Ultra-structural studies were conducted on root nodules of Samanea saman (Jacq.) Merr. collected from trees growing under natural conditions. Nodules were distributed singly as well as in clusters on the main and lateral roots. Mature nodules were elongated, branched and coralloid. Root hair curling was found but infection threads could not be observed. Rhizobia entered through the epidermis and moved intercellularly through the cortical region. Mature nodules of S. saman could be differentiated into meristem, cortex, vascular tissue and bacteroid tissue. The latter showed both infected and non-infected cells mixed together. Vascular bundles were inversely collateral and distributed around the bacteroid tissue. The bacteroids were enclosed in peribacteroid membrane in groups and showed prominent granules of polyhydroxybutyrate in their cytoplasm. Mycorrhizal hyphae were also observed along with rhizobia in the bacteroid tissue. S. saman with dual rhizobial and mycorrhizal infection is a potential tree for plantation in arid soils of Pakistan.
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