Guide: Connecting Root Traits to Functions

• A growing interest in connecting root traits to functions has identified significant patterns.
• Several root traits can be involved in a single plant function, and the importance of trait contribution will differ depending on species, biomes, seasons, environment, and soil types.
• The widely studied morphological and architectural root traits are root length density, root diameter and area, root branching density, root hair length and density, root mass fraction, vertical root distribution, specific root length, maximum rooting depth, and mycorrhizal association.

Tools like the minirhizotrons have advanced the study of plant belowground components. Root traits can be morphological, physiological, anatomical, chemical, or biological, and a single root trait can help in more than one plant’s functioning. The soil depth and fertility, environment, species, biomes, plant ontogeny, and season moderate the importance of each root trait for a function. Freschet et al. (2021) explored how root morphology and architecture play a role in connecting root traits to functions, particularly in areas like nutrient and water uptake, microbial interactions, plant anchorage, and drought resistance.

1. Root Length Density

Root length density is defined as the length of roots in a unit volume of soil. It is critical for plant growth and performance. This trait increases the root’s spatial coverage of soil. The main plant functions it is linked to is plant anchorage in all plants.

• Herbaceous species: More root length density prevents vertical uprooting in herbaceous species, especially during grazing by large herbivores. As root length increases to a critical point, it is difficult for plants to be uprooted from the soil; they snap. This root trait is important across soil horizons and depths.
• Trees: Higher root length density offers resistance to overturning in tree species subject to strong lateral wind. Regardless of the root type, tap, lateral, or sinker, an increase in root length density stabilizes trees and prevents overturning.

2. Root Diameter and Area

Diameter variation between roots and along the root can be associated with its length and affects structure. Root diameter is crucial for root elongation. Root diameter and length are also used to calculate root surface area.

Figure 1: “The relationship between root surface area and Ca uptake by cucumber plant,” Tachibana & Ohta (1983). (Image credits: DOI: 10.1080/00380768.1983.10434642)

Both root diameter and surface are essential for root development:

• Water and nutrient uptake: Root surface area influences the uptake of water and nutrients and ion influx; see Figure 1.
• Root penetration: Roots with more diameter can infiltrate deeper in hard soils.
• Plant anchorage: Higher primary and fine root areas increase soil-to-root contact stabilizing soils. As a result, trees are less likely to be toppled during heavy winds.

3. Root Branching Density

Root branching density is the number of lateral root branches per unit length of a root. It is a critical root architecture trait. There is considerable variation in root branching density depending on species or form, for example, tap or adventitious roots. It can determine the number of roots a plant has. Root branching density has several plant functions and is listed below.

• Soil occupancy: Root numbers and branching patterns determine a plant’s soil area. Plants with high branching density have more roots in a volume of soil, increasing local soil exploitation and keeping competing roots away. Higher branching density occurs in nutrient-rich microsites.
• Deep soil exploration: The root branching angle of large roots increases root growth deeper into soils and explores more soil volume when faced with competition.
• Nitrogen and water acquisition: Plants with low branching density favor soil exploration to look for mobile resources like water and nitrogen.
• Phosphorus acquisition: Dauciform and cluster roots are specialized to access phosphorus in nutrient-poor soils.
• Plant anchorage: Root branching density and the number of roots improve plant anchorage. As root branching density increases, the force required to uproot a plant increases. More lateral roots increase the weight of soil enmeshed by roots, and more root length increases pull-out resistance.
• Drought resistance and avoidance: By improving the exploration of soil volume for water, laterally and vertically, root density determines drought resistance and tolerance.
• Belowground dispersal: The lateral growth of roots determines how far root rhizomes and stolon spread, to produce new ramets, in clonal species.

4. Root Hair Length and Density

Root hair density is the number of root hairs per unit root length and has several crucial plant functions.

• Soil occupancy: Root hair length and density increase a plant’s coverage of soil volume and can improve plant anchorage.
• Phosphorus acquisition: Root hairs are more critical for acquiring phosphorus than a mycorrhizal association. Root hair traits are also crucial for the uptake of other immobile nutrients like zinc, iron, and manganese.
• Increases nitrogen uptake rate: Root hairs are more important for accessing immobile nutrients. However, they can also improve mobile nitrogen uptake by improving the total root surface area for soil exploration and reaching beyond the rhizosphere around the primary roots.
• Water uptake: By increasing contact of roots with soil particles covered with water, root hairs increase water access and uptake.
• Resistance and avoidance of drought: Root hair traits help plants against drought by improving water uptake.
• Protection against pathogens and herbivory: Root hairs provide a habitat for natural enemies of herbivores and nematodes and reduce biological stress. Also, root hairs surrounding the main roots prevent pathogens and herbivores from reaching and damaging the root epidermis.
• Microbial association: Root hair traits are also crucial for determining the amount of root exudates excreted into the soil, which shapes the microbial population in the rhizosphere and helps in mineralization and increasing nutrient availability for plants.

5. Root Mass Fraction

Root mass fraction is the biomass allotted to belowground plant components and determines plant performance. It can indicate the energy the plant is putting into root length to increase plant-soil interaction.

• Soil exploration and nitrogen acquisition: Plants can prioritize root functions by increasing the investment into the entire root system or specific parts. For example, increase root length to reach more soil volumes to look for nitrogen.
• Drought resistance and avoidance: Biomass allocation can be determined by environmental conditions. During dry periods, plants will increase the relative allotment of biomass resources to grow more extensive root systems to access water to survive.
• Plant anchorage: A plant that invests less in roots risks being uprooted due to herbivory or storms.

6. Vertical Root Distribution

Vertical root distribution (VRD) is the distribution of root-density biomass along the soil profile at different depths. VRD is unknown for most plant species, even though this architectural parameter is crucial to understanding plant functions like water and nutrient uptake, competition, and plant-soil interactions.

• Resource acquisition: Finding and absorbing resources like water and nutrients depends on the soil depth where plants have the most roots.
• Root mass fraction: A plant’s VRD will determine the resources a plant has to allocate for the belowground parts. Ecosystems’ temperatures, precipitation patterns, and soil features are essential in shaping general trends.
• Plant anchorage: The root architecture and depths will determine how a plant can resist uprooting. Deeper roots are better, as anchorage force is proportional to their length.

7. Specific Root Length

Specific root length (SRL) is the root length ratio to the dry mass of fine roots. Plants with high SRL grow longer roots for a given mass, have shorter root lifespans, and access more nutrients and water. However, low diameter and tissue density can also provide a high SRL.

• Soil occupancy: Since SRL indicates increased root length, there are more roots for exploring and exploiting the soil per unit biomass.
• Nitrogen acquisition: Since SRL increases the depths to which roots grow, it can increase nitrogen uptake in many cases. However, this effect of SRL is not ubiquitous.
• Resistance to uprooting: High SRL improves anchorage with longer and thinner roots, while low SRL gives less stable anchorage due to shorter and thicker roots.
• Drought resistance and avoidance: Higher SRL increases water uptake to alleviate drought effects.

8. Maximum Rooting Depth

The maximum depth to which plants grow varies across biomes, species, and plant habits. Rooting depth is more than initially held views. Global averages of maximum rooting depths for trees are 7.0m, for shrubs 5.1m, and for herbaceous plants, it is 2.6m.

• Soil occupancy and resource acquisition: The maximum rooting depth refers to the soil depths from which plants can absorb water and nutrients in dry years. It is the soil layers that a plant can colonize to access resources.
• Drought resistance and avoidance: Deep roots for plants help avoid or resist drought effects.

9. Mycorrhizal Association

Around 90% of plants form symbiotic associations with mycorrhiza, where roots provide carbohydrates in return for increased acquisition of nutrients, protection against pests, and stress tolerance. The mycorrhizal hyphal length enhances soil occupancy and extends a plant’s exploration and exploitative capacity to improve plant productivity.

• Nitrogen acquisition: Mycorrhizal fungi can absorb nitrogen in a form usable by plants even when the compounds are available at low concentrations.
• Phosphorus acquisition: Arbuscular Mycorrhizae (AM) can mineralize and extract phosphorus from soils and transfer them to the plant roots.
• Biocontrol of pathogens and herbivory: AM can change microbial community composition to one favorable for plants by increasing beneficial microbes. These microbes compete for resources and space with pathogens. The beneficial microbes also can control nematode populations through parasitism.
• Improve soil structure and water retention capacity: Mycorrhizal fungi improve soil structure by aggregating and binding soil particles. Therefore, plant roots can penetrate easily in the soil. Furthermore, the soil can retain more water, improving plant-water relations.

Conclusion

Minirhizotrons like CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager can measure the morphological root traits and indirectly measure some physiological parameters like root elongation, lifespan, and turnover, which are also crucial root traits.

The plant functions of various root traits are not entirely understood. The most widely used root traits are not necessarily the most crucial for plant functioning. Other challenging-to-measure root traits, like physiological and chemical traits, can also be essential for a plant’s function. Understanding root traits’ connection to plant functioning is vital to regulate nutrient levels and improving crop productivity and forest functioning.

Source

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Freschet, G. T., Roumet, C., Comas, L. H., … Stokes, A. (2021). Root traits as drivers of plant and ecosystem functioning: Current understanding, pitfalls and Future Research Needs. New Phytologist, 232(3), 1123–1158. https://doi.org/10.1111/nph.17072

Paez-Garcia, A., Motes, C., Scheible, W.-R., Chen, R., Blancaflor, E., & Monteros, M. (2015). Root traits and phenotyping strategies for plant improvement. Plants, 4(2), 334–355. https://doi.org/10.3390/plants4020334

Pagès, L. (2019). Analysis and modeling of the variations of root branching density within individual plants and among species. Frontiers in Plant Science, 10. https://doi.org/10.3389/fpls.2019.01020

Pierret, A., Moran, C.J., Mclachlan, C.B., et al. (2011).  Measurement of root length density in intact samples using x-radiography and image analysis. Image Analysis & Stereology, 19(2), 145-149. ISSN 1854-5165. https://doi.org/10.5566/ias.v19.p145-149.

Saengwilai, P., Strock, C., Rangarajan, H., Chimungu, J., Salungyu, J., & Lynch, J. P. (2021). Root hair phenotypes influence nitrogen acquisition in maize. Annals of Botany, 128(7), 849–858. https://doi.org/10.1093/aob/mcab104

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Tachibana, Y., & Ohta, Y. (1983) Root surface area, as a parameter in relation to water and nutrient uptake by cucumber plant. Soil Science and Plant Nutrition, 29 (3), 387-392, DOI: 10.1080/00380768.1983.10434642

Wu, Q., Pagès, L., & Wu, J. (2016). Relationships between root diameter, root length and root branching along lateral roots in adult, field-grown maize. Annals of Botany, 117(3), 379–390. https://doi.org/10.1093/aob/mcv185