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Evaluation of Sugarcane x Saccharum spontaneum Progeny for Biomass Composition and Yield Components

^a Yunnan Sugarcane Research Institute, Yunnan Academy of Agricultural Sciences, Eastern Lingquan Road 363, Kaiyuan, Yunnan 661600, China
^b CSIRO Plant Industry, Davies Laboratory, PMB, PO Aitkenvale, Townsville, Qld. 4814 Australia
^c Guangzhou Sugar Industry Research Institute, 10 Shiliuguang Road, Guangzhou, Guangdong, 510316, China
^d CSIRO Plant Industry, Queensland Bioscience Precinct, 306 Carmody Road, St. Lucia, Qld 4067 Australia. This research was part funded by the Australian Centre for International Agricultural Research

^* Corresponding author (Phillip.Jackson{at}csiro.au).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Saccharum spontaneum L. has contributed important traits to modern sugarcane (S. spp. L.) cultivars such as adaptation to environmental stress and ratooning ability. There is interest in further use of S. spontaneum in sugarcane improvement for sugar or energy-from-biomass production systems. In this study, parents and progeny from 43 biparental crosses between sugarcane and S. spontaneum clones were evaluated in field trials in China and Australia, along with several commercial cultivars. The S. spontaneum clones were from diverse geographic origins in China. Measurements were made on biomass composition (% dry matter, brix and pol in juice and cane, purity, fiber content) and yield components. Moderate to high (>0.7) broad-sense heritabilities and high genetic variances were observed for most traits. About half the total genetic variance was retained as among-family variance for the biomass composition traits, but this proportion was generally <25% for biomass yields. Midparent values in an independent trial predicted biomass composition traits reasonably well (generally, r > 0.6), but less so for cane and biomass yield (0 < r < 0.4). Genetic correlations between performance of families evaluated in different countries were strong, providing preliminary evidence that results in one country could be used for identifying elite families in the other. Strategies for efficient development and selection of elite clones from S. spontaneum are suggested.

Abbreviations: YSRI, Yunnan Sugarcane Research Institute, HSBS, Hainan Sugarcane Breeding Station

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
THE CONTRIBUTION of Saccharum spontaneum L. in imparting a range of important traits to modern sugarcane (S. spp. L.) cultivars, such as adaptation to environmental stresses, ratooning performance, resistance to diseases and pests, and general vigor, is widely recognized (e.g., Panje, 1972, Roach 1977). Until the early 20th century, cultivated sugarcane varieties in most parts of the world consisted mainly of S. officinarum clones, collected from Papua New Guinea and Indonesia. In the early 20th century, breeders in India and Indonesia initiated programs that utilized interspecific hybrids derived from crosses between S. officinarum and S. spontaneum (Daniels and Roach 1987). The initial interspecific hybrids were crossed back to S. officinarum clones or other hybrids to retain sufficiently high sugar content, in a process that was termed nobilization by sugarcane breeders (Bremer 1961).

Saccharum spontaneum is a highly variable species found growing wild throughout Asia and parts of Africa between 8° S and 40° N (Brandes et al., 1939; Mukherjee, 1950). The species is characterized by a number of traits, most of which contrast with S. officinarum, such as low sucrose content, high fiber levels, and strong ratoon growth after harvesting, as well as general vigor and adaptation to a range of environmental stresses. Over 30 cytotypes have been reported with chromosome numbers ranging from 2n = 40 to 128 (Panje and Babu, 1960). Morphologically, S. spontaneum clones vary considerably, from short bushy types to types with large stalks >5 m in height. Clones have also been found growing in widely diverse habitats ranging from tropical jungles, deserts, swamps, and altitudes over 3000 m with temperatures well below freezing (Brandes et al., 1939; Warner and Grassl, 1958; He et al., 1999). Only a relatively small number of S. spontaneum clones were used in the early interspecific crosses. Current sugarcane cultivars around the world and parental material in modern sugarcane breeding programs, which remain based largely on derivatives from the interspecific hybrids in the early 20th century, therefore can be traced back to a limited number of original clones (Arceneaux, 1967; Roach 1989). Brown et al. (1969), Roach (1989), and others have contrasted the small number of S. spontaneum clones utilized in modern sugarcane breeding programs with the large number and diversity available in germplasm collections.

Following the interspecific hybridization in sugarcane breeding programs in the early 20th century, there was rapid progress in sugarcane improvement. By the 1960s, however, some sugarcane breeders were uneasy about possible reduced rate of breeding progress and the limited sampling of basic germplasm in breeding programs relative to the diversity available (Roach 1989). This motivated a recommencement of interspecific hybridization in several countries in the 1960s. These efforts have continued until today to varying degrees in a range of breeding programs, with a large portion of these efforts focused on using S. spontaneum (Berding and Roach, 1987; Miller and Tai, 1992).

In general it is difficult to gauge precisely what the impact or success of introgression breeding programs in sugarcane has been in recent decades. A number of important cultivars from these efforts were bred in Australia, the United States, China, Barbados, Argentina, and other countries. Some highly successful examples include the use of the S. spontaneum clone Mandalay in Australia, leading to many cultivars through the parent QN66-2008 and the highly productive cultivar LCP85-384 grown in Louisiana (Milligan et al., 1994). However, it has also been noted that much effort has not led to commensurate commercial successes (Stalker, 1980; Berding and Roach, 1987). The process of introgression in sugarcane breeding is therefore traditionally a long-term and risky investment. The time and risk factors have clearly acted to reduce the level of resources devoted in most sugarcane breeding programs to introgression breeding despite general agreement among sugarcane breeders of its potential value.

We recently (2002) commenced a breeding and research program aimed at utilizing S. spontaneum. In addition to the reasons motivating past breeding efforts by other sugarcane breeders, two other factors were involved. First, we hypothesize that DNA markers may be usefully applied to improve the effectiveness of introgression breeding in sugarcane compared with past efforts. This has been suggested for plant breeding generally (e.g., Lee, 1995; Tanksley and Nelson 1996; Tanksley and McCouch, 1997). We are currently investigating the application of DNA markers for introgression breeding in several case-study populations in sugarcane. Concurrent with this research, we are generating a range of progeny populations derived from crosses between sugarcane (S. officinarum or commercial cultivars) and diverse sources of S. spontaneum. Progeny from the best populations identified will subsequently be available for further cycles of breeding and selection using traditional phenotypic based evaluation and/or DNA marker assisted methodology currently under assessment.

A second motivating factor was the potential application of sugarcane genotypes more closely related to S. spontaneum than current modern sugarcane cultivars for future energy production systems. Sugarcane is being increasingly considered and used as a low-cost feedstock for renewable energy production, both for liquid fuel such as ethanol, and for electricity generation. Furthermore, investment and progress being made to develop commercially viable processes for conversion of ligno-cellulosic biomass to liquid biofuels (e.g., USDOE, 2006), coupled with potentially increased value of renewable energy in the future, may lead to a demand for low-cost biomass feedstock crops within 5 to 15 yr. Sugarcane genotypes closely related to S. spontaneum, and which retain some desirable features of S. spontaneum such as adaptation to adverse (e.g., dry) environments and strong ratoon growth, may offer cost-competitive options compared with other candidate species for such production systems. Some promising early results have been reported (e.g., Irei et al., 2006; Kennedy, 2006).

In this paper, we describe results from several field experiments that may be used to help direct future selection among populations derived from S. spontaneum. The experiments involved small plots, typical of those used routinely in sugarcane breeding programs in early stages of selection. Generally, in small plot sugarcane trials, measurements relating to biomass composition such as brix, dry matter content, and sucrose content are considered reliable. Measurements relating to cane yield may be affected by intergenotypic competition and need to be interpreted cautiously but are of some predictive value of relative performance in pure stands (Jackson and McRae, 2001). The results reported here provide an indication of (i) the extent to which performance of S. spontaneum, and sugarcane parental clones, measured in germplasm collections, predicts performance of their progeny; (ii) the relative proportion of variation retained among families (based on biparental crosses) versus variation within families for several important traits, and the implications for developing efficient selection among large clonal populations derived from S. spontaneum; and (iii) the size of genetic correlations for family performance between environments in Australia and China, and a preliminary indication of the relevance of selection trial results between continents.

These results are used to provide suggestions for efficient strategies to develop commercially valuable sugarcane cultivars from new sources of S. spontaneum for sugar production or biomass production systems.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Plant Material
Crosses between either S. officinarum clones or commercial sugarcane clones and S. spontaneum were made in China in 2002 and 2003 at Yunnan Sugar Research Institute and Hainan Sugar Experiment Station. A list of crosses used, and the source province of the S. spontaneum clone collected in China, is shown in Table 1 .

Field Experiments
Three field experiments were conducted as follows.

Parent Evaluation Experiment
The parent evaluation experiment was conducted to determine the extent to which measurements made in clones maintained in a germplasm collection could be used to predict relative performance of their progeny. Conditions of growth and maintenance in the current study followed similar procedures as used for the Chinese national sugarcane germplasm collection, at Yunnan Sugar Research Institute (YSRI), Kaiyuan, Yunnan Province, China (23°N, 101°E). This includes growing clones as spaced plants and repropagating every 3 to 4 yr. All clones used for crosses made at YSRI (30 out of 43 crosses; Table 1) were planted into a field experiment on 7 Mar. 2005 at YSRI. A randomized complete block design was used, with three blocks. Each clone was planted as a single plant, on a square grid with 5 m of spacing between plants. Plants were allowed to tiller freely. Fertilizer was added equivalent to a rate of 345 kg ha^–1 N, 144 kg ha^–1 P, and 90 kg ha^–1 K. Flood irrigation was applied six times during growth to reduce moisture stress. Sampling and measurements were made from 28 to 30 Nov. 2005.

Progeny Evaluation Trial in China
In China 15 clones were taken at random from each of the 30 crosses made at YSRI (Table 1), and setts (short pieces of stem) from each of these 450 clones were planted into a field experiment on 7 Mar. 2005. The experiment was laid out in a randomized complete block design with two replicates. Setts from each clone were planted end on end into a single row plot of 1.5 m in length in each of two blocks. The inter-row spacing was 1 m. In addition to the progeny, three plots of each female parent (either commercial clone or S. officinarum clone) were also included in each block. Cultivation and crop management followed conventional commercial practices, which included application of 345 kg ha^–1 N, 144 kg ha^–1 P, and 90 kg ha^–1 K. Flood irrigation on six occasions to avoid moisture stress. Sampling and measurements were made on 21 to 29 Nov. 2005 following procedures described below.

Progeny Evaluation Trial in Australia
In Australia 30 clones were taken at random from each of the 43 crosses shown in Table 1. Fifteen of the clones were included in each of two field experiments planted at Kalamia Estate near Ayr, Queensland (19° S, 147° E) on 2 Oct. 2005 and at Macknade Experiment Station, near Ingham, Queensland (18° S, 145° E) on 5 Nov. 2005. The experiment was laid out in a randomized complete block design with two blocks, and each progeny clone was grown in one plot in each block. At least 16 plots of two standard commercial cultivars grown in each region were also included in each block for comparison with the experimental clones. In each trial, uniform crop establishment was ensured through pregermination of single-eye setts (pieces of stem with a single internode and single bud or "eye") in small pots, followed by transplanting the resulting plants into the field at approximately 6 wk later. Each individual plot consisted of five plants spaced about 0.4 m apart, with approximately 1.1 m between the ends of plots and row spacing of 1.5 m. Cultivation and crop management followed conventional commercial practices in each of these regions, which included application of 100 kg ha^–1 N, regular furrow irrigation at Kalamia Estate to avoid moisture stress, and rainfed conditions at Macknade Experiment Station. Sampling and measurements were made on 10 to 14 June and 5 to 9 July for the Kalamia Estate and Macknade Experiment Station trials, respectively.

Measurements
Stalk number was determined in all trials by counting all stalks >1 m high in each plot at approximately 6 mo following planting. A sample of stalks that were >1 m in height was taken at random for estimating stalk weights and for stalk biomass composition measurements described below. In all cases, a 6-stalk sample (or 12-stalk for clones with extremely low stalk weight) was used for both weights and composition measurements. Cane yield was determined from the product of stalk number and stalk weight.

Measurements of dry matter percentage of cane, fiber content, and brix and pol in juice were made on the samples of stalks obtained from each plot in the field trials. For the Australian trials, samples obtained from 8 clones (out of 15) in each family from each the two trials (i.e., 43 families x 8 clones/family/site x 2 sites = 688 clones), with measurements made in each replicate of each clone sampled (giving a total of 1376 plots sampled). In China samples were taken from all 15 clones in each family (30 families x 15 clones per family = 450 clones). In China samples were combined across replicates in the progeny evaluation trial, but a set of 30 random clones was sampled and measured in each replicate to obtain an estimate of experimental error variance. This estimate was considered appropriate for applying to the combined sample measurements in statistical analysis, since prior experience indicated that variance in laboratory measurements is extremely small compared with differences between replicates in field trials.

The methods used for estimating brix and pol components in cane closely followed those routinely used in sugarcane breeding programs (BSES, 1984), while the methods used for fiber and dry matter contents followed the press method described by Tanimoto (1964). Brix percentage (g solute per 100 g of solution) was measured on extracted juice using a refractometer. Pol percentage (g sucrose as estimated by polarimetry per 100 g of juice) was estimated from

[1]

This formula was derived from regression analysis of data in Table 2 in BSES (2001).

[2]

Slightly different but directly comparable methods were used in China and Australia for measuring the above components, as well as dry matter content in cane and fiber content. In China, all stalks sampled (and weighed) were crushed through a small mill that extracted approximately 60% of juice in the commercial cultivars. Brix and pol in juice were measured on the extracted juice as given above. The bagasse (i.e., material remaining after crushing of the cane) was weighed and then dried in an oven to determine moisture content. Dry matter percentage in cane was estimated from the following:

[3]

where WSB is the weight of sample of stalks before crushing (g), WSA is the weight of bagasse (i.e., after crushing) (g), and DMB =is the dry matter content of bagasse (%). Brix in cane (%) was estimated from

[4]

Pol in cane (%) was estimated from the same formula, substituting pol in juice for brix in juice. Fiber content (%) was estimated from DM% in cane minus brix in cane (%).

In Australia the sampled stalks were fibrated using a Jeffco cutter grinder (Jeffries Ltd, Dry Creek, SA, Australia), and 400 g of the fibrated sample was crushed using a hydraulic press. Brix and pol in juice were measured on the extracted juice as described above. The pressed sample of bagasse was weighed before and after oven drying. The same formula was used as in China for estimating dry matter percentage, except that 400 g used for WSB.

Total dry matter in each plot was estimated by the product of dry matter percentage and cane yield.

Analysis of Data
Data were analyzed using the SAS statistical package (Release 8.2, SAS Institute, Cary, NC). For both the progeny and parent evaluation experiments, analyses of variance were first done for all data (excluding data from check cultivars and parents) in each individual experiment for each attribute, assuming the following model:

[5]

where µ, b_j, g_i, and e_ij are the grand mean, block effect, genotype effect, and error effect, respectively. Genotypes and blocks were considered to be random effects, with genotypes generating variance component _g² and genotype x block interaction variance generating error variance component _e². Variance components for genotypes and error were estimated from the following expectations of mean squares: _e² + r_g² for genotype mean square and _e² for error mean square, where r = number of replicates = 2.

For the trial in China, where samples were combined across replicates, broad-sense heritabilities (h_b²) for each trait were determined from h_b² = _g²/_p², where _g² is genetic variance, and _p² is phenotypic variance (Falconer and Mackay, 1996). Phenotypic variance was determined from _p² = _g² + _e²/r. The genetic coefficient of variation (GCV, %) was determined from GCV (%) = _g/µ.

Genetic variance was further partitioned into variance among-families and within-families, using the following model:

[6]

where µ, b_j, f_i, c_ik, and e_ij are the grand mean, block effect, family effect, progeny within-family effect, and error effect, respectively. Families, progeny within families, and blocks were considered to be random effects, with families and progeny within families generating variance components _f² and _p², respectively. Variance components for families, progeny within families and error variance were estimated from the following expectations of mean squares: _e² + r_p² + rn _f² for family mean square, _e² + r_p² for progeny within families mean square, and _e² for error mean square, where r is the number of replicates = 2, and n is the number of progeny per family (=15 in experiment in China and 8 in experiments in Australia).

Repeatability on a family mean basis (h_fam²) was determined from

[7]

The genetic correlation between family means (r_fam) at different experiments was determined from

[8]

following Falconer and Mackay (1996), with the assumption that error effects were independent (r = 0) between experiments.

Phenotypic correlations between parent performance (based on means across replicates) in the parent evaluation experiment at Kaiyuan and family means at each of the progeny evaluation experiments (based on means of all progeny in all replicates) were determined for both female and male parents separately, as well as for midparent values.

	RESULTS

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Means and Analysis of Variance in Progeny Evaluation Trial
Mean performance of all progeny clones at each of the three field experiments are shown in Table 2. As expected, purity levels and levels of brix and pol in both juice and in cane were lower in the progeny than those of the commercial sugarcane cultivars included for comparison. However, performance of the highest-ranked clones approached levels recorded in the cultivars for these traits. Average levels of dry matter and fiber percentage for the progeny clones were greater than the cultivar checks by about 20 and 100%, respectively.

Average cane yield and biomass yields of the progeny clones were similar to the sugarcane cultivar checks, but the mean levels of the top 5% of clones were considerably higher (Table 2). The latter results must be treated cautiously, however, since, as indicated earlier, competition effects may affect results of individual clones in small plots such as those used in this study. Nonetheless, these results, combined with relatively high levels of broad-sense repeatability (see below) indicate that individual clones may be selected with performance levels for biomass yields in small plots well exceeding those of the cultivar checks.

While cane yields on average were similar between the progeny and sugarcane cultivars, the relative contribution of the two components making up cane yield, stalk number, and stalk weight differed dramatically. In the progeny clones, stalk numbers on average were about threefold higher, and stalk weight threefold less, relative to the sugarcane cultivars.

Partitioning of Variance among and within Families
The relative proportion of total genetic variance retained among and within the families for most traits was generally consistent across the three different sites (Table 3 ). Highly significant (P < 0.01) variation due to both families and to clones within families were observed for all traits measured. In general, for most traits, variation due to families was at least half as large as variance within families, with this proportion being largest for fiber, dry matter percentage, purity, and stalk weight. However, for cane yield and biomass yield, variation due to families accounted for lower proportions of total variance among genotypes at all sites (Table 3). These results suggest that family-based selection would be moderately effective for most traits, but more effective for biomass composition traits than for cane and biomass yield.

Comparisons were made of progeny derived from crosses where the female parent was either S. officinarum or commercial-type sugarcane clones. Overall, there were only small, although statistically significant (P < 0.05), differences for average performance of progeny generated from these two types, and these differences reflected average differences in performance overall between these two classes of parents observed in the parent evaluation trial (Table 7 ). Progeny derived from the commercial-type parents had on average higher levels of brix, pol, fiber, purity, and stalk number, and lower stalk weight, while there were no significant differences in cane yield and biomass.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The high correlations observed between the female (commercial sugarcane or S. officinarum) parent emphasize the high importance of using high-performing female clones in crosses with S. spontaneum. The lower correlations observed between performance of the S. spontaneum parents and their progeny suggest that there may be limited gains from selection among candidate S. spontaneum parents in experimental trials such as that used in this study, compared with selection of high-performing S. officinarum or commercial sugarcane parents. The lack of a strong relationship between parent and progeny for the S. spontaneum parents differs to a moderately strong relationship found in the study by Roach (1977). One possible explanation for the results in the current study could be the relatively small genetic variance observed among the S. spontaneum clones used as parents for pol and brix levels, compared with the S. officinarum or commercial sugarcane parents (Table 5). However, variation among the parent clones were not reported by Roach (1977), so comparisons cannot be made to help support this potential explanation of the differing results.

By contrast, the results in the current study indicate the importance of the S. spontaneum parent in determining both stalk number and stalk weight of progeny. These results may be of little practical significance, however, since the effects of neither component dominates in determination of cane or biomass yields, with both the latter components being only weakly related to parent performance.

It is well established that there is normally 2n chromosome transmission from S. officinarum when it is crossed with S. spontaneum or Saccharum spp. hybrids with S. spontaneum included in the genome (Bremer, 1922, cited in Roach, 1977; Roach 1968). Because of this pattern of chromosome transmission, it has been suggested that the use of S. officinarum as the female parent, rather than commercial sugarcane (with a component of S. spontaneum included in the genome) in crossing with S. spontaneum to be desirable because of faster return to high sucrose levels (Roach, 1977; Berding and Roach, 1987). However, in this study, the results do not support this hypothesis. In fact, the reverse trend appeared. The results obtained here suggest that the use of high-performing, high-sucrose, commercial hybrid parents in crossing with S. spontaneum would be preferable to the use of S. officinarum in initial crosses, presuming an aim of maximizing sucrose content and economic value.

This study provides the first report to our knowledge of genotype x environment interactions between different continents in sugarcane-related germplasm. The high genetic correlations observed for the biomass composition traits between countries clearly suggest that selection trial results in one country would be of some relevance to the other. This has clear implications for designing collaborative programs or selection systems spanning effort across countries.

Pol and purity levels in these progeny clones were, as expected, lower than the commercial cultivars included in the trials for comparison, but some clones performing at levels similar to the commercial cultivars were also identified. For cane yield and biomass yields, clones with considerably higher biomass levels relative to commercial sugarcane were identified. However, as emphasized above, results relating to cane and biomass yields should be interpreted cautiously at this stage since competition effects may be important in the small plots used in this study; further evaluation in large plots is required to substantiate these results. Nevertheless, the performance levels in biomass coupled with large predicted gains from selection suggest that some clones generated from crosses between commercial sugarcane and S. officinarum could offer opportunities for providing high-yielding biomass crops. In addition, based on prior knowledge about the contribution of S. spontaneum to sugarcane breeding, it may be expected that strong ratooning ability and adaptation to a range of environmental stresses may be expected to be a feature of some first cross progeny involving S. spontaneum. These traits were not tested in the current study but should be a priority for further evaluation.

The collective results obtained in this study point to the following broad strategy to exploit S. spontaneum clones for developing cultivars for future sugar or biomass production systems.

1. S. spontaneum should be crossed to sugarcane clones with high performance for all commercially valuable traits, especially sucrose content. There would seem to be little or no advantage in crossing S. spontaneum to S. officinarum as has been previously suggested.
   
2. High-performing families for biomass composition traits of interest could be initially identified on the basis of performance of a sample of 10 to 20 clones per family in small plots with limited (1–2) replication per clone.
   
3. Evaluation of a large numbers of individual clones from selected families for biomass composition traits in small plots, followed by evaluation of selected clones (based on biomass composition traits) in multirow plot trials for accurate evaluation of cane and biomass yields.
   

Development of optimal selection criteria and indices in the above scheme is important and should be a priority of future research to optimize selection systems. Information not generated in this study but needed for developing such indices include estimates of the relative economic value of changes in levels of different candidate traits (e.g., dry matter percentage, pol or sucrose content in cane, biomass yields) and of the genetic correlation between biomass production in small plots (subject to competition effects) and larger plots or pure stands.

	ACKNOWLEDGMENTS

 
This work was funded in part by the Australian Centre for International Agricultural Research (ACIAR), and the CRC for Sugarcane Industry Innovation through Biotechnology, Australia. We acknowledge with thanks the competent assistance of numerous field and technical staff from YSRI, HSBS, CSIRO, BSES and CSR, Ltd., in conducting the field trials and collecting samples for this research.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication November 26, 2007.

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