Heterosis Studies for Ethanol Yield and Its Related Traits in F 1 Hybrids of Sweet Sorghum [ Sorghum bicolor (L.) Moench]

Sweet sorghum [ Sorghum bicolor (L.) Moench], a specific type of sorghum, has been considered a potentially valuable source for biofuel production because of its high energy convertion efficiency. Many characteristics such as green stalk yield, stalk sugar content, stalk juice extractability and grain yield have been proved as major contributors to its economic superiority. Although heterosis is well established in grain and forage sorghum [ Sorghum bicolor (L.) Moench], reports of heterosis in sweet sorghum are limited to results from grain sorghum × sweet sorghum hybrids. Thirty hybrids, derived from a line × tester trial, were evaluated in kharif season during 2010 along with their parents and the checks. Heterosis over mid parent, better parent and standard checks viz., CSH 22SS, PAC 52093 and NSSV 13 was studied for bioethanol yield and its component characters. Based on standard heterosis the hybrids viz., NSS 1007A × CSV 19SS and RS 1220A × SSV 74 are found highly suitable for heterosis breeding as they exhibited significant positive standard heterosis for ethanol yield and its contributing traits.

Growing sweet sorghum hybrid with high cane juice, sweetness and the total fermentable sugars is considered to be a highly efficient in producing ethanol. In USA, the sorghum production has tripled since the adoption of hybrid sorghum cultivars and exploitation of hybrid vigour in conjunction with intensive management practices. The heterosis or hybrid vigour is the expressions of the F1 hybrid over its parents. Heterosis in sorghum was first observed in 1927, but commercial exploitation was not possible until the discovery of cytoplasmic genetic male sterility system by Stephens and Holland in 1954. The substantial magnitude of standard heterosis for all the traits related to ethanol production (plant height: up to 46.9%, stem girth: up to 5.3%, total soluble solids (%): up to 7.4%, millable stalk yield: up to 1.5% and extractable juice yield: up to 122.6%) further supports breeding for heterosis for genetic enhancement of sweet sorghum (Sankarapandian et al. 1994). However, for quantitative traits like total biomass, fresh stalk yield, juice yield and sugar yield the progress has been *Corresponding author email: rani.chapara@gmail.com hindered due to lack of understanding in the inheritance and complex relationship among themselves. Most of the sweet sorghum varieties mature between 115 and 125 days during rainy season. Stalks can be harvested along with grain. The green cane yield varies from 30 to 50 tons ha-1, and grain yield from 0.8 to 2.0 tons ha-1 with a brix value of 16 to 20%. Sweet sorghum varieties and hybrids bred at NRCS, India has potential to produce biomass up to 48 tons ha-1 and 1.5 to 2.9 tons ha-1 grain with brix value of 14 to 18%. The juicy stalks of sweet sorghum can be used for preparation of syrup, jaggery and fuel grade ethanol. Sweet sorghum has ability to yield 40-45 tons ha-1 millable cane and 1-1.5 tons ha-1 grain, and an average brix of 18.4%. The juice has a minimum of 12% sucrose and at least 15% total fermentable sugar (Ratnavathi, C. V. 2008.). Ethanol, the finished product of fermentation has high commercial value. It is a "clean burning fuel" with high octane rating and the existing automobile engines can be operated with petrol blended with 20% ethanol (80% petrol) without engine modifications.
At present, very few hybrids of sweet sorghum are released. In 2005, ICRISAT (India) recommended 8 pure lines sweet sorghum to public namely NTJ2, SPV422 (ICSV574), SPV1411, ICSR93034, ICSV93046, ICSV700, S35, and E36-1. In 2008, it was reported that two pure lines, SSV84 and CSV19 and one hybrid, CSH 22 (NSSH104) were used in the research on a potential energy crop for bio-fuel production in India. Hence, to choose the high potential sweet sorghum for growing, the objective of this research was to estimate the extent and nature of heterosis among F1 hybrids for ethanol yield and its related traits.

Material and Methods
Thirty F1 hybrids were produced by crossing five lines with six testers in a L × T mating design during rabi 2009. These hybrids along with there parents were planted in a randomized complete block design (RCBD) in three replications during kharif at experimental farm, Rajendranagar, Hyderabad, Mahatma Phule Krishi Vidyapeeth, Rahuri and Centre for Plant Breeding and Genetics farm, Coimbatore. Each entry was sown in two rows of 4m length with a spacing of 60 cm between rows and 15 cm between plants. Five competitive plants were selected at random from each replication for recording the observations on brix per cent (using an Atago PAL-1 digital hand -held pocket refractometer with automatic temperature compensation ranging from 0 to 50oC at the hard dough stage).
At physiological maturity stage, total biomass were recorded on selected plants by weighing leaves, stems and panicles in kilograms while fresh stalk yield were recorded by removing panicles and leaves along with sheath and later converted to t ha-1. Further, juice extraction was done on an electrically operated three-roller stalk crusher with a minimum of three passings of the selected stalk measured in kilograms and later converted into litres to get juice yield in L ha-1. The total soluble solids and ethanol yield were calculated by using formula given by Corleto and Cazzato (1997), as reported by Reddy et al. (2005). Ethanol yield (L ha-1) = [Total sugar yield (t ha-1) / 5.68] × 3.78 × 1000 × 0.8

Statistical Analysis
The analysis was done using INDOSTAT software. The error variances in the trials conducted in three locations were homogeneous, as revealed by Bartlett's test (Bartlett 1937), providing statistical validity to carry out combined ANOVA.

Estimation of Heterosis:
Heterosis of F1 over mid parent (MP) and better parent (BP) were calculated as per Turner (1953) and Hayes et al. (1955).

i. Mid Parental Heterosis / Relative Heterosis (RH)
Heterosis over mid parental value (HMP) was estimated using the following formula. Estimates of heterosis were tested for significance at error degrees of freedom as suggested by Turner (1953
Higher level heterosis in a cross always represents genetically more diverse parents than those crosses, which show little or no heterosis.
From the results, an appreciable level of heterosis over standard checks and better parent was evident for the characters under study. The hybrids, viz., NSS 8A × CSV 19SS and NSS 10A × CSV 19SS for total biomass and fresh stalk yield; NSS 8A × SSV 74 for total biomass, fresh stalk yield and juice extraction per cent; NSS 1007A × CSV 19SS for fresh stalk yield, juice yield, juice extraction per cent and ethanol yield; RS 1220A × RSSV 120 for total biomass, fresh stalk yield and juice extraction per cent and RS 1220A × SSV 74 for fresh stalk yield, juice yield and ethanol yield exhibited significant positive standard heterosis over at least one check. The hybrid, NSS 1007A × CSV 19SS could be suggested for commercial exploitation of heterosis as it exhibited significant and positive standard heterosis for ethanol yield and its contributing traits. In the present investigation, the hybrids viz., RS 1220A × SSV 84 for total soluble solids, NSS10A × CSV19SS and RS 1220A × RSSV 120 for total biomass, RS 1220A × SSV 74 and NSS 8A × CSV19SS for fresh stalk