Aims Selection for optimal root system architecture (RSA) is important to ensure genetic gains in the sustainable production of wheat (Triticum aestivum L.). Here we examine the hypothesis that past wheat breeding has led to changes in RSA and that future breeding efforts can focus directly on RSA to improve adaptation to target environments. Methods We conducted field trials using diverse wheat varieties, including modern and historic UK varieties and non-UK landraces, tested under contrasting tillage regimes (non-inversion tillage versus conventional ploughing) for two trial years or different seeding rates (standard versus high rate) for one trial year. We used field excavation, washing and measurement of root crowns (‘shovelomics’) to characterise RSA traits, including: numbers of seminal, crown and nodal roots per plant, and crown root growth angle. Results We found differences among genotypes for all root traits. Modern varieties generally had fewer roots per plant than historic varieties. On average, there were fewer crown roots and root angles were wider under shallow non-inversion tillage compared with conventional ploughing. Crown root numbers per plant also tended to be smaller at a high seeding rate compared with the standard. There were significant genotype-by-year, genotype-by-tillage and genotype-by-seeding-rate interactions for many root traits. Conclusions Smaller root systems are likely to be a result of past selection that facilitated historical yield increases by reducing below-ground competition within the crop. The effects of crop management practices on RSA depend on genotype, suggesting that future breeding could select for improved RSA traits in resource-efficient farming systems.

28Aims 29 Selection for optimal root system architecture (RSA) is important to ensure genetic gains in the sustainable production 30 of wheat (Triticum aestivum L.). Here we examine the idea that past wheat breeding has led to changes in RSA and 31 that future breeding efforts can focus directly on root traits to improve adaptation to a target environment. 32 Methods 33We conducted three field trials using diverse wheat varieties, including modern and historic UK varieties and non-UK 34 landraces, tested under contrasting tillage regimes (non-inversion tillage versus conventional ploughing) or different 35 seeding rates (standard rate versus high rate). We used field excavation, washing and measurement of root crowns 36 ('shovelomics') to characterise RSA traits, including: numbers of seminal, crown and nodal roots per plant, and crown 37 root growth angle. 38 Results 39We found large differences among genotypes for all root traits. Modern varieties generally had fewer roots per plant 40 than historic varieties. There were fewer crown roots and root angles were wider, on average, under shallow non-41 inversion tillage compared with conventional ploughing. Crown root numbers per plant also tended to be smaller at a 42 high seeding rate compared with the standard rate. There were significant genotype-by-year, genotype-by-tillage and 43 genotype-by-seeding-rate interactions for many root traits. 44 Conclusions 45Smaller root systems is likely to be a result of past selection and may have facilitated historical yield increases by 46 reducing below-ground competition within the crop. The effects of crop management practices on RSA depend on

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