• Energy Research

Plant allelochemicals released into the soil can significantly impact the performance of associated plant species thereby affecting their competitive ability. Soil microbes can potentially affect the interaction between plant and plant chemicals by degrading the allelochemicals. However, most often plant-plant chemical interactions are studied using filter paper bioassays examining the pair-wise interaction between a plant and a plant chemical, not taking into account the potential role of soil microorganisms.To explore if the allelopathic effects on a grass by the common thyme monoterpene "carvacrol" are affected by soil microorganisms. Seedlings of the grass Agrostis capillaris originating from 3 different thyme sites were raised in the greenhouse. Seedlings were grown under four different soil treatments in a 2*2 fully factorial experiment. The monoterpene carvacrol was either added to standard greenhouse soil or left out, and soil was either sterilized (no soil microorganisms) or not (soil microorganisms present in soil). The presence of carvacrol in the soil strongly increased mortality of Agrostis plants, and this increase was highest on sterile soil. Plant biomass was reduced on soil amended with carvacrol, but only when the soil was also sterilized. Plants originating from sites where thyme produces essential oils containing mostly carvacrol had higher survival on soil treated with that monoterpene than plants originating from a site where thyme produced different types of terpenes, suggesting an adaptive response to the locally occurring terpene.The study shows that presence of soil microorganisms can alleviate the negative effect of a common thyme monoterpene on the performance of an associated plant species, emphasizing the role of soil microbes in modulating plant-plant chemical interactions.

AbstractInvestment in reproduction and growth represent a classic tradeoff with implication for life history evolution. The local environment can play a major role in the magnitude and evolutionary consequences of such a tradeoff. Here, we examined the investment in reproductive and vegetative tissue in 40 maternal half‐sib families from four different populations of the herb Plantago coronopus growing in either a dry or wet greenhouse environment. Plants originated from populations with an annual or a perennial life form, with annuals prevailing in drier habitats with greater seasonal variation in both temperature and precipitation. We found that water availability affected the expression of the tradeoff (both phenotypic and genetic) between reproduction and growth, being most accentuated under dry condition. However, populations responded very differently to water treatments. Plants from annual populations showed a similar response to drought condition with little variation among maternal families, suggesting a history of selection favouring genotypes with high allocation to reproduction when water availability is low. Plants from annual populations also expressed the highest level of plasticity. For the perennial populations, one showed a large variation among maternal families in resource allocation and expressed significant negative genetic correlations between reproductive and vegetative biomass under drought. The other perennial population showed less variation in response to treatment and had trait values similar to those of the annuals, although it was significantly less plastic. We stress the importance of considering intraspecific variation in response to environmental change such as drought, as conspecific plants exhibited very different abilities and strategies to respond to high versus low water availability even among geographically close populations.

Summary How species coexist is a central question in ecology. Niche differentiation can prevent two species from excluding each other by competition. However, this interaction can vary in space because of internal factors such as intraspecific genetic variation, or external ones such as the presence of a third species. While these effects have been repeatedly observed, their joint effect on the outcome of competition has rarely been considered. We experimentally investigated how a dominant ecosystem engineer (the shrub Thymus vulgaris) affects interactions between two co‐occurring and closely related annuals (Medicago minima and M. rigidula). We ask first whether the outcome of their interaction depends on the genetic identity of the competitors, and in particular whether intraspecific interaction differs between genetically related versus unrelated conspecifics. Secondly, we ask whether coexistence of the two species is modified by the presence of thyme plants altering local soil conditions. We grew genotypes collected in nature of both annual species in inter‐ and intraspecific competition experiments using soil collected directly underneath thyme plant (thyme soil) and soil collected away from thyme plants (no‐thyme soil). We found that thyme soil affected interactions in a way that shifted the species’ rank. Crucial for this result was the division of intraspecific competition between genetically related and unrelated individuals. Intraspecific competition was significantly less detrimental to kin than to unrelated conspecifics. In highly structured plant populations, such effects could favour local dominance of a patch by one species. However, this kin effect occurred for each species only in its preferred soil (thyme vs. no thyme), promoting a stabilizing coexistence mechanism at the landscape level. Synthesis. This study shows that the importance of intraspecific competition relative to interspecific competition can be highly dependent on the genotype identity of intraspecific competitors and of the local environment in which the interaction occurs. The local environment can itself be modified by the presence of a third species in the community. These results emphasize how, in order to understand the overall problem of species coexistence, it can be insightful to divide this into smaller local‐scale problems of coexistence.

Meeting report on the symposium ‘New solutions to monitor plants, pollinators and their interactions in a changing world’, held at Collège de France and Muséum national d'Histoire naturelle, Paris, France, 23–24 May 2024 International audience

Knowledge of the effect of plant secondary compounds (PSCs) on belowground interactions in the more diffuse community of species living outside the rhizosphere is sparse compared with what we know about how PSCs affect aboveground interactions. We illustrate here that PSCs from foliar tissue, root exudates, and leaf litter effectively influence such belowground plant-plant, plant-microorganism, and plant-soil invertebrate interactions. Climatic factors can induce PSC production and select for different plant chemical types. Therefore, climate change can alter both quantitative and qualitative PSC production, and how these compounds move in the soil. This can change the soil chemical environment, with cascading effects on both the ecology and evolution of belowground species interactions and, ultimately, soil functioning.

Recent studies have shown that plant allelochemicals can have profound effects on the performance of associated species, such that plants with a history of co-existence with "chemical neighbour" plants perform better in their presence compared to naïve plants. This has cast new light on the complexity of plant-plant interactions and plant communities and has led to debates on whether plant communities are more co-evolved than traditionally thought. In order to determine whether plants may indeed evolve in response to other plants' allelochemicals it is crucial to determine the presence of genetic variation for performance under the influence of specific allelochemicals and show that natural selection indeed operates on this variation. We studied the effect of the monoterpene carvacrol-a dominant compound in the essential oil of Thymus pulegioides-on three associated plant species originating from sites where thyme is either present or absent. We found the presence of genetic variation in both naïve and experienced populations for performance under the influence of the allelochemical but the response varied among naïve and experienced plant. Plants from experienced populations performed better than naïve plants on carvacrol soil and contained significantly more seed families with an adaptive response to carvacrol than naïve populations. This suggests that the presence of T. pulegioides can act as a selective agent on associated species, by favouring genotypes which perform best in the presence of its allelochemicals. The response to the thyme allelochemical varied from negative to neutral to positive among the species. The different responses within a species suggest that plant-plant interactions can evolve; this has implications for community dynamics and stability.