Photosynthesis in a fast-changing world

Previous efforts to improve photosynthesis have mostly focussed on fully sunlit leaves at the top of crop canopies, whilst the lower canopy leaves have received much less attention.

The leaves in the lower canopy are regularly exposed to highly fluctuating light conditions. It is critical that these leaves can respond quickly when light conditions change. For example, if the upper canopy leaves move after a gust of wind leaving the lower canopy leaves exposed to direct sunlight, faster induction of photosynthesis will be advantageous to assimilate as much carbon as possible.

Dr William Salter, together with Dr Andrew Merchant at the University of Sydney and research collaborators Dr Richard Richards at CSIRO and Dr Tom Buckley at the University of California, Davis, for the first time identified significant variation in the speed of photosynthetic induction across wheat genotypes. The research, published in the Journal of Experimental Botany, used novel measurement techniques to investigate the activation of photosynthetic reactions after a switch from low to high light.

These measurements revealed activation of the carbon fixing enzyme Rubisco to be the 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%.

Strategic plant breeding to speed up Rubisco activation in the slowest genotype to match that in the fastest genotype could increase daily net carbon gain by up to 3.4%. If a wider number of genotypes were investigated, this increase could be even more substantial.

Dr Salter is now running a follow up experiment to investigate photosynthetic induction across twenty chickpea genotypes. This work will incorporate laboratory analysis of photosynthetic induction kinetics, detailed measurement of the canopy light environment in the field and subsequent modelling simulations to fully understand this important trait in chickpea.

Professor Margaret Barbour, Dr Arjina Shrestha and Dr Salter are also investigating other plant physiological properties of chickpea, including plant water use and carbon balance as part of the ARC Legumes for Sustainable Agriculture Industrial Transformation Hub. Chickpea has a number of key advantages over cereal grains; including better drought tolerance and the ability to fix atmospheric nitrogen.

From a nutritional and economic standpoint, chickpeas are a very good source of protein so offer growers a better return than cereal grains. It has also been bred less intensively than the likes of wheat and rice so the potential for yield improvement is much greater. Dr Salter and his colleagues hope that their work will contribute to a growing understanding of chickpea physiology and that this knowledge will benefit future plant breeding efforts.

This research has been conducted as part of the International Wheat Yield Partnership and Legumes for Sustainable Agriculture (LSA) Research Hub, and was funded by the Grains Research and Development Corporation.