Fine root

Fine roots collectively comprise the majority of total length of a root system in many perennial and annual plants. As they age and develop, their function shifts from primarily acquiring soil resources to transporting materials to other parts of the plant body. The primary function of a fine root can be determined based on its functional characteristics. Certain characteristics of fine-root growth and physiology are highly plastic, however, allowing a plant's roots to respond to the nature of the local soil environment. Fine roots have been shown to respond to soil nutrient patches. The effect of these responses on a plant's nutrient uptake is unclear. Features that appear to be lateral branch scars have been observed on fine roots, indicating that some fine roots are deciduous. == Classification ==

Traditionally, fine roots are defined as plant roots with a diameter of two millimeters or less. This size-based definition is arbitrary, as it does not clearly or logically define fine roots based on anatomy, morphology, physiology, and/or function. Because fine-root traits like diameter vary by species, and research examining the function of different root sizes in different species is limited, diameter-based classes of fine roots are mostly arbitrary and complicate cross-species comparisons. Order-based classes This classification system assigns an order number to a root based on that root's position in the branching hierarchy of the root system, and is based on the Horton-Strahler scheme for ordering stream tributaries. The most distal segments of the root system (unbranched root segments that end in root tips) are classified as first-order roots. When two roots of the same order converge, the root that results from their combination is assigned the next highest root order (so two first-order roots merge to form a second-order root). When two root segments of different orders meet, the resulting root is given the higher order of the two roots that merged (so a second-order and a first-order root combine to form a second-order root). This classification system is common in modern root research, as many studies have shown that significant differences in fine-root traits can be detected when distinguishing roots by order. Traits that have been shown to increase with root order include root diameter, life span, and secondary growth, while root nitrogen content, mycorrhizal colonization, and turnover have shown decreases with increasing root order. Function-based classes By this system, fine roots are classified as either absorptive fine roots or transport fine roots. Absorptive fine roots mostly function in acquiring soil resources and comprise the most distal segments of a root system (lower-order segments). Transport fine roots result from the merging of absorptive fine roots and are therefore higher in root order. Primarily, transport fine roots transport plant materials and support plant structure, but may also store plant materials. These functional classes can often be distinguished visually in trees, but not in crops. == Ecology ==

Mycorrhizal associations In trees, fine roots are generally exclusively or dominantly colonized by a single mycorrhizal type, either arbuscular mycorrhizae or ectomycorrhizae. Material cycling In terrestrial environments, fine roots absorb water and nutrients from soil, and return such resources to the soil upon death and decomposition. Soil stability depends on root tensile strength. Root tensile strength increases with decreasing root diameter, so fine roots are stronger than coarse roots. == See also ==