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Résumé :
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Salivary sheath distribution of the Russian wheat aphid, Diuraphis noxia (Mordvilko), on 'Arapahoe' (susceptible) and 'Halt' (resistant) wheat, Triticum aestivum L., leaves were compared. D. noxia produced both single and multiple branched salivary sheaths on resistant and susceptible wheat leaves. Three types of salivary sheath initiation sites (i.e., interor intracellularly on epidermal cells, or through leaf stomata) and sheath termination sites (i.e., mesophyll, vascular tissue, or both) in susceptible and resistant wheat leaves also sere recorded. Although the number of aphids on the leaves before staining was not significantly different, Arapahoe seedlings showed higher leaf rolling and chlorosis ratings than Halt. The majority of aphid salivary sheaths was on the rolled leaves within chlorotic streaks on Arapahoe plants, whereas on Halt leaves the majority of the sheaths was surrounded by chlorotic spots. Salivary sheaths also were found without showing chlorotic symptoms on Arapahoe and Halt plants. D. noxia produced more salivary sheaths through leaf stomata on Halt than on Arapahoe, although the total number of salivary sheaths on Arapahoe was not different from halt. The number of salivary sheaths in either mesophyll or vascular tissues was not different between the 2 wheat cultivars, but more salivary sheaths terminated in vascular bundles than in mesophyll tissues in both Arapahoe and Halt. Although the aphids produced single and multiple branches of salivary sheaths on the 2 cultivars, a higher percentage of the multiply branched sheaths terminated in both vascular and mesophyll tissues on Arapahoe than on Halt. The salivary sheaths on Halt also were longer than on Arapahoe in both mesophyll and vascular tissues. The sheaths terminating in vascular bundles were longer than the sheaths terminating in mesophyll tissues in both cultivars. Implications of this study in relation to understanding the dynamic nature of aphid damage symptom development processes were discussed.
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