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Flag Leaf Over-Rated in Yield Equation

15 August 2011

AUSTRALIA - New Grains Research and Development Corporation (GRDC)-supported research shows the cereal plant's flag leaf may not be the yield influencer previously thought.

NSW Department of Primary Industries (NSW DPI) researchers Neil Fettell and Alan Bowring told growers and advisers at the recent round of GRDC Updates that assumptions made from northern hemisphere research may not be applicable to Australia’s northern grains region.

“Current industry estimates of leaf contribution to grain filling originate from the United Kingdom, under conditions markedly different to the northern grains region,” Dr Fettell said.

“Field trials using carbon and leaf desiccation methods suggest that the contribution of the flag leaf may be half the value currently used,” he said.

“Ear and stem photosynthesis appear to be of greater importance in Australia, as do water soluble carbohydrates stored prior to grain filling, particularly under post-anthesis water stress.

“Earlier disease management (tillering through to flag leaf emergence) might be more important than maintaining flag leaf area late in grain filling in this environment.”

GRDC-supported research shows the contribution of the flag leaf to eventual grainfill may be over-rated when it comes to influencing yield. Ear and stem photosynthesis may be of greater importance in Australia.

The findings are important this season which is expected to see higher than normal disease pressure on winter crops due to last year’s wet season.

Dr Fettell says an understanding of the physiology of crop growth and grain filling can help in agronomic management of cereal crops.

In particular, foliar disease management can be enhanced by knowledge of disease severity and the economic benefit of spray timing in relation to the protection and retention of green leaf area.

“With diseases such as stripe rust estimated to cost the Australian wheat industry A$127 million annually, an understanding of the impact of leaf loss on yield is crucial,” he said.

“In particular, the estimation of individual leaf and stem contribution to yield plays a critical role in assessing potential economic impact and management decisions relating to disease control.”

Dr Fettell said grain yield could be considered as the balance between the supply of assimilates such as carbohydrates and proteins and the capacity of the grains to accumulate these assimilates, often referred to as the “source” and the “sink” respectively.

“Improvements in grain yield over time have come mainly from increases in grain number per unit area, rather than in grain size,” he said.

“This relationship plus evidence from GRDC-funded sink-source manipulation experiments suggests that where environmental conditions are favourable (cool, high daily radiation, no disease, adequate water and nitrogen), yields are limited by sink capacity rather than assimilate supply.

“However, these conditions are rare in the Australian wheat belt where grain yield and quality are frequently reduced by a lack of assimilates for grain filling.”

Dr Fettell said using desiccation over two seasons, the average contribution of the flag leaf to grain yield was 23 per cent.

He says further trials are needed but this work suggests a review of disease management, agronomic and breeding programs within the northern region of Australia may be needed if further results are consistent.


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