Australia: The Land Where Time Began

A biography of the Australian continent 

These are specialised roots in which there are densely packed, hairy rootlets that occur in longitudinal rows along parent lateral foots. The surface area of the root hairs comprise up to 90 % of the entire surface of a proteoid root, but the volume is only about 10 % of that of the entire root. This root type, that are characteristic of Proteaceae, carries out the same function as the mycorrhizal symbionts in most other plants. They are only present during the wet season, developing rapidly, but are short-lived. As the soil dries, the hairs lengthen, crossing gaps between soil particles. They produce mucigels that block the gaps between soil particles that have multiple functions, maintaining connection with soil moisture, and providing a substrate for the maintenance of the soil microbes that solubilise the minerals. Evidence has been found that the induction of proteoid root development is dependent on a co-factor produced by the bacteria, but the mechanism had still to be worked out at the time of writing. The production of extra long root hairs increases rapidly when nutritional levels drop, especially phosphorus.

It has been found that in a 9-month-old Leucadendron laureolum (from South Africa) growing in sand 40 % the root mass was of proteoid type, with an increase in total surface area of up to 15 times. A large increase in chemical exchange results from greatly increased surface area of ultimate roots, a proliferation of fine rootlets and root hairs leading to greatly increased nutrient uptake. Potassium uptake is increased by about 13 times that in normal roots have been recorded, and it is at least twice as much as in normal roots. It is energetically expensive to take up nutrients, phosphorous is preferentially moved to young proteoid roots, where there is also some inorganic phosphate storage, that is available for later mobilisation for shoot growth. Some water is exuded from the surface of proteoid roots at night.

Plants such as Viminaria juncei (a legume) have adopted proteoid roots as a means of coping with heavier soils that are waterlogged in winter. They also produce pneumatophores (aerating tissues), as are produced by mangroves, to increase oxygen supply to the roots. The pneumatophores are connected to aerenchyma (aereating tissue) that occurs in the lower parts of the stem, the roots and the root nodules. This allows the legume to continue fixing nitrogen even when the soil is waterlogged.

Sources & Further reading

1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003