Impact of breeding innovations in canopy architecture and function on yield formation in winter wheat

Abstract

Wheat (Triticum aestivum L.) is one of the most important staple crops worldwide and there is an urgent need to develop high-yielding and resilient new cultivars and to elevate the breeding progress. This thesis presents a retrospective analysis of the breeding progress of the last 50 years aiming to identify innovations in physiological traits with great relevance for future breeding. Intercepting radiation and the radiation use efficiency (RUE) determine biomass production, which is the more promising influencing factor for total grain yield in comparison to biomass partitioning (harvest index). The first objective within this thesis was to discover the genetic variation in relative light interception and RUE and to understand the underlying architectural and physiological functions of the canopy determining the source of assimilate production. From another perspective, grain yield formation can also be seen as a constant interplay between sink and source components, as the assimilates produced by the source are allocated to the sink organs of the crop. Therefore, the second objective was an in-depth analysis of the interdependencies between sinks and sources and possible limiting factors within that network. Furthermore, a genetic analysis of the physiological and yield related traits was performed. The identification of the genetic regions relevant for the source compartments, which partly enabled the yield increase and their potential effect, was the third objective of this thesis. During three experimental seasons, canopy traits were assessed in the field by measuring the relative leaf chlorophyll content (via SPAD measurements), the proportion of green leaf material, light interception and leaf area index (LAI) non-destructively. Using these traits, relative light interception, RUE, green canopy duration (GCD), green leaf area integral and the light extinction coefficient were derived. The field trials were conducted with 220 cultivars of which 174 represent the German breeding history. For the evaluation of the dependencies of the canopy parameters and final grain yield, correlations and causal effects between the variables were investigated. Additionally, the progress of each parameter with the year of release of each cultivar was assessed to detect relevance of the traits during breeding progress. A genome wide association study (GWAS) with single markers and additionally with chromosomal segments (haploblocks) was performed to detect co-evolutionary processes between sink and source traits and detect causal genetic regions. The broad-sense heritability of all measured and derived physiological traits ranged from 7-66%, with the highest values for RUE, SPAD and the average LAI and the lowest for the extinction coefficient. Relative light interception and RUE were identified as two independent traits, which showed high explanatory power for grain yield (30% and 64%, respectively). III Previous studies already indicated the importance of RUE for future breeding progress, but to our knowledge, this is the first study representing more details regarding the underlying traits and concrete causal agents (SPAD and GCD). Investigating the breeding progress, we found that grains per spike showed the most pronounced progress in the breeding history besides total yield (0.45% per year). This trait showed the strongest correlation with final yield among the yield components (r= 0.54). However, the variation of grains per spike was significantly associated with the variation of SPAD and GCD. The investigations of the network of sinks and sources substantiate the relevance of SPAD and GCD, which both explained substantial variation in grain yield (40% and 42%, respectively). However, the physiological link between the sink and sources lies at stages relevant for grains per spike in contrary to the expected association between grain weight and the canopy persistence. Our results suggest that the potential longevity of the green canopy is predetermined at the time point when the number of grains is fixed. The GWAS underpinned the association of breeding progress in canopy longevity as we observed a shift in allele frequencies. Furthermore, highly significant associations were observed for single marker effects of which some were overlapping with high haplotype variances. Especially a region spanning over 40 Mbp on chromosome 6A was associated with canopy height and also parameters describing the canopy architecture, persistence and thereby light interception. The results present important findings which can be applied in the network of genomic, phenomics and crop modelling and provide new application possibilities for large scale field phenotyping.