To arrest protein decline in wheat, researchers are measuring the impact of nitrogen application timing on leaf and grain protein content and composition.

Key message

• The impact of different nitrogen application timings on wheat grain protein content and composition is being studied in WA to determine whether there are genetic differences in wheat lines that could be of value.

Maximising protein content in wheat has been the subject of considerable research, but protein level is only part of the story. Protein composition – or the type of proteins produced – is important for wheat quality and end-use markets.

To date, much of the research on grain protein content has focused on soil nitrogen cycling, fertiliser application strategies and uptake into the plant.

Protein composition

While split applications of nitrogen fertiliser have often been considered the most effective at boosting grain protein content, researchers are only just beginning to understand how different application strategies affect protein composition and end-use quality.

Following uptake, nitrogen can be converted into various protein compounds that are either stored as plant biomass or converted into grain. These compounds can be broken down multiple times and recycled in different ways.

This process – known as futile cycling – can be highly energy inefficient. Preliminary University of Western Australia (UWA) glasshouse research found that as much as 25 per cent of stored plant proteins can be recycled over and over, but the cost to yield and final grain protein content is not yet known.

European research has shown that the timing of nitrogen can affect the final composition of plant proteins.

To evaluate the potential impact of nitrogen timing on protein content and composition in Australian wheat, GRDC has invested in a postgraduate PhD research project at UWA.

Researchers hope that a better understanding of the biology of grain protein production could help increase protein levels in WA wheat.

Embedded within the School of Molecular Sciences, the research will be the first to measure these processes in the field in Australian soils and varieties.

Field studies

The initial field trial at UWA’s Shenton Park Field Station in 2022 evaluated the impact of nitrogen application timing on plant protein content in the leaf and grain of six genetically diverse wheat varieties.

It is possible that there could be useful genetic differences in how the wheat lines process plant proteins.

Nitrogen fertiliser was applied at three timings, comparing a time-of-sowing application with split applications either six or 12 weeks after sowing. Protein content and, more importantly, composition was analysed from grain and flag leaf samples collected 10, 20 and 30 days after anthesis.

Along with some additional data collected at Merredin in 2022, the results will inform the basis for a second trial at Merredin in 2023.

Alongside this work, detailed glasshouse and field studies will track nitrogen-15 applied directly to wheat heads to better understand the turnover of individual proteins during the process of futile cycling.

While the process is energy-intensive, it is not known whether it is an essential part of maximising grain protein quality.

Nitrogen-15 is a stable isotope of nitrogen used to determine crop fertiliser use efficiency. It is also used to quantify the amount of nitrogen that crops can acquire from the atmosphere through a process known as biological nitrogen fixation.

Ultimately, a better understanding of the biological function of plant protein cycling and the implications for protein quality could be useful either for making on-farm nitrogen application decisions, or in the breeding of wheat lines that produce a highly desirable combination of grain proteins.

More information: Samantha Harvie, samantha.harvie@research.uwa.edu.au