Limitations of photosynthate sources and sinks at time of grain filling in an Australian hard wheat cultivar.

Introduction
An estimated two-thirds of the worlds population depend on wheat in their diet (Alam et al. 2008). In
Australia, wheat (Tritium sp.) alone had an export value of $5.5 billion in 2010-2011 and is therewith the most valuable grain crop for Australia (ABARES 2011). To remain competitive as a wheat exporter in future, improving wheat yields seems imperative, hence improving the understanding of physiological processes during different plant-growth phases, may help to find and select specific factors that limit grain yield and ultimately help to breed better performing cultivars (Richards 1996). One area of research is dedicated to the better understanding of the limits of sources (plant organs such as leaves that produce and export photosynthates) and sinks (all plant parts such as roots or seeds that mostly store those photosynthates) after anthesis or during grain filling (Bingham et al. 2009; Fischer et al. 1977). Grain filling is a complex process involving formation, translocation, partitioning and accumulation of photosynthates in interaction with the sink-source capacities and preferences under different environments and growth stages (Felekari et al. 2014).
Past research indicated that the wheat grain as a sink is limited by the capacity of its sources during grain filling (Ma et al. 1990; Alam et al. 2008), however data had a high variance reflecting the different genotypes used and the diversity of experimental techniques and the wide spectrum of conditions these studies have been undertaken (Bowring 2010). Although the harvest index (ratio of yield to total above ground biomass) has increased over the years due to plant breeding (Taiz et al. 2015) the continuous goal of the industry is to further increases production values, also by increasing plant physiology knowledge and in particular the partitioning (amount distributed into different sinks) of photosynthates. The aim of this experiment was to identify source-sink limitations of an Australian wheat cultivar and test the hypothesis that, at stage of grain filling, sources are in fact the dominant limitation.

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Conclusion
The results of the tested wheat cultivar T. aestivum cv. Meering, which was tested post anthesis, was indeed limited by its photosynthate producing sources which confirms this common hypothesis. The spike of this cultivar showed a high source strength which was equally important as a source as FL1 and FL2 combined.
However as highlighted by Bowring (2010) the physiological processes are complex and data of past and current research is highly variable, hence for future research it would be prudent to include genotype, season and climate conditions into each analysis so breeders are able to narrow down common factors.

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