Drought-tolerant chickpeas that are highly efficient at extracting water from the soil are on the drawing board, with researchers employing novel technology to speed their search of available genetic material.
Dr Helen Bramley, a senior lecturer at the University of Sydney, and her research team have developed a new way to quickly screen chickpea genotypes for water-use efficiency - with the aim of reducing the time taken to release drought-tolerant chickpea varieties to growers.
"The traditional approach involves inserting aluminium access tubes into the soil to measure soil water content using a neutron probe," Dr Bramley says.
"But installing the tubes and taking the measurements at different depths is time-consuming and labour-intensive."
For example, Dr Bramley says installing 40 probes to a depth of 1.5 metres and recording soil water contents just once during the season took one of her team an entire day.
Precautions are also needed in handling the device because of the radioactive source, which means it requires a licensed operator. In addition, she says, measurements at shallow depths are prone to errors.
Accordingly, the team was keen to find a more accurate, efficient and non-destructive method for gathering water-use data across a large population of trial plots at critical periods.
"I saw the University of Sydney Professor of Digital Agriculture and Soil Science Alex McBratney give a presentation that described various technologies for measuring the soil properties, which included electromagnetic induction (EMI)," Dr Bramley says.
"EMI measures the apparent electrical conductivity (ECa) of soil - or how salty the soil is.
"Because there is generally a relationship between ECa and moisture content, I asked Professor McBratney if it was possible to adapt the technology to measure soil water use in chickpea plots.
"He said it could be possible."
Subsequently, the team used an EM38-MK1 sensor - a one-metre-long instrument capable of collecting ECa data. A model, calibrated against neutron probe measurements, was then developed to calculate available soil water for different depths within the soil.
At the end of 2017, a proof-of-concept experiment was established using 36 different chickpea genotypes. Some were rain-fed, while others were irrigated because of dry seasonal conditions.
"Using the EM38 sensor, we were able to calculate water-use for the plants in every plot, as well as at different soil depths after a rainfall event," Dr Bramley says.
"Being able to measure moisture at different depths allowed us to pinpoint where in the soil the plants were extracting water from."
As a consequence, Dr Bramley says the technology has enabled the team to identify the chickpea genotypes with deeper roots - which may be one of the traits important for drought tolerance.
"Using neutron probe data over the 2017 season, we discovered water use at podding is critical for chickpea yield, but this was on a limited number of genotypes," she says.
"Accordingly, we wanted to confirm this over a larger population of diverse genotypes using the EMI technology."
In a scientific paper, Dr Bramley and her colleagues proved the EMI water-use concept at the plot level and suggested that the method, coupled with physiology data, was a useful way to pinpoint drought-tolerance mechanisms and screen genetic material for effective water use.
Using neutron probe data over the 2017 season, we discovered water-use at podding is critical for chickpea yield, but this was on a limited number of genotypes.
During 2018, the researchers - including PhD student Omar Murad (co-supervised by The University of Sydney's Professor in Soil-Landscape Modelling Budiman Minasny) - employed the system every two weeks over almost 300 trial plots to track water use during the growing season.
They further improved the throughput of the measurements using an EM38-MK2 loaned by Dr Tim Weaver from CSIRO. Using Dr Weaver's EM38 sensor, the number of measurements taken was cut by half.
A prototype buggy, called the BrEM38, was developed to ease the task of taking thousands of water-use measurements throughout the year.
The buggy, which operates at different heights, was constructed by University of Sydney technical officer Chris Bramley entirely from plastic to fit over the plots and run on wheels to improve the speed of data collection. No metal was used in the construction because it could interfere with the EMI measurements.
In 2019, Dr Bramley and the team scaled-up the system to take water-use measurements across an entire mapping population. This is a group of chickpea lines that are the offspring of two parents with contrasting drought-tolerance traits. These include traits such as growth rate, biomass production and seed number per plant.
The genetic make-up and traits of this chickpea population are known, so the 2019 experiment aimed to describe the observable characteristics (phenotype) of the genotypes to determine their water-use efficiency.
"Once we have this phenotypic data we associate that with the genotypic data to develop qualitative trait loci (or QTLs) to speed the breeding process," Dr Bramley says.
"We could then give this molecular marker to breeders if, for example, they want to produce a variety that can be grown on stored soil moisture with deeper or more efficient roots at podding that produce higher yields."
The benefit of the BrEM38, according to Dr Bramley, is that multiple soil cores do not have to be taken to determine where the roots are in the soil.
This is an advantage because soil cores and washing roots from the soil is highly labour-intensive and can damage plants if done during the season.
She says it is also difficult to determine whether the roots are from the current year's trial or a previous year.
It also allows water use to be monitored accurately multiple times during the growing season.
If particular chickpea genotypes warrant closer investigation, the researchers may carry out experiments in a laboratory built underground (rhizotron) to better understand the form and mechanisms of the root architecture and hydraulic traits associated with more efficient water use.
A robotic future
Going forward, Dr Bramley hopes the process of collecting water-use data can be further automated using robotics.
"We hope to build something that will drive itself through 2000 to 3000 plots so we don't need to push the buggy," she says.
"I've been in talks with the University of Sydney's robotics department and we can potentially automate the task to simultaneously collect other phenotypic data as well as water-use."
The next step, she says, is determining whether chickpeas with early vigour use too much water early in the season, leaving less for podding, which is critical for yield.
"My vision is that we will soon have multiple robots operating at various locations across Australia that continuously collect this data from the field," she says
GRDC Research Code: US00083
More information: Dr Helen Bramley, University of Sydney, 0477 738 776, email@example.com