Until the early 1990s, low "falling number" and poor starch viscosity in wheat were invariably attributed to pre-harvest sprouting or inherently poor starch properties. That was until the discovery of late-maturity alpha-amylase (LMA).
It has now been established that about 20 percent of current varieties and advanced breeding lines are susceptible to LMA. Researchers are now working to minimise the release of varieties susceptible to this defect.
LMA is a defect similar to preharvest sprouting, in that amylase enzymes capable of degrading starch become active during mid-to-late grain filling. Growers will not observe any damage but the result is a low falling number, rendering the grain unsuitable for processing.
Unlike sprouting, LMA is triggered by low temperatures rather than rainfall. The distribution of enzyme within the grain is distinctly different, there is no splitting of the seed coat or growth of the germ, and the defect is found only in particular varieties.
A series of experiments by Dr Kolumbina Mrva, of the University of Adelaide, has established the general details of the environmental trigger for LMA and identified the genetic controls for this defect.
As with pre-harvest sprouting, LMA is induced by environmental conditions. Through her research Dr Mrva has established that LMA is triggered by cool temperatures occurring in a critical window that starts about a month after flowering.
"The danger period for the development of late-maturity alpha-amylase is between 25 and 35 days after flowering," Dr Mrva reports. "Exposing a susceptible wheat variety to cool temperatures will stimulate the production of high pI alpha-amylase isozymes in the wheat grain."
Further research will be carried out in this project on the duration of the cool treatment, the range of effective temperatures and whether a temperature differential is required.
Danger signs: Dr Kolumbina Mrva and Dr Daryl Mares inspect bread wheat lines in the late-maturity alpha-amylase screening trials. Photo: Emma Leonard
This new knowledge of these environmental triggers underpins Dr Mrva"s rigorous screening trials that indicate that about 20 percent of current varieties and advanced breeding lines are susceptible to LMA and that most Australian, and a number of international, breeding programs are affected.
The screening process consists of growing replicated mini-plots of each line in a field trial where precautions are taken to avoid biotic and abiotic stress. As each ear flowers it is tagged to identify the flowering date.
Twenty-six days later the stem is cut at ground level and the whole tiller is put into a cool shock treatment for a week in a controlled-environment growth cabinet, cycling between 12°C and 18°C.
"At this stage of maturity the tiller does not require an external source of nutrients, so the cut stem is placed in water and the ear continues to mature as it would in the paddock. The only difference is we are in control of the temperature."
The stem is then transferred to a warm controlled-environment growth chamber, where it remains until maturity.
The grain is harvested and the level of alpha-amylase in the grain is measured using an ELISA antibody test developed by Dr Mrva in collaboration with CSIRO.
This specific ELISA test provides a rapid assessment of the level of alpha-amylase and is available to breeders through the Value Added Wheat CRC should they wish to set up in-house screening.
Dr Mrva"s work has resulted in the identification of two genes that control LMA. These are located on separate chromosomes and work independently, but if both are present the level of susceptibility is increased.
Molecular markers linked to both genes have been identified although these are not yet at the stage of being diagnostic for LMA.
"We are hoping that in the not too distant future breeders will have all the tools they require to assess new lines for susceptibility to this defect."
As LMA is triggered by low temperatures during maturity it is a defect of greater concern in varieties developed for the southern and some western grain production regions.
However, some varieties are grown in several regions. For example Kennedy, a popular and otherwise high-quality variety, was released in Queensland and northern NSW where the risk of the defect being expressed is very low. Popular with growers because of the premium APH classification, Kennedy has spread into cooler central and southern NSW, where expression of the trait has led to low falling number in some deliveries, potentially damaging Australia"s reputation for delivering high quality grain.
All parts of the grain chain agree that LMA must be eliminated from the Australian wheat industry.
The dilemma for industry is whether all LMA-susceptible varieties should be removed from the marketplace immediately - as end users and marketers would prefer - or, as wheat breeders argue, growers should be allowed to reap the benefits of otherwise valuable varieties where the risk to markets is low.
Is the industry mature enough to manage the release and adoption of susceptible varieties, so that growers benefit from higher yield and disease resistance but at the same time make sure Australia"s markets are protected from damaged grain?
At a meeting held in September as part of the Wheat Breeders Assembly, marketers, breeders, researchers and the GRDC formed a working group to formulate guidelines for an industry-agreed approach to the management of LMA.
For more information:
Dr Kolumbina Mrva 08 8303 7480, firstname.lastname@example.org
GRDC Research Code: UA00037, program 1
Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.