Flag leaf influence overrated

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Ear and stem photosynthesis in cereals could have a greater effect on grain-fill and yield than flag leaf emergence.

Field trials show the contribution that flag leaf emergence in cereals makes to grain-fill, and ultimately yield, could be overestimated.

NSW Department of Primary Industries (DPI) researcher Neil Fettell told growers and advisers at recent GRDC Updates that the importance attributed to the flag leaf in the Northern Hemisphere is misplaced in Australia.

“Current industry estimates of flag leaf contribution to grain-filling originate from the UK, where conditions are markedly different to Australia’s northern grains region,” Dr Fettell said.

Field trials using carbon and leaf desiccation methods suggest that the emergence of the flag leaf may provide about half the influence widely believed in the northern grains region, he said.

“Ear and stem photosynthesis appear to be of greater importance in Australia, along with water-soluble carbohydrates stored by the plant prior to grain-filling, particularly under conditions of water-stress after flowering,” he said. 

“In this environment, disease management from tillering through to flag leaf emergence might be more important than the flag leaf for grain-filling.”

Dr Fettell said understanding the physiology of crop growth and grain-filling is particularly important this season because of high disease pressure following high rainfall last year.

He said management could be improved by knowledge of foliar diseases and optimum spraying times to protect and retain the green leaf area in cereal crops.

“Estimation of how the individual leaf and stem contribute to yield plays a major part in assessing the potential affects of disease and the control measures required,” he said.

Dr Fettell said grain yield could be considered as the balance between the “source” of energy or assimilates, such as proteins and carbohydrates, and the crop’s ability to accumulate or “sink” them.

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

This pattern, along with GRDC-funded sink-source experiments, suggest that yields are limited by sink capacity, rather than assimilate sources when conditions are favourable.

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

Dr Fettell said the research suggested northern growers could benefit from a revised approach to agronomy, disease management and breeding, but further trials were needed.

More information: Neil Fettell, 0427 201 939, neil.fettell@une.edu.au; www.grdc.com.au/ICN00009

GRDC Project Code IGN00009

Region North