Nieves Martín-Robles, Javier Morente-López, Grégoire T. Freschet, Hendrik Poorter, Catherine Roumet, Rubén Milla
Plant domestication involves selection, modification and long-term use of wild plant species with traits regarded as favorable by humans. Major changes commonly associated with artificial selection include increased yield in the organs of interest (e.g. seeds or fruits), stronger apical dominance, and loss of seed dispersal and seed dormancy mechanisms. However, crop plants have not only been shaped by artificial selection but also by natural selection pressures. Natural selection under agricultural conditions, which differ from wild habitats in the availability of resources, or the intensity and frequency of disturbance, might have led to adaptations in above and belowground traits. Aboveground, the consequences of natural selection include decreased herbivore defense, higher stomatal densities on the upper side of leaves and increased nitrogen and phosphorus concentration in leaves. Some of the aboveground consequences of crop evolution (high growth rates and leaf nitrogen content) are typical of fast-growing resource-acquisitive strategies. Theoretical and empirical evidence based on aboveground traits suggest that domesticated species have fast acquisitive strategies, either as a consequence of pre-adaptations to the agricultural environment and/or by evolution under cultivation. In this study we investigated whether domesticated plants show root trait values 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 (or wild progenitors) for domestication.
Our comparative analysis revealed that none of the root traits reacted to domestication in accordance with evolution towards faster-growth strategies. Root traits changed during most of the 30 domestication processes surveyed here, but this occurred in diverse directions, depending on the crop species, and irrespective of evolutionary history and functional group affiliations, or of variability in the domestication processes. The diversity of responses to domestication encountered here emphasizes the importance of a comparative focus on multiple crops . Finally, the less dense and thicker roots of crop wild progenitors suggests that the roots of the wild species selected by early farmers were already adapted to fertile and disturbed conditions, thereby supporting the Dump Heap hypothesis (early plant domestication occurred by plants that had passed through the human digestive tract growing in fertile middens). Thus, the adaptation of roots to fertile soil appears to be largely determined by the choice of wild species by the first farmers rather than by further evolution under domestication. These results have important implications for our understanding of resource acquisition strategies of crop roots and portend applied approaches to develop improved cultivars.