Fine root

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A fine root is most commonly defined as a plant root that is two millimeters or less in diameter. [1] Fine roots may function in acquisition of soil resources (eg. nutrients, water) and/or resource transport, making them functionally most analogous to the leaves and twigs in a plant's shoot system. [1] Fine-root traits are variable between species and responsive to environmental conditions. [2] [3] 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 fungi and play a role in many ecosystem processes like nutrient cycles and soil reinforcement. [4] [2]

Contents

Overview

Fine roots collectively comprise the majority of total length of a root system in many perennial and annual plants. [5] As they age and develop, their function shifts from primarily acquiring soil resources to transporting materials to other parts of the plant body. [2] [1] [6] The primary function of a fine root can be determined based on its functional characteristics. [1]

The traits of a plant species's fine roots are thought to be indicative of that species's evolved strategy for soil resource acquisition. [7] 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. [2] [3] Fine roots have been shown to respond to soil nutrient patches. [8] [7] Responses include the lengthening of root segments and increased total length of fine roots, increased initiation of lateral roots, and increased branching. [8] [9] The effect of these responses on a plant's nutrient uptake is unclear. [8] In multiple ecosystem types and forest stand ages, fine-root biomass has been found to decrease with increasing soil depth. [7] Similarly, fine-root nitrogen concentration has been shown to be lower in deeper soil. [7] These shifts may reflect vertical changes in the nature of soil, as shallow soils may have greater available nutrient content than deep soils. [10] [11] Features that appear to be lateral branch scars have been observed on fine roots, indicating that some fine roots are deciduous. [2]

Classification

Traditionally, fine roots are defined as plant roots with a diameter of two millimeters or less. [1] This size-based definition is arbitrary, as it does not clearly or logically define fine roots based on anatomy, morphology, physiology, and/or function. [2] [5] 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. [1]

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. [1] Roots with a diameter less than 0.5 millimeters have been termed 'very fine roots'. [2] [12] Because fine-root traits [13] 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. [6] [1] 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. [14]

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. [15] 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). [2] 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). [2] 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. [1] 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. [1]

Orders assigned to stream segments using the Horton-Strahler ordering scheme. Root orders are assigned using this scheme. Strahler-stream-order.png
Orders assigned to stream segments using the Horton–Strahler ordering scheme. Root orders are assigned using this scheme.

Function-based classes

By this system, fine roots are classified as either absorptive fine roots or transport fine roots. [1] Absorptive fine roots mostly function in acquiring soil resources and comprise the most distal segments of a root system (lower-order segments). [1] 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. [1] These functional classes can often be distinguished visually in trees, but not in crops. [14]

Douglas-fir fine roots associate with ectomycorrhizal fungi. Ectomycorrhizae002.jpg
Douglas-fir fine roots associate with ectomycorrhizal fungi.

Ecology

Mycorrhizal associations

In trees, fine roots are generally exclusively or dominantly colonized by a single mycorrhizal type, either arbuscular mycorrhizae or ectomycorrhizae. [2]

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. [2] [16]

Material cycling

Flax roots associate with arbuscular mycorrhizal fungi. Arbuscular mycorrhiza microscope.jpg
Flax roots associate with arbuscular mycorrhizal fungi.

In terrestrial environments, fine roots absorb water and nutrients from soil, and return such resources to the soil upon death and decomposition. [1] Fine roots also release exudates, including labile carbon, during life processes and turnover. This directly affects soil organic carbon pools, and indirectly affects them by stimulating microbial activity. [1] Therefore, fine roots play a role in water, carbon, and nutrient cycles of terrestrial ecosystems. [1] In forest carbon and nutrient cycles, the formation, death, and decomposition of fine roots can account for 20-80% of total net primary production. [6]

Soil reinforcement

Plant roots support soil, which stabilizes sloped landscapes and limits soil erosion. Root size properties, including diameter, influence the mechanical reinforcement of a slope. [4] Soil stability depends on root tensile strength. Root tensile strength increases with decreasing root diameter, so fine roots are stronger than coarse roots. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Root</span> Basal organ of a vascular plant

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 surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.

<span class="mw-page-title-main">Mycorrhiza</span> Fungus-plant symbiotic association

A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.

<span class="mw-page-title-main">Arbuscular mycorrhiza</span> Symbiotic penetrative association between a fungus and the roots of a vascular plant

An arbuscular mycorrhiza (AM) is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscules. Arbuscular mycorrhiza is a type of endomycorrhiza along with ericoid mycorrhiza and orchid mycorrhiza .They are characterized by the formation of unique tree-like structures, the arbuscules. In addition, globular storage structures called vesicles are often encountered.

<span class="mw-page-title-main">Ericoid mycorrhiza</span> Species of fungus

The ericoid mycorrhiza is a mutualistic relationship formed between members of the plant family Ericaceae and several lineages of mycorrhizal fungi. This symbiosis represents an important adaptation to acidic and nutrient poor soils that species in the Ericaceae typically inhabit, including boreal forests, bogs, and heathlands. Molecular clock estimates suggest that the symbiosis originated approximately 140 million years ago.

<span class="mw-page-title-main">Cluster root</span>

Cluster roots, also known as proteoid roots, are plant roots that form clusters of closely spaced short lateral rootlets. They may form a two- to five-centimetre-thick mat just beneath the leaf litter. They enhance nutrient uptake, possibly by chemically modifying the soil environment to improve nutrient solubilisation. As a result, plants with proteoid roots can grow in soil that is very low in nutrients, such as the phosphorus-deficient native soils of Australia.

<span class="mw-page-title-main">Soil respiration</span> Chemical process produced by soil and the organisms within it

Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna.

<i>Rhizophora apiculata</i> Species of tree

The tall-stilt mangrove belongs to the Plantae kingdom under the Rhizophoraceae family. R. apiculata is distributed throughout Australia, Guam, India, Indonesia, Malaysia, Micronesia, New Caledonia, Papua New Guinea, the Philippines, Singapore, the Solomon Islands, Sri Lanka, Taiwan, the Maldives, Thailand, Vanuatu, and Vietnam. Rhizophora apiculata is called ‘bakhaw lalaki,’ in the Philippines, "Thakafathi ތަކަފަތި" in the Maldives, 'Đước' in Vietnam, Garjan in India, as well as other vernacular names.

<span class="mw-page-title-main">Forest dieback</span> Stand of trees losing health and dying

Forest dieback is a condition in trees or woody plants in which peripheral parts are killed, either by pathogens, parasites or conditions like acid rain, drought, and more. These episodes can have disastrous consequences such as reduced resiliency of the ecosystem, disappearing important symbiotic relationships and thresholds. Some tipping points for major climate change forecast in the next century are directly related to forest diebacks.

<span class="mw-page-title-main">Mycorrhizal network</span> Underground fungal networks that connect individual plants together

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<span class="mw-page-title-main">Mycorrhizal fungi and soil carbon storage</span>

Soil carbon storage is an important function of terrestrial ecosystems. Soil contains more carbon than plants and the atmosphere combined. Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial decomposers, and soil minerals affect this pool of carbon, it is recently coming to light that mycorrhizal fungi—symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5 to 20% of total plant carbon uptake, and in some ecosystems the biomass of mycorrhizal fungi can be comparable to the biomass of fine roots. Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden. Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.

<span class="mw-page-title-main">Ectomycorrhiza</span> Non-penetrative symbiotic association between a fungus and the roots of a vascular plant

An ectomycorrhiza is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. The mycobiont is often from the phyla Basidiomycota and Ascomycota, and more rarely from the Zygomycota. Ectomycorrhizas form on the roots of around 2% of plant species, usually woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine and rose families. Research on ectomycorrhizas is increasingly important in areas such as ecosystem management and restoration, forestry and agriculture.

Biomass partitioning is the process by which plants divide their energy among their leaves, stems, roots, and reproductive parts. These four main components of the plant have important morphological roles: leaves take in CO2 and energy from the sun to create carbon compounds, stems grow above competitors to reach sunlight, roots absorb water and mineral nutrients from the soil while anchoring the plant, and reproductive parts facilitate the continuation of species. Plants partition biomass in response to limits or excesses in resources like sunlight, carbon dioxide, mineral nutrients, and water and growth is regulated by a constant balance between the partitioning of biomass between plant parts. An equilibrium between root and shoot growth occurs because roots need carbon compounds from photosynthesis in the shoot and shoots need nitrogen absorbed from the soil by roots. Allocation of biomass is put towards the limit to growth; a limit below ground will focus biomass to the roots and a limit above ground will favor more growth in the shoot.

Malcolm Colin Press is a British ecologist, professor and Vice-Chancellor of Manchester Metropolitan University (MMU), in the United Kingdom.

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<span class="mw-page-title-main">Ectomycorrhizal extramatrical mycelium</span>

Ectomycorrhizal extramatrical mycelium is the collection of filamentous fungal hyphae emanating from ectomycorrhizas. It may be composed of fine, hydrophilic hypha which branches frequently to explore and exploit the soil matrix or may aggregate to form rhizomorphs; highly differentiated, hydrophobic, enduring, transport structures.

<span class="mw-page-title-main">Root microbiome</span>

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Dark septate endophytes (DSE) are a group of endophytic fungi characterized by their morphology of melanized, septate, hyphae. This group is likely paraphyletic, and contain conidial as well as sterile fungi that colonize roots intracellularly or intercellularly. Very little is known about the number of fungal taxa within this group, but all are in the Ascomycota. They are found in over 600 plant species and across 114 families of angiosperms and gymnosperms and co-occur with other types of mycorrhizal fungi. They have a wide global distribution and can be more abundant in stressed environments. Much of their taxonomy, physiology, and ecology are unknown.

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Orchid mycorrhizae are endomycorrhizal fungi which develop symbiotic relationships with the roots and seeds of plants of the family Orchidaceae. Nearly all orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont.

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  17. ↑ Zhang, Chao-Bo; Chen, Li-Hua; Jiang, Jing (February 2014). "Why fine tree roots are stronger than thicker roots: The role of cellulose and lignin in relation to slope stability". Geomorphology. 206: 196–202. doi:10.1016/j.geomorph.2013.09.024. ISSN   0169-555X.
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