Root traits of herbaceous crops: Pre‐adaptation to cultivation or evolution under domestication?

Abstract Agricultural fields are commonly characterized by high nutrient and water availabilities, which are favourable for plant growth. Such conditions might promote the evolution of resource‐acquisitive strategies. We asked whether crop plants show root traits typical of resource‐acquisitive strategies and whether this strategy is primarily a result of their evolution under domestication or of the early selection of successful candidates for domestication. We studied a set of 30 crop species and their wild progenitors. We set up a greenhouse experiment to measure five root traits: root thickness, root tissue density, specific root length (SRL), root mass fraction (RMF) and root length ratio. In addition, we compiled data from other wild herbaceous species, growth in similar conditions to this experiment, to place the root traits of our crops in the context of wider botanical variation. Wild progenitors had thicker and less dense roots, with higher RMF and lower SRL, than other wild herbs. Thicker and less dense roots are indicative of fertile soils, which suggest that wild progenitors could have been adapted for success in agricultural conditions. Additionally, we found that domestication generally increased total plant dry mass, but none of the root traits evolved consistently towards a more resource‐acquisitive strategy after domestication across all species. Root trait values differed between progenitors and crop species for most pairs surveyed, but this occurred in diverse directions depending on crop species. Such differences were independent of phylogeny, functional group or variability in the domestication processes, such as timing of the domestication event or organ under focal artificial selection. Our comparative study revealed that the root phenotype exhibited by wild progenitors (thick roots with low density and SRL), when compared with other wild herbs, was in accordance with plants typical of fertile habitats. However, none of the root traits reacted to domestication in accordance with evolution towards faster growth strategies. Thus, the adaptation of crop root phenotypes to the fertile conditions of agricultural fields might be largely determined by early choices of wild species, rather than by further evolution under domestication. A plain language summary is available for this article.