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Sugar Tech paper

  1. 1. 1 23 Sugar Tech An International Journal of Sugar Crops and Related Industries ISSN 0972-1525 Volume 14 Number 3 Sugar Tech (2012) 14:237-246 DOI 10.1007/s12355-012-0166-9 Combining Ability and Heterosis over Environments for Stalk and Sugar Related Traits in Sweet Sorghum (Sorghum bicolor (L.) Moench.) A. V. Umakanth, J. V. Patil, Ch. Rani, S. R. Gadakh, S. Siva Kumar, S. S. Rao & Tanmay Vilol Kotasthane
  2. 2. 1 23 Your article is protected by copyright and all rights are held exclusively by Society for Sugar Research & Promotion. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication.
  3. 3. RESEARCH ARTICLE Combining Ability and Heterosis over Environments for Stalk and Sugar Related Traits in Sweet Sorghum (Sorghum bicolor (L.) Moench.) A. V. Umakanth • J. V. Patil • Ch. Rani • S. R. Gadakh • S. Siva Kumar • S. S. Rao • Tanmay Vilol Kotasthane Received: 18 January 2012 / Accepted: 1 June 2012 / Published online: 22 June 2012 Ó Society for Sugar Research & Promotion 2012 Abstract Till date only one sweet sorghum hybrid CSH 22SS has been released for general cultivation in India and the current levels of new hybrids are unable to surpass this hybrid. The objective of this study was to assess the general and specific combining abilities of eight parents and 16 hybrids respectively at three semi arid locations by following a line 9 tester mating design. Significant differences among environments, testers, environments 9 testers and environ- ments 9 line 9 tester effects were observed for all traits suggesting the environmental influence on testers and the interactions. The variance component estimates of specific combining ability (SCA) were greater than that of general combining ability (GCA) for total biomass, juice extraction and grain yield indicating the non-additive control of genetic variation while the GCA variance was higher than the SCA variance for fresh stalk yield, juice yield, brix content, total sugar yield and computed bioethanol yields indicating additive gene action. Among females, DMS 28A for fresh stalk, juice and grain yields and DMS 25A for brix content were promising. Rio was a potential male parent for fresh stalk yield, total sugar content, computed bioethanol and grain yields. These parents can be exploited to address eth- anol production from juice without compromising on grain yields. The best hybrids for total biomass, fresh stalk yield, juice yield, juice extraction, total sugar content and com- puted bioethanol yields were DMS 13A 9 Rio and DMS 23A 9 RS 647 and after adequate testing across many locations, these hybrids are recommended for commercial exploitation for ethanol production. Keywords Sweet sorghum Á Biomass Á Brix Á Combining ability Á Ethanol Introduction Sorghum [Sorghum bicolor (L.) Moench] is the fifth major cereal crop in the world and is the principal dry land coarse cereal grown in semi-arid environments of India covering an area of 7.53 million hectares, with a production of 7.25 million tons at a productivity of 962 kg/ha (Anony- mous 2011). In the form of sweet sorghum, it has the capa- bility to influence and improve the rural livelihoods in India due to its potential industrial use for bioethanol production. The national biofuel policy of 2009 aims at promoting bio- fuels to meet India’s energy needs in an environmentally- sustainable manner, while reducing its import dependence on fossil fuels. The policy also proposed an indicative target of 20 per cent blending of ethanol by 2017 from the current 10 per cent ethanol blending with petrol. The traditional route of ethanol production through sugarcane molasses would not be meeting this huge demand because of the dif- ficulties in increasing the sugarcane area beyond the current 4.4 million ha in the country. Therefore, renewable sources of energy in the form of other biofuel crops would be promising options in view of the emerging trends in inter- national energy markets as well as indigenous strengths. Sweet sorghum has been used for nearly 150 years to pro- duce concentrated syrup with a distinctive flavor (Schaffert A. V. Umakanth (&) Á J. V. Patil Á Ch. Rani Á S. S. Rao Á T. V. Kotasthane Directorate of Sorghum Research, Rajendranagar, Hyderabad 500030, Andhra Pradesh, India e-mail: umakanth@sorghum.res.in S. R. Gadakh Sorghum Research Station, MPKV, Rahuri, Maharashtra, India S. Siva Kumar Department of Millets, TNAU, Coimbatore, Tamil Nadu, India 123 Sugar Tech (July-September 2012) 14(3):237–246 DOI 10.1007/s12355-012-0166-9 Author's personal copy
  4. 4. 1992). The stem juice of sweet sorghum is rich in fermen- tative sugar and is a desirable material for alcoholic fer- mentation. Further, the stillage from sweet sorghum after the extraction of juice has higher biological value than the bagasse from sugarcane when used as fodder for animals, as it is rich in micronutrients and minerals. The sweet sorghum bagasse is as good as the stover for the intake and body weight gain in the animals when used as live-stock feed. Apart from these, stillage contains similar levels of cellulose as sugarcane bagasse, therefore has a good prospect as a raw material for pulp product (Srinivasa Rao et al. 2009). The bagasse or residue can also be used to cogenerate power of about 3.2–3.4 MW/ha (Gururaj et al. 2010) for every hectare of crop. Besides these uses, the whole plant biomass can also be used as a substrate for production of ligno cellulosic ethanol. Sweet sorghum was suggested as the best alternative feedstock for bio ethanol production (Dayakar Rao et al. 2004; Shukla et al. 2006). Sweet sorghum is the best alternative raw material to supplement the use of sugarcane in ethanol production according to a pilot study conducted by Vasantdada Sugar Institute. At 5,600 l per hectare per year (from two crops, at 70 tons per hectare of millable stalk per crop at 40 l per ton), the ethanol production from sweet sorghum compares well with the 6,500 l per ha per crop for sugarcane (at 85–90 tons per hectare of millable cane per crop at 75 l per ton) (Anonymous 2004). Techno-economic feasibility studies have shown that the cost of alcohol production from sweet sorghum was Rs 1.87 less than that from molasses. This conclusion was based on the prevailing prices of molasses during that period. In addition to sweet stalk, an average grain yield of 1.5–2.0 t/ha can be harvested which can be used as food or feed (Dayakar Rao et al. 2004). Concerted research efforts during last two decades at Directorate of Sorghum Research and its cooperating centres in different State Agricultural Universities under National Agricultural Research System and at ICRISAT have resulted in excellent sweet sorghum varieties for use in ethanol pro- duction by the sugar industries/alcohol distilleries and for use as green/dry fodder. However, till date only one sweet sorghum hybrid CSH 22SS has been released (in the year 2005) for general cultivation in India and the current yield levels of new hybrids are unable to surpass this hybrid. This necessitates the identification of new hybrid parents with good combining ability for different traits of interest. In hybrid oriented breeding programmes, the knowledge of combining ability of the parents and the inheritance of the traits is important (Itai et al. 2010). This information helps in optimizing the breeding strategy, either selection when general combining ability (GCA) effects are impor- tant; inbreeding followed by hybrid breeding when specific combining ability (SCA) effects are predominant; or selection followed by hybridization if both are important; because GCA effects are attributed to preponderance of genes with additive effects and SCA indicates predomi- nance of genes with non-additive effects (Kenga et al. 2004; Mutengwa et al. 1999). Studies have shown both GCA and SCA to be important in many sorghum traits including grain yield (Haussmann et al. 1999; Tadesse et al. 2008; Yu and Tuinstra 2001). The objective of this study was to determine the combining ability of 8 parents for stalk and sugar related traits. The study envisaged assessing the general combining ability of parents and specific combining ability of hybrids by following a line 9 tester (L 9 T) mating design and the experiment was conducted at 3 diverse locations as significant geno- type 9 environment interaction effects have been reported in sorghum (Chapman et al. 2000). Materials and Methods Plant Material Four cytoplasmic genetic male sterile lines (DMS 13A, DMS 23A, DMS 25A and DMS 28A) used as females (A-lines) were crossed on to each of the four male-fertile lines (N 98, Rio, RSCN 5008 and RS 647) in line 9 tester fashion to produce 16 F1 hybrids during post rainy season of 2009 at Directorate of Sorghum Research (DSR), Hyderabad. The males included two sweet stalked temperate lines (N 98 and Rio) and two sub-tropical breeding derivatives (RSCN 5008 and RS 647) in sweet stalk background with high stalk yields while the females had higher sugar content (Table 1). The parental line selection criteria were based on characters contributing to increased stalk and sugar yields. Experimental Sites The 16 F1 hybrids, their corresponding eight parents and three checks were evaluated at three different locations in three different states of India with semi-arid environments viz., Directorate of Sorghum Research (DSR) farm, Hy- derabad, Andhra Pradesh (latitude 17°190 N, longitude 78°230 E), Centre for Plant Breeding and Genetics farm, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu (latitude 11°00 N, longitude 76°550 E) and Sorghum Research Station farm, Mahatma Phule Krishi Vidyapeeth, Rahuri, Maharashtra (latitude 19°200 N, longitude 74°380 E) during rainy season of 2010. The experiment was con- ducted in a Randomized Block Design with three replica- tions at all the three locations. Each entry was raised in two rows of 4 m length with 60 9 15 cm spacing. Recom- mended agronomic practices were followed throughout the crop season. Atrazine (1.0 kg/ha of active ingredient) was applied immediately after sowing. A basal fertilizer dose of 238 Sugar Tech (July-September 2012) 14(3):237–246 123 Author's personal copy
  5. 5. 42 kg/ha N, 42 kg/ha P2O5 was applied just before sow- ing, in the 2nd week of June 2010, and a topdressing of 46 kg/ha N was applied 1 month after germination (floral initiation stage) in the third week of July 2010. In each rep- lication, observations were recorded on 10 randomly selec- ted competitive plants. At physiological maturity, data was recorded on following traits. Traits Studied Total fresh biomass (t/ha): At physiological maturity, all the plants in net plot with their leaves, stems, and panicles were weighed in kilograms and then converted into tons per hectare. Fresh stalk yield (t/ha): The leaves plus sheath and pan- icles were removed from the plants carefully and the weight of the fresh (millable) stalk yield in the net plot was recorded in kilograms and later converted into tons per hectare. Juice yield: The extraction of juice from 10 randomly selected plants was done on an electrically operated three- roller stalk crusher with a minimum of three passings of the fresh (millable) stalk so that the last drop of juice came out from the stems. Juice extracted was measured in kilograms and later converted into litres per hectare. Brix content: It is a measure of the mass ratio of soluble solids to water, is a widely used approximation for sugar content and is reported as a trait in the rest of the text. The brix values from the composite juice were recorded °brix, using an Atago PAL-1 digital hand-held pocket refrac- tometer (with automatic temperature compensation ranging from 0 to 50 °C) at the hard dough stage. Juice extraction (%): The juice extractability in percent was calculated using the data of total weight of ten fresh stalks Juice extraction %ð Þ ¼ Juice weight=Fresh stalk yieldð Þ½ Ã100Š: Total Sugar yield (t/ha): Total sugar yield was calculated on the basis of following formula (Reddy et al. 2005). Total soluble sugar %ð Þ=100ð Þ Â Juice yield kl=hað Þ Grain yield: All the panicles were collected from the net plot, threshed, dried and the dry weight was recorded in kilograms at 14 % seed moisture content and later converted into kilograms per hectare. Computed ethanol yields: The ethanol yields were cal- culated based on the total sugar yield (Smith and Buxton 1993). Statistical Analysis Analysis of variance for combining ability was carried out using mean values across environments (Kempthorne 1957) to test the significance of differences among the genotypes including crosses and parents (Snedecor and Cocharan 1967; Panse and Sakhatme 1964). The sum of squares for hybrids was further partitioned into variation due to lines, testers and line 9 tester interactions. The mean squares due to lines and testers were tested against the mean squares due to line 9 tester, and the latter were tested against the pooled error. The mean squares due to environment 9 line and environment 9 tester were tested against the mean squares due to environment 9 tester 9 line, and the latter was tested against the pooled error. Estimate of GCA variances (r2 GCA) and SCA variances (r2 SCA) were obtained (Singh and Chaudhary 1977). Mid-parent heterosis and better parent heterosis were estimated and tested by working out the standard errors (Hays et al. 1955). Results Combined analyses of variance for eight characters mea- sured over three environments are presented in Table 2. Significant differences among environments, testers, envi- ronments 9 testers and environments 9 line 9 tester effects were observed for all the characters studied sug- gesting that the testers and the interactions for these traits were influenced by the environment. The line 9 tester Table 1 Details of parental sorghum lines used to generate 16 hybrids Line no Name Fertility status Origin Role in crosses Principal selection criteria 1 DMS 13A CMS India Female High sugar, high stalk yield 2 DMS 23A CMS India Female High sugar, high stalk yield 3 DMS 25A CMS India Female High sugar 4 DMS 28A CMS India Female High sugar 5 N 98 CMF USA Male High sugar 6 Rio CMF USA Male High sugar, high stalk yield 7 RSCN 5008 CMF India Male High sugar, high stalk yield 8 RS 647 CMF India Male High sugar, high stalk yield CMF cytoplasmic male fertile, CMS cytoplasmic male sterile Sugar Tech (July-September 2012) 14(3):237–246 239 123 Author's personal copy
  6. 6. effect was also significant for all the traits except brix content indicating the existence of genetic diversity in the material tested. The line effects were non-significant for total biomass while the environment 9 line effect was not significant for total sugar content, grain yield and com- puted bioethanol yields. The importance of the source of variation is indicated by the relative magnitude of variance components. The vari- ance component estimates of SCA were greater than that of GCA for total biomass, juice extraction and grain yield (Table 3) indicating the non-additive control of genetic variation for these traits. On the contrary, the GCA vari- ance was higher than the SCA variance for fresh stalk yield, juice yield, brix content, total sugar yield and com- puted bioethanol yields indicating the presence of additive gene action. In addition, the ratio of the mean square components associated with variance of GCA and SCA was in negative direction and much less than the theoretical maximum of unity for most of the traits studied. There was significant interaction of variance due to SCA with envi- ronment for all the characters studied except juice extrac- tion (%). The selection of parental lines for hybrid programs was the main objectives of this study. Thus, the estimates of the general combining ability (gi) of a parent provide impor- tant indicators of its potential for generating superior lines. A low gi estimate, whether positive or negative, indicates that the mean of a parent in crossing with the other, does not differ greatly from the general mean of the crosses. On the other hand, a high gi estimate indicates that the parental mean is superior or inferior to the general mean (Kenga et al. 2004). Estimates of GCA effects for different traits viz., total biomass, fresh stalk yield, juice yield, brix content, juice extraction, total sugar content, grain yield and computed bioethanol yields for the eight parents used in this study are presented in Table 4. DMS 28A was the most promising female parent for most of the traits like fresh stalk yield, juice yield, juice extraction and grain yield with highly significant and positive GCA effects but the GCA effects for brix content were in the negative direction. The female DMS 23A was the best general combiner for total biomass while DMS 25 A was good general combiner for total biomass, brix content and juice extraction. On the other hand, the female DMS 13A showed significant and nega- tive GCA effects for total biomass, juice yield, juice extraction and grain yield. Among the male parents (testers), N 98 was the best general combiner for juice extraction. However it was not a good combiner for total biomass, fresh stalk and grain yields. Rio was the best combiner for most of the important traits related to biofuel production like fresh stalk yield, brix content, total sugar content, grain yield and computed Table2Pooledanalysisofvarianceforcombiningabilityinline9testeranalysisofsweetsorghum SourceDegree offreedom Totalbiomass (t/ha) Freshstalkyield (t/ha) Juiceyield(l/ha)Brixcontent (%) Juiceextraction (%) Totalsugar yield(t/ha) Grainyield (kg/ha) Computedethanol yields(l/ha) Environments223,083.50***8,072.28***559,950,080.00***1,098.26***3,337.04***13.54***14,414,082.00***3,799,307.75*** Parents(line)310.41119.44***21,784,970.00***14.70***48.27***0.37***1,467,017.88**103,104.55*** Parents(testers)3436.77***116.18***59,796,116.00***84.78***797.57***1.59***2,451,725.25***440,501.72*** Line9testereffect9355.10***117.37***7,493,865.00**4.39125.42***0.32***2,980,356.25***81,521.52*** Env9parents(L)686.07***59.33***7,247,019.50**6.45*18.50***0.09408,080.2821,093.05 Env9parents(T)6238.49***69.29***21,139,448.00***33.68***236.18***0.27***1,174,779.25***75,233.90*** Env9L9Teffect18215.87***32.87***9.06***11.01***0.33***17,732,490***93,143.59***517,586.50* Error13815.272.161.182.410.052,284,184.5013,742.74286,545.00 *,**,***SignificantatPB0.05,0.01and0.001,respectively 240 Sugar Tech (July-September 2012) 14(3):237–246 123 Author's personal copy
  7. 7. bioethanol yields. The testers RSCN 5008 and RS 647 were promising for total biomass only. It was observed that on a pooled basis, significant and positive SCA effects for total biomass were shown by 5 hybrids viz., DMS 23A 9 RS 647, DMS 13A 9 Rio, DMS 13A 9 RSCN 5008, DMS 25A 9 N 98 and DMS 25A 9 Rio. These hybrids showing significant and positive SCA effects were also among the best in per se perfor- mance (Table 5). With respect to fresh stalk yield, DMS 13A 9 Rio and DMS 23A 9 RS 647 exhibited significant and positive SCA effects and the trend in per se perfor- mance was similar to total biomass. However, the hybrid DMS 28A 9 Rio which was the top fresh stalk yielder showed insignificant negative effects. For juice yield, the hybrid DMS13A 9 Rio recorded significant and positive SCA effects and it also excelled in biomass traits too. DMS 28A 9 Rio and DMS 28A 9 RSCN 5008 were the other hybrids with high mean juice yields but insignificant and positive SCA effects. Out of 16 hybrids, only one hybrid DMS 13A 9 RSCN 5008 demonstrated significant and positive SCA effect for brix content. DMS 25A 9 Rio recorded the highest brix content in the trial but had shown a non-significant SCA effect which was in the positive direction. Six hybrids viz., DMS 23A 9 N 98, DMS 25A 9 RSCN 5008, DMS 28A 9 N 98, DMS 13A 9 Rio, DMS 25A 9 Rio and DMS 25A 9 RS 647 showed desirable SCA effects for juice extraction. All these six hybrids recorded higher juice yields. However it was gratifying to note that three hybrids exhibited positive SCA effects while the other three have shown negative SCA effects for juice yields which were non-significant. For total sugar content, the hybrids DMS 13A 9 Rio and DMS Table 3 Estimates of variance components as reference to the prevailing gene action Source Total biomass (t/ha) Fresh stalk yield (t/ha) Juice yield (l/ha) Brix content (%) Juice extraction (%) Total sugar yield (t/ha) Grain yield (kg/ha) Computed ethanol yields (l/ha) r2 Environments 221.788** 94.873*** 9,905,079.170*** 13.693*** 34.368** 0.215*** 133,643.140 ** 60,204.159*** r2 gca -10.982 1.301 227,523.945 0.329 4.624 0.001 60,166.904 643.923 r2 sca 15.469 -3.412 -1,137,624.929 -0.736 10.283** -0.001 273,641.090 *** -1,291.342 r2 gca/r2 sca -0.709 -0.381 -0.200 -0.447 0.449 -1.666 0.219 -0.498 r2 gca 9 environments 50.998* -0.273 -305,541.301 -0.429 8.408 -0.000 9,403.367 -441.428 r2 sca 9 environments 66.718*** 45.969*** 5,083,964.079*** 2.985*** 10.092 0.090*** 84,664.377 * 26,077.082 *** *, **, *** Significant at P B 0.05, 0.01 and 0.001, respectively Table 4 Pooled estimates of general combining ability effects of parents in sweet sorghum S. No. Parents Total biomass (t/ha) Fresh stalk yield (t/ha) Juice yield (l/ha) Brix content (%) Juice extraction (%) Total sugar yield (t/ha) Grain yield (kg/ha) Computed ethanol yields (l/ha) Females 1. DMS 13A -5.278*** 0.035 -566.174* 0.092 -5.376*** -0.029 -414.896*** -16.333 2. DMS 23A 3.028*** -1.354* -421.229 -0.322 -0.234 -0.082* -157.785 -44.083* 3. DMS 25A 2.806*** -0.326 121.993 0.544* 3.858*** 0.043 104.521 20.278 4. DMS 28A -0.556 1.646** 865.410** -0.314 1.752*** 0.068 468.160*** 40.139 S.E (gi) 0.660 0.0531 262.498 0.239 0.268 0.039 85.568 20.352 S.E (gi-gj) 0.934 0.751 371.22 0.33 0.379 0.055 121.012 28.78 Males 1. N 98 -5.472*** -2.826 *** -861.104 0.328 3.722*** -0.043 -408.285*** -22.889 2. Rio -0.778 3.618*** 33.868 0.669 ** -1.481*** 0.188*** 454.882*** 95.278*** 3. RSCN 5008 3.139*** -1.021 826.785 -0.119 -1.190*** -0.06 169.965 -28.667 4. RS 647 3.111*** 0.229 0.451 -0.878 *** -1.051*** -0.085* -216.563* -43.722* S.E (gi) 0.660 0.053 262.498 0.239 0.268 0.039 85.568 20.352 S.E (gi-gj) 0.934 0.751 371.22 0.33 0.379 0.055 121.012 28.78 *, **, *** Significant at P B 0.05, 0.01 and 0.001, respectively Sugar Tech (July-September 2012) 14(3):237–246 241 123 Author's personal copy
  8. 8. Table5Estimatesofspecificcombiningabilityeffectsandmeansofcrossesinsweetsorghum S. No. CrossesTotal biomass (t/ha) MeanFreshStalk Yield(t/ha) MeanJuiceyield (l/ha) MeanBrix(%)MeanJuice extraction (%) MeanTotalsugar yield(t/ha) MeanGrainyield (kg/ha) MeanComputed ethanolyields (l/ha) Mean 1DMS13A9N 98 2.55632-0.25723237.1467,555.770.50815.6-0.308320.0320.95404.535*2,63810.444502 2DMS 13A9Rio 6.917***413.521***281,132.563*9,617.77-0.52214.91.451**290.335***1.49-844.410***2,252171.167***781 3DMS 13A9RSCN 5008 6.611***450.16025-420.0767,215.331.011*15.7-1.330*26-0.0850.82102.7292,915-38.000447 4DMS13A9RS 647 -1.25037-5.424***21-949.6326,729.66-0.997*12.90.18728-0.282***0.6337.1462,763-143.611***327 5DMS23A9N 98 -5.528***321.13223818.2018,281.77-0.10014.65.062***420.0180.8919.3132,51013.750477 6DMS 23A9Rio 0.66743-5.090***23-873.8267,756.330.42515.4-3.602***29-0.157*0.94493.035**3,847-77.861504 7DMS 23A9RSCN 5008 -3.139*43-0.45123-926.4656,853.88-0.55313.7-0.32732-0.0760.78-255.2712,814-41.361416 8DMS23A9RS 647 8.000***544.410***29982.0908,806.330.22813.7-1.133*320.215**1.04-257.0762,426105.472*548 9DMS25A9N 98 5.694***430.88223-194.2437,812.55-0.54415-6.852***350.0041-84.1042,669-2.389525 10DMS 25A9Rio 5.111***470.54930-292.6048,880.770.32516.21.373*38-0.0931.13-511.826**3,105-42.556603 11DMS 25A9RSCN 5008 -5.583***41-1.25723519.7578,843.330.12515.24.359***410.1101.08547.868**3,87955.278577 12DMS25A9RS 647 -5.222***41-0.17425-32.9108,334.550.09414.41.120*38-0.0210.9348.0632,993-10.333497 13DMS28A9N 98 -2.722*31-1.75723-861.1047,889.110.13614.82.098***41-0.0540.97-339.7432,777-21.806526 14DMS 28A9Rio 2.13941-0.9793033.8689,950.66-0.22814.80.77835-0.0851.17863.201***4,843-50.750615 15DMS 28A9RSCN 5008 2.111451.54928826.7859,893.77-0.58313.7-2.702***320.0511.06-395.326*3,30024.083566 16DMS28A9RS 647 -1.528411.188290.4519,111.330.67514.2-0.174340.0871.07-128.1323,18148.472575 S.Em±1.3211.063524.9970.4770.5360.078171.13740.705 *,**,***SignificantatPB0.05,0.01and0.001,respectively 242 Sugar Tech (July-September 2012) 14(3):237–246 123 Author's personal copy
  9. 9. 23A 9 RS 647 showed significant and positive SCA effects apart from higher mean sugar contents. DMS 13A 9 Rio and DMS 23A 9 RS 647 were the only crosses to exhibit significant and positive SCA effects for com- puted bioethanol yields. However, the hybrids DMS 28A 9 Rio and DMS 25A 9 Rio were among the top mean ethanol yielders though they have recorded insig- nificant and negative SCA effects. For grain yield, DMS 28A 9 Rio, DMS 25A 9 RSCN 5008 and DMS 23A 9 Rio displayed significant SCA effects in positive direction. Heterosis Mid-parental heterosis and better parental heterosis for important traits like total biomass, juice yield, brix content and computed ethanol yield were studied. Mid-parental heterosis The mid-parental heterosis for total biomass, juice yields and computed ethanol yields are depicted in Fig. 1. For total biomass and juice yields, 11 out of 16 hybrids exhibited significant and positive heterosis. The hybrid DMS 23A 9 RS 647 recorded 62 % heterosis for total biomass while DMS 23A 9 N98 followed by DMS 13A 9 N 98 exhibited 79 and 69 % heterosis respectively for juice yield. With respect to brix content, only one hybrid DMS 28A 9 RS 647 has shown significant heter- osis up to 9 % while five hybrids recorded significant heterosis in negative direction. Stem sugar heterosis values up to 7.39 % in Texas, USA were reported (Corn 2008). Significant and positive heterosis for computed ethanol yields ranged from 26 % in DMS 25A 9 RS 647–98 % in DMS 23A 9 RS 647. Better parent heterosis Six hybrids have shown significant and positive better parent heterosis ranging between 12 to 41 % for total biomass. The hybrid DMS 23A 9 RS 647 which exhibited significant mid parent heterosis for total biomass also registered significant heterosis (41 %) in positive direction (Table 6). For juice yield, seven out of 16 hybrids have shown significant and positive better parent heterosis. The hybrid DMS 28A 9 RSCN 5008 registered 60 % signifi- cant heterosis. For brix content, 12 hybrids exhibited sig- nificant heterosis in negative direction. Better parent heterosis for stem brix up to 45 % was observed in hybrids (Itai et al. 2009). Eight out of 16 hybrids have shown significant and positive better parent heterosis for computed bioethanol yields which ranged between 19 and 86 %. The hybrids viz., DMS 23A 9 RS 647 (86 %), DMS 28A 9 RS 647 (69 %), DMS 23A 9 N98 and DMS 28A 9 RSCN 5008 (62 %) have shown more than 60 % better parent heterosis for this trait. Fig. 1 Mid-parent heterosis for a total biomass, b. juice yield and c computed ethanol yields in sweet sorghum hybrids Sugar Tech (July-September 2012) 14(3):237–246 243 123 Author's personal copy
  10. 10. Discussion The mean squares due to environment, entries and their interactions were significant indicating the genotypic diversity and their responses to environment. Partitioning of the mean squares into variations attributable to testers and line 9 testers showed that variation within each group with environment was significant for most of the traits. Similar to the present investigations, in a study on com- bining ability and heterosis in sweet sorghum germplasm, significant site 9 hybrid interaction effects for stem brix, stem biomass weight ha-1 and stem brix-juice index were reported (Itai et al. 2009). In a study on tropical sorghums across four environments, significant mean squares due to environment, entries and environment-entries interactions were observed (Kenga et al. 2004). Testers were more variable than lines in a line 9 tester study across locations and years in forage sorghum (Mohammed and Mohamed 2009) and in sweet sorghum (Indhubala et al. 2010) similar to the present study. The significance of line 9 tester effect for most of the traits except brix content suggests the presence of high heterotic responses for these traits. Mean stem °brix and stem °brix-juice index were not significantly (P B 0.05) different between the parents, hybrids and the standard check variety in a study on combining ability and heterosis of sorghum germplasm for stem sugar traits under off-season conditions in tropical lowland environments (Itai et al. 2009) which is similar to the present findings. The estimates of variances due to combining ability revealed the significance of both additive (fresh stalk yield, juice yield, brix content) and the non-additive type of gene actions (total biomass and grain yield) for important traits. This implies that improvement for these traits can be achieved through both selection and hybridization. More- over a definitive separation of additive, dominance and non-additive genetic effects for these traits requires eval- uation of additional sets of genetic material. The total biomass yields among the hybrids ranged from 26 to 54 t/ ha in the present investigation and this trait is of paramount importance in breeding sweet sorghum for biofuel pro- duction. Similar to the present study, stem biomass yields of 47.9, 46.4 and 39.5 t ha-1 for cultivars Wray, Keller and Rio, respectively were reported from in-season evaluations in Indonesia (Tsuchihashi and Goto 2004). Heterosis breeding could be exploited for increasing the biomass yields owing to the importance of non-additive gene action in determining this character. The other important biofuel trait, i.e., brix content ranged from 12.9° to 16.2° in the hybrids and was controlled by additive gene action and further gains for this trait can be achieved through selec- tion. Earlier studies (Tsuchihashi and Goto 2004; Woods 2000) have demonstrated stem sugar concentrations of between 14.0 and 18.5° brix with specialized sweet sor- ghum cultivars. Stem °brix values of about 13 under dry- land production were reported in Indonesia (Tsuchihashi and Goto 2004). The interaction effect of SCA variance with that of environment was significant for most of the characters studied. The predominant role of non-additive gene action for plant height, stem girth, total soluble solids, millable sweet-stalk yield and extractable juice yield was Table 6 Better parent heterosis in 16 sweet sorghum hybrids Hybrid Total biomass (tons/ha) Juice yields (l/ha) Brix (%) Computed ethanol yields (l/ha) DMS 13A 9 N 98 -16.52** 42.14** -10.69** 54.62** DMS 13A 9 Rio -34.81** -2.34 -18.02** 6.33 DMS 13A 9 RSCN 5008 16.52** 24.92** 1.95 28.25* DMS 13A 9 RS 647 -4.06 2.72 -16.00** 0.72 DMS 23A 9 N 98 -16.18** 48.02** -16.54** 62.32** DMS 23A 9 Rio 7.18 -21.24** -15.09** -31.38** DMS 23A 9 RSCN 5008 12.43* 18.66 -10.93* 19.33* DMS 23A 9 RS 647 41.33** 34.42** -0.48 86.43** DMS 25A 9 N 98 15.09** -10.46 -14.12** -0.90 DMS 25A 9 Rio 17.68** -9.82 -10.87** -17.80* DMS 25A 9 RSCN 5008 7.99 1.35 -9.09* 8.89 DMS 25A 9 RS 647 8.88 -4.48 -13.80** -6.33 DMS 28A 9 N 98 -21.61** 27.42* -15.14** 54.42** DMS 28A 9 Rio 1.93 1.04 -18.63** -16.21* DMS 28A 9 RSCN 5008 11.91* 59.80** -11.07* 62.23** DMS 28A 9 RS 647 2.77 39.08** -2.38 68.94** *, ** Significant at P B 0.05 and 0.01, respectively 244 Sugar Tech (July-September 2012) 14(3):237–246 123 Author's personal copy
  11. 11. observed in a study on heterosis and combining ability for juice yield related characteristics in sweet sorghum (Sankarapandian et al. 1994) indicating the importance of heterosis breeding for improving these traits. The identification of new hybrid parents with good combining ability for different traits of interest was one of the important objectives of the present study. For total biomass, DMS 23A and DMS 25A among female parents and RSCN 5008 and RS 647 among males are potential parents for improving the total biomass as these parents exhibited high and significant GCA values. These parents can also be utilized for development of feedstock material for the production of 2nd generation biofuels apart from their use for production of hybrids to address biofuel from sweet sorghum juice. For fresh stalk yield, DMS 28A from female group and Rio among males were promising while the former one was a good combiner for juice yield and grain yield also and these can be used in hybrid making to address ethanol production from juice without compro- mising on grain yields. For brix content, DMS 25A among female parents and Rio among males were potential parents and thus can be exploited for improving the brix content. It was gratifying to note that Rio had also exhibited signifi- cant and positive GCA effects for total sugar content, computed bioethanol and grain yields. It was observed that for various characters, high combiners were the male parents. In this study, the hybrids DMS 13A 9 Rio (for total biomass, fresh stalk yield, juice yield, juice extraction, total sugar content and computed bioethanol yields), DMS 23A 9 RS 647 (for total biomass, fresh stalk yield, total sugar content and computed bioethanol yields), DMS 13A 9 RSCN 5008 (for total biomass and brix content), DMS 13A 9 N 98, DMS 23A 9 Rio, DMS 25A 9 RSCN 5008 and DMS 28A 9 Rio (for grain yield) exhibited significantly higher SCA effects. It is evident that most of the hybrids promising for various traits were observed to be constituted from hybrids with both or one parent exhibiting significant GCA effects and produced hybrids with higher SCA effects. The significance of both GCA and SCA effects suggesting both additive and non-additive gene effects for grain yield was observed (Itai et al. 2009). The heterosis levels observed in this study could also explain the high biomass, juice and computed ethanol yields observed for hybrids as compared to the parents. Most of the hybrids which recorded significantly positive heterosis also recorded higher SCA effects. Parental selection for crop improvement programmes cannot be based on SCA effects alone, but in association with hybrid means and GCA effects of the parents involved (Marilia et al. 2001). It is prudent to consider only those hybrids between parents with positive and significant GCA effects because genetic gain is realized in the presence of sufficient additive variances. The interaction effect of SCA variance with that of environment was significant for all the characters except juice extraction. This implies that the environment significantly influenced the expression of non- additive gene effects. The observation of significant envi- ronmental influences on SCA effects is consistent with reports that genotype 9 environment interaction is impor- tant in sorghum (Panse and Sakhatme 1964; Chapman et al. 2000; Yu and Tuinstra 2001). Therefore, it is necessary to conduct multi-location testing for GCA and SCA to select the best parents and potential hybrids (Itai et al. 2010) before deploying specific sweet sorghum hybrids in dif- ferent environments for commercial cultivation which ultimately benefit the poor farmers of the semi-arid tropics. Conclusions The parents viz., DMS 28A among females for fresh stalk, juice and grain yields, Rio among males for fresh stalk yield were identified as promising for biomass traits as they have shown positive and significant GCA and in combi- nation large SCA effects. For quality traits, DMS 25A among female parents and Rio among males were found to be promising as potential parents for brix content while the latter had also exhibited significant and positive GCA effects for total sugar yield and computed bioethanol yields. All these parents can be used in sweet sorghum cultivar development programs to address ethanol pro- duction from juice without compromising on grain yields and offer solution to the ongoing food vs fuel debate. The study also demonstrated the significance of both additive and the non-additive type of gene actions for important traits. Most of the hybrids which recorded significantly positive heterosis also recorded higher SCA effects. Fur- ther the study identified the hybrids DMS 13A 9 Rio and DMS 23A 9 RS 647 with significantly higher SCA effects for sweet sorghum productivity traits. These hybrids would be recommended for further testing across many locations in the semi-arid target production environments for ethanol production. Acknowledgments Authors are grateful to the Indian Council of Agricultural Research for financial support and the staff at Rahuri and Coimbatore locations for their assistance in running the sweet sor- ghum trials. References Anonymous. 2004. 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