Oxidative responses of resistant and susceptible cereal leaves to symptomatic and nonsymptomatic cereal aphid (Hemiptera : Aphididae) feeding

The impact of the leaf-chlorosis-elicitiug Russian wheat aphid, Diuraphis n oxia (Mordvilko), and the nonchlorosis-eliciting bird cherry-oat aphid, Rho palosiphum padi (L.), feeding on D. noxia-susceptible and -resistant cereal s was examined during the period (i.e., 3, 6, and 9 d after aphid infestati on) that leaf chlorosis developed. After aphid number, leaf rolling and chl orosis ratings, and fresh leaf weight were recorded on each sampling date, total protein content, peroxidase, catalase, and polyphenol oxidase activit ies of each plant sample were determined spectrophotometrically. Although R , padi and D. noxia feeding caused significant increase of total protein co ntent in comparison with the control cereal leaves, the difference in total protein content between R. padi and D. noxia-infested leaves was not signi ficant. Although R. padi-feeding did not elicit any changes of peroxidase s pecific activity in any of the four cereals in comparison with the control leaves, D. noxia feeding elicited greater increases of peroxidase specific activity only on resistant 'Halt' wheat (Triticum aestivum L.) and suscepti ble 'Morex' barley (Hordeum vulgare L.), but not on susceptible 'Arapahoe' and resistant 'Border' oat (A:ena sativa L.). D. noxia-feeding elicited a n inefold increase in peroxidase specific activity on Morex barley and a thre efold on Halt wheat 9 d after the initial infestation in comparison with co ntrol leaves. Furthermore, D, noxia feeding did not elicit any differential changes of catalase and polyphenol oxidase activities in comparison with e ither R. padi feeding or control leaves. The findings suggest that D. noxia feeding probably results in oxidative stress in plants. Moderate increase of peroxidase activity (approximately threefold) in resistant Halt compared with susceptible Arapahoe wheat might have contributed to its resistance t o D. noxia, whereas the ninefold peroxidase activity increase may have poss ibly contributed to barley's susceptibility. Different enzymatic responses in wheat, barley, and oat to D. noxia and R, padi feeding indicate the cere als have different mechanisms of aphid resistance.