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Claudia Fassio, Ricardo Cautin, Alonso Pérez-Donoso, Claudia Bonomelli and Mónica Castro

Root morphological traits and biomass allocation were studied in 2-year-old ‘Duke 7’ avocado (Persea americana) trees propagated using seedling and clonal techniques. The plants either were or were not grafted with the scion ‘Hass’. Whole tree excavation 1 year after planting revealed that the propagation technique affected the root growth angle of the main roots (third order roots), the root length density (defined as the total length of roots per volume of soil), and the number of first and second order roots present. The root system of clonal trees showed a typical morphology of rooted cuttings, with a crown of roots originating from a relatively short stem, resulting in a shallow root system. Clonal trees, compared with seedlings, produced main framework roots with shallower angles and more fine roots (first and second order roots) that increased the root length density (defined as the total length of roots per volume of soil). Nongrafted seedlings exhibited a main taproot and lateral roots with narrow angles that penetrated deeper into the soil and increased the aboveground biomass but had a lower root-to-shoot ratio than nongrafted clonal trees. The grafting of both clonal and seedling trees resulted in similar root architecture and revealed that grafting significantly decreased the soil volume explored and the shoot and root biomass. Although both root systems were shallow, grafted clonal trees had a higher root-to-shoot ratio than grafted seedlings. In this study, a distinct class of roots with large diameter and unbranched growth was more abundant in the root systems of clonal trees. These types of roots (previously undescribed in avocado trees), called pioneer roots, may enhance soil exploration in clonal trees.