Legumes lend a hand

By Eammon Conaghan

Australia’s needy crops are involved in a subterranean arm wrestle with soil – for access to the phosphorus that underpins cell division and energy production during plant development.

So far, soil is winning and Australian dirt is becoming filthy rich, having banked up to $10 billion worth of phosphorus in compounds that cannot be used by most crop plants.

A GRDC-supported University of WA postgraduate researcher and Department of Agriculture collaborator, Mohammad Nuruzzaman, says some muscle from the legume family may help address the phosphorus inequity between soil and plant.

“Plant roots only take up phosphorus as water-soluble phosphate, which is unstable and reacts quickly with elements in the soil, such as iron, aluminium and calcium, to form stable, insoluble compounds, which are not absorbed by the roots of most crop species, including wheat,” he explains.

“However, it seems that some legumes may be equipped to take up existing soil-bound phosphorus, after trials showed that chickpea, field pea, faba bean and lentils responded less vigorously than wheat when phosphorus fertiliser is applied.”

Mr Nuruzzaman says this muted response is probably due to the ability of these legumes to extract phosphorus from the soil that corresponds to their lesser need for fertiliser. It is thought that carboxylates exuded by the legumes’ roots separates phosphorus from the bonds, which normally prevent its absorption by plants.

Collaborating on the GRDC project, which is supervised by UWA’s Professor Hans Lambers and Dr Erik Veneklaas, is ARC funded postgraduate researcher Stuart Pearse. Mr Pearse has found that phosphorus liberating legumes moderate carboxylate exudation – in cases where ample phosphorus fertiliser is applied.

Also working on the project, international postgraduate researcher Madeleine Wouterlood has discovered that this relationship is not so evident in the field, where rhizosphere carboxylate concentrations alter only slightly.

“This implies that there was no simple relationship between available phosphorus and root exudation rates,” she says. However, an observed increase in carboxylate concentrations during the phosphorus-hungry early stages of plant development, may indicate their connection with phosphorus extraction.

Although the relationship between carboxylate exudation and soil phosphorus levels is yet to be understood, Professor Lambers says that those legumes which exude the most carboxylates do improve phosphorus availability, for themselves and subsequent crops.

“Our study showed that wheat grew better after legumes than after wheat, demonstrating their residual benefit in promoting phosphorus uptake.”

Professor Lambers says faba bean, albus lupin and chickpea have been the most effective.

For more information:
Professor Hans Lambers, 08 6488 7381

GRDC Research Code: UWA 370, program 4