Long-term research on avocado roots has not been performed in detail, mainly due to the difficulty and time required for these types of studies. However, at present, the use of genotypes with efficient root systems appears to be a key crop trait for sustainable intensification of agricultural production; consequently, research in this area has become more important (Gewin, 2010; Gregory et al., 2013).
The results of recent studies on the morphological characteristics of root systems have provided insight into the relationship between root architecture and the resulting increased efficiency in the use of important resources, such as water and nutrients, and improved productivity of cultivated crop species. For example, in soils with low phosphorus levels, the genotypes of common bean (Phaseolus vulgaris), which have shallow root systems, exhibit better development and productivity than genotypes with deeper rooting architecture (Henry et al., 2010; Ho et al., 2004; Rubio et al., 2001). The importance of root architecture for plant productivity relies on the fact that soil resources, such as water and nutrients, are unevenly distributed or are subjected to localized depletion. Thus, knowledge of the root system of a plant species is essential to exploit these resources (López-Bucio et al., 2003; Lynch, 1995).
The propagation technique used has been shown to influence the root architecture of crops. Recent studies of jatropha (Jatropha curcas) by Severino et al. (2011) indicated that seedling propagation generated a more vigorous root system than propagation through stem cuttings. In peach (Prunus persica) trees, a comparison between the air layering method and traditional tree propagation from seeds demonstrated that the rootstock genotype and the propagation method affected productivity and peach fruit quality (Picolotto et al., 2010).
Limited data are available regarding the effects of grafting on root system development in grafted commercial fruit tree crops. Firth et al. (2003) documented a significant effect of grafting on the root architecture of macadamia (Macadamia tetraphylla) trees. Nongrafted trees had long taproots (120 cm) and grafted trees exhibited short taproots (40 cm), with most of the fibrous root system located near the soil surface.
In commercial avocado orchards, both seedling and clonally propagated rootstocks are used. Commercial nursery production has primarily involved the use of seedling rootstocks, but due to the great heterozygosity of avocado trees, vegetative propagation techniques have been developed and used to improve the uniform tolerance to poor soil conditions and increase productivity (Schaffer et al., 2013). Clonally propagated rootstocks are the result of grafting the desired rootstock onto a seedling nursery tree, etiolating and rooting the desired rootstock, and then grafting the desired scion variety onto the rooted etiolated shoot (Frolich and Platt, 1972). Due to the importance of root architecture to tree physiology and productivity, the goal of this research was to quantify the effects of this unique clonal propagation technique and grafting on avocado root architecture and biomass allocation.
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