Phytochemical Changes during Recurrent Selection for Resistance to the European Corn Borer
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Feeding performance of herbivorous insects is influenced by host plant nutritional quality which can be Improved for insect resistance by artificial selection. This study was conducted to determine which biochemical constituents in maize (Zea mays L.) change during recurrent selection for resistance to first-and second-generation European corn borer (ECB) [Ostrinia nubilalis (Hubner)]. Four cycles of selection (C0, C2, C4, and C5) from the BS9 population were field grown, artificially and naturally infested with ECB, and the following tissues sampled for biochemical analysis: immature and mature leaf blade, leaf sheath, rind, node, and pith, Tissue was analyzed for percent protein, DIMBOA [2,4-dihydroxy-7-methoxy 2H-1,4-benzoxazin-3 (4H)-one], fiber, and cell wall-bound phenolics, which included p-coumaric acid (pCA), ferulic acid (FA), cyclobutane dimers (CBD), and diferulic acid (DFA). Leaf and stalk toughness were also determined and showed significant increases over cycles of selection. Protein content was lowest in stalk. tissues with advanced cycles having Lower levels, but Leaf protein content did not differ significantly, DFA reached high levels in the rind (0.85 mg/g) and leaf sheath (1.35 mg/g) tissues, and increased significantly in immature leaf tissue (0.55-1.02 mg/g) over cycles of selection and may serve to fortify tender whorl tissue. Number of tunnels per stalk was negatively correlated,vith DFA content in the pith (r = -77, P = 0.02). Microspectrophotometer determinations of epidermal cell wall absorbance for leaf blade and rind tissue showed increased absorbances (23 and 27%, respectively) in the spectral region characteristic of phenolic acids over cycles of selection. Phenolic acids, in particular DFA, have increased over cycles of selection to render maize tissue more resistant through fortification of cell walls, especially in leaf and rind epidermal tissue.
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