Measuring Fine Roots in Fruit Crops: Finding Order in Functional Variation

Sustained production in horticultural crops depends on fine roots to supply water and nutrients over the life of a planting.  However the linkages of root measurements to root function are often arbitrary.  Commonly experiments that investigated root processes used traditional methods that characterize fine roots as those with diameters of ≤ 2 mm; however, these methods do not account for variation in fine-root diameter across species or fine-root specialization for absorption or transport.  Therefore, the objective of this research was to examine the efficacy of using a more functional approach based on root branching order to predict root function in economically important fruit crops.  Fine roots of apple (Malus domestica), peach (Prunus persica), grape (Vitus vinifera), and sour orange (Citrus aurantium) were sampled in the United States, and Asian pear (Pyrus pyrifolia) and jujube (Ziziphus jujuba) roots were sampled in China. Fine-root branches were sorted into orders one through five, and morphological and anatomical traits such as root diameter, specific root length (SRL), mycorrhizal colonization, C:N ratio, and the presence of cork periderm and secondary vascular tissue were assessed. Across all species, lower order roots (1 through 3) had no development of secondary vascular tissue or cork periderm, low C:N ratios, and presence of mycorrhizal fungal colonization, which suggests that these roots are specialized for absorption of water and nutrients. In contrast, fourth and fifth order roots that were often less than 2 mm in diameter typically had development of cork periderm and secondary vascular tissue, high C:N ratios, and little or no mycorrhizal colonization, which suggests that these roots are specialized for transport of water and nutrients.  These results suggest that the use of root order is an effective predictor of fine root function and is useful to group fine roots into two distinct classes: absorptive and transport.  Classifying root function based on root order can be used to draw meaningful conclusions about root foraging efficiency, production, and turnover, which have implications for whole-plant carbon allocation and root system responses to common horticultural practices such as fertilizer application and irrigation.