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The different plant strategies in the use of resources can be aligned on an economics spectrum, resulting from the trade-off between maximizing growth and resource capture (acquisitive strategy) versus the conservative strategy, with more protected organs of longer life span. Within this context, it is of great relevance to know the amount of photosynthate inverted by the plant on the construction of fine roots to explore the soil and uptake water and nutrients.

          A research team from the University of Cordoba, in collaboration with IRNAS-CSIC, CEBAS-CSIC and the University of Brandenburg-Cottbus, has studied the components of the construction costs (carbon, minerals and nitrogen concentrations) of the fine roots (< 2mm diameter) for 60 Mediterranean woody species.           The type of association with symbiotic microbes influenced on the contribution of different components in the root construction costs. Thus, the concentration of minerals was more important for the roots of plants associated with ectomycorrhizal fungi (EcM), while the organic nitrogen was dominant in roots associated with rhizobial N-fixing species. Therefore, the specific resource-use strategy associated to each type of symbiont modulate the trade-off between structural and metabolic compounds, during the building of roots.

Environmental conditions also influenced on the root construction costs. In nutrient-poor soils the energy spent per unit of root mass by plants was greater than the root cost in more fertile and favourable conditions. Plant species able to tolerate stress caused by water and nutrient scarcity often have a conservative strategy in the use of resources, with more costly roots of longer life span.   

In conclusion, the root construction cost is a key functional trait for terrestrial plants, related with their strategies in the use of resources, the type of associations with symbiotic microorganisms, and with the availability of soil water and nutrients.

This study has been published on line the 7 February 2021 in the Journal of Ecology:

de la Riva EG, Prieto I, Marañón T, Pérez‐Ramos IM, Olmo M, Villar R (2021). Root economics spectrum and construction costs in Mediterranean woody plants: The role of symbiotic associations and the environment. Journal of Ecology (in press). https://doi.org/10.1111/1365‐2745.13612

Root functions are multiple and essential for plant growth and survival. They include nutrient and water acquisition, resource storage, support of symbiotic soil microbes and anchorage. At the ecosystem level, they contribute to soil structure and to the carbon and nutrient cycles.

Researchers of the IRNAS-CSIC, in collaboration with the University of Seville and the University of Cordoba, have studied the variation of 27 root traits (morphological, chemical and isotopic) in seven tree species planted in the Guadiamar Green Corridor (Seville, Spain). Three of the species are deciduous: white poplar (Populus alba), narrow-leafed ash (Fraxinus angustifolia) and hackberry (Celtis autralis). While the other four species are evergreen: stone pine (Pinus pinea), holm oak (Quercus ilex), wild olive (Olea europaea) and carob (Ceratonia siliqua).

The main variation trend observed in the root traits supports the “root economics spectrum” hypothesis. There is a trade-off between tree species having lighter roots and higher length per mass unit, which maximizes soil resource acquisition and thus they grow faster on favourable conditions. On the contrary, tree species with denser roots and lower specific length tend to have resource conservation and slower growth, on adverse conditions.

Besides the main trend, there are other dimensions reflecting the root multifunctionality. 1) The root carbon concentration was not correlated with the morphological variables. 2) the ability to bind trace elements (for example, Pb and Cd) to root cells tends to be associated with the plant tolerance to soil contamination by metals. 3) The fractionation of 15N in roots is a time-integrated trait of mycorrhizal mediated nutrition.

In general, roots and leaves are functionally coordinated. The fast acquisition and processing of water and nutrients by the roots must be coupled with fast acquisition and processing of carbon by leaves. However, there was strong discordance between roots and leaves, with regards to the accumulation of several trace elements.

Soil conditions, and metal contamination in particular, affect morphological and chemical traits of tree roots. There are reciprocal interactions and feedback processes between soil and tree roots.

A remarkable physiological trait for these tree roots is the capacity to diminish the availability of trace elements into the soil, in special of those potentially toxic (Cd, Pb, As). We recommend planting trees with high “phytostabilisation” potential to remediate metal-contaminated soils.

This study has been published in the April issue of the journal Plant and Soil:

Marañón, T., Navarro-Fernández, C. M., Gil-Martínez, M., Domínguez, M. T., Madejón, P., Villar, R. (2020). Variation in morphological and chemical traits of Mediterranean tree roots: linkage with leaf traits and soil conditions. Plant and Soil, 449: 389-403.