A fine root is most commonly defined as a plant
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
that is two millimeters or less in diameter.
Fine roots may function in acquisition of soil resources (eg. nutrients, water) and/or resource transport, making them functionally most analogous to the
leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
and
twig
A twig is a thin, often short, branch of a tree or bush.
The buds on the twig are an important diagnostic characteristic, as are the abscission scars where the leaves have fallen away. The color, texture, and patterning of the twig bark are ...
s in a plant's
shoot system.
Fine-root traits are variable between species and responsive to environmental conditions.
Consequently, fine roots are studied to characterize the resource acquisition strategies and competitive ability of plant species. Categories of fine roots have been developed based on root diameter, position in a root system's branching hierarchy, and primary function. Fine roots are often associated with
symbiotic
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
fungi and play a role in many ecosystem processes like nutrient cycles and soil reinforcement.
Overview
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.
The traits of a plant species's fine roots are thought to be indicative of that species's evolved strategy for soil resource acquisition.
Certain characteristics of fine-root growth and physiology are highly
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
, 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.
Responses include the lengthening of root segments and increased total length of fine roots, increased initiation of lateral roots, and increased branching.
The effect of these responses on a plant's nutrient uptake is unclear.
In multiple ecosystem types and forest stand ages, fine-root biomass has been found to decrease with increasing soil depth.
Similarly, fine-root nitrogen concentration has been shown to be lower in deeper soil.
These shifts may reflect vertical changes in the nature of soil, as shallow soils may have greater available nutrient content than deep soils. Features that appear to be lateral branch scars have been observed on fine roots, indicating that some fine roots are
deciduous
In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...
.
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.
Early studies that used this definition assumed that all roots in the two millimeter size class are functionally alike, but many successive studies have shown that roots in this size class can have different demographic and functional traits.
Within the two millimeter size class, roots can be highly variable in characteristics and function. To account for this, root biologists have begun to define subcategories of fine roots based on root diameter, position in the root branching hierarchy, and function.
Diameter-based classes
As a group, fine roots are most consistently defined by the diameter cutoff of two millimeters. In recognition of the variety of root traits and functions within this category and the relationship between diameter and function, smaller diameter classes have been used in recent research. Studies focusing only on roots that function in resource acquisition have examined roots under one millimeter or 0.5 millimeters.
Roots with a diameter less than 0.5 millimeters have been termed 'very fine roots'.
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.
For example, two-millimeter-diameter fine roots may occur in trees, but would be very large roots in many annual and perennial species of crops.
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
mycorrhiza
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
l type, either
arbuscular mycorrhiza
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. (N ...
e or
ectomycorrhiza
An ectomycorrhiza (from Greek ἐκτός ', "outside", μύκης ', "fungus", and ῥίζα ', "root"; pl. ectomycorrhizas or ectomycorrhizae, abbreviated EcM) is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobi ...
e.
Competition
Plants often compete with surrounding plants for root space and resources. A plant's ability to compete, and strategy for competition, can be determined by examining the traits, abundance, distribution, and functions of fine roots and their associated mycorrhizas.
Material cycling
![Arbuscular mycorrhiza microscope](https://upload.wikimedia.org/wikipedia/commons/8/86/Arbuscular_mycorrhiza_microscope.jpg)
In terrestrial environments, fine roots absorb water and nutrients from soil, and return such resources to the soil upon death and decomposition.
Fine roots also release
exudates
An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding or exudation.
''Exudate'' is derived from ''exude'' 'to ooze' from Latin ''exsūdāre'' 'to (ooze out) sweat' (''ex-'' 'out' and ''sūdāre'' 'to ...
, including labile carbon, during life processes and turnover. This directly affects soil organic carbon pools, and indirectly affects them by stimulating microbial activity.
Therefore, fine roots play a role in
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
,
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
, and
nutrient
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
cycles of terrestrial ecosystems.
In forest carbon and nutrient cycles, the formation, death, and decomposition of fine roots can account for 20-80% of total net
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
.
Soil reinforcement
Plant roots support soil, which stabilizes sloped landscapes and limits soil
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
. Root size properties, including diameter, influence the mechanical reinforcement of a slope.
Soil stability depends on root tensile strength. Root tensile strength increases with decreasing root diameter, so fine roots are stronger than coarse roots.
[{{Cite journal, last1=Zhang, first1=Chao-Bo, last2=Chen, first2=Li-Hua, last3=Jiang, first3=Jing, date=February 2014, title=Why fine tree roots are stronger than thicker roots: The role of cellulose and lignin in relation to slope stability, journal=Geomorphology, volume=206, pages=196–202, doi=10.1016/j.geomorph.2013.09.024, issn=0169-555X]
See also
*
Absorption of water
In higher plants water and minerals are absorbed through root hairs which are in contact with soil water and from the root hairs zone a little the root tips.
Active absorption
Active absorption refers to the absorption of water by roots with th ...
*
Mycorrhiza
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
*
Plant nutrition
Plant nutrition is the study of the chemical elements and compounds necessary for plant growth and reproduction, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element i ...
*
Plant physiology
Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (bi ...
*
Rhizosphere
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microor ...
*
Root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
References
External links
Fine-Root Ecology Database
Plant roots