What is Root Architecture?

• The root system architecture is defined using root morphological, topological, and geometric parameters.
• Two classification systems developed by Yen et al. (1987) and Fitter et al. (1991) are widely used.
• Several other classifications for root architecture also exist.
• Root system forms vary between and within species and are based on the plant’s soil conditions at the growth site. Root form or architecture can influence various plant functions such as nutrient and water acquisition, anchorage, and interactions with other species. It is a trait that is rigorously controlled through plant breeding to improve water use efficiency and stress tolerance. This article outlines the most common classifications of root systems used by scientists to optimize crop production.

What is Root Architecture?

The root system architecture refers to the spatial distribution and arrangement of the roots in the soil, defined by root topology and geometry. Root architecture covers the arrangement of most of the root system, except root hairs.

According to Lynch (1995), several concepts are used to describe root architecture, including morphology, topology, geometry, and distribution of roots.

• Root morphology describes the surface characteristics of a single root axis and includes root traits like diameter, root cap, surface undulations, and pattern of daughter roots.
• Root topology refers to the non-metric hierarchical order in which the root axes are connected to determine the branching patterns in the roots. Root topology also reflects the branching status of each axis.
• Root geometry is based on branch lengths, interbranch distances, root tip length, branching angles, and radial direction.
• Root distribution refers to the presence of roots and is measured by traits such as root length, biomass, and distance from the stem. Distance from neighboring plants is also considered in root distribution.

Root architecture is complex and can vary within a species, different growth stages, seasons,  or in various parts of the root system. Soil properties, such as bulk density, drainage, mechanical resistance, aeration, pH, and concentrations of minerals and toxins, will influence root growth. For example, soil compaction reduces root elongation but increases the diameter of the roots.

• Roots are very responsive to their environment and optimize resource acquisition.
• Tradeoffs exist between root form and function. Benefits in resource acquisition, growth, and productivity offset the cost of producing more roots.
• Plants exhibit significant variations in their root responses to stress.
• Root architectural traits are controlled by multiple genes.
• Root architecture effects extend also to plant communities and ecosystems by influencing nutrient cycling, water budgets, hydrology, and landscape stability.

The plasticity in root architecture makes it challenging to classify roots, but also offers opportunities for improving crop productivity.

Root System Development

Figure 1: Types of embryonic and adventitious roots, Bianco and Kepinski (2018). ( Image credits: https://doi.org/10.1093/jxb/ery390).

The variation in root forms starts from their origin. The first roots to emerge are embryonic, the primary and seminal roots, which can develop lateral roots. Adventitious roots arise from non-root tissue. They can be junction roots that originate at the root-shoot junction, crown roots arising from nodes below the soil surface, brace roots growing from nodes above the ground, stem roots from internodes, and hypocotyl roots; see Figure 1. In monocots, the embryonic roots do not persist after the initial stages, and crown and brace roots form the root system. In dicots, the embryonic primary roots and their laterals form the architecture.

Several hormones are involved in influencing root branching, like auxin, cytokinin, and ethylene. Auxin is the primary hormone that also initiates the growth of the two types of roots. Root branching is a critical factor that determines the distribution of roots in the architecture.

Root Architecture Classification Systems

Several classification systems for root architecture exist. In this article, we discuss three central systems.

1. Eleven root system types identified by Kutschera et al.

Based on root topology and geometry, two classification systems are used:

2. Two root types proposed by Fitter et al.
3. Five types of root systems were proposed by Yen et al., which is the most commonly used classification system.

Root Systems by Kutschera

Figure 2: The types of root systems identified by Kutschera et al., according to Lichtenegger et al. (1997). (Image credits: https://archive.org/details/stapfia-49-001-331/page/n65/mode/2up)

The types of root architecture proposed by Kutschera et al. (1960, 1982, 1992) are one of the oldest systems. They described 11 types, and these are as follows:

1. String type: A long string-like taproot with short lateral roots, as in common thistle (Eryngium campestre). (Types 1-3 in Figure 2)
2. Strand-conical: Strong taproot and lateral roots at the tip that form a cone pointed to the topsoil, as in Chondrilla juncea. (Type 4 in Figure 2).
3. Cylindrical: Large taproot with strong lateral roots and vertical sinkers that are uniformly distributed in an arc that ends suddenly; for example, pasque flower (Pulsatilla pratensis). (Types 5 and 6 in Figure 2).
4. Cylindrical-conical: Similar to cylindrical with a strong taproot and laterals, but the deep roots taper at the end, with longer roots at the center, for example, German broom (Genista germanica). (Type 7 in Figure 2).
5. Inverted conical: The roots are shaped like an inverted cone, with most of the biomass in shallow soils, where the lateral spread is wide at the top and less at the bottom; for example, fenugreek (Trigonelia balansae). (Types 8 and 9 in Figure 2).
6. Mushroom-shaped: The Upper laterals spread widely, while the taproot continues down to send more vertical laterals in the deeper soils to give an inverted mushroom-shaped root system; for example, clover (Trifolium trichocephalum). (Type 10 and 11 in Figure 2).
7. Dumbbell-shaped: The taproots form two areas of profuse lateral growth, one at the top and one at the bottom; for example, sandy sand (Onosma arenarium). (Type 12 in Figure 2).
8. Ovate: The root system is not deep and resembles an inverted conical root system, as seen in species such as black-leaf catchfly (Silene otites). (Type 13 in Figure 2).
9. Umbrella-shaped: The shallow root system has several downward laterals that end at the same depth, forming an umbrella shape, for example, in the stemless primrose (Primula acaulis). (Type 14 in Figure 2).
10. Cup-shaped: The laterals grow upward in an arch in marsh and peat plants; for example, broad-leaved marsh orchid (Dactylorhiza majalis). (Type 15 in Figure 2).
11. Disc-shaped: The root system grows in the topsoil, spreading laterally to form a plate or disc, for example, common heather (Calluna vulgaris). (Type 16 in Figure 2).

Kutschera et al. have tried to classify the root systems, taking into consideration the climatic conditions and soil types where the plants are found, to connect the root architecture with environmental factors.

Fitter’s System

Fitter et al. (1991) proposed a system of two extreme types of branching patterns based on root topology- herringbone branching and dichotomous branching, as shown in Figure 3.

In this system, the nodes from which the lateral arises, as well as the length of links or the internode/branch distance, are crucial. Another critical factor is whether the link ends at the exterior or is an internal link that continues to the next node.

The links are ordered (or numbered, starting with one at the top). The number of external links gives the magnitude, and the number of links in the longest root from base to tip provides the altitude used to calculate the topology index.

Figure 3: “Schematic representation of Fitter’s link-based parameters to describe root topology: (a) Herringbone branching; (b) Dichotomous branching. A link is defined as a piece of root between two branching points (interior link) or between a branch and a meristem (exterior link). The length between two branching points is link length” Pang et al. 2024. (Image credits: https://www.mdpi.com/1999-4907/15/4/585)

Herringbone branching: In this form, the branches occur only on the central taproot. The laterals are all external links and do not produce further branches.

Dichotomous branching: This is a complex branching pattern where branching occurs on the primary roots as well as laterals. In this root system, the laterals produce both internal and external links.

The fitter’s system is based on a primary root, with first-order and second-order laterals.

Yen’s Root Architecture Systems

Figure 4: Five branching patterns of plant root system architectures proposed by (Yen1987). (a) H-type root system; (b) V-type root system; (c) R-type root system; (d) VH-type root system; (e) M-type root system, Wang et al. 2024. (Image credits: https://doi.org/10.1007/s11104-025-07333-6)

Yen’s classification is similar to Fitter’s in its use of root numbers, length, diameter, and branching points (nodes) and angles. Yen et al. studied the root systems of 108 species and identified seven types based on maximum root depth, lateral root extent, and root distribution pattern.

However, based on root shapes, Yen et al. (1987) classified the roots into five major types based on effects used for soil stability, binding, and wind resistance; see Figures 4 and 5. These five types are:

1. H-type: Horizontal root systems have roots that grow horizontally and spread wide. Most of the distribution (>80% of biomass) is within moderate depths of 60 cm.
2. R-type: Right root systems have obliquely growing roots and have a wide spread. Lateral roots are found in some cases. These root systems are deep, with only 20% of growth in the top 60 cm.
3. VH-type: These root systems have a firm tap root and several sturdy laterals branching profusely at low angles. The root depth is moderate to deep, with 80% of biomass in the top 60 cm of soil.
4. V-type: Vertical root systems have near-vertical roots with a narrow spread and few laterals. These are also moderate to deep systems.
5. M-type: Massive root systems have roots that branch and grow in many directions. The longest root is deep, but 80% of roots are found in the top 30 cm of the soil.

VH types are ideal for slope stabilization and wind resistance. H and M types are suitable for soil reinforcement. V types are wind resistant.

One study considered first-order roots horizontal roots if the angle was between 0° ≤ θ < 30°, roots growing at an angle of 30° ≤ θ < 60° were inclined roots, and vertical roots had an angle between 60° ≤ θ ≤ 90°.

Classifying Root Architecture

Earlier, destructive sampling was necessary to find the root architecture type due to the classification system used and the lack of non-destructive tools. The underground nature and opaque qualities of soil make non-destructive monitoring a challenge. Root architecture classification systems based on root topology, geometry, and morphology are easier to use in combination with modern non-destructive tools, such as minirhizotrons. For example, CID BioScience Inc. offers two handheld root scanners, the CI-600 In-Situ Root Imager and the CI-602 Narrow Gauge Root Imager, which can be used with installed transparent root tubes.  The camera can scan root growth and structure at various depths up to 120 cm to produce high-resolution images. The camera can be rotated to take 360° images of roots. Root number, length, diameter, area, angle, and volume are easy to estimate by analyzing the images with the accompanying software RootSnap! These parameters provide the necessary information to identify the root architecture of plants.

Contact us at CID BioScience to learn more about our root imagers for root analysis.

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