An analysis of the costs and benefits of physiological integration between ramets in the clonal perennial herb Glechoma hederacea

Summary The costs and benefits, measured in terms of dry weight, of physiological integration between clonal ramets, were analysed in two experiments conducted on the clonal herb Glechoma hederacea. Firstly, integration between consecutively-produced ramets was examined in an experiment in which stolons grew from one set of growing conditions (either unshaded or shaded and either nutrient-rich or nutrient-poor) into conditions in which light or nutrient level was altered. Comparisons were made between the dry weight of the parts of the clones produced before and after growing conditions were changed, and the dry weights of the corresponding part of control clones subjected to constant growing conditions. In a second experiment, integration between two distinct parts of G. hederacea clones was investigated. In this experiment clones were grown from two connected parent ramets and the parts of the clone produced by each parent ramet were subjected independently to either nutrient-rich or nutrient-poor conditions. Ramets in resource-rich conditions provided considerable physiological support to those in resource-poor conditions. This was measured as a dry weight gain compared with the weight of the corresponding part of the control clones growing in resource-poor conditions. However, when stolons grew from resource-poor conditions into resource-rich conditions, there was no similar evidence of the resourcepoor ramtes receiving support from resource-rich ramets. Physiological integration did not result in dry weight gains when this would have necessitated basipetal translocation of resources.Because of the predominantly acropedal direction of movement of translocates in G. hederacea, the structure of the clone was important in determining the effectiveness of integration between ramets. Where physiological integration was effective, the cost to the supporting ramets in terms of dry weight was insignificant. Physiological integration allows clones to maintain a presence in less favourable sites with insignificant cost to ramets in favourable sites, thereby reducing the probability of invasion by other plants, and providing the potential for rapid clonal growth if conditions improve. Integrated support of ramets in unfavourable conditions also enables the clone to grow through unfavourable sites, thus increasing the probability of encountering more favourable conditions by wider foraging.     

Causes and consequences of sectoriality in the clonal herb Glechoma hederacea

The causes of sectoriality and consequences for clone behaviour are examined using data from the stoloniferous herb Glechoma hederacea. The proximal causes of physiological integration patterns are investigated using anatomical studies, acid fuchsin dye to reveal patterns of xylem continuity between ramets, and ¹⁴C as a label to reveal quantitative photoassimilate translocation patterns in the phloem. Dye movement in the xylem was acropetal and sectorial, and the sectoriality was determined by phyllotaxy. Patterns of ¹⁴C-labelled photoassimilate allocation were qualitatively similar to those of xylem based resources, although there was some basipetal movement of photoassimilate. The patterns of physiological integration and independence between ramets are shown to be governed by rules which depend on vascular continuity and discontinuity between ramets. Physiological support to stolon apices results in acquisition of relative branch autonomy (branches become semi-autonomous integrated physiological units, IPUs).This paper evaluates whether observed physiological integration patterns may be modified by altering normal source-sink relationships or by modifying environmental conditions. An experiment using different defoliation intensities, and different defoliation patterns at the same overall intensity, demonstrated that the precise positions of leaves removed from a clone had unique consequences for its subsequent development. Individual ramets of a given clone may be located in microhabitats of differing quality. An experiment in which competition was either present or absent throughout the space occupied by the clone, or patchy in distribution, showed that G. hederacea did not respond to competition at the whole clone level. Instead, connected stolons (IPUs) responded independently to local competition. Sectoriality may promote the restriction of lethal, localised environmental factors within the affected IPU. A study investigating the uptake and translocation of zinc by clones revealed that quantified patterns of zinc distribution resembled patterns of ¹⁴C movement in the phloem, and that there was no significant transport of zinc from one stolon to another.Although sectorial patterns of resource movement in G. hederacea can be modified in the short term, in the long-term, physiological integration may not allow this species to integrate the effects of environmental heterogeneity. A mobile clonal species with a high growth rate and relatively short-lived ramets, such as G. hederacea, is likely to benefit from a semi-autonomous response to patch quality at the level of the stolon, since the alternative of widespread intra-clonal support may increase the residence time of the clone in unfavourable pathches.     

The effects of rooting frequency and position of rooted ramets on plasticity and yield in a clonal species: an experimental study with Glechoma hederacea

Clonal plants produce numerous ramets that can be distributed over a considerable area. Resources are translocated between ramets, especially when they occupy microsites of different quality, or places where leaves or roots cannot be deployed. It is common for a proportion of the ramets of clones and clonal fragments to lack roots. We conducted a greenhouse study using clonal fragments of Glechoma hederacea to examine the effects of differences in the number and position of rooted ramets on yield and plasticity of clonal fragments. We hypothesized that (1) mass of roots and root mass ratio would increase as the number of rooted ramets decreased, (2) plasticity in rooted ramets would buffer the clonal fragment against reduction in yield as the number of rooted ramets declined, (3) ramet plasticity in response to the absence of rooting, and the beneficial effects of this plasticity, would be greater when older ramets were rooted. The same yield was achieved in clonal fragments with only one out of four ramets rooted as in clonal fragments with all four of their ramets rooted, regardless of whether rooting was confined to older or younger ramets. Plasticity in biomass allocated to roots was greater in older rooted ramets succeeded by unrooted ramets than in younger rooted ramets preceded by unrooted ramets. Modular plasticity, involving both direct responses to local conditions, and indirect responses to the conditions experienced by connected modules, buffered performance against variation in rooting ability, enabling clonal fragments to maintain their yield and lateral expansion even when a high proportion of their ramets lacked roots.     

Pollen limitation and distance-dependent fecundity in females of the clonal gynodioecious herb Glechoma hederacea (Lamiaceae)

Summary Pollen movement is often restricted in natural populations, and insufficient pollination is a potential constraint on sexual reproduction in outcrossing species. Seed-set should decrease with increased distance from the pollen source in outcrossing plants. This prediction was tested using females of the clonal, gynodioecious herb Glechoma hederacea in three natural populations. In controlled pollinations, both hermaphrodites and females had similar high percentages of fruit-set and seed-set. In a natural population where a female clone was isolated from the nearest hermaphroditic clone by c. 100 m, fruit-set was low (1%). In another population where hemaphroditic clones were rare and female clones had a patchy distribution, fruit-and seed-set in females were pollen-limited and decreased with increased distance from the nearest pollen source. The estimated mean pollen dispersal distance was 5.9 m when calculated on fruit-set and 5.3 m when calculated on seed-set. The most frequent pollinators were bumblebees. The mean and median distances moved by pollinators between ramets were 0.13 m and 0.05 m. In a third population where female clones were isolated from the nearest hermaphrodites by more than 200 m, fruit-set was 0%. After introduction of 16 hermaphroditic ramets in the center of the female clone, fruit-set varied between 0% and 100% in individual female ramets. Fruit-set decreased with increased distance from the pollen source. The mean and median pollen movement distances were 1.06 m and 0.54 m.     

Heterogeneous distribution of soil nutrients increase intra-specific competition in the clonal plant Glechoma hederacea

Small-scale heterogeneity strongly affects plant fitness and many ecological processes, and it can significantly influence the growth of individual plants, populations and communities. Generally, clonal species achieve significantly more growth when essential resources are patchily distributed than when resources are uniformly distributed. In this study, we aim to determine the effect of spatial heterogeneity in soil resources on intraspecific competition in the clonal plant Glechoma hederacea. We report the outcomes of a greenhouse experiment where high and low densities of plants were exposed to patchy and uniform distribution of nutrients. Our results showed that patchy distribution of resources exacerbated intra-specific competition between clonal systems. We found a reduction of total mass of clonal systems growing at high-density, especially under patchy conditions. Patchy distribution of resources conduct to high concentration of resources located in small areas, and as consequence increase the competition interaction between plants. This study demonstrates that full understanding of plant–plant competitive interactions requires consideration of spatial heterogeneity in nutrient supply.     

Clonal integration ameliorates the carbon accumulation capacity of a stoloniferous herb,Glechoma longituba,growing in heterogenous light conditions by facilitating nitrogen assimilation in the rhizosphere

Background and Aims Enhanced availability of photosynthates increases nitrogen (N) mineralization and nitrification in the rhizosphere via rhizodeposition from plant roots. Under heterogeneous light conditions, photosynthates supplied by exposed ramets may promote N assimilation in the rhizosphere of shaded, connected ramets. This study was conducted to test this hypothesis.Methods Clonal fragments of the stoloniferous herb Glechoma longituba with two successive ramets were selected. Mother ramets were subjected to full sunlight and offspring ramets were subjected to 80 % shading, and the stolon between the two successive ramets was either severed or left intact. Measurements were taken of photosynthetic and growth parameters. The turnover of available soil N was determined together with the compostion of the rhizosphere microbial community.Key Results The microbial community composition in the rhizosphere of shaded offspring ramets was significantly altered by clonal integration. Positive effects of clonal integration were observed on NAGase activity, net soil N mineralization rate and net soil N nitrification rate. Increased leaf N and chlorophyll content as well as leaf N allocation to the photosynthetic machinery improved the photosynthetic capability of shaded offspring ramets when the stolon was left intact. Clonal integration improved the growth performance of shaded, connected offspring ramets and whole clonal fragments without any cost to the exposed mother ramets.Conclusions Considerable differences in microbial community composition caused by clonal integration may facilitate N assimilation in the rhizosphere of shaded offspring ramets. Increased N content in the photosynthetic machinery may allow pre-acclimation to high light conditions for shaded offspring ramets, thus promoting opportunistic light capture. In accordance with the theory of the division of labour, it is suggested that clonal integration may ameliorate the carbon assimilation capacity of clonal plants, thus improving their fitness in temporally and spatially heterogeneous habitats.     

Differential effects of clonal integration on performance in the stoloniferous herb Duchesnea indica, as growing at two sites with different altitude

Clonal integration may be adaptive and enhance the genet performance of clonal plants. Degree of clonal integration may differ between different environments . Here, a container experiment was used to determine how clonal integration affected the performance of the stoloniferous herb Duchesnea indica at two sites with different altitude along the transitional zone between the Qinghai-Tibet plateau and the Sichuan basin of Southwest China. In the experiment, the stolon between partially shaded two ramets experienced severing and intact treatments.We predicted that clonal integration would increase performance of whole clonal fragments and their shaded clonal parts at both sites. In both arctic and alpine environments, clonal plants may form highly integrated plant units (group of ramets).We predicted again that the reduction due to stolon severing in performance of whole clonal fragments and their shaded clonal parts would be greater at the site with high altitude than one with low altitude. The results indicated that the benefit for the shaded clonal parts and whole clonal fragments due to clonal integration was only observed at the site with high altitude. The results suggest that the performance of Duchesnea indica tends to be more responsive to the stolon severing at the site with high altitude than one with low altitude and support the second prediction. In addition, the effects of conditions of the sites and clonal integration on local morphological traits of ramets may be adaptive, five morphological traits of ramet-level (length of petiole, mean stolon internode length, specific petiole weight, specific stolon internode weight and specific leaf area) were investigated. Altogether, the results suggest that clonal integration might help D. indica plants to successfully inhabit the high-altitude habitat of the Qinghai-Tibet plateau of Southwest China, providing new evidences for the notion that clonal integration is an adaptive trait in stressful environments.     

The Structure and Development of the Vegetative Shoot Apex in Glechoma hederacea L

The development of the vegetative shoot apex of Glechoma hederaceahas been followed through a double plastochron. During this period the apex grows from c. 20 to c. 260 µin height and c. 100 to c. 300 µ in width, whilst thepair of leaves inserted at the apex base increase from o toc. 600µ in height. The width of the apex and height ofthese leaves are directly related to apex height. Some variationoccurs in the average maximal dimensions of the apex with plastochronnumber but no regular increase or decrease in these dimensionsis apparent. Both a tunica-corpus organization and cytohistological zonationis visible within the apex throughout a double plastochron.The central initiation zone shows little change in size or appearanceduring this period but the rib and flank meristems grow considerablyand undergo some differentiation. The boundaries of these zonesare not sharply defined, but normally the rib meristem givesrise to the pith, and the flank meristem forms the epidermis,cortex, and provascular tissue. The provascular tissue differentiatesacropetally and in continuity with that in the axis below.     

Environmental heterogeneity and clonal growth: a study of the capacity for reciprocal translocation in Glechoma hederacea L.

Clonal fragments of Glechoma hederacea L. (Lamiaceae) were subjected to environments in which light and nutrients were supplied with a strictly negative association in space, i.e. when one of these resources was in ample supply the other was scarce. Treatments were chosen to simulate environments in which clones grew either within homogeneous conditions or across patch types (heterogeneous conditions). The hypothesis was tested that reciprocal translocation (i.e. exchange of both nutrients and assimilates) between connected groups of ramets would increase biomass production of clones growing under heterogeneous conditions compared to that of clones growing in homogeneous conditions. A cost-benefit analysis was carried out to test this hypothesis. Results suggested that reciprocal translocation did not occur at the structural scale considered in this experiment; no evidence was found for a significant effect on whole clone biomass of assimilate and/or nutrient translocation between clone parts experiencing contrasting levels of resource supply. It is suggested that predominantly acropetal movement of resources and the pattern of integrated physiological unit formation in G. hederacea are the main properties responsible for the lack of mutual physiological support between connected clonal fragments growing in differing habitat conditions. These properties are expected to promote clonal expansion and the exploitation of new territory, rather than sustaining clone parts in sub-optimal patches of habitat for prolonged periods of time.     

The effects of ingesting plant hormones on schistosomiasis in mice: An experimental study

A number of studies have shown that primates monitor and select plant species in their diet as a function of their secondary compound composition. The possibility also exists that secondary compounds, when used in appropriate concentrations, may have a beneficial, medicinal aspect. In this regard, synthesis of a variety of data suggested a connection between selection of Balanites by Ethiopian baboons, and the distribution of schistosomiasis. To test the hypothesis that the secondary compounds in Balanites might be selected for ‘medicinal purposes’, we conducted an experiment on the effect of adding the active principle, diosgenin, to the food of schistosome-infected mice. The hypothesis was that this steroidal saponin might alter the host's hormonal milieu, making a less hospitable environment for the adult schistosomes. Sacrifice of the mice showed diosgenin-fed animals to have an augmented rather than decreased response to the disease. However, the data support the developing literature that shows that the host's hormonal environment has a major effect on the parasitic diseases they are subject to, and that the hormonal environment can be dramatically influenced by the secondary compounds in the diet.     

Phenotypic plasticity and mechanical stress: biomass partitioning and clonal growth of an aquatic plant species

Mechanical stresses from wind, current or wave action can strongly affect plant growth and survival. Survival and distribution of species often depend on the plant's capacity to adapt to such stresses, particularly when amplified by climatic variations. Few studies have dealt with plastic adjustments in response to mechanical stress compared to resource stress. We hypothesized that mechanical stress should favor plastic adjustments that result in increased biomass production in zones protected from the stress and that altered growth patterns should be reversible after mechanical stress removal. Here we measured plastic adjustments in morphological traits and clonal architecture for an aquatic clonal species (Berula erecta) under two contrasting mechanical stresses in the field-standing vs. running water. Reversion of the morphological changes was then assessed using transplants in standing water. In the case of mechanical stress, size reduction, biomass reallocation within clones (higher allocations to clonal growth and to belowground organs), and a more compact growth form (reduced spacer lengths) contributed to reducing the damage risk. The removal of mechanical stress induced compensatory growth, probably linked to the production of low density tissues. However, most patterns of dry mass partitioning induced by current stress were not reversed after stress removal.     

Shifting effects of physiological integration on performance of a clonal plant during submergence and de-submergence

Background and Aims

Submergence and de-submergence are common phenomena encountered by riparian plants due to water level fluctuations, but little is known about the role of physiological integration in clonal plants (resource sharing between interconnected ramets) in their adaptation to such events. Using Alternanthera philoxeroides (alligator weed) as an example, this study tested the hypotheses that physiological integration will improve growth and photosynthetic capacity of submerged ramets during submergence and will promote their recovery following de-submergence.

Methods

Connected clones of A. philoxeroides, each consisting of two ramet systems and a stolon internode connecting them, were grown under control (both ramet systems untreated), half-submerged (one ramet system submerged and the other not submerged), fully submerged (both ramet systems submerged), half-shaded (one ramet system shaded and the other not shaded) and full-shaded (both ramet systems shaded) conditions for 30 d and then de-submerged/de-shaded for 20 d. The submerged plants were also shaded to very low light intensities, mimicking typical conditions in turbid floodwater.

Key Results

After 30 d of submergence, connections between submerged and non-submerged ramets significantly increased growth and carbohydrate accumulation of the submerged ramets, but decreased the growth of the non-submerged ramets. After 20 d of de-submergence, connections did not significantly affect the growth of either de-submerged or non-submerged ramets, but de-submerged ramets had high soluble sugar concentrations, suggesting high metabolic activities. The shift from significant effects of integration on both submerged and non-submerged ramets during the submergence period to little effect during the de-submergence period was due to the quick recovery of growth and photosynthesis. The effects of physiological integration were not found to be any stronger under submergence/de-submergence than under shading/de-shading.

Conclusions

The results indicate that it is not just the beneficial effects of physiological integration that are crucial to the survival of riparian clonal plants during periods of submergence, but also the ability to recover growth and photosynthesis rapidly after de-submergence, which thus allows them to spread.     

Genetic effects on the biomass partitioning and growth of Pisum and Lycopersicon

We examined a series of eight pea genotypes differing in three naturally occurring allelic mutations, i.e., af (afila), st (stipules reduced), and tl (tendril-less) and three species, five cultivars, and one interspecific hybrid of tomato differing in SP (SELF-PRUNING) allele composition to determine whether different phenotypes ontogenetically express different biomass partitioning patterns compared to the isometric partitioning pattern and an interspecific 3/4 scaling "rule" governing annual growth with respect to body mass. The slopes and "elevations" (i.e., α and log β, respectively) of log-log linear regression curves of bivariate plots of leaf, stem, and root dry mass and of annual growth vs. total body mass were used to assess pattern homogeneity. The annual growth of all pea and tomato phenotypes complied with the 3/4 growth rule. The biomass partitioning patterns of all tomato phenotypes were statistically indistinguishable from the isometric pattern as were those of the pea wild type and three single-mutant genotypes. However, significant departures from the isometric (and pea wild type) biomass allocation pattern were observed for three genotypes, all of which were homozygous for the af allele. These results open the door to explore the heritability and genetics underlying the allometry of biomass partitioning patterns and growth.     

Short‐term fitness benefits of physiological integration in the clonal herb Hydrocotyle peduncularis

Abstract We test whether physiological integration enhances the short‐term fitness of the clonal herb Hydrocotyle peduncularis (Apiaceae, R. Brown ex A. Richards) subjected to spatial variation in water availability. Our measures of fitness and costs and benefits are based on the relative growth rate of fragmented genets. Physiological integration over a gradient in soil moisture resulted in a highly significant net benefit to genet growth of 0.015 g g^?1 day^?1. This net benefit represents a significant enhancement of the average fitness of fragmented genets spanning the moisture gradient relative to the average of those growing in homogeneous moist or dry conditions. Sections of genet fragments growing in dry conditions in spatially heterogeneous treatments had significantly higher growth than the sections they were connected to that were growing in moist conditions. Within fragments, older (parent) sections growing in moist conditions experienced significant costs from connection to younger (offspring) sections growing in dry conditions. In contrast, offspring sections with ample water did not experience any costs when connected to parent sections growing in dry conditions. However, the net benefit of physiological integration was similar for parent and offspring sections, suggesting that parent and offspring sections contributed equally to the net benefit of physiological integration to genet growth and short‐term fitness.     

Climate change effects on plant biomass alter dominance patterns and community evenness in an experimental old‐field ecosystem

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change‐induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO₂], air temperature, and precipitation in a constructed old‐field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO₂], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.