Clearer picture of sprouting resistance

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Simulated summer showers are providing new information about varietal factors that contribute to sprouting resistance.

Photo of petri dishes

The germination index uses the speed at which individual grains sprout in prepared Petri dishes to assess a variety’s sprouting resistance.

PHOTO: Kevin Young

Western Australian field trials and rain-simulation data are being combined with germination index ratings to more accurately define the susceptibility of wheat to pre-harvest sprouting, allowing growers to avoid the highest-risk varieties.

Research agronomist Kevin Young, working with the Department of Food and Agriculture, WA, says sprouting is a recognised problem for growers in several WA regions, particularly in coastal areas such as Esperance, Albany and Geraldton.

Predictions of increased summer rain in other regions as a result of climate change could lead to sprouting becoming a more widespread issue.

Mr Young’s project – now in its third and final year – aims to build on the germination index, which has traditionally been used to predict sprouting resistance by assessing grain dormancy, or how susceptible different varieties are to germination.

It uses grains separated from the heads of wheat, which are sprouted in Petri dishes. However, Mr Young says he believes the physiology of the plants also contributes to sprouting resistance.

For instance, some varieties ‘nod’ when ready for harvest, a feature that helps rain run off the ripened heads. Awnless varieties and those that have a waxy coating, or tightly adhering glumes, also demonstrate increased resistance to wetting – and therefore sprouting – which results in less rain damage compared with other varieties.

“EGA Eagle RockPBR logo is a good example of a variety that has only average resistance to sprouting, based on the germination index, but which consistently maintains grain quality despite harvest rainfall. It performs far better than other awned varieties that have a similar germination index,” Mr Young says.

Esperance research

As part of his research in 2012, 40 wheat varieties were sown at Esperance Downs Research Station, with seven different sowing dates from May until August, to provide grain maturity across varying conditions. There were also 40 varieties in the 2013 trial, including newer experimental lines, and five different sowing times ranging from late April to early July.

“By sampling across multiple sowing dates we can harvest varieties of different maturities that have matured at the same time. It is important to attempt to remove variations in maturity as a factor when comparing lines for sprouting tolerance.” 

Mr Young explains the process: “We cut samples from twin-row plots in the field, complete with stalks, and stand them in a rain simulator 10 days after maturity. We subject them to four different levels of rainfall, for four different periods.

“Basically, we are simulating what would happen to a grower’s crop if it was dry and ready to harvest, and it rained,” he says. “The plants are then dried out, threshed and the grain analysed for soundness.”

To measure the extent of damage from the ‘rain’ he uses the internationally recognised falling number test. A low falling number in rain-affected grain indicates the grain has started to sprout, and the starch has started to break down. For growers, the ultimate impact of sprouting is grain downgraded to feed status.

As the falling number is the ultimate indication of marketable quality, this is used as the basis of the new sprouting-tolerance-rating system in the annual Wheat Variety Guide for WA, which combines the results from the field trials, rain simulation, falling number test and the grain germination index.

FIGURE 1 2012 germination index for best, worst and average Western Australian wheat varieties.

2012 Germination index for Western Australian wheat varieties

On a scale of one to nine, the higher the sprouting tolerance rating, the more likely a variety is to maintain a high falling number – or, the more sprouting resistant it is.

The highest resistance rating in commercial varieties in WA is six, for EGA Eagle RockPBR logo, LongReach EnvoyPBR logo, EstocPBR logo, LongReach ScoutPBR logo and Clearfield WHT STLPBR logo. However, Mr Young believes there is room for improvement.

Several of these commercial varieties already incorporate improved genetics for sprouting resistance identified by Dr Daryl Mares from the University of Adelaide, who also conducts the germination index test on new varieties on an annual basis. 

The lower the score on the index, the greater the grain dormancy and the less likely it is to sprout.

During the past two decades, Dr Mares has identified white-grained landraces from China and South Africa with strong resistance to pre-harvest sprouting and has transferred the resistance to semi-dwarf, locally adapted cultivars, which have been supplied to plant breeders for incorporation in new varieties.

Based on the germination index, the best genetics from South Africa, incorporated into SUN325B, provide a seven-fold improvement in tolerance over the popular variety GladiusPBR logo. The best Chinese genetics in the line 50213/Cunn.799 provide further improvement, as does P07.683, which combines genes from both Chinese and South African varieties (Figure 1).

Dr Mares says several genes are responsible for sprouting resistance and the more of these that are incorporated into new varieties, the better the resistance will be. However, adding more genes increases the expense of developing new varieties.

He says with the current focus on yields, it is uncertain whether further improvements in sprouting resistance will become a priority for plant breeders.

Mr Young’s field trials are also trying to identify the extent to which agronomic and seasonal factors may contribute to sprouting, although he admits this is a complex and sometimes controversial area to venture into.

“Our primary objective remains helping growers identify the most susceptible varieties so that they can avoid them, and the most resistant varieties, so that they can give them a go. But no variety is totally resistant. If you leave any wheat variety in a paddock wet enough, for long enough, it will sprout,” Mr Young says.

The falling number test

The falling number test is the internationally recognised standard for assessing sprouting damage in wheat. Sprouting affects the quality of products made from the flour.

To assess sprouting damage, grain is ground to a powder and mixed with distilled water in a test tube, which is then heated. The starch from the wheat will gelatinise, making the mixture viscous.

When a stirrer is dropped into the test tube, the length of time it takes the stirrer to reach the bottom, in seconds, is the falling number. The more viscous the mixture, the longer this takes, the higher the falling number and the higher the wheat quality. The more sprouting damage there is, the greater the alpha-amylase activity, the less viscous the mixture and the lower the falling number.

Figure 2 shows the falling number of the wheat varieties EGA Eagle RockPBR logo (the current commercial variety with the highest resistance), Westonia (sprouting susceptible), MacePBR logo (the most widely grown variety in Western Australia) and one of Dr Daryl Mares intermediate resistant breeding lines, DM167, for a range of simulated rain treatments.

FIGURE 2 Falling number of the wheat varieties EGA Eagle RockPBR logo, Westonia, MacePBR logo compared with DM167 for a range of simulated rain treatments.

Falling number of wheat varieties in simulated rain treatments

More information:

Kevin Young
08 9083 1160
kevin.young@agric.wa.gov.au

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GRDC Project Code UA00130, SDI00011