The effects of shell size and coil orientation on reproduction in female hermit crabs, Clibanarius vittatus

We investigated the effects of shell coil orientation and shell size on reproduction in field populations of the hermit crab, Clibanarius vittatus. Females were collected in the intertidal in Beaufort, NC. Shell parameters were measured and size (cephalothorax length) and reproductive status were determined for 70 females occupying Busycon shells. Crabs were categorized as berried (eggs on the pleopods), mature ovaries, or non-reproductive (no eggs). For berried females, the number of eggs was recorded. By offering a separate group of females access to empty shells, it was possible to calculate optimal shell size and the deficit in shell size for field-collected animals.Females that were berried were in shells closer to the optimal shell size than females with mature ovaries, both for shell weight and shell volume. And females with mature ovaries were in shells that were closer to the optimal size than females that were non-reproductive. For both categories of females without eggs on the pleopods, the majority of females were in shells that were too big (in weight and internal volume). While the percentage of berried females did not differ between dextral (Busycon carica) and sinistral (Busycon sinistrum) shells, the non-reproductive females had a much smaller deficit in volume in sinistral shells compared to dextral shells. For berried females, there was no relationship between the magnitude of their shell deficit and the number of eggs carried. Our results suggest that reproduction is inhibited when females occupy shells sufficiently greater than the optimal shell size.     

Modelling the contribution of arbuscular mycorrhizal fungi to plant phosphate uptake

In this paper, we investigate the role of arbuscular mycorrhizal fungi in plant phosphorus nutrition. We develop a mathematical model which quantitatively assesses the contribution of external fungal hyphae to plant phosphate uptake.We derive an equation for solute uptake by a growing fungal mycelium which we couple with a model for root uptake. We analyse the model using nondimensionalization and numerical simulations.Simulations predict that removal of phosphate from soil is dominated by hyphal uptake as opposed to root uptake. Model analysis shows that the depletion zones around hyphae overlap within 8 h and that the transfer between fungus and root is a critical step for the behaviour of phosphorus within the mycelial phase. We also show that the volume fraction of mycelium is negligibly small in comparison to other soil phases.This is the first model to quantify the contribution of mycorrhizal fungi to plant phosphate uptake. A full data set for model parametrization and validation is not currently available. Therefore, more complete sets of experimental measurements are necessary to make this model more applicable.     

Modelling applicability of fractal analysis to efficiency of soil exploration by roots

BACKGROUND AND AIMS: Fractal analysis allows calculation of fractal dimension, fractal abundance and lacunarity. Fractal analysis of plant roots has revealed correlations of fractal dimension with age, topology or genotypic variation, while fractal abundance has been associated with root length. Lacunarity is associated with heterogeneity of distribution, and has yet to be utilized in analysis of roots. In this study, fractal analysis was applied to the study of root architecture and acquisition of diffusion-limited nutrients. The hypothesis that soil depletion and root competition are more closely correlated with a combination of fractal parameters than by any one alone was tested. MODEL: The geometric simulation model SimRoot was used to dynamically model roots of various architectures growing for up to 16 d in three soil types with contrasting nutrient mobility. Fractal parameters were calculated for whole roots, projections of roots and vertical slices of roots taken at 0, 2.5 and 5 cm from the root origin. Nutrient depletion volumes, competition volumes, and relative competition were regressed against fractal parameters and root length. KEY RESULTS: Root length was correlated with depletion volume, competition volume and relative competition at all times. In analysis of three-dimensional, projected roots and 0 cm slices, log(fractal abundance) was highly correlated with log(depletion volume) when times were pooled. Other than this, multiple regression yielded better correlations than regression with single fractal parameters. Correlations decreased with age of roots and distance of vertical slices from the root origin. Field data were also examined to see if fractal dimension, fractal abundance and lacunarity can be used to distinguish common bean genotypes in field situations. There were significant differences in fractal dimension and fractal abundance, but not in lacunarity. CONCLUSIONS: These results suggest that applying fractal analysis to research of soil exploration by root systems should include fractal abundance, and possibly lacunarity, along with fractal dimension.     

VARIABILITY IN CELL SIZE,NUTRIENT DEPLETION,AND GROWTH RATES OF THE SOUTHERN OCEAN DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE) AFTER PROLONGED IRON LIMITATION1

Diatoms are the main primary producers in the Southern Ocean, governing the major nutrient cycles. Fragilariopsis kerguelensis (O’Meara) Hust. is the most abundant diatom species in the Southern Ocean and its paleo‐oceanographic record is frequently used to reconstruct the past position and nutrient characteristics of the Antarctic polar front. Here we report on the responses of F. kerguelensis on prolonged exposure to a range of iron concentrations, allowing a characterization of morphological and nutrient‐depletion changes in relation to iron status. Under iron limitation, F. kerguelensis grew slower, cells became smaller, chains became shorter, and the nutrient‐depletion ratios changed. Prolonged exposure to iron limitation caused F. kerguelensis to decrease its surface area and volume 2‐fold, and to increase its surface‐to‐volume ratio by 25%. With the decrease in growth rates, silicon (Si) and phosphorus (P) depletion per cell remained fairly constant, but when normalized per surface area (Si) or per cell volume (P), depletion increased. In contrast, nitrogen (N) depletion per cell decreased significantly together with the decrease in growth rates but was constant when normalized per cell volume. The different response in Si, P, and N depletion resulted in changes in the nutrient‐depletion ratios, most notably in the Si:N ratio, which significantly increased, and in the N:P ratio, which significantly decreased with decreasing growth rates. It is concluded that under iron limitation, variation in cell size and/or nutrient depletion ultimately can cause changes in oceanic biogeochemical nutrient cycles. It enables the use of cell size of F. kerguelensis as a paleo‐oceanographic proxy.     

<Superscript>15</Superscript>N-nitrate-labelling demonstrates a size symmetric competitive effect on belowground resource uptake

Strong hints exist that belowground competition is generally size-symmetric. While this has frequently been shown by use of integrative indicators like growth or biomass, resource-focussed approaches are still lacking, especially those investigating the competitive effect. Here, we present a correlation between neighbour plants’ root sizes and their competitive effect on their target plants’ nitrate uptake. This was derived from a controlled field experiment where intra- and interspecific combinations of five different herbaceous species from nutrient poor sand ecosystems were examined in an additive design. Short-term pulses of ¹⁵N-labelled nitrate were applied between competing pairs of plant individuals. The sizes of neighbour root systems had high explanatory power for the competitive effect on target plants’ nitrate uptake. Equally important, a curve fitting approach revealed that the competitive effect based on ¹⁵N-uptake matched predictions of a size-symmetric interaction. With 66% of the variation in competitive effect on nitrate uptake explained by root system size, the degree to which root size results in a belowground overlap of zones of influence is crucial. Within this overlap, further attributes like architecture or uptake capacity may be important. Our data represent experimental support for a size symmetric competitive effect for a specific belowground resource. Since this is not consistent with an overproportional size advantage when mobile soil resources are limiting, it suggests that the survival of small individuals or species should be facilitated by the symmetric nature of belowground competitive effects.     

Whorl overlap and the economical construction of the gastropod shell

A model is used to show how changes in the amount of overlap between successive whorls alters the ratio of the area of shell material to the volume enclosed. An optimum amount of overlap which minimizes this ratio is shown to exist.
Measurements on the shells of a number of British gastropods show that the actual amount of overlap is always greater than the optimum. It is concluded that selection for economy is being opposed by some other factor such as shell strength, shape or aperture size.     

Comparing different approaches to calculate the effects of heterogeneous root distribution on nutrient uptake: a case study on subsoil nitrate uptake by a barley root system

Simulation models of nutrient uptake of root systems starting with one-dimensional single root approaches up to complex three-dimensional models are increasingly used for examining the interacting of root distribution and nutrient uptake. However, their accuracy was seldom systematically tested. The objective of the study is to compare one-dimensional and two-dimensional modelling approaches and to test their applicability for simulation of nutrient uptake of heterogeneously distributed root systems giving particular attention to the impact of spatial resolution. Therefore, a field experiment was carried out with spring barley (Hordeum vulgare L. cv. Barke) in order to obtain data of in situ root distribution patterns as model input. Results indicate that a comparable coarse spatial resolution can be used with sufficient modelling results when a steady state approximation is applied to the sink cells of the two-dimensional model. Furthermore, the accuracy of the model was clearly improved compared to a simple zero sink approach assuming both near zero concentrations within the sink cell and a linear gradient between the sink cell and its adjacent neighbours. However, for modelling nitrate uptake of a heterogeneous root system a minimum number of grid cells is still necessary. The tested single root approach provided a computational efficient opportunity to simulate nitrate uptake of an irregular distributed root system. Nevertheless, two-dimensional models are better suited for a number of applications (e.g. surveys made on the impact of soil heterogeneity on plant nutrient uptake). Different settings for the suggested modelling techniques are discussed.     

Comparison of nutrient uptake between three‐dimensional simulation and an averaged root system model

Abstract

We present a new numerical approach describing nutrient uptake in three dimensions. Dynamic boundary conditions are considered at the individual root surfaces within a root system. As an example, we compare the three‐dimensional simulation results of phosphate uptake by a young maize root system to the corresponding effective solution. We show that the two solutions are similar concerning phosphate uptake and the size of the depletion zones. The presented approach makes it possible to verify simplifications that are made in the development of effective models. Furthermore, it is possible to extend existing models by including spatial heterogeneities that will increase our understanding of rhizosphere processes.     

Nutritional implications for sexual cannibalism in a sexually dimorphic orb web spider

Models predicting mechanisms driving sexual cannibalism in spiders with sexual size dimorphism (SSD) often assume that spiders use post‐copulatory food to channel nutrients into eggs and fecundity is altered through changes in clutch size or egg mass. I tested these assumptions using an orb web spider with extreme SSD, Argiope keyserlingi. I fed mated female spiders prey of either high protein‐low energy or low protein‐high energy composition. I measured egg energy density; a measure of the relative volumes of yolk and albumen. I predicted that if A. keyserlingi increase their egg energy density upon feeding on prey of a specific nutrient composition, they could enhance their fecundity by investing in more energy dense eggs. However, if the egg energy densities are dissimilar to their post‐copulatory prey they must be drawing energy from their somatic reserves to invest in eggs. In a further experiment I allowed female spiders to mate with and cannibalize males to determine if cannibalism induces similar effects on egg energy density. Male spider protein energy ratio was measured and found to resemble the high protein‐low energy prey. I found disagreement between the composition of post‐copulatory food and eggs in both experiments. Additionally, spiders fed high protein‐low energy prey lost weight indicating that they draw on their energy reserves to invest in eggs. I thus concluded that spiders that feed on high protein‐low energy prey or on males increase their egg energy density and, possibly, fecundity. However, the nutrient content of the prey or males cannot provide for investment in eggs. The energy invested in eggs is drawn from somatic reserves, probably induced by an as yet undescribed physiological trigger.     

A theoretical explanation of the Piper-Steenbjerg effect

The relation between plant yield and plant nutrient concentration is sometimes found to be negative, a phenomenon called the Piper-Steenbjerg (PS) effect. A model was used to examine the underlying causes of the PS effect, and the conditions under which it is most likely to occur. The model uses the nutrient productivity concept for plant growth and a nutrient uptake equation in which root growth rate and external nutrient concentration determine the uptake rate. The study suggests that the PS effect occurs when the fast growth of plants grown in an initially higher nutrient medium eventually leads to a more rapid depletion of external nutrients than the slow growth of plants grown in an initially lower nutrient medium. The fast growth of plants combined with a rapid decrease of nutrient uptake leads to a fall in plant nutrient concentration. When these large plants with very low nutrient concentrations are compared with the smaller, slow-growing plants, a PS effect may be found depending on the time at which the plants are harvested, and on the range of initial values of the external nutrient content. When it occurs, the effect is greatest when the depletion volume per unit new root (V_d) is lowest, and when the mobility of nutrients in the medium is highest (α=1). The results are sufficiently general to apply to a variety of nutrients, plant species and growth media.     

Can birds count eggs in their nests?

Sensory mechanisms controlling avian clutch size have diversified into distinct types, according to the nature of the input that is used to disrupt the growth of ovarian follicles and hence halt egg‐laying. In an article on brood parasitism, Lyon (2003) claimed that female American Coots Fulica americana can reduce their clutch size on the basis of visual cues in response to eggs laid in their nests by other females; in this species, therefore, egg counting would be used to control clutch size. After a close examination of the physiological determination of clutch size in American Coots, I show that seven of 17 parasitized clutches were smaller than the range controlled through the mechanism using an input to disrupt follicular growth (7–10 eggs per clutch). My reanalysis suggests that American Coots are incapable of adjusting clutch size via counting and re‐asserts that a species that can count eggs has yet to be found among birds that rely upon their own body heat for incubation.     

The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model

We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r ² between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability. This revised version was published online in August 2006 with corrections to the Cover Date.     

Root Gravitropism and Below-ground Competition among Neighbouring Plants: A Modelling Approach

Competition for nutrients among neighbouring roots occurs whentheir individual depletion volumes overlap, causing a reductionin nutrient uptake. By exploring different spatial niches, plantswith contrasting root architecture may reduce the extent ofcompetition among neighbouring root systems. The main objectivesof this study were: (1) to evaluate the impact of root architectureon competition for phosphorus among neighbouring plants; and(2) to compare the magnitude of competition among roots of thesame plant vs. roots of neighbouring plants. SimRoot, a dynamicgeometric model, was used to simulate common bean root growthand to compare the overlap of depletion volumes. By varyingthe gravitropism of basal roots, we simulated three distinctroot architectures: shallow, intermediate and deep, correspondingto observed genetic variation for root architecture in thisspecies. Combinations of roots having the same architectureresulted in more intense inter-plant competition. Among them,the deep-deep combination had the most intense competition.Competition between deep root systems and shallow root systemswas only half that of deep root systems competing with otherdeep root systems. Inter-plant root competition increased assoil diffusivity increased and the distance among plants decreased.In heterogeneous soils, co-localization of soil resources androots was more important in determining resource uptake thaninter-plant root competition. Competition among roots of thesame plant was three- to five-times greater than competitionamong roots of neighbouring plants. Genetic variation for rootarchitecture in common bean may be related to adaptation todiverse competitive environments. Copyright 2001 Annals of BotanyCompany Root architecture, phosphorus, competition, common bean, Phaseolus vulgaris L. nutrient uptake, gravitropism     

Mechanical constraints on aperture form in gastropods

Gastropod apertures reflect expanded states of their mantle edge under variable boundary conditions. The apertures are divided into two groups: apertures without distinct overlap zones (e.g., whorl overlap) and those with overlap zones. Each group follows a unique morphological rule. Apertures without overlap zones are generally circular in outline. Apertures with overlap zones are either elongated perpendicularly to the overlap zone or inflated abapically. Moreover, the latter abapically inflated apertures are generally accompanied by a straight section anterior to the overlap zone along the columellar axis (columellar part). Numerical analysis of an elastic double membrane tube whose main frame simulates the gastropod mantle indicates that these morphological rules are the products of mantle edge expansion under the condition that the head-foot mass presses against the mantle edge in both the overlap zone and the columellar part. The mantle edge in these two zones is thus in a completely or partly fixed boundary condition at the moment of shell growth. The rest of the mantle edge is free to expand either symmetrically or asymmetrically. It is hypothesized that the head-foot mass is a driving force for regulating the pattern of shell coiling and apertural shape.     

Estimates of root system topology of five plant species grown at steady-state nutrition

Results from a controlled growth-analysis experiment were used to illustrate some methods for measuring and describing root system topology. The experiment was performed in a nutrient solution system with an exponential nutrient supply and steady-state growth, to achieve well-defined levels of whole-plant nutrient status. Five naturally coexisting grassland species were included: The slow-growing forbs Polygala vulgaris L. and Crepis praemorsa (L.) F. L. Walth., and the grass Danthonia decumbens (L.) DC. were compared with the more common, fast-growing grasses Agrostis capillaris L. and Dactylis glomerata L. The most marked difference in morphological indices was a much higher specific root length in the grasses than in the forbs, which implies thinner roots. In contrast to the conclusions of previous studies, an index of the topology for the grasses was very similar to that for the forbs. The specific root lenght and link length apparently vary more between species and nutrient levels than topology does, and may therefore be more ecologically important. The only clear plastic response to growth-limiting nitrogen supply was a markedly increased link length in P. vulgaris. There were also indications that nitrogen limitation led to more herringbone-like root systems in P. vulgaris and C. praemorsa. In general, there was a clear tendency for the estimates of topology to change with plant size, which may make many topological indices, especially those based on regression slopes, very difficult to interpret. Until interactions with plant size, other morphological parameters and among-plant competition can be properly understood, the relevance of root topology for plant performance remains unclear. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of experimental calcium availability,egg parameters and laying order on Great Tit Parus major eggshell pigmentation patterns

Many bird species lay eggs speckled with protoporphyrin‐based spots, however, for most of them the function of eggshell spotting is unknown. A plausible hypothesis is that protoporphyrin might have a structural function in strengthening the eggshell and is therefore deposited when calcium is scarce. In this study, we experimentally provided Great Tit Parus major females with supplemental calcium to examine its effect on the protoporphyrin‐based maculation of their eggs. In addition, we studied variation in eggshell pigmentation patterns in relation to other egg parameters and laying order. Calcium‐supplemented females laid larger eggs but shell thickness was not significantly affected by the treatment. Calcium supplementation may reduce the time and energy females devote to searching for calcium‐rich material, so that they can collect more nutrients and so lay larger eggs. Furthermore, pigment darkness was associated with egg volume and shape, which suggests that female quality and environmental food availability may also influence the shell pigmentation pattern. Within clutches, later‐laid eggs had larger and darker spots that were distributed more unevenly on the shell surface. This within‐clutch pattern could be explained by the increase in egg volume and egg shape and a decline in shell thickness with egg‐laying order, which characteristics were all related to shell‐spotting pattern. Eggs with a coronal ring had thinner shells, but pigment intensity and spot size were not related to shell thickness. Thus, our results suggest that concentrated spotting distribution may have a mechanical function, supporting the structural‐function hypothesis.     

The architecture of Norway spruce ectomycorrhizae: three-dimensional models of cortical cells, fungal biomass, and interface for potential nutrient exchange

Gathering realistic data on actual fungal biomass in ectomycorrhized fine root systems is still a matter of concern. Thus far, observations on architecture of ectomycorrhizae (ECMs) have been limited to analyses of two-dimensional (2-D) images of tissue sections. This unavoidably causes stereometrical problems that lead to inadequate assumptions about actual size of cells and their arrangement within ECM's functional compartments. Based on extensive morphological investigations of field samples, we modeled the architectural components of an average-sized Norway spruce ECM. In addition to our comprehensive and detailed quantitative data on cell sizes, we studied actual shape and size, in vivo arrangement, and potential nutrient exchange area of plant cortical cells (CCs) using computer-aided three-dimensional (3-D) reconstructions based on semithin serial sections. We extrapolated a factual fungal biomass in ECMs (Hartig net (HN) included) of 1.71 t?ha^?1 FW (0.36 t?ha^?1 DW) for the top 5 cm of soil for an autochthonous, montane, optimum Norway spruce stand in the Tyrolean Alps. The corresponding potential nutrient exchange area in ECMs including main axes of ECM systems, which is defined as the sum of interfaces between plant CCs and the HN, amounts to at least 3.2?×?10⁵?m²?ha^?1. This is the first study that determines the contribution of the HN to the total fungal biomass in ECMs as well as the quantification of its contact area. Our results may stimulate future research on fungal below-ground processes and their impact on the global carbon cycle.     

A root is a root is a root? Water uptake rates of Citrus root orders

Knowledge about the physiological function of root orders is scant. In this study, a system to monitor the water flux among root orders was developed using miniaturized chambers. Different root orders of 4‐year‐old Citrus volkameriana trees were analysed with respect to root morphology and water flux. The eight root orders showed a broad overlap in diameter, but differences in tissue densities and specific root area (SRA) were clearly distinguishable. Thirty per cent of the root branch biomass but 50% of the surface area (SA) was possessed by the first root order, while the fifth accounted for 5% of the SA (20% biomass). The root order was identified as a determinant of water flux. First‐order roots showed a significantly higher rate of water uptake than the second and third root orders, whereas the fourth and fifth root orders showed water excess. The water excess suggested the occurrence of hydraulic redistribution (HR) as a result of differences in osmotic potentials. We suggest that plants may utilize hydraulic redistribution to prevent coarse root desiccation and/or to increase nutrient acquisition. Our study showed that the novel ‘miniature depletion chamber’ method enabled direct measurement of water fluxes per root order and can be a major tool for future studies on root order traits.     

Correlations Between the Dimensions of Different Zones of Grass Root Apices, and their Implications for Morphogenesis and Differentiation in Roots

The sizes of different zones within root apices of nine speciesof grass were estimated, and statistically significant correlationswere found between certain of them. The volume of the cap isrelated to the volume of the meristem of the root proper. Thecortical and stelar portions of the meristem are also related,and their lengths and volumes correlate with the volume of thequiescent centre. The volume of the quiescent centre also correlateswith the length of the zone in which periclinal divisions arefound in the inner cortex; these divisions generate the rowsof cells across the cortex. Diameter of the procambial cylinder,quiescent centre volume and vascular complexity are related,though from correlations alone it is not possible to say whetherone of these characters directly influences another as has beensuggested by other workers. All the zones within the root apexprobably form a tightly-integrated developmental unit. Root structure seems to be independent of cell size, thoughcell size correlates with nuclear DNA content which is a species-specificfeature. Gramineae, meristems, morphogenesis, root apices     

Fractal analysis of the root architecture of Gliricidia sepium for the spatial prediction of root branching,size and mass: model development and evaluation in agroforestry

Based on fractal and pipe model assumptions, a static three-dimensional model of the Gliricidia sepium root system was developed, in order to provide a basis for the prediction of root branching, size and mass in an alley cropping system. The model was built from observations about the topology, branching rules, link length and diameter, and root orientation, provided by in situ and extracted root systems. Evaluation tests were carried out at the plant level and at the field level. These tests principally concerned coefficients α and q –- the proportionality factor α between total cross-sectional area of a root before and after branching, and allocation parameter q that defines the partitioning of biomass between the new links after a branching event –- that could be considered as key variables of this fractal approach. Although independent of root diameter, these coefficients showed a certain variability that may affect the precision of the predictions. When calibrated, however, the model provided suitable predictions of root dry matter, total root length and root diameter at the plant level. At the field level, the simulation of 2D root maps was accurate for root distribution patterns, but the number of simulated root dots was underestimated in the surface layers. Hence recommendations were made to improve the model with regard to α and q. This static approach appeared to be well suited to study the root system of adult trees. Compared with explicit models, the main advantage of the fractal approach is its plasticity and ease of use. This revised version was published online in June 2006 with corrections to the Cover Date.