ComBio 2003: The hard truth about salt resistance

'Durum' means 'hard' in Latin -- but for wheatgrowers who have tried to grow high-value, high-protein durum wheats in the semi-arid regions of Victoria, SA an WA semi-arid zone, durum wheats are in the too-hard basket.

Used in noodles and pasta, durum wheats command premium prices in export market, typically $20 to $40 per tonne more than prime hard bread wheats. Unfortunately, durum wheats have poor salt tolerance, and often sulk in the mallee wheatlands because of seasonal subsoil salinity.

Plant physiologist Dr Rana Munns told ComBio 2003 that CSIRO Plant Industry breeders have developed a durum wheat that rivals bread wheat in salt tolerance. Unfortunately, it was consigned to the incinerator when wheat streak virus turned up in CSIRO Plant Industry's Black Mountain glasshouses and trial plots early this year.

The variety was developed from seed of a salt-tolerant land race grown in Persia (now Iran) several centuries ago. CSIRO geneticists discovered it when screening 70 land races chosen at random from thousands of wheats stored in the Australian Winter Cereal Collection in Tamworth.

Bread wheats are hexaploids, endowed with full complements of chromosomes from three different wild ancestors; durum wheats lack the D genome, from goat grass, which confers salt tolerance.

The CSIRO team screens seedlings for salt tolerance by adding salt to the growth medium, then sampling a single leaf blade 10 days later to measure its sodium level.

After an initial false lead -- a DNA probe revealed a promising carthlicum durum wheat with high salt tolerance as a misidentified bread wheat -- the CSIRO team found three salt-tolerant durum land races, the best of which came from a region in old Persia known to have high levels of subsoil salt.

Munns said current Australian durum wheats excludes only 95 per cent of dissolved salt in their root zone; the new durum line, re-created by CSIRO breeders after the wheat streak episode, rejects 99 per cent of dissolved salts, comparable with highly tolerant bread wheat.

Molecular geneticist Dr Evans Lagudah subsequently found a DNA marker so tightly linked to the salt-tolerance trait that it recovers 100 per cent of salt-tolerant seedlings.

Munns says the first-generation cross grows to nearly 2 metres; the next task is to install the dwarfing gene present in all modern high-yielding varieties.

Transient subsoil salinity has been identified as a serious problem in the mallee wheatlands relatively recently. The mallee lands normally receive 250-400mm of rain annually, most of it in autumn and winter.

Munns said subsoil salt remains below the root zone, but when the water table rises with winter rain, it approaches the surface. As the crop pumps the water down again in spring and early summer, the excluded salt concentrates in the root zone, where it is washed deeper when rain returns after summer.

"If you're trying to grow durum wheat in salt-affected areas, salt is not the only problem," says Munns. "You're going to get drought, boron toxicityand acidity, so a new durum wheat with salt tolerance alone isn't going to make a huge difference.

"But using DNA markers, we can 'pyramid' multiple traits much faster and more cheaply than we could using conventional breeding and selection methods."

Munns said CSIRO will multiply its salt-tolerant breeding lines this summer. "We'll definitely have seed ready to test at a larger number of sites next year."