Doctor of Philosophy / Department of Agronomy / P. V. Vara Prasad / Sweet sorghum (Sorghum bicolor L. Moench) is an important bioenergy crop. There is a wide array of genetic diversity in sweet sorghum germplasm collections. However, information on traits associated with sugar yield, optimum harvesting time for maximum sugar yield, effects of abiotic stresses on sugar yield is scarce. The objectives of the present study were: to identify traits that are associated with sugar yield, to determine the optimum harvesting time for maximum sugar yield and to understand the physiological responses of different sweet sorghum genotypes to drought and high temperature. In order to meet these objectives, five independent field and greenhouse studies were conducted. Field experiments were conducted using 280 sweet sorghum germplasm and were evaluated for 2 years. From this study, 30 genotypes representing high and low sugar yielders were selected for the subsequent experiment. We observed a significant variation in physiological, morphological and sugar yield traits associated with biofuel production. In the selection experiment, investigations on the morphological, physiological attributes helped to identify those characters which influence or limit sugar yield in the sweet sorghum. Another field study was conducted to optimize the harvesting time for obtaining highest sugar and juice yields in sweet sorghum. Sweet sorghum variety M81E was harvested at ten growth stages. Our results suggest that the optimum time for harvesting of sweet sorghum cultivar M81E is between milk and hard dough stages when highest sugar yield was observed. Studies on different levels of water stress were studied under greenhouse conditions. Four sweet sorghum genotypes (Awanlek, Smith, Tracy and Wray) were subjected to three water stress treatments (100% pot capacity (PC); 70% PC and 30% PC) for 20 days at early seed filling (Milk) stage. The results showed that genotypes differed significantly for all growth and yield, biochemical and physiological traits. Severe water stress significantly decreased juice and sugar yields by decreasing net photosynthetic rate, transpiration rate, stomatal conductance and sucrose content in the stem juice. Genotypes Tracy and Wray produced significantly highest brix, stem fresh weight, juice and sugar yield under both irrigated and water stress conditions. In another greenhouse study, we quantified the effects of drought, high temperature, and their combinations on growth, physiology and yield of sweet sorghum genotypes. The same four genotypes above were subjected to four treatments, T1 - control, T2 - drought stress, T3 - high temperature stress and T4 - combination of drought and high temperature for 16 days after anthesis. The result showed that significant difference was observed for growth and yield traits, physiological traits and non-reducing and total sugar content in juice for genotypes and treatments. Among the genotypes Tracy recorded higher juice and sugar yield. Among the various treatments, combination of drought and high temperature was found to be more deleterious in reducing most of the biofuel traits followed by drought and high temperature stress. The above studies gave significant findings with regards to the identification of superior sweet sorghum germplasm, their tolerance capacity to different abiotic stresses, which allows better selection for the use of bioenergy production.

Doctor of Philosophy / Department of Agronomy / Tesfaye Tesso / Sweet Sorghum [Sorghum bicolor (L.) Moench] is a type of cultivated sorghum grown primarily for its sugar-rich stalks. Because of its high fermentable sugar content, the crop is widely recognized as an alternative feedstock source for bio-fuel production. The extent to which stalk sugar accumulation occurs may be determined by several factors including the sink size. Grain is the most important sink in sorghum and other grain crops. Three experiments were conducted in this study to determine the extent to which the grain sink can reduce sugar accumulation in the stalks, to test and validate a genetic system that allows development of sterile sweet sorghum hybrids, and to assess the potential of sugar-rich hybrids to overcome stalk rot diseases. The first experiment, based on 22 sweet sorghum genotypes, was undertaken to study the effect of eliminating the grain sink (removing the head prior to anthesis) on stalk juice yield, sugar accumulation, and biomass. The data showed that the grain sink had a significant effect on all traits measured. Elimination of the grain sink significantly increased oBrix % (17.8%), dry biomass (27.8%), juice yield (23.9%), and total sugar yield (43.5%). The second experiment was aimed at validating the role of A3 genetic male sterility system for producing sterile sweet sorghum hybrids. Ten sweet sorghum pollinator lines of variable sugar content were selected among the entries included in the previous experiment. The lines were crossed to four A1 and A3 cytoplasmic male sterile (CMS) lines using a Design II mating scheme. The A3 females did not have effective restorers so that the hybrids were expected to be sterile. The parental lines and corresponding hybrids were evaluated for biomass production, oBrix, juice and sugar yield using a randomized complete block design. All A3 hybrids were sterile and did not produce seed when heads were covered prior to pollination. The effect of grain sink represented by the A1 vs. A3 CMS were highly significant for Brix%, biomass, juice, and sugar yield. Comparison of parents vs. crosses component was highly significant, indicating marked heterosis effect for the traits. Both general (GCA) and specific (SCA) combining ability effects were also significant for all traits, indicating the role of both additive and dominance genetic effects in the inheritance of the characters. Earlier studies have shown positive relationships between stalk sugar concentration and stalk rot disease resistance in sorghum. Thus, the objective of the third experiment was to study the effects of the CMS mediated differential accumulation of stalk sugar on severity of charcoal rot disease caused by Macrophomina phaseolina. The experiment provided an opportunity to test the effect of variable stalk sugar in the same genetic backgrounds. The data indicated that hybrids produced from A3 cytoplasm were more resistant to charcoal rot (7.1cm lesion length) compared to those produced from the A1 hybrids (9.5 cm lesion length). The enhanced resistance of hybrids with higher sugar yield could have significant agronomic advantage in sugar based bio-fuel feedstock production.