Abstract
Root system characteristics determine soil space exploration and resource acquisition, and these characteristics include competitive traits that increase individual fitness but reduce population performance. We hypothesize that crop breeding for increased yield is often a form of “group selection” that reduces such “selfish” traits to increase population yield. To study trends in root architecture resulting from plant breeding and test the hypothesis that increased yields result in part from group selection on root traits, we investigated root growth and branching behavior in a historical sequence of wheat (Triticum aestivum) cultivars that have been widely grown in northwestern China. Plants were grown in gel‐filled chambers to examine growth angles, numbers, and lengths of seminal roots, and in soil‐filled chambers under eight soil resource levels for fractal analysis of root system architecture. Yield in field was evaluated at standard and low planting densities. Newer cultivars produced higher yields than older ones only at the higher sowing density, showing that increased yield results from changes in competitive behavior. Seminal root number and growth angles were negatively correlated with yield, while primary seminal root length was positively correlated with yield. Roots of higher‐yielding modern varieties were simpler and less branched, grew deeper but spread less laterally than modern varieties. The fractal dimension of root branching was negatively correlated with the yield of cultivars at all resource levels. Root:shoot ratio was negatively correlated with yield under high soil resource levels. The results are consistent with the hypothesis that the success of wheat breeding for higher yields over past 100 years in northwestern China has been in part due to unconscious group selection on root traits, resulting in smaller, less branched, and deeper roots, suggesting a direction for further increases in crop yield in the future.
Highlights
Natural selection increases the fitness of individuals, but this does not always maximize population performance, because “selfish” traits or behaviors, which damage population performance, are often selected (Denison, Kiers, & West, 2003; Weiner, Du, Zhang, Qin, & Li, 2017)
To study trends in root architecture resulting from plant breeding and test the hypothesis that increased yields result in part from group se‐ lection on root traits, we investigated root growth and branching behavior in a his‐ torical sequence of wheat (Triticum aestivum) cultivars that have been widely grown in northwestern China
The results show that the relationships between yield, root:shoot ratio, and harvest index were influenced by soil resource levels (Tester & Langridge, 2010)
Summary
Natural selection increases the fitness of individuals, but this does not always maximize population performance, because “selfish” traits or behaviors, which damage population performance, are often selected (Denison, Kiers, & West, 2003; Weiner, Du, Zhang, Qin, & Li, 2017). Donald (1968) pro‐ vided a list of the desirable shoot architectural characteristics for what he called a cereal “ideotype” for intensive production: short stem, few, small, erect leaves, a large, and erect ear He implied that these traits will benefit population yield in monoculture at the ex‐ pense of individual performance in a more diverse plant population or community. We ask whether changes in several root architectural traits over 110 years of wheat breeding in northwest‐ ern China are consistent with the hypothesis that cultivar evolution will show increased population yield at the expense of some traits favored by natural selection.
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