Size-asymmetric competition and size-asymmetric growth in a spatially explicit zone-of-influence model of plant competition

Size-asymmetric competition among plants is usually defined as resource pre-emption by larger individuals, but it is usually observed and measured as a disproportionate size advantage in the growth of larger individuals in crowded populations (“size-asymmetric growth”). We investigated the relationship between size-asymmetric competition and size-asymmetric growth in a spatially explicit, individual-based plant competition model based on overlapping zones of influence (ZOI). The ZOI of each plant is modeled as a circle, growing in two dimensions. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. We grew simulated populations with different degrees of size-asymmetric competition and at different densities and analyzed the size dependency of individual growth by fitting coupled growth functions to individuals. The relationship between size and growth within the populations was summarized with a parameter that measures the size asymmetry of growth. Complete competitive symmetry (equal division of contested resources) at the local level results in a very slight size asymmetry in growth. This slight size asymmetry of growth did not increase with increasing density. Increased density resulted in increased growth asymmetry when resource competition at the local level was size asymmetric to any degree. Size-asymmetric growth can be strong evidence that competitive mechanisms are at least partially size asymmetric, but the degree of size-asymmetric growth is influenced by the intensity as well as the mode of competition. Intuitive concepts of size-asymmetric competition among individuals in spatial and nonspatial contexts are very different.     

Supply pre-emption, not concentration reduction, is the mechanism of competition for nutrients

Concentration reduction theory is the leading theory regarding the mechanism of competition for nutrients in soils among plants, yet it has not been rigorously tested. Here we used a spatially explicit, fine-scale grid-based model that simulated diffusion and plant uptake of nutrients by plants in soil to test whether concentration reduction theory was appropriate for terrestrial plant competition for nutrients. In the absence of competition, increasing the rate of diffusion allows a plant to maintain positive growth rates below the lowest average concentration to which it can reduce nutrients in soil solution (R*). As such, differences among plants in the reduction of soil moisture, which here primarily affects nutrient diffusion, can cause R* to predict competitive success incorrectly. The stronger competitor for nutrients captures the largest proportion of the nutrient supply by ensuring nutrients contact its roots before those of a competitor. Although the metric derived from concentration reduction theory, R*, might have predictive power for competitive outcomes in terrestrial ecosystems, this evidence suggests that plants outcompete other plants for nutrients by pre-empting the supply, not reducing the average concentration.     

Early-season effects of supplemented solar UV-B radiation on seedling emergence, canopy structure, simulated stand photosynthesis and competition for light

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV-B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of field study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-season measurements showed significant UV-B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5–7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while midseason Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6–7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV-B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering competitive interactions for light.     

Carbon allocation and competition maintain variation in plant root mutualisms

Plants engage in multiple root symbioses that offer varying degrees of benefit. We asked how variation in partner quality persists using a resource‐ratio model of population growth. We considered the plant's ability to preferentially allocate carbon to mutualists and competition for plant carbon between mutualist and nonmutualist symbionts. We treated carbon as two nutritionally interchangeable, but temporally separated, resources—carbon allocated indiscriminately for the construction of the symbiosis, and carbon preferentially allocated to the mutualist after symbiosis establishment and assessment. This approach demonstrated that coexistence of mutualists and nonmutualists is possible when fidelity of the plant to the mutualist and the cost of mutualism mediate resource competition. Furthermore, it allowed us to trace symbiont population dynamics given varying degrees of carbon allocation. Specifically, coexistence occurs at intermediate levels of preferential allocation. Our findings are consistent with previous empirical studies as well the application of biological market theory to plantroot symbioses.     

Calibration of minirhizotron readings against root length density data obtained from soil cores

The minirhiozotron (MR) root observation method was studied versus root length density (RLD) obtained from soil cores. Two plant species, acacia (Acacia saligna) and wheat (Triticum aestivum L.) were grown in a 1-m³ container on Silt Loam (Typic Torrifluvent) and on fine dune sand (Typic Torripsamment), respectively. Roots of both plants were measured periodically by the two methods. The MR observation tubes (MROT) were inserted, either vertically or at 45°. The correlation between the number of roots obtained by the MR and RLD was significant for the entire profile. However, an appreciable error in root estimation by the MR root observation method at the upper 10-cm soil might occur. No significant difference was obtained from MROT oriented vertically or at 45°. The differences between the correlation coefficients of the two methods were not significant, for both plants and soils, indicating that this correlation expresses the geometry of the two measurement systems, not affected by plant or soil types. We concluded that the MR method may be used as an in situ, non-destructive root measuring method with reasonable confidence. This revised version was published online in June 2006 with corrections to the Cover Date.     

疏叶骆驼刺根系对土壤异质性和种间竞争的响应

近年来, 植物根系对土壤异质性的响应和植物根系之间的相互作用一直是研究的热点。过去的研究主要是针对一年生短命植物进行的, 而且多是在人工控制的温室条件下进行的。而对于多年生植物根系对养分异质性和竞争的综合作用研究很少。该文对塔里木盆地南缘多年生植物疏叶骆驼刺(Alhagi sparsifolia)根系生长对养分异质性和竞争条件的响应途径与适应策略进行了研究, 结果表明: (1)在无竞争的条件下, 疏叶骆驼刺根系优先向空间大的地方生长, 即使另一侧有养分斑块存在, 其根系也向着空间大的一侧生长; (2)在有竞争的条件下, 疏叶骆驼刺根系生长依然是优先占领空间大的一侧, 但是竞争者的存在抑制了疏叶骆驼刺的生长, 导致其枝叶生物量和根系生物量都明显减少(p < 0.01), 而养分斑块的存在促进了疏叶骆驼刺根系的生长; (3)疏叶骆驼刺根系的生长不仅需要养分, 也需要足够的空间, 空间比养分更重要; (4)有竞争者存在的时候, 两株植物的根系都先长向靠近竞争者一侧的空间, 即先占据“共有空间”。研究结果对理解植物根系觅食行为和植物对环境的适应策略有重要意义。     

Testis size,epididymis weight,and sperm competition in rhesus macaques

The intensity of sperm competition is often measured using the gonadosomatic index (testes/body weight). But sperm competition could be mediated more by size of the epididymis than by size of the testicles, and little information is available on the relationship between testicular and epididymal size. We found that both organs were positively correlated in size among male rhesus macaques. Body weight accounted for over 70% of the variance in testicle size and volumetric estimates of testicle size accurately reflected testicle weight. We conclude that methods for ascertaining testicle size are accurate, but the covariation in size between testicles and epididymis will hamper understanding of the physiological mechanisms involved in sperm competition in primates. © 1993 Wiley-Liss, Inc.     

Competition both drives and impedes diversification in a model adaptive radiation

Competitors are known to be important in governing the outcome of evolutionary diversification during an adaptive radiation, but the precise mechanisms by which they exert their effects remain elusive. Using the model adaptive radiation of Pseudomonas fluorescens, we show experimentally that the effect of competition on diversification of a focal lineage depends on both the strength of competition and the ability of the competitors to diversify. We provide evidence that the extent of diversification in the absence of interspecific competitors depends on the strength of resource competition. We also show that the presence of competitors can actually increase diversity by increasing interspecific resource competition. Competitors that themselves are able to diversify prevent diversification of the focal lineage by removing otherwise available ecological opportunities. These results suggest that the progress of an adaptive radiation depends ultimately on the strength of resource competition, an effect that can be exaggerated or impeded by the presence of competitors.     

How changing root system architecture can help tackle a reduction in soil phosphate (P) levels for better plant P acquisition

The readily available global rock phosphate (P) reserves may run out within the next 50–130 years, causing soils to have a reduced P concentration which will affect plant P uptake. Using a combination of mathematical modelling and experimental data, we investigated potential plant‐based options for optimizing crop P uptake in reduced soil P environments. By varying the P concentration within a well‐mixed agricultural soil, for high and low P (35.5–12.5 mg L^?1 respectively using Olsen's P index), we investigated branching distributions within a wheat root system that maximize P uptake. Changing the root branching distribution from linear (evenly spaced branches) to strongly exponential (a greater number of branches at the top of the soil) improves P uptake by 142% for low‐P soils when root mass is kept constant between simulations. This causes the roots to emerge earlier and mimics topsoil foraging. Manipulating root branching patterns, to maximize P uptake, is not enough on its own to overcome the drop in soil P from high to low P. Further mechanisms have to be considered to fully understand the impact of P reduction on plant development.     

小麦浆片在开花过程中的结构与作用观察

小麦可育株系和杂种F1花期早,开花张角大,开闭花持续时间短,其浆片吸胀快,体积大,增重幅度大,浆片解剖结构有3个部分;表皮层细胞外壁厚,有角质层,无气孔（器）;基本组织薄壁细胞（GPC）含丰富的原生质,在浆片吸胀和萎缩过程中,GPC体积增大至内膜系统破裂,细胞自溶后内溶物撤离;维管束分散于基本组织中,由导管,筛管薄壁细胞所组成。     

Growth,abscisic acid content,and carbon isotope composition in wheat cultivars grown under different soil moisture

Changes in dry matter accumulation and allocation, abscisic acid content and carbon isotope composition of three wheat cultivars from dry, middle and wet climate regions were recorded at full maturity after exposure to different watering regimes (100, 50 and 25 % field capacity). Compared with the wet climate cultivar, the dry climate cultivar showed lower stem height, total leaf area, total dry biomass and total grain dry mass, and higher root/shoot ratio, abscisic acid content and carbon isotope composition under all watering regimes. Both water-limited treatments significantly reduced leaf growth and increased dry matter allocation into the roots leading to a significant raise of root/shoot ratio in all cultivars tested. In addition, drought affected morphological and physiological properties more in the dry climate cultivar than in the wet climate cultivar.     

Effect of sodium fluoride on the root apex border cells in one-day-old wheat seedlings

The number of border (scaled off) cells (BCs) was determined in the root apex of 1-day-old wheat (Triticum aestivum L.) seedlings. Microscopic examination of cytological root preparations showed that in 24 h the number of BCs in the gel sheath of the root apex was 30–40 per root. When the gel sheath was preparatively removed, their number per root increased twice. It is assumed that the subpopulation of BCs directly associated with the root apex differs from the subpopulation of BCs freely accommodated in the gel sheath. The number of BCs was the same in the roots with low and high natural growth rates. NaF (1–20 mM) suppressed growth of wheat seedling roots; the viscosity of the gel sheath increased (by 3–5 times), and the number of BC rose with the most pronounced increment in the size of the BC subpopulation directly associated with the root apex. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 530–538.