominent species.

Saccharum has also been successfully crossed with Sorghum and Zea mays. Several earlier reports of hybridisation during the 1930s with bamboo, Sorghum etc. could not be substantiated.

Nobilisation of cane

Sugarcane (Saccharum spp.) is one of the crops for which interspecific hybridization has provided a major breakthrough in its improvement. Modern commercial sugarcane varieties (Saccharum hybrids, 2n=100-130) are derived from interspecific hybridization. Nobilisation refers to the crossing of the wild cane, S. spontaneum, to the noble cane S. officinarum, and further backcrossing of progenies to the latter, and includes the planned introgression of the other Saccharum species and related genera into the noble cane. Initial work on hybridisation and selection were restricted to intercrossing the S. officinarum clones (both typical and atypical) and limited success was obtained through intervarietal crosses, but improved vigour and resistance to many diseases became possible only after inter specific crosses were attempted. Kobus in 1897 crossed a S. barberi clone ‘Chunnee’ with S. officinarum and by backcrossing the progeny to the S. officinarum for dilution of the traits from the barberi clone obtained ‘sereh’ disease resistant varieties. This led to increased interest in interspecific hybrids were renewed.

In India, Barber in 1912 crossed ‘Vellai’ a S. officinarum clone with a S. spontaneum clone (2n = 64 ) found locally and obtained several promising clones starting with Co 205 which became the first interspecific hybrid to become a commercial success in India. Co 205 replaced the officinarum and the barberi varieties in cultivation in North India, on account of its hardiness and ability to withstand abiotic stresses better. In addition to introducing disease resistance in the noble background, nobilisation produced unexpected gains in general vigour, increased cane and sugar yields, adaptability to stress conditions and ratooning ability. The early success of interspecific hybridization led to the intercrossing of other species to produce tri­species hybrids that proved very successful in subtropical areas in India. Subsequently clones of S. barberi were nobilised to produce clones which were intercrossed to S. spontaneum clones. Some of these early trispecific hybrids became commercially successful in subtropics and in several other countries as well. Though earlier attempt for sugarcane breeding was to improve the yield, however, the quality of the cane was also given equal importance owing to industrial relevance of the cane produced.

References

Daniels, J. and Roach, T. (1987) Taxonomy and evolution. In: Sugarcane Improvement Through Breeding. Heinz, D.J. (ed.) Elsevier, Amsterdam, pp. 7-84.

Daniels, J., Smith, P., Paton, N. and Williams, C.A. (1975) The origin of the genus Saccharum. Sugarcane Breeders Newsletter 36, 24-39.

Mukherjee, S.K. (1957) Origin and distribution of Saccharum. Botanical Gazette 119, 55-61.

Genetics of Sugarcane Quality Traits and Breeding for Quality Improvement

K.K.Vinod

The quality traits in sugarcane depend on the end use of the cane in the industry rather than a common list of quality traits.  The main parameters are listed in the table below.

Major quality traits in sugarcane

Industry	Quality traits
Sugar Industry	Sugar recovery Sucrose content in juice Purity coefficient of juice
Distillery	Higher juice volume Higher total sugars
Paper Industry	Higher cellulose Lower Ash content Lower Lignin content Lower Pith content or higher Fibre/pith ratio Lower colouring matter in bagasse Higher L/D Ratio of ultimate fibres

The novel idea of using the wild species Saccharum spontaneum (as male with S. officinarum as female) in sugarcane breeding with the objectives of incorporating gene complexes for biotic and abiotic stresses and for high biomass production resulted in a tremendous level of useful genetic variability including that of better cane quality, sugar content and incorporation of rationing ability. This paved the way for revolutionising sugarcane agriculture and sugar industry not only in India, but also in most of the sugarcane growing countries of the world. Most, if not all, of the present day varieties world-over have the Coimbatore bred canes in their genealogy.

In India, Co 419 released in 1933 became the most popular variety in tropical India and was rightly hailed as the 'wonder cane' the world-over. Co 997 and Co 1148, released in 1950s, became the ruling varieties in Andhra Pradesh and North India respectively. Co 1148 remained the most predominant variety for over four decades in sub-tropical belt and was hailed the 'Wonder Cane of North India'. Co 62175 became the most sought after variety by jaggery farmers owing to its heavy yield. Varietal evaluation conducted across seasons and different months of juice analysis helped in the identification of varieties with high sucrose levels such as Co 7204, Co 7704, CoA 7601, CoC 671, Co 8336, Co 8338 etc.

In the USA, evaluation of F₁ crosses between commercial hybrids and Miscanthus and Erianthus (as male parents) indicated that thin stalk and low sucrose content were strongly influenced by Miscanthus and Erianthus. However, mean sucrose content and purity of F₂ and BC₁ seedlings of crosses between commercial hybrids and Miscanthus spp., Miscanthidium spp. and Erianthus spp. were markedly improved over that of F₁ hybrids, but mean stalk diameter was still very small.

In Taiwan, sugar content in a Saccharum × Miscanthus hybrid showed a steady increase in successive nobilized generations. Clones selected from F₁ Saccharum × Miscanthus hybrids were productive in fibre yields, with better pulp sheet as compared to a commercial control. Some clones from BC₁ crosses involving sugarcane and sorghum had sucrose content equivalent to commercial controls and proved to be early maturing (TSRI, 1995-96).

In sugarcane breeding, the available evidences indicates that selection of parents based on their juice quality would be successful in increasing the juice quality of the succeeding generation, but selection of parents followed by progeny testing would seem to be necessary for increasing cane yield. Studies on milling characters showed that starch content in juice, percentage of reducing sugars, fibre percent fresh weight has high heritabilities and hence both parental selection and individual selection would be effective for these characters. Roach (1968) found highly significant male × female interaction effects for yield and sucrose in S. officinarum × S. spontaneum crosses. Heterosis was observed for early growth, stalk height, cane yield, flowering, pollen production and level of reducing sugars.

Inbreeding depression for components of cane yield viz., stalk diameter, stalks per plant, and stalk length was reported but there was no apparent depression for brix. This suggested that most of the genetic variance for brix be additive in nature.  However, it is important that non-additive genetic variance for cane yield and its components is required in sugarcane. This indication is supported by the few studies that have provided estimates of additive and non-additive genetic variances (Hogarth, 1987).

Studies on 14 crosses between commercial hybrids (female) and S. spontaneum (male) and found families to differ significantly for stalk number, stalk diameter and cane quality characters. Narrow-­sense heritability estimated from the partitioning of variances between and within family ranged from relatively low for dry matter % cane (Brix % cane + fibre % cane) (0.17) to moderately low for stalk number and fibre % cane (0.39 and 0.38, respectively) and moderately high for cane quality characters (0.67-0.90) and stalk diameter (0.75). These estimates were generally higher than those obtained from commercial × commercial crosses. The distribution of Brix, sucrose content, juice purity and fibre content in crosses between commercial hybrids and S. spontaneum clones were continuous and controlled by polygenes, and additive genetic variance was more important than dominance genetic variance for all four characters.

In experiments using diallel crosses, specific combining ability (indicating non-additive genetic variance) has generally been greater than general combining ability for both cane yield and sugar content as measured by brix. However, due to the possibility of self-pollination in such crosses, these results should be interpreted cautiously.

Estimates of heritability and clonal repeatability on an individual basis for three milling characters were reported in sugarcane.

Heritability and clonal repeatability of milling characters in sugarcane

Character	Heritability	Clonal Repeatability
Starch (log ppm)	0.53 +/- 0.14	0.58+/-0.04
Reducing sugars % extract	0.31 +/- 0.13	0.57+/-0.04
Fibre % fresh weight	0.34 +/- 0.15	0.72+/-0.03

This indicated that progress from selection of parents should have some value, particularly for starch content. Progress from clonal selection should be very good for all characters even on an individual basis. Studies on the inheritance of ash in juice showed that for ash percent in juice, 95% of the genetic variance was additive. Thus, there should be no difficulty in breeding for this character, if it is deemed desirable. However, the authors pointed out that selection based on ash content might reject many varieties with good agronomic characters.

Increasing fibre content of commercial varieties but still maintaining an acceptable sugar content was thought to be an attractive economic option that could cut down on imported coal and also reduce CO₂ emissions. In the case of fibre percent fresh weight. 80% of the genetic variance is additive, and the character is moderately heritable on an individual basis (h,² = 0.45), indicating that selection of parents for low or high fibre content should be effective in producing the progeny with desirable fibre content.

Evaluation of S. spontaneum, S. robustum, S. sinense, S. officinarum, commercial hybrids and Erianthus spp. for cane quality components, Brix % cane (total solids of juice), pol % cane (apparent sucrose), fibre % cane and dry matter % cane, by the method described by Saint Antoine and Froberville (1964). A ratio of pol : fibre (PF) was derived from the pol % dry matter and fibre % dry matter. Juice purity was also derived from the ratio of Brix % cane and pol % cane. These studies revealed high levels of fibre and dry matter content in S. spontaneum clones as compared with S. officinarum. Commercial hybrids ranked very close to S. officinarum, and the Erianthus group ranked very close to S. robustum. Brix % cane and pol % cane were the highest in S. officinarum and commercial hybrids whereas sucrose content of the Erianthus group was even lower than that of S. spontaneum. The PF ratio showed equal partitioning of dry matter between pol and fibre in the S. officinarum and commercial clones.

Many correlations between agronomic and milling characters were found in sugarcane. The genotypic correlation between cane yield and fibre percent was 0.57 and for sucrose percent and fibre was –0.76. Significant positive correlations between fibre content and brix was also reported. A very weak correlation between fibre content and brix in juice as well as negative correlations between brix and ash were also reported in sugarcane. This indicated that selection for brix might produce varieties with lower ash content.

With further developments, microsatellite markers could soon become a tool for genetic fingerprinting, genetic mapping and to assist in selecting specific, genetically diverse parents for use in introgression. The development of marker assisted selection would have a major potential use in introgressing a single quantitative trait locus (QTL) or multiple QTL from an ancestral species into commercial cane. The search for molecular markers for sucrose genes to assist in breeding and selection could reduce the number of backcrosses necessary to recover the recipient genotype near to the donor target genes.