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HighlightA high level of QTL coincidences was found for the traits that determine individual grain weight in wheat: carpel size, grain dry matter and water accumulation, and grain morphology.

Background and Aims Plant roots growing underground are critical for soil resource acquisition, anchorage and plant-environment interactions. In wheat (Triticum aestivum), however, the target root traits to improve yield potential still remain largely unknown. This study aimed to identify traits of seedling root system architecture (RSA) associated with yield and yield components in 226 recombinant inbred lines (RILs) derived from a cross between the bread wheat Triticum aestivum 'Forno' (small, wide root system) and spelt Triticum spelta 'Oberkulmer' (large, narrow root system). Methods A 'pouch and wick' high-throughput phenotyping pipeline was used to determine the RSA traits of 13day-old RIL seedlings. Two field experiments and one glasshouse experiment were carried out to investigate the yield, yield components and phenology, followed by identification of quantitative trait loci (QTLs). Key Results There was substantial variation in RSA traits between genotypes. Seminal root number and total root length were both positively associated with grains m-2 , grains per spike, above-ground biomass m-2 and grain yield. More seminal roots and longer total root length were also associated with delayed maturity and extended grain filling, likely to be a consequence of more grains being defined before anthesis. Additionally, the maximum width of the root system displayed positive relationships with spikes m-2 , grains m-2 and grain yield. Ten RILs selected for the longest total roots exhibited the same effects on yield and phenology as described above, compared with the ten lines with the shortest total roots. Genetic analysis revealed 38 QTLs for the RSA, and QTL coincidence between the root and yield traits was frequently observed, indicating tightly linked genes or pleiotropy, which concurs with the results of phenotypic correlation analysis. Conclusions Based on the results from the Forno  Oberkulmer population, it is proposed that vigorous early root growth, particularly more seminal roots and longer total root length, is important to improve yield potential, and should be incorporated into wheat ideotypes in breeding.

Abbreviations: chl, chlorophyll; Chl accum , accumulated chlorophyll content; Chl loss , duration of rapid chlorophyll loss; Chl per , duration of chlorophyll persistence; Chl tot , total duration of chlorophyll persistence and loss; o Cd, degree day; GA, green area; GA accum , accumulated green area; GA loss , duration of rapid green area loss; GA per , duration of green area persistence;GA tot , total duration of green area persistence and loss; GF, grain filling; GFR, grain filling rate; H 2 , broad sense heritability; LOD, logarithm of the odds; Max chl, maximum chlorophyll content; Max CLR, maximum chlorophyll loss rate; Max GALR, maximum green area loss rate; MWC, maximum water content of grain; QTL, quantitative trait locus; RIL, recombinant inbred line; TGW, thousand grain weight; t max , the time at maximum grain filling rate; t mwc , the time at maximum water content; WAR, grain water absorption rate; WLR, grain water loss rate. 2 AbstractThe physiological process of how anthesis time and leaf senescence patterns affect individual grain weight of wheat has only been partially elucidated. In this study, a recombinant inbred line mapping population of bread wheat (Triticum aestivum L. 'Forno') and its relative spelt (Triticum spelta L. 'Oberkulmer'), contrasting for phasic development and leaf senescence kinetics, was used to understand the physiological and genetic relationships among anthesis time, leaf senescence, grain filling processes, and individual grain weight. Phenotypic measurements were taken in the field over two growing seasons. The results showed that earlier anthesis and delayed leaf senescence were associated with larger grains. Furthermore, early anthesis and delayed but fast leaf senescence promoted grain filling rate (but shortened its duration), grain water absorption rate and maximum grain water content, while individual grain dry matter and water accumulation displayed strong relationships with individual grain weight. A total of 118 significant quantitative trait loci (QTL) were identified in this mapping population, including six QTL for anthesis dates, 24 for flag leaf senescence, 69 for grain filling traits, and 19 for individual grain weight. Frequent QTL coincidences between these traits were observed on chromosomes 2A, 3B, 4A, 4DL, 5A, 5B, 5DL and 7B. Analysis of allelic effects confirmed the above physiological relationships. Therefore, anthesis time and leaf senescence affect individual grain weight at least partly through their effects on individual grain dry matter and water accumulation, resulting from pleiotropy or tight gene linkages. Slightly early anthesis, and delayed but fast leaf senescence, can be used to maximize individual grain weight and yield potential in wheat.

Tillering is under the control of both genetic factors and R:FR. Genetic variation in tillering and tolerance to low R:FR can be used to optimize tillering patterns for yield improvement in wheat.

Transformation from q to Q during wheat domestication functioned outside the boundary of threshability to increase yield, grains m, grain weight and roundness, but to reduce grains per spike/spikelet. Mutation of the Q gene, well-known affecting wheat spike structure, represents a key domestication step in the formation of today's free-threshing, economically important wheats. In a previous study, multiple yield components and spike characteristics were associated with the Q gene interval in the bread wheat 'Forno' × European spelt 'Oberkulmer' recombinant inbred line population. Here, we reported that this interval was also associated with grain yield, grains m, grain morphology, and spike dry weight at anthesis. To clarify the roles of Q in agronomic trait performance, a functional marker for the Q gene was developed. Analysis of allelic effects showed that the bread wheat Q allele conferred free-threshing habit, soft glumes, and short and compact spikes compared with q. In addition, the Q allele contributed to higher grain yield, more grains m, and higher thousand grain weight, whereas q contributed to more grains per spike/spikelet likely resulting from increased preanthesis spike growth. For grain morphology, the Q allele was associated with reduced ratio of grain length to height, indicating a rounder grain. These results are supported by analysis of four Q mutant lines in the Chinese Spring background. Therefore, the transition from q to Q during wheat domestication had profound effects on grain yield and grain shape evolution as well, being a consequence of pleiotropy.

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Abbreviations: ADM, accumulated dry matter; CGR, crop growth rate; cM, centi-Morgan; o Cd, degree days; GS39, Growth Stage 39 (the time at full flag leaf emergence); HI, harvest index; H 2 , broad sense heritability; LI, light interception; LOD, logarithm of the odds; QTL, quantitative trait loci; RIL, recombinant inbred line; RUE, radiation use efficiency; SFI, spike fertility index; SGD, spike growth duration; SGR, spike growth rate; SPI, spike partitioning index; TGW, thousand grain weight; WSC, water soluble carbohydrate. Artwork information AbstractThe preanthesis period in wheat is critical for growth of plant organs including leaves, stems, spikes and roots. However, the roles of the preanthesis biomass accumulation of plant and plant organs in yield determination are only partially elucidated, and the underlying genetic basis remains largely unknown. This study aimed to understand the physiological and genetic relationships between preanthesis biomass accumulation and yield determination. In a mapping population of bread wheat (Triticum aestivum 'Forno') and its relative spelt (Triticum spelta 'Oberkulmer') contrasting for biomass, the dry weight of above-ground whole shoots and different organs, and leaf area, were analysed at GS39 (full flag leaf emergence) and anthesis. Yield components (thousand grain weight, grains per spike, final shoot biomass and grain weight per spike) and plant height were measured at maturity,