4. Discussion:Sweet sorghum (Sorghum biocolor (L.) Moench) showed high radiation use efficiency (Curt et al., 1998), high biomass and sugar yields (Gnansounou et al., 2005, Mamma et al., 1995), low N fertilizer rate and irrigation water use (Evans JM, Cohen 2009), wide adaptability, and tolerance to drought and salinity (Yu et al.
, 2008). Various studies have been carried out on the energy potential of sweet sorghum and recognized the crop as one of the most potential feed stocks for biofuels (Gnansounou et al., 2005, Yu et al., 2008, Zhao et al., 2008, Liu et al., 2008, Mei et al., 2008).It is important that sweet sorghum has a long enough harvest periods for bioprocessing of carbohydrates to biofuel. In a previous study, we found that the variability for the production of ethanol by various sweet sorghum genotypes in a laboratory fermentor. Five Sweet Sorghum (Sorghum bicolor) L. Moench) genotypes were evaluated for ethanol production from stalk juice (Keller, SSV84, Wray, NSSH 104 and BJ 248) (Ratnavathi et al., 2010). In the present study, eight genotypes of sweet sorghum have been investigated with respect to sugar composition (% Brix, % total sugars, reducing sugars and sucrose, data not shown) and sugar accumulation in four different crop growth stages viz before flowering, milky, physiological maturity and maturity periods in three seasons viz.
, kharif, rabi and summer during 2011 and 2012.The various characters representing sugar composition, brix %, total soluble sugars, reducing sugars and sucrose were initiated after flowering in all the genotypes. They are very low at before flowering stage in all genotypes. Usually in sweet sorghum the sugar synthesis starts at flowering stage of the plant.
The increase in sugar content was compared in all types of sorghum. In sweet sorghum genotypes the increase in overall sugar composition and individual sugar composition were relatively higher compared to forage and grain sorghum genotypes up to physiological maturity stage and the sugar content increased at maturity stage in forage and grain sorghums. Qazi et al. (2012) pointed out that contribution of varieties, stage, and internode position was significant for the variation in sugar content. Recent reports suggesting that Wray showed a highly significant increase in Brix percentages between anthesis and physiological maturity, indicating increased carbohydrate partitioning to the stem. Overall, on a per plant basis, Wray exhibited an approximately 24-fold greater abundance of stem solutes than Macia (Bihmidine et al., 2015).Initially reducing sugars are high at before flowering stage and again they decreased till physiological maturity and increased at maturity stage in all types of sorghums. This shows that the sucrose synthesis and accumulation continues up to physiological maturity stage in all kinds of sorghum, however, sweet sorghum has accumulated more sucrose at physiological maturity stage. In general, hexoses favor cell division and expansion, whereas Sucrose favors differentiation and maturation (Wobus and Weber, 1999; Weschke et al., 2003; Borisjuk et al., 2003). In the maturing storage of most plants, division and expansion are very little. Therefore, in matured sweet sorghum stems, the high ratio of Suc/hexoses is used for differentiation and maturation (Liu yang et al., 2013).Reduction in the reducing sugars at physiological maturity stage showed that accumulation of sucrose reaches maximum at physiological maturity stage in sweet sorghum. In an earlier study by Ratnavathi et al (2005), the increase in sucrose observed from milky stage of the grain to grain maturity stage. This indicated that stable sucrose in sweet sorghum is available at grain maturity stage. In maturity stage, the physical activity decreases, metabolism slows down with less demand for hexose. Sucrose in the upper internode is used to store more rather than broken down into hexose.Significant interactions between variety and internode position were seen for total sugar accumulation at the pre-panicle emergence and grain filling stages. Experiments showed that reducing sugars constituted most of the total sugars accumulated in sweet sorghum at the first internode during pre-panicle emergence stage and grain filling stages, which could be required for sustaining growth of the panicle, as has been reported in wheat and barley (Xue et al., 2008). The two enzymes sucrose phosphate synthase were studied in leaf, stem and juice tissue of sorghum genotypes at different crop growth stages whereas the enzyme variability among genotypes was studied in 12 genotypes of which 8 genotypes are sweet sorghum, 2 genotypes each are from forage and grain sorghum. Leaf tissue contained less SuSy activity as compared to the stem and juice in all stages and all genotypes. The synthetic enzyme activities at all stages of the crop was correlated with the respective brix, total sugars, reducing sugars and sucrose content to see the association of these two characters and data not shown. The sucrose synthase activity in stem tissue is significantly correlated to the brix, total sugar content and sucrose in the juice. The SuSy activity in juice tissue is highly significant to the brix, total sugars and sucrose content of the juice. The SPS activity in stem tissue is significantly correlated to the brix, total sugars and sucrose in the juice where as the SPS activity in juice is highly significant to the brix, total sugars and sucrose content of the juice.Sucrose accumulation is regulated by the sequential action of sucrose phosphate synthase (SPS) and SUSY (Hatch et al. 1963), and the roles of these two enzymes during various developmental stages of sugarcane are well established. Verma et al., 2011 reported that SPS activity is high in high sugar cultivars compared to low sugar cultivars at all developmental stages. SPS activity was positively correlated with sucrose and negatively correlated with hexose sugars. However, SUSY activity was negatively correlated with sucrose and positively correlated with hexose sugars. This showed that the sucrose synthase and sucrose phosphate synthase activity are a biochemical marker for sucrose accumulation. In Sugarcane study by You-Qiang Pan (2009) also the sucrose synthase was positively correlated with sugar content. There are many other studies on sucrose accumulation in other crops. Since sugarcane and sorghum belong to the same gramineae family and sucrose accumulation is compared with sugarcane. The SuSy and SPS activities of leaf tissue are positively correlated to the juice parameters except reducing sugars. Reducing sugars are negatively correlated to the synthetic enzymes of all three tissues. The first report of differential expression profiling of SPS, SuSy and SAI in intergeneric hybrids involving sugarcane and sorghum opens the possibility for production of novel hybrids with improved sucrose content and with early maturity. This study describes the positive association of SPS and SuSy and negative association of SAI on sucrose accumulation (Ramalashmi et al.,2014). Ghate et el.,(2016) reported that sugar and starch content, activities of sugar metabolism enzymes and levels of their expression were studied in the 3rd (source) leaf from panicle and the 5th (sugar storing) internode of the three lines, in irrigated plants and in plants exposed to a brief drought exposure at the panicle emergence stage.However in sweet sorghum, though carbon is partitioned with panicle formation, sucrose synthesis seems to be very active after grain physiological maturity as the sugars are retranslocated to stem for sucrose accumulation. The potential of sucrose accumulation per unit time in sweet sorghum was much higher as compared to the accumulation rate in sugarcane. However studies for high sucrose accumulating genotype are more required for making this crop a successful bio-energy crop. For engineering sorghum stem composition for improved conversion to biofuels and bio-products, the stem transcription profile resource and the genes and regulatory dynamics were identified by Makline (2016). Hiroshi et al., (2016) reported that transcriptome profiling by RNA-seq, categorization using phylogenetic trees, analysis of chromosomal synteny, and comparison of amino acid sequences between SIL-05 (a sweet sorghum) and BTx623 (a grain sorghum) helped in elucidating the expression of key SWEET (sugars will eventually be exported transporters) genes involved in sucrose accumulation of sorghum. It is evident that performance of sweet sorghum is attributed to both genetic make-up and environment (Rono et al., 2016).
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