Pre-harvest sproutingLet dormant genes lieStronger genes sought from wild cousins

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After a decade of GRDC supported research, progress is being made in understanding the causes of grain quality defects - pre-harvest sprouting, late-maturity alpha-amylase and black point - that regularly take the gloss off an otherwise profitable crop.

In the October issue of Ground Cover, Mike Perry defined these quality defects related to weather damage and explained their impact.

In this special three-page report, Emma Leonard explores the research that is uncovering the causes of the defects and new insights that biotechnology is bringing to the problems - and the answers.

Pre-harvest sprouting, late-maturity alpha-amylase and black point are grain quality defects controlled by independent genes, triggered by different weather conditions. All result in wheat being downgraded at receival. The incidence and the severity of loss are variable, making it difficult to select improved varieties and to predict the risk of downgrading.

Both sprouting and late-maturity alpha-amylase result in high levels of alpha-amylase activity in the grain, which is indicated by a low falling number. This high alpha-amylase activity results in the breakdown of starch during processing and end-product quality problems such as discolouration and sticky dough that collapses when baked.

Black point-affected grains contain a pigment in the seed coat overlying the germ that can contaminate the flour with black specks during milling, making it unacceptable for pasta and noodle products.

Today"s highly mechanised and automated bakeries and pasta factories require uniform raw materials and even low levels of sprouting or late-maturity alpha amylase introduce unacceptable variability into the flour. Similarly, our export markets expect grain to match very high quality specifications.

A considerable research investment by the GRDC over the past three years has led to advances in technology that can help wheat breeders to screen breeding populations for lines with greater tolerance to these defects. However, the path to new cultivars is not as direct as growers might assume.

Over the past 20 years Associate Professor Daryl Mares has researched the problem of pre-harvest sprouting. His work has focused primarily on bread wheats including those used for Asian noodles, but it also now embraces durum wheat, as all three can suffer downgrading.

Tolerance to pre-harvest sprouting is intrinsically linked with dormancy, so a key step in the development of more tolerant lines has been to unravel the controls for dormancy.

Dr Mares has established that in wheat there are at least two genes that influence dormancy levels. One gene is expressed in the embryo or "germ" of the seed. This is a critical component that has to be present if any level of seed dormancy is to develop.

This gene appears to make the grain sensitive to the plant hormone abscisic acid, which inhibits germination at the time of crop maturity.

Alone it gives an intermediate level of dormancy, especially in Australia"s cooler southern wheat-growing regions. The "embryo gene" can also interact with the second gene, and possibly other minor genes, that Dr Mares has found to be involved in dormancy.

Expressed in the seed coat alone, this second gene has little or no effect on dormancy, but in combination with the embryo gene a more robust and stable dormancy is produced. This level of dormancy is essential in varieties targeted for Queensland and northern NSW, and would be highly desirable in southern Australia as well.

Having identified good sources of sprouting tolerance, the next steps in the research included movement of the dormancy genes into locally adapted germplasm, and development of screening techniques that are independent of environmental factors.

Progress was slow until a screening system was developed that allowed accurate and reproducible identification of lines carrying the dormancy genes.

Screening is a laborious process (see "Stronger genes sought from wild cousins", below) but provided the accurately characterised gene pool for researchers Kevin Williams and Judy Cheong of the SA Research and Development Institute and Molecular Plant Breeding CRC to examine in the search for molecular markers.

To date the program has identified "flanking molecular markers" which are on either side of, but a short distance from, the embryo dormancy gene. Following this breakthrough, it is hoped that further progress will be rapid and that more diagnostic markers for the embryo gene and markers for the seed coat gene will soon be identified.

Australian plant breeders are already incorporating into breeding programs improved germplasm and knowledge that has come from Dr Mares research. Under the terms of the research agreement for this project, new markers and improved germplasm are available to all Australian wheat-breeding programs.

Australian commercial varieties are generally regarded as being susceptible or very susceptible to sprouting; that is they have essentially no dormancy at harvest-ripeness.

A small number, however, do appear to contain versions of the embryo gene giving moderate tolerance (Sunlin), or moderate susceptibility (Spear and CascadesVariety protected under the Plant Breeders Rights Act 1994).

The introduction of stronger dormancy genes identified by Dr Mares, into new varieties will considerably reduce their susceptibility to pre-harvest sprouting yet have no residual impact on the germination percentage of the variety when sown in the following autumn.

"Even the protection supplied by the "double" dormancy gene combination breaks down naturally within three to four weeks of crop maturity," he says.

"In Australia, on average, there is about six months between harvest and sowing so the window of protection will have disappeared completely by the time germination is required."

PBR Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.

Listen also: Harvest Radio: Weather Damage

Until the early 1980s, there was no active selection in Australia for lines that have a low susceptibility to preharvest sprouting.

In the white grain wheat varieties grown in Australia any level of tolerance has been difficult to find and Dr Mares resorted to working with "Landrace" wild types of wheat.

"We screened over 10,000 lines of white grained wheats held at the Australian Winter Cereals collection and a single South African line offered the best tolerance," he says. "Since then we have sourced other lines with tolerance from China.

"We found it necessary to understand and then control a number of environmental factors including weather conditions and soil constraints as well as stresses such as disease that all influenced sprouting."

In particular, Dr Mares and colleagues established that the amount of rain and the timing in relation to crop maturity are crucial drivers for sprouting.

Rain that falls in the lead-up to harvest, especially 10 to 15 days before full maturity, reduces the level of tolerance remaining at harvest-ripeness.

At the time, this rain usually does not cause measurable damage but predisposes the crop to greater damage than expected should it receive further rainfall after maturity.

The greater the amount of rain the greater the reduction in tolerance and the more likely that the grain will sprout. (Figure 1).

Dr Mares" in-field screening plots, housed in a bird-protected field site at the Waite Campus, University of Adelaide, can be irrigated, and during the critical later phases of maturation can be covered to protect against rainfall.

This seems a paradox, but to compare varieties with different growth habits and ripening times, it is necessary to ensure all plants experience the same conditions.

Grain from the wheat grown in the screening program is collected and germination tested to establish the level of dormancy.

In addition to this screening of released varieties and breeder lines targeted for release, some resource is allocated to screening breeders" parent lines and exotic germplasm imported from CIMMYT and other international breeding programs. Information on parents and breeding lines is returned to breeders while data on varieties released and in commercial production will be made available to growers.

Figure 1: What is the impact of delaying harvest? Four simulated rainfall treatments, 0mm, 22mm, 57mm and 73mm were applied to wheat in the 20 days before harvest maturity followed by a standard wetting treatment applied at harvest or 10 or 20 days after harvest, to simulate the impact of delaying harvest.

Figure 1: What is the impact of delaying harvest?

This figure shows several things:

Susceptibility continued to increase after harvest ripeness. The amount of damage was greater when the samples were left for 10 or 20 days before applying the wetting treatment.

 

 

 

 

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