Morphological characterization and identification of molecular markers for dwarfism genes in Sorghum bicolor L. [Moench]

English: Sorghum is a grain crop with enormous potential in meeting the nutritional needs of people in developing countries, as well as optimising current meat and poultry production in the rest of the world. Sorghum has been overlooked in recent years and is not utilised to its full potential, especially in African countries. The conversion of exotic lines in terms of height and maturity holds great potential for the development of improved varieties. The genes controlling height or dwarfism (dw) have also been found to enhance drought tolerance making it an important trait to be incorporated for sorghum improvement in semi-arid areas. However, little information is available on the genetic and physiological basis of height. In this study, near-isogenic lines developed for different height classes were characterised (Chapter 2). Significant differences were observed in plant height, internode length and flag leaf length between different height classes as previously hypothesised. However, the current study also identified significant differences in flag leaf width, panicle weight and number of panicle branches between the different height classes, in contrast to previous reports comparing only two height classes. Furthermore, this is the first report that plant height is correlated to flag leaf length, flag leaf width, panicle weight, leaf sheath length and panicle length. The exogenous application of gibberellic acid (GA) did not result in a final height increase, as expected from previous reports (Chapter 3). However, it was observed that GA application may affected the growth response in the different height classes. This suggests that dwarf genes probably do not affect gibberellic acid biosynthesis but rather may act as a negative regulator in response to GA and that other genes, possibly the maturity genes determine the response to GA. This is the first report of using SSRs on near-isogenic lines for different height classes to identify specific dwarfism genes (Chapter 4). Potential markers were identified for the dW1, dW2 and DW3 genes in sorghum. These markers also allowed the deduction of genotypes for the different height classes. Linkage group A was confirmed to contain loci affecting plant height. These markers will facilitate sorghum breeding and reduce the time needed for line conversion. The AFLP technique was effective in identifying a total of 164 possible markers for the different dwarfism genes (Chapter 5). Markers were identified for recessive and dominant dwarfism alleles at dW1, dW2 as well as dW3. Furthermore, the genetic distance observed between the different near-isogenic height classes and the parental lines indicate that a bias exists in the retention of an unexpectedly large donor segment, possibly introduced through the selection of height and maturity characteristics. Future research resulting from the present study includes further investigation of the interaction between the dwarfism and maturity genes, as well as the verification of SSR and AFLP markers in genetic material with confirmed genotypes in order to incorporate these markers into international breeding programmes. The use of these markers in conversion programmes will allow more cost effective and accelerated sorghum improvement.