Soluble and cell wall bound phenolic-mediated insect resistance in corn and sorghum

This thesis examined the role of phenolic substances in resistance to insect pests of maize and sorghum. Eight landraces of sorghum collected in Ethiopia were assessed for methanol soluble phenolic content (as catechin equivalents) by the Prussian blue procedure and storage insect resistance in standardized tests with Sitophilus oryzae and were found to show significant variation by genotype. Significant inverse linear relationships were also found between resistance parameters such as weight loss of grain, the Dobie index of susceptibility, number of eggs laid and progeny emerged and the phenolic content of the grain (r2 = 0.85, 0.55, 0.46 and 0.52 respectively). The results suggest that the soluble phenolic content, which has been shown to consist primarily of proanthocyanidins, can be used as a leading indicator of resistance.; Host plant resistance in maize, Zea mays L. to the southwestern corn borer (SWCE), Diatraea grandiosella, and the sugarcane borer (SCB), Diatraea saccharalis, was investigated in relation to bound phenolics in the leaf cell wall.{09}A mapping population was developed from CML131 x CML 67 as the susceptible and resistant parents, respectively. Recombinant inbred lines were rated for leaf feeding damage after artificial infestation. Leaf tissue toughness, fiber content and gravimetric soluble metabolites (GSM) were quantified. A significant negative correlation was found between tissue toughness and leaf feeding damage (r = −0.37, P = 0.001) with the SCB in 1997A. Fiber and GSM were not significantly correlated to leaf feeding damage. Cell wall bound phenolics were investigated as possible mechanisms for elevated levels of tissue toughness. Both cell wall bound p-coumaric acid and ferulic acid were significantly and negatively correlated to leaf feeding damage. Diferulic acid (DFA), which cross-links cell wall arabinoxylans was found to be significantly correlated to tissue toughness in 1997A. Also, significant negative correlations were found between DFA and leaf feeding damage for both insects in 1996B and 1997A. Leaf nitrogen levels were investigated as a nutritional basis for resistance. Nitrogen levels were positively correlated with leaf feeding damage, lower levels of nitrogen resulted in reduced feeding. Approximately 40% of the variation in leaf feeding damage for SCB could be accounted for by nitrogen, DFA and ferulic acid content in leaf tissue.; Putative phytochemical traits for insect resistance were further investigated with quantitative traits loci (QTL) mapping. Five out of eight QTLs for 5-5 ′ DFA were located at the same position or close to QTLs for SWCB and SCB leaf feeding damage. Eight QTLs for 5-5′ DFA could explain 44% of the phenotypic variation for this dimer. QTLs for p-coumaric acid and ferulic acid were also associated with QTLs for leaf feeding damage. QTLs for cell wall bound phenolics linked with QTLs for leaf tissue toughness in four chromosome locations. (Abstract shortened by UMI.)...