The canopies of chickpeas are the subject of new research that scientists hope will one day lead to the development of varieties that use available sunlight more efficiently for grain production.
During photosynthesis, plants use sunlight to convert water and carbon dioxide into oxygen and carbohydrates. The rate of photosynthesis is often constrained by the amount of sunlight available, with shaded leaves in the lower canopy contributing less than fully sunlit leaves.
University of Sydney postdoctoral research fellow, Dr William Salter, University of Sydney postdoctoral research associate, Dr Arjina Shrestha, and University of Sydney professor, Margaret Barbour, are exploring chickpeas to boost photosynthesis and carbohydrate production. Carbohydrates are used for plant growth and grain production.
Dr Salter says previous research to improve photosynthesis focused mostly on fully sunlit leaves at the top of crop canopies, with lower canopy leaves receiving far less attention.
"Lower canopy leaves are regularly exposed to fluctuating light conditions," he says.
"It is critical these leaves respond quickly when light conditions change."
For example, he says, if a plant's upper canopy leaves move after a gust of wind, leaving the lower canopy leaves exposed to direct sunlight, a plant that quickly starts the process of photosynthesis will convert more carbon dioxide and water into carbohydrates than one that is slower to induce photosynthesis.
"If we can improve whole canopy photosynthesis, this will result in chickpeas acquiring more carbon, which has a much better chance of ending up in the grain and increasing yield," he says.
Previous GRDC-invested work by Dr Salter, together with Dr Andrew Merchant, at the University of Sydney, and research collaborator Dr Tom Buckley, at the University of California, Davis, identified - for the first time - significant variation in the speed of photosynthetic induction across wheat genotypes.
Their research used novel measurement methods to investigate the activation of photosynthetic reactions after a switch from low to high light.
Read more: High-tech tools drive wheat yield push
The measurements showed activation of the carbon-fixing enzyme Rubisco to be the main limiting process of photosynthesis in fluctuating light. Subsequent canopy modelling simulations revealed that slow activation of Rubisco reduces daily net carbon gain by up to 15 per cent in wheat.
Now, Dr Salter, in collaboration with Dr Shrestha and Professor Barbour, is running follow-up experiments to investigate photosynthetic induction in promising chickpea genotypes.
Their research, through the Australian Research Council Hub for Legumes for Sustainable Agriculture - with GRDC investment - has already yielded the development of novel three-dimensional (3D) imaging technology to better understand how chickpeas respond to changing light.
Dr Salter came up with the idea after reading University of Nottingham papers that used 3D scanning to research wheat and rice.
"I decided to build a 3D scanner in my spare time," he says.
"It worked well so we developed it further into a fully functioning research tool."
The tool uses open-source hardware and software normally used to teach children how to build objects using code, which means the technology is inexpensive and easy to use.
Dr Salter says the 3D scanner consists of cameras and a motorised turntable driven by a micro-controller.
A plant is placed on the turntable and three cameras capture still images at set intervals for a full rotation. The process takes just six minutes.
The next step, Dr Salter says, is to generate 3D data points (point clouds) from the images using the open-source Visual SFM software on a Windows computer.
"Non-green points are removed using an automated script in MeshLab," he says.
"Another automated script in CloudCompare scales the point cloud and removes statistical outliers."
Finally, Dr Salter says, the chickpea's surfaces are reconstructed using a ball pivoting algorithm to produce a model that can be used to acquire detailed data on plant architecture, such as leaf area and canopy light environment.
He says accurate leaf area estimates were previously difficult to obtain without destructively harvesting plants.
"It was time-consuming and labour-intensive because chickpeas have many branches and lots of small leaves," he says.
"With our 3D scanner we have estimated total leaf area of chickpeas with an almost 1:1 relationship against destructively harvested measurements."
Another benefit, Dr Salter says, is that measurements can be made during plant development to screen for traits such as early vigour or early senescence (death).
"In future work we also hope to pick up when pods start to set," he says.
The next step for the researchers is to screen chickpeas with the scanner to explore diversity in canopy architecture traits.
Dr Salter says promising chickpea lines and commercial varieties will be grown in full sun and aligned at commercial row spacings.
Other work will seek to understand what happens to photosynthetic enzymes when a leaf returns to darkness.
"When a leaf goes into light, the photosynthetic enzyme Rubisco takes time to activate and this activation time varies across genotypes," Dr Salter says.
"When the leaf returns to the dark, it is largely unknown how long Rubisco takes to deactivate or whether there is variation in this process. Our new work seeks to address this gap."
The researchers then hope to analyse the photosynthetic activation, deactivation and 3D architectural data together to understand how best to improve whole canopy photosynthesis.
Dr Salter says the end goal is to have chickpeas that intercept and use the maximum amount of light available.
"Ultimately, we hope a better understanding of the genetic and physiological mechanisms that control chickpea photosynthesis will yield molecular markers for breeders to develop varieties that are more efficient at photosynthesis to improve grain production," he says.
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GRDC Research Code US00083