After the seed number is determined, a reduction in source limitation can be achieved physiologically by extending the canopy longevity, or the capacity to stay green. This indicates a negative effect of source limitation around anthesis on the formation of the sink size, namely grain number. Furthermore, the crop growth rate around anthesis could also be associated with grain number ( Bancal, 2008 Guo et al., 2018). by shading of leaves around anthesis) reduces grain number per spike ( Wang et al., 2003). It has been shown that limitation of source strength (e.g. The physiological factors relevant for source limitation at anthesis are 1) the canopy leaf area, which maximizes light interception, and 2) photosynthetic capacity per leaf area which maximizes the utilization of light energy for production of plant mass.
Around anthesis, spikelet fertility is affected and the physiological process of floret abortion could be activated, thereby reducing the grain number. In the very early phase, from double ridge to terminal spikelet stages, a source limitation may result in a reduced spikelet number ( Guo et al., 2018).
The interplay between sinks and sources has an impact on yield formation which becomes apparent from double ridge stage (first spikelet ridges visible) to the end of grain filling. Source size is related to the production of photo-assimilates, namely the size, the photosynthetic capacity, and the duration of the leaf area, which drive spike development and grain filling ( Jagadish et al., 2015). Sink size of the developing yield organs is determined by the number of spikes per unit area, grains per spike, and a specific sink size per grain (resulting in TGW).
The formation of different yield components can be interpreted as the result of the interplay between sinks and sources. The final grain yield of winter wheat is the result of the number of grains and the grain weight which are determined by the primary yield components: spike number per unit area, grain number per spike, and thousand grain weight (TGW) ( Fischer, 2011). For future breeding we suggest to choose parental lines with high grain numbers per spike on the sink side, and high photosynthetic activity around anthesis and canopy duration on the source side, and to place emphasis on these traits throughout selection. Our results suggest that, 1) the potential longevity of the green canopy is predetermined at the time point that the number of grains is fixed 2) a co-evolution of source and sink strength during the breeding history contribute to the yield formation of the modern cultivars. Surprisingly, despite significant breeding progress in GCD and other source components, they showed no direct influence on thousand grain weights, indicating that grain filling was not limited by the source strength. Breeding progress mainly resulted from an increase in grain numbers per spike, a sink component, whose variations were largely explained by the photosynthetic activity around anthesis, a source component. In addition, genetic associations of QTL for the traits were assessed with genome-wide association studies. Field trials were conducted over three years with 220 cultivars representing the German breeding history of the past five decades. This study analyzed the breeding progress of sink and source components and their relationships to yield components. Furthermore, there is little information on how the source traits have been modified during the breeding history of winter wheat. green canopy duration, GCD) and sink (e.g. However, the physiological limitations of yield from source (e.g. Optimizing the interplay between sinks and sources is of crucial importance for breeding progress in winter wheat. 2Department of Plant Breeding, IFZ Research Centre for Biosystems, Land Use and Nutrition, Justus Liebig University, Giessen, Germany.1Vegetable Systems Modelling Section, Institute of Horticultural Production Systems, University of Hannover, Hannover, Germany.
Carolin Lichthardt 1, Tsu-Wei Chen 1*, Andreas Stahl 2 and Hartmut Stützel 1
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