Studies on genetic diversity, genotype by environment interaction, combining ability and farmers’ perception on sweet sorghum (sorghum bicolor (l.) Moench)

Sweet sorghum (Sorghum bicolor (L.) Moench) is a cultivated sorghum recognized as potential alternative source of bio-fuel due to its high fermentable sugar content in the stalk. Sweet sorghum has elicited the interest of breeders due to its capacity to provide renewable energy products such as biofuel, industrial commodities, food and animal feed. The constraints for its large scale cultivation are the limited availability of genotypes suited to different agro-climatic conditions. There is limited information about the combining ability, gene action and genetic effects on stem sugar and biomass traits which is required in formulating appropriate strategies for developing super performing sweet sorghum varieties. Sweet sorghum gene pool creation has also not received much attention mainly because it is not considered to be among important crops in Kenya, and the pedigree information is incomplete. Furthermore, there is also lack of information about the perceptions of resource-limited, small-scale farmers on the potential of sweet sorghum and feasibility of its utilization in Kenya. Therefore, the objectives of this study were to: (i) determine farmers perceptions on sweet sorghum and potential of its utilization in Kenya (ii) investigate the influence of genotype by environment interaction on sugar and biomass production of sweet sorghum (iii) assess genetic diversity and relationship among a collection of sweet sorghum germplasm by simple sequence repeats (SSR) markers and (iv) determine the combining ability in respect of stem sugar and biomass traits in sweet sorghum. A survey was undertaken in Western and Coastal regions of Kenya to determine farmers’ perception on sweet sorghum and feasibility of its utilization. Descriptive and inferential statistical tests were used to analyze the data. It was observed that 72.9% of the respondents were male, 95.7% married and 46% had formal education. Most of the farmers (73.0%) cultivated below 2 acres of farmland. About 40% of the respondents were aware of existence of sweet sorghum varieties while 50% of them were aware of sweet sorghum processing technologies. The study revealed that farmers appreciate the potential of sweet sorghum and existence of capacity for its exploitation. To assess of genetic diversity and relationship among a collection of sweet sorghum germplasm using simple sequence repeats (SSR) markers, eighty six sweet sorghum cultivars from Argentina, Brazil, Kenya (ICRISAT and Moi University), United States of America and Zambia were genotyped with 11 SSR markers that generated 86 alleles with an average of 8 alleles per locus. Polymorphism information content (PIC) value was 0.53 indicating a moderate diversity with a range of 0.09–0.89. The variability among the populations was low at 3 % but amounted to 22% and 75 % within individual genotypes and among individuals respectively. Clustering analysis based on the genetic similarity (GS) grouped the 86 sweet sorghum genotypes into 2 distinct clusters. The study also revealed the genetic relationship of cultivars with unknown parentage to those with known parentage. Information generated from this study can be exploited to select parents for hybrid development to maximize sugar content and total biomass and for development of segregating populations to map genes controlling sugar content in sweet sorghum. To investigate the influence of genotype by environment interaction on sugar and biomass production of sweet sorghum, field experiments were conducted to evaluate sweet sorghum genotypes in Western Kenya during the 2011, 2012 and 2013 rainy season of April to August at Alupe, Kibos, Homa Bay and Spectre International farm. The materials used in the study consisted of sixteen sweet sorghum genotypes and two sorghum genotypes sourced from ICRISAT and KARI. The treatments were laid out in a Randomized Complete Block Design (RCBD) and replicated three times. Data were collected on sorghum traits in accordance with the procedure outlined in the ICRISAT sorghum descriptor. The study revealed that genotype by environment interaction had significant influence on most of the traits. This indicates that selection for plant height, girth, brix juice, juice volume and stalks weigh cannot be carried out across the four environments, suggesting that selection for these traits have to be carried separately in each of the four environments. High performance demonstrated by genotypes IESV 93046 and IS2331 for stem brix and stem biomass shows their potential for exploitation for ethanol production. On the combining ability study, an investigation was carried out to assess the combining ability and nature of gene action in respect of sugar yield and its attributing traits in 25 new hybrids of sweet sorghum developed by crossing five (5) high sugar lines with five (5) low sugar lines in a North Carolina II mating design and grown in alpha lattice with two replications during long rains of April to July 2014 in western Kenya. The variance among the lines in respect of their general combining ability (GCA) was highly significant for Brix and plant height at 90 days. Specific combining ability (SCA) variance was relatively higher in magnitude for grain weight and plant height indicating predominance of non-additive gene action in the genetic control of these traits. GADAM, MALON and PAISANO among the females and IESV93036, IS2331 and NTJ 2 among males were identified as good general combiners indicating their ability in transmitting additive genes in the desirable direction to their progenies. The best hybrids for total biomass and total sugar content were GADAMxIESV93036, GADAMxIS2331 and MALONxIS2331 and after adequate testing in many locations across the target production environments, these hybrids can be recommended for commercial exploitation for ethanol production. Overall, the study showed that development of sweet sorghum cultivars and hybrids is feasible and genotypes identified as potential cultivars can be exploited for ethanol production.