Structural Aspects of Cluster Root Development and their Possible Significance for Nutrient Acquisition inGrevillea robusta(Proteaceae)

Abstract

Light microscopy (LM), scanning (SEM) and transmission electron microscopy (TEM) were used to study structure and function of cluster roots in Gre illea 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.# 1996 Annals of Botany Company

Cluster root development in Grevillea robusta (Proteaceae). I. Xylem, pericycle, cortex, and epidermis development in a determinate root

“…They occur in most members of the Proteaceae, a number of legumes, and in members of the Casuarinaceae, Myricaceae, Betulaceae and Moraceae, and are thought to be involved in phosphate acquisition (for references see Dinkelaker, Hengeler & Marschner (1995)). In Grevillea robusta they are produced at set distances along lateral roots (Skene et al, 1996), and are thought to improve the acquisition of phosphate and other nutrients (Dinkelaker et al, 1995). These roots are a useful system for studying root initiation and development.…”

“…Cluster rootlets are the definitive example. In G. robusta, the rootlet grows for a period, then the meristem differentiates and at this point root hairs are formed (Skene et al, 1996). Following this, exudation of organic acids begins (H. Marschner, pers.…”

“…seedlings (Provenance Loitokitok, Kenya Forestry Seed Centre) were grown in Vermiperl2 (medium grade from William Sinclair Hort. Ltd., Firth Road, Lincoln, UK)\Silvaperl2 (graded horticultural perlite from Silvaperl Ltd., Albion Works, Ropery Road, Gainsborough, Lincs., UK) (3 : 1), under controlled conditions (Skene et al, 1996). Following 16 wk of growth on vermiculite, plants were examined for the presence of cluster roots.…”

“…We have used both transverse and longitudinal serial sections in an attempt to build a true picture of events, as recommended by Clowes (1994). Cluster rootlets in Grevillea robusta are determinate (Purnell, 1960, Skene et al, 1996. There is a closed meristem, and while active, it appears to be composed of a single tier of cells (Fig.…”

“…This situation might therefore differ from that in Azolla (Gunning, 1978), where a reduction in numbers of plasmodesmata might gradually isolate the meristem (but see van Bel & Oparka (1995) on drawbacks of plasmodesmatal counts), but here, the root tip often abscises eventually. In the cluster root, rootlets remain physiologically active, in that a globular exudate is not released from the epidermis until determination (Skene et al, 1996). In old rootlets (30 d after initiation), cortical cells have few starch granules (Fig.…”