Cellular and Histological Changes Induced byPhytophthora cinnamomiin a Group of Plant Species Ranging from Fully Susceptible to Fully Resistant

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Abstract
The pattern of invasion and the histological changes in primary root tissues after infection by Phytophthora cinnamomi are described from the following species: Xanthorrhoea australis, X. resinosa, Themeda australis, Eucalyptus marginata, E. sieberi, and Acacia melanoxylon (susceptible); A. pulchella, E. calophylla, E. maculata, Gahnia radula, Juncus bufonius, Zea mays, and Triticum aestivum var. cappelle (resistant). Zoospores germinated on and penetrated the roots of all species, and lesions formed within 8-16 hr after invasion at 20-24.degree.C. Root growth ceased within 24 hr of inoculation but resumed within 48 hr in resistant species, usually from a lateral branch. In susceptible species, progressive symptom development included water soaking of tissues, lesion extension through the root to the hypocotyl, and root death. This was accompanied by wilting and chlorosis of the leaves, die-back of shoots, and plant death. Sporulation occurred between 24 and 72 hr after inoculation on all but the most resistant species. In resistant species, lesions were contained well before they extended to the hypocotyl. Deposition of phenolic materials, granulation of the cytoplasm, shrinkage of the protoplast, and cell wall distortion and disruption accompanied infection in all species. Lignification of cell walls, deposition of phenolics, and formation of callosic papillae were more commonly observed in the resistant species but occurred in some susceptible species. No specific anatomical feature or histological change was consistently associated with resistance. Although the species examined were classed as either resistant or susceptible on the basis of their field response, examination of the anatomical and histological changes that followed infection showed a gradient in which the fully susceptible and most resistant types represent the extremes.