Cluster roots

Root clusters are an alternative to mycorrhizae for acquiring nutrients in impoverished soils. These have been defined as densely packed groupings of determinate rootlets which develop, grow, then cease growth and undergo a synchronous burst which liberates an exudation of organic acids. These exudations last for about two days but in this time achieve a localized reduction in soil pH creating an increase in the pool of phosphate and iron available for plant uptake (Skene, 1998, 2003). Cluster roots were originally observed in the Proteaceae where they occur in almost all species and were thought to be a particular characteristic of this family; they were therefore termed proteoid roots. They are now known to be much more widespread both taxonomically and geographically and are particularly prevalent in the non-mycorrhizal plants that inhabit nutrient-deficient soils as in the South African Fynbos and Australia. Recently, they have been shown to occur on temperate species (Fig. 3.37) such as lupin (Lupinus albus) (Hagstrom etal, 2001), bog myrtle (Myrica gale), alder (Alnus incana, A. rubra, A. glutinosa), and sea buckthorn (Hippophae rhamnoides). The spatial clustering of the roots and the synchrony in their development and the large exudative burst are remarkable and have been suggested as analogous to a cavalry charge, where a concentrated and sudden attack prevents bacteria from metabolizing the exudate completely before it can enhance root nutrient uptake (Skene, 1998).

In some Australian sites there appears to be a definitive association in habitats rich in Proteaceae between zones of root proliferation and ferricrete layers in lateritic soils. This has led to the intriguing hypothesis that certain Australian lateritic and related oligotrophic soils may have been partly derived biologically from soluble iron-rich complexes generated following secretion of low-molecular weight organic acids by phosphate-absorbing specialized cluster roots of proteaceous plants (Pate et al., 2001). Subsequent precipitation of the iron is postulated as leading to development of laterites after consumption of the organic components of the complexes by soil bacteria.

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