Structural aspects of cluster root development and their possible significance for nutrient acquisition in Grevillea robusta (Proteaceae)

Light microscopy (LM), scanning (SEM) and transmission electron microscopy (TEM) were used to study structure and function of cluster roots in Grevillea robusta . These roots were developed during growth of G. robusta seedlings in modified Hoagland's solution lacking phosphate. Cluster rootlets formed root hairs, basipetally, only after completing their determinate development. The rootlet hairs branched in two ways and some had apical swelling. Rootlets with hairs produced two different forms of exudate, one fibrous and the other globular in nature. The fibrous material appeared to be synthesised in the cortical cells. It is released by exocytosis from the epidermis. Rootlet hairs produced only fibrous exudate. They attached firmly to pieces of vermiculite. The significance of cluster roots is discussed within the context of patchy soil resources.