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1. Competition for resources has long been considered a prevalent force in structuring plant communities and natural selection, yet our understanding of the mechanisms that underlie resource competition is still developing. 2. The complexity of resource competition is derived not only from the variability of resource limitation in space and time and among species, but also from the complexity of the resources themselves. Nutrients, water and light each differ in their properties, which generates unique ways that plants compete for these resources. 3. Here, we discuss the roles of supply pre-emption and availability reduction in competition for the three resources when supplied evenly in space and time. Plants compete for nutrients by pre-empting nutrient supplies from coming into contact with neighbours, which requires maximizing root length. Although water is also a soil resource, competition for water is generally considered to occur by availability reduction, favouring plants that can withstand the lowest water potential. Because light is supplied from above plants, individuals that situate their leaves above those of neighbours benefit directly from increased photosynthetic rates and indirectly by reducing the growth of those neighbours via shade. In communities where juveniles recruit in the shade of adults, traits of the most competitive species are biased towards those that confer greater survivorship and growth at the juvenile stage, even if those traits come at the expense of adult performance. 4. Understanding the mechanisms of competition also reveals how competition has influenced the evolution of plant species. For example, nutrient competition has selected for plants to maintain higher root length and light competition plants that are taller, with deeper, flatter canopies than would be optimal in the absence of competition. 5. In all, while more research is needed on competition for heterogeneous resource supplies as well as for water, understanding the mechanisms of competition increases the predictability of interspecific interactions and reveals how competition has altered the evolution of plants.

Plant competition determines the diversity and species abundance of natural communities as well as potential yields in agricultural systems. Understanding the mechanisms of plant competition is instrumental to understanding plant performance in true vegetations. In this review, we will address various components of competition between plant individuals with a specific focus on molecular aspects. As plant—plant interactions during competition are multiple and complex, we will focus here on a restricted set of examples of plant traits that are thought to enhance their performance during competition. To respond to competition by neighbours, plants first need to detect these competitors in a reliable way. We discuss the various ways of molecular detection of competition through light-quality signals, nutrient levels, soluble root exudates and volatile organic compounds emitted by neighbouring plants. Once perceived, these signals are translated into responses such as shade avoidance, root foraging and allelopathy. We integrate the various molecular patterns of signal detection and subsequent plant responses, both above- and below-ground and including their interaction. We outline research strategies towards creating a general, mechanistic understanding of how plants increase their performance during competition.

The spatial arrangement of nutrients and neighbours in soil influences plant growth and reproduction. Plants often respond to such stimuli through plasticity in root proliferation (root mass per soil volume), or the breadth of their root system. Here, we asked how plants adjust nutrient foraging strategies when grown alone or with neighbours. We asked (i) Does root proliferation into nutrient‐rich patches when plants are grown alone predict root proliferation when plants are grown with neighbours? (ii) What factors (nutrients or neighbours) best predict the probability of root placement at different soil locations? (iii) How does the spatial distribution of nutrients alter the degree to which neighbours suppress plant growth? To answer these questions, we grew four grassland species either as individual plants or in competition, in patchy or patch‐free soil, in a factorial design. We used genomic DNA to identify the spatial distribution of roots of each species when plants were grown in mixtures. The root foraging behaviour of individuals grown alone did not consistently predict behaviour in mixture. Specifically, (i) the behaviour of individually grown plants predicted behaviour of competing plants inside patches, but not in background soil. We observed over‐proliferation of roots in background soil relative to what was expected from plants grown alone. (ii) Neighbours were consistently the most important variable for predicting the placement of roots in soil and caused either an increase in root system breadth, or no change relative to alone. (iii) If a species experienced growth suppression when grown in competition, individuals experienced this more severely in patchy soil compared to patch‐free soil. Synthesis. Game theoretic models have predicted that under interspecific competition, over‐proliferation of roots in the presence of neighbours might occur for some species but not others. Our data are consistent with these predictions but more work is needed. Nutrient foraging studies have primarily focused on plants grown alone or assumed that plants do not respond separately to neighbours and nutrients. Our data call these practices into question and contribute to a growing understanding that plants integrate information about both nutrients and neighbours when placing roots in soil.

Breeders work to adapt winter wheat genotypes for high planting densities to pursue sustainable intensification and maximize canopy productivity. Although the effects of plant-plant competition at high planting density have been extensively reported, the quantitative relationship between competitiveness and plant performance remains unclear. In this study, we introduced a shoot competitiveness index (SCI) to quantify the competitiveness of genotypes and examined the dynamics of nine competitiveness-related traits in 200 winter wheat genotypes grown in heterogeneous canopies at two planting densities. Higher planting densities increased shoot length but reduced biomass, tiller numbers, and leaf mass per area (LMA), with trait plasticity showing at least 41% variation between genotypes. Surprisingly, genotypes with higher LMA at low density exhibited greater decreases under high density, challenging expectations from game theory. Regression analysis identified tiller number, LMA, and shoot length as key traits influencing performance under high density. Contrary to our hypothesis, early competitiveness did not guarantee sustained performance, revealing the dynamic nature of plant-plant competition. Our evaluation of breeding progress across the panel revealed a declining trend in SCI (R² = 0.61), aligning with the breeding objective of reducing plant height to reduce individual competitiveness and increase the plant-plant cooperation. The absence of historical trends in functional traits and their plasticities, such as tiller number and LMA, suggests their potential for designing ideal trait-plasticity for plant-plant cooperation and further crop improvement.

1. Plant competition has been studied for decades. Yet, it is still an elusive concept that means different things to different people, is resistant to direct study and is shrouded in semantic and statistical complexity. We still lack basic information about many competitive mechanisms, processes and outcomes and their relationship to other ecological processes, and about how local interactions between individuals are propagated through communities. We suggest here that two critical issues have been overlooked in previous studies. 2. First, there is a need for direct measurements of the process of competition as opposed to indirect mechanisms of competitive outcomes. Biomass has become the 'industry standard' for measuring competition, but we suggest that biomass cannot provide unambiguous insights into plant competition because it is the product of too great a range of factors and processes. 3. Second, the use of a single measure of competition at an arbitrarily assigned end point of an experiment misses much of the complexity of dynamic interactions between competing plants and can lead to erroneous interpretations. Here we suggest approaches to handle these difficulties, using new techniques or the application of well-known methods in a novel way. We also provide examples of systems or questions where the improved understanding these approaches could bring would be of particular benefit. 4. Ultimately, we suggest the need for a major shift in the way in which we consider and measure plant competition to identify broadly agreed rules for variation in its importance, its role in different communities and habitats, and how and whether it influences or drives patterns of species diversity and abundance.

1. Plants recognize their kin and respond to growing with relatives with changes in functional traits. Here, we integrate competition and evolutionary theory to evaluate these changes. 2. We draw parallels between the definitions and empirical measurement of competitive effect and competitive response from competition theory, and the costs and benefits of altruistic and selfish behaviours from kin selection theory. 3. Do plants compete less with relatives, as these parallels suggest? While functional traits respond to the presence of relatives or strangers, no study has directly demonstrated that plants are less competitive with siblings. 4. However, there are empirical challenges in identifying the competitive value of traits that are measured destructively, such as root allocation. 5. If these challenges can be addressed, kin recognition responses can offer new insights into plant competition.

1. Reproductive efficiency (the efficiency of conversion of resources from vegetative tissue to reproductive output) is a central to our understanding of reproductive allocation and the evolution of reproductive strategies in plants. Plant strategy theory predicts that reproductive efficiency should decrease under competition. Short-lived semelparous species are not predicted to evolve under competition and therefore should not express adaptive responses to the presence of competitors. Long-lived iteroparous species are predicted to delay reproduction in favour of growth and resource acquisition in the presence of competitors. I use life-history theory to advance a prediction that reproductive efficiency increases under competition in both short-lived semelparous and potentially longer-lived iteroparous species. 2. Contrary to the predictions of plant strategy theory, short-lived semelparous species are frequently observed to live in highly competitive environments. Further, iteroparous species under intense competition may die long before they reach competitive dominance or an optimal size for reproduction. 3. I surveyed the literature for studies on plant species including measurements of vegetative and reproductive allocation in high and low (or no) competition treatments. 4. Across species, relative reproductive efficiency (reproductive efficiency under high competition/reproductive efficiency under low competition) significantly increased with increasing competition intensity. 5. Patterns of allocation to reproduction under competition support the existence of a competitive annual strategy and a reproductive perennial strategy. Under these strategies, short-lived semelparous species and long-lived iteroparous species express high reproductive efficiency under competition as an adaptation to high neighbour density. In addition, some species also expressed patterns of allocation to reproduction consistent with plant strategy theories. 6. Under this interpretation, I predict that competitive strategies, where plants delay reproduction in competitive environments to gain competitive superiority, are favoured not under intense competition but under modest competition. Including a life-history interpretation in reproductive efficiency under competition provides a much needed predictive framework for strategies of reproduction observed across species.

This study investigated effects of plant density and arbuscular mycorrhizal (AM) colonization on growth and phosphorus (P) nutrition of a cultivar of wheat (Triticum aestivum) that often shows early AM-induced growth depressions. Two experiments were conducted. Expt 1 had three plant densities and one soil P concentration. Expt 2 had two plant densities and two P concentrations. Plants were grown in calcareous P-fixing soil, inoculated with Glomus intraradices or Gigaspora margarita, or noninoculated (nonmycorrhizal (NM)). Glomus intraradices colonized well and caused a growth depression only in Expt 1. Gigaspora margarita caused large growth depressions in both experiments even though it colonized poorly. The results showed that growth depressions were mitigated by changes in relative competition for soil P by NM and AM plants, and probably by decreasing carbon costs of the fungi. The different effects of the two fungi appear to be attributable to differences in the balance between P uptake by the fungal pathway and direct uptake via the roots. These differences may be important in other AM symbioses that result in growth depressions. The results show that mycorrhizal growth responses of plants grown singly may not apply at the population or community level.

BACKGROUND AND AIMS: Tillering has a significant effect on canopy development and, hence, on resource capture, crop growth and grain yield in sorghum. However, the physiological basis of tillering and its regulation by environmental effects are not fully understood. The objective of this study was to understand and quantify the environmental effects on tillering in sorghum using a carbohydrate supply-demand framework. METHODS: A series of five experiments with a wide range of radiation and temperature conditions was conducted and details of the tillering responses of a single representative hybrid were monitored. The concept of internal plant competition for carbohydrate was developed for analysis of these responses. KEY RESULTS: Tiller appearance was highly synchronized with main shoot leaf appearance, with a consistent hierarchy for tillering across environments. The main environmental effect was on the frequency of tiller appearance, in particular of the lower-rank tillers. This explained some of the observed environmental differences in the onset of tillering. A generalized index of internal plant competition, which took account of plant assimilate supply and demand (S/Dindex) during the critical period for tillering, explained most of the variation in maximum tiller number observed across the five experiments. CONCLUSIONS: This result was consistent with the hypothesis that internal plant competition for assimilates regulates tillering in sorghum. Hence, the framework outlined has a predictive value that could provide the basis for dynamic simulation of tillering in crop growth models.

In order to fit population ecological models, e.g., plant competition models, to new drone-aided image data, we need to develop statistical models that may take the new type of measurement uncertainty when applying machine-learning algorithms into account and quantify its importance for statistical inferences and ecological predictions. Here, it is proposed to quantify the uncertainty and bias of image predicted plant taxonomy and abundance in a hierarchical statistical model that is linked to ground-truth data obtained by the pin-point method. It is critical that the error rate in the species identification process is minimized when the image data are fitted to the population ecological models, and several avenues for reaching this objective are discussed. The outlined method to statistically model known sources of uncertainty when applying machine-learning algorithms may be relevant for other applied scientific disciplines.