Abstract
Plants adopt a variety of life history strategies to succeed in the Earth's diverse environments. Using functional traits which are defined as “morphological, biochemical, physiological, or phonological” characteristics measurable at the individual level, plants are classified according to their species’ adaptative strategies, more than their taxonomy, from fast growing plant species to slower‐growing conservative species. These different strategies probably influence the input and output of carbon (C)‐resources, from the assimilation of carbon by photosynthesis to its release in the rhizosphere soil via root exudation. However, while root exudation was known to mediate plant‐microbe interactions in the rhizosphere, it was not used as functional trait until recently. Here, we assess whether root exudate levels are useful plant functional traits in the classification of plant nutrient‐use strategies and classical trait syndromes? For this purpose, we conducted an experiment with six grass species representing along a gradient of plant resource‐use strategies, from conservative species, characterized by low biomass nitrogen (N) concentrations and a long lifespans, to exploitative species, characterized by high rates of photosynthesis and rapid rates of N acquisition. Leaf and root traits were measured for each grass and root exudate rate for each planted soil sample. Classical trait syndromes in plant ecology were found for leaf and root traits, with negative relationships observed between specific leaf area and leaf dry matter content or between specific root length and root dry matter content. However, a new root trait syndrome was also found with root exudation levels correlating with plant resource‐use strategy patterns, specifically, between root exudation rate and root dry matter content. We therefore propose root exudation rate can be used as a key functional trait in plant ecology studies and plant strategy classification.
Highlights
Plants are able to colonize a broad diversity of environments due to a variety of different strategies to manage nutrient acquisition, use and conservation
These resource-use strategies can be explained by physiological characteristics which are measurable as plant functional traits, defined as “morphological or physiological characteristics measurable at the individual level, from the cell to the whole-organism level, without reference to the environment or any other level of organization which impact fitness indirectly via their effects on growth, reproduction and survival” (McGill, Enquist, Weiher, & Westoby, 2006; Violle et al, 2007)
Leaf and root traits provided a good separation of our plant species along a gradient of plant resource-use strategies, with conservative grasses (S. caerulea (SC) and F. paniculata (FP) at one end, and a range of exploitative grasses on the other end (D. glomerata [DG], B. erectus [BE], A. odoratum [AO] and T. flavescens [TF]; Figure 1a)
Summary
Plants are able to colonize a broad diversity of environments due to a variety of different strategies to manage nutrient acquisition, use and conservation. The “leaf economics spectrum” (Wright et al, 2004) describes such relationships by highlighting the link between the specific leaf area (SLA) or the leaf nitrogen concentration (LNC), and a spectrum ranging from fast-growing plant species (i.e. exploitative species) with higher photosynthetic capacity and rapid rates of N acquisition (Aerts & Chapin, 1999), to slow- growing plant species (i.e. conservatives species) associated with the conservation of nutrients in thick leaf tissues (Aerts & Chapin, 1999; De Deyn, Cornelissen, & Bardgett, 2008; Osnas, Lichstein, Reich, & Pacala, 2013; Personeni & Loiseau, 2004; Reich, 2014; Wright et al, 2004) These trade-offs between leaf acquisition and conservation of resources has been suggested to occur for root traits (Birouste, Kazakou, Blanchard, & Roumet, 2012; Fort et al, 2013; Mommer & Weemstra, 2012; Roumet, Urcelay, & Dıaz, 2006), and recent studies demonstrated the value of root traits as indicators of ecosystem functions (Grassein et al, 2015; Roumet et al, 2016) and soil functions (Cantarel et al, 2015; Moreau et al, 2015). Plant functional traits and root exudation rate were measured for each plant’s rhizosphere after one week of 13CO2 labeling
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