Effect of physiological integration in self/non-self genotype recognition on the clonal invader Carpobrotus edulis

Aims Biological invasions represent one of the most important threats to the conservation of biodiversity; however, the mechanisms underlying successful invaders remain unsolved. Many of the most aggressive invaders show clonal growth, and capacity for clonal integration has been pointed out recently as an important trait explaining the success of invasive plants. We aim to determine the role of physiological integration in the capacity for self/non-self genotype recognition in the clonal invader Carpobrotus edulis and the implications of this capacity for the expansion of this aggressive invader.Methods We used connected and severed ramets of identical or different genotype and we determined the capacity for self/non-self recognition by comparing changes in biomass partitioning to avoid competition for resources between pairs of ramets.Important findings Physiological integration allowed self/non-self genotype recognition in the invader C. edulis. Results showed a significant effect of physiological integration on the biomass allocated to roots by genetically identical ramets: older ramets specialize in acquisition of soil-based resources and younger ramets specialize in lateral expansion. This specialization could be considered a form of division of labour, which reduce intra-genotype competition. This is the first evidence that division of labour could be interpreted as a form of self/non-self recognition between genetically identical ramets. Capacity for self/non-self discrimination could contribute to increase the colonization capacity of the aggressive invader C. edulis. This is the first study showing an association between self/non-self recognition and invasiveness in a clonal plant.     

Two types of division of labour in clonal plants: benefits, costs and constraints

Clonal plants spread vegetatively within their habitats by forming rooted ramets on stolons or rhizomes. Each of these ramets is capable of an independent existence after establishment. Nevertheless, ramets remain physically connected by stolon or rhizome internodes for variable periods of time, thereby allowing for resource movement and signal transduction within clones.Interconnected ramets of clonal plants, though potentially independent and totipotent, can specialize functionally in the performance of limited numbers of tasks such as the uptake of resources from above- vs below-ground sources, carbohydrate storage, vegetative spread and sexual reproduction. Such specialization and cooperation is comparable to a division of labour in economic systems or in colonies of social animals. The ecological significance of division of labour in clonal plants may be found in the increased efficiency of entire clones in exploiting their environments.Two different types of division of labour in clonal plants will be discussed in this review. The first type is an environmentally-induced specialization of ramets in the uptake of locally abundant resources (plastic division of labour), which can be found in several stoloniferous species. Evidence exists that this response increases resource uptake in spatially heterogeneous environments. The second type of division of labour, which occurs mainly in rhizomatous species, relates to a developmentally-programmed specialization and cooperation between interconnected ramets. This response pattern is thought to enhance plant performance by restricting the number of tasks for individual ramets and thereby significantly increasing the efficiency of task performance. In some plants, such an inherent division of labour is likely to contribute to nutrient extraction from poor and unpredictably variable sources.In this article not only benefits but also potential costs and constraints on division of labour in clonal plants are shown. The aim is to provide a review of existing knowledge and to develop concepts and hypotheses for future research.     

Do Clonal Plants Show Greater Division of Labour Morphologically and Physiologically at Higher Patch Contrasts?

Background

When growing in reciprocal patches in terms of availability of different resources, connected ramets of clonal plants will specialize to acquire and exchange locally abundant resources more efficiently. This has been termed division of labour. We asked whether division of labour can occur physiologically as well as morphologically and will increase with patch contrasts.

Methodology/Principal Findings

We subjected connected and disconnected ramet pairs of Potentilla anserina to Control, Low, Medium and High patch contrast by manipulating light and nutrient levels for ramets in each pair. Little net benefit of inter-ramet connection in terms of biomass was detected. Shoot-root ratio did not differ significantly between paired ramets regardless of connection under Control, Low and Medium. Under High, however, disconnected shaded ramets with ample nutrients showed significantly larger shoot-root ratios (2.8∼6.5 fold) than fully-lit but nutrient-deficient ramets, and than their counterparts under any other treatment; conversely, fully-lit but nutrient-deficient ramets, when connected to shaded ramets with ample nutrients, had significantly larger shoot-root ratios (2.0∼4.9 fold) than the latter and than their counterparts under any other treatment. Only under High patch contrast, fully-lit ramets, if connected to shaded ones, had 8.9% higher chlorophyll content than the latter, and 22.4% higher chlorophyll content than their isolated counterparts; the similar pattern held for photosynthetic capacity under all heterogeneous treatments.

Conclusions/Significance

Division of labour in clonal plants can be realized by ramet specialization in morphology and in physiology. However, modest ramet specialization especially in morphology among patch contrasts may suggest that division of labour will occur when the connected ramets grow in reciprocal patches between which the contrast exceeds a threshold. Probably, this threshold patch contrast is the outcome of the clone-wide cost-benefit tradeoff and is significant for risk-avoidance, especially in the disturbance-prone environments.     

Clonal Integration of Fragaria orientalis in Reciprocal and Coincident Patchiness Resources: Cost-Benefit Analysis

Clonal growth allows plants to spread horizontally and to experience different levels of resources. If ramets remain physiologically integrated, clonal plants can reciprocally translocate resources between ramets in heterogeneous environments. But little is known about the interaction between benefits of clonal integration and patterns of resource heterogeneity in different patches, i.e., coincident patchiness or reciprocal patchiness. We hypothesized that clonal integration will show different effects on ramets in different patches and more benefit to ramets under reciprocal patchiness than to those under coincident patchiness, as well as that the benefit from clonal integration is affected by the position of proximal and distal ramets under reciprocal or coincident patchiness. A pot experiment was conducted with clonal fragments consisting of two interconnected ramets (proximal and distal ramet) of Fragaria orientalis. In the experiment, proximal and distal ramets were grown in high or low availability of resources, i.e., light and water. Resource limitation was applied either simultaneously to both ramets of a clonal fragment (coincident resource limitation) or separately to different ramets of the same clonal fragment (reciprocal resource limitation). Half of the clonal fragments were connected while the other half were severed. From the experiment, clonal fragments growing under coincident resource limitation accumulated more biomass than those under reciprocal resource limitation. Based on a cost-benefit analysis, the support from proximal ramets to distal ramets was stronger than that from distal ramets to proximal ramets. Through division of labour, clonal fragments of F. orientalis benefited more in reciprocal patchiness than in coincident patchiness. While considering biomass accumulation and ramets production, coincident patchiness were more favourable to clonal plant F. orientalis.     

Physiological integration increases the survival and growth of the clonal invader <Emphasis Type="Italic">Carpobrotus edulis</Emphasis>

Clonal growth seems to be a common trait for many of the most aggressive invasive plant species. However, little research has been conducted to determine the role of clonality in the successful invasion of new areas by exotic species. Carpobrotus edulis (L.) N.E. Br. is a mat-forming succulent plant, native to South Africa that is invasive in coastal dunes of Australia, New Zealand, USA and Southern Europe. Although Carpobrotus edulis is a clonal plant, there is no information on the role of clonality for the invasion by this species, therefore the objective of this study was to test whether or not physiological integration improves the performance of C. edulis invading coastal sand dunes. To do that, a 6-month field experiment was designed in which the stolon connections between the apical ramets and the C. edulis mats were severed to prevent physiological integration. This treatment was applied to ramets growing under high and low competition with the native species. Apical ramets with intact stolon connections were used as control. Integration improved the survivorship and growth of apical ramets, both in high and low competition. Connected ramets showed a more pronounced increase of clonal growth (estimated as stolon length) during the experimental period and a higher total biomass and number of ramets at the completion of the experiment. In terms of survivorship, the benefit of integration was greater under high competition. Physiological integration can therefore be considered an important factor in the invasiveness of C. edulis, both in open space and in direct competition with the native plants.     

Effects of clonal integration in the expansion of two alien <Emphasis Type="Italic">Carpobrotus</Emphasis> species into a coastal dune system – a field experiment

What makes a plant a successful invader is one of the most interesting questions in modern ecology. Comparative studies including congeners differing in invasiveness are a straightforward approach to detect potential traits explaining invasions. In this experiment we studied the importance of clonal integration and the capacity to buffer fragmentation in the expansion of two stoloniferous invaders, Carpobrotus edulis, considered more invasive, and Carpobrotus acinaciformis, considered less invasive. In particular we aim to determine whether differences in these clonal traits may explain differences in invasiveness between both species. We report evidence that clonal integration favour the expansion of the two exotic clonal species into a sand dune system. Benefit derived from clonal integration by itself does not explain differences in invasiveness between theses two exotic species. However, our result indicates that the greater invasiveness of C. edulis could be explained by a higher capacity to buffer the negative effect of fragmentation in comparison with C. acinaciformis. To elucidate the real contribution of clonal traits in plant invasions, new comparative studies should be conducted including more clonal species.     

Clonal integration supports the expansion from terrestrial to aquatic environments of the amphibious stoloniferous herb Alternanthera philoxeroides

Effects of clonal integration on land plants have been extensively studied, but little is known about the role in amphibious plants that expand from terrestrial to aquatic conditions. We simulated expansion from terrestrial to aquatic habitats in the amphibious stoloniferous alien invasive alligator weed ( Alternanthera philoxeroides ) by growing basal ramets of clonal fragments in soils connected (allowing integration) or disconnected (preventing integration) to the apical ramets of the same fragments submerged in water to a depth of 0, 5, 10 or 15 cm. Clonal integration significantly increased growth and clonal reproduction of the apical ramets, but decreased both of these characteristics in basal ramets. Consequently, integration did not affect the performance of whole clonal fragments. We propose that alligator weed possesses a double-edged mechanism during population expansion: apical ramets in aquatic habitats can increase growth through connected basal parts in terrestrial habitats; however, once stolon connections with apical ramets are lost by external disturbance, the basal ramets in terrestrial habitats increase stolon and ramet production for rapid spreading. This may contribute greatly to the invasiveness of alligator weed and also make it very adaptable to habitats with heavy disturbance and/or highly heterogeneous resource supply.     

The effect of stolon fragmentation on the colonization of clonal invasive Carpobrotus edulis in a coastal dune system: a field test

Disturbances usually initiate processes of fragmentation in clonal plants, with the consequent division into portions of different size. The ability of these portions to survive and regrow after fragmentation plays an important role in the maintenance of populations and the colonization of new environments. In this field experiment we aim to determine the importance of stolons as reserve organs in the colonization of a coastal sand dune by a clonal invader. We simulated an event of fragmentation of clones of an aggressive invader into portions with short and long stolon sizes. Our results showed a reduction of biomass allocation to roots in the long stolon treatment that was balanced by an increase in the above‐ground growth; consequently, the area colonized by the invader was greater. We report evidence that stolons can contribute to buffering stressful conditions and allow expansion of the invader into a natural coastal sand dune.     

Clonal plant allocation to daughter ramets is a simple function of parent size across species and nutrient levels

The evolution of clonal growth is a widespread phenomenon among plant species, characterized by the production of genetically identical clonal fragments (ramets) via rhizomes or stolons that form an interconnected clonal organism (genet). Clonal plant species are known to differ in their investment into ramet production, and exhibit considerable variation in ramet morphology both within and among species. While patterns of resource allocation are thought to be linked to a number of plant characteristics, many analyses are limited by uncertainty in how clonal plants determine the morphology and resources allocated to new ramets. In this study, we attempted to discern what aspects of parent ramets best predicted resource allocation to new daughter ramets, and the relationship between resource allocation and daughter ramet rhizome morphology. We grew two sedge species, Schoenoplectus tabernaemontani and Eleocharis elliptica, in a greenhouse under two levels of fertilizer addition. By harvesting daughter ramets that had initiated stem production, yet remained aphotosynthetic, we were able to isolate parental investment into non-independent daughter ramets at a point where daughter ramet spacer length became fixed. Our results indicate that parent ramets allocated a non-linear proportion of parent rhizome biomass to the production of daughter ramets. Moreover, this relationship was unaffected by environmental nutrient availability. Daughter ramet biomass, in turn, was strongly correlated with daughter ramet spacer length. These observations shed light on key processes governing clonal growth in plants, and their potential application in unifying allocational and morphological perspectives to explore the fitness implications of variability in clonal growth.     

Functional specialization of ramets in Scirpus maritimus – Splitting the tasks of sexual reproduction, vegetative growth, and resource storage

Clonal plant species can be considered as populations of interconnected ramets which are basically identical in form and function, and potentially independent from each other. Experimental studies and field observations suggest that an intra-clonal specialization of ramets with different roles (division of labour) can increase the performance of clonal systems under heterogeneous conditions. This paper explores structural and functional variation in the emergent macrophyte Scirpus maritimus, which forms ramets that specialize in three main activities: sexual reproduction, photosynthetic assimilation and vegetative growth, and reserve storage. The main question asked in this study is whether such specialization is a developmentally programmed syndrome in this species, and whether environmental conditions can alter the pattern of ramet differentiation along rhizome systems.We analyzed clonal fragments collected from a population in the field, and grew clones individually in pots of two sizes to simulate different degrees of crowding and shoot density. Specialization of ramets was largely predictable from their position along the rhizome system, indicating that specialization is an inherent feature (developmentally programmed) of clone ontogeny in S. maritimus. In the field, sexual ramets were always situated at the base of rhizome systems, vegetative ramets were in first and intermediate positions, and shoot-less storage ramets were almost always formed distally on rhizomes (terminal ramets). In the pot experiment flowering ramets were observed in all positions along rhizome systems, suggesting that specialization for sexual reproduction shows a plastic response to environmental conditions.S. maritimus can adjust the relative numbers of ramets with and without above-ground shoots when grown in different shoot densities, i.e. the frequency of individual ramets responsible for a certain functional or developmental process can be adjusted to environmental conditions and internal needs. In S. maritimus, the density-dependant regulation of storage versus vegetative growth and sexual reproduction may represent a mechanism to limit shoot competition in crowded populations.     

Patchy habitats, division of labour and growth dividends in clonal plants

Natural habitats are patchy in quality. in clonal plants, resource-acquiring structures often occupy sites that differ in quality. Clonal plants can display division of labour in resource-acquisition duties, manifested as local specialization by ramets, which enhances acquisition of each resource from sites of greatest abundance. Physiological integration can re-distribute resources internally from sites of acquisition to clone parts sited where the same resources are scarce. Recent research is showing that such specialization and resource sharing is a highly efficient strategy for acquiring resources and that it can result in considerably greater growth when resources are heterogeneously distributed than when the same quantity of resources is distributed homogeneously.     

克隆植物中的劳动分工及其生态学效应

劳动分工是经济进步的发动机,克隆植物也具有与经济学相类似的劳动分工现象。环境异质性、分株专化与合作以及分株潜在的生长独立性是克隆植物劳动分工发生的基本条件。根据发生条件可以把克隆植物劳动分工分为环境诱导型和遗传型两种。克隆植物能够通过利用形态或生理可塑性和生理整合、劳动分工机制实现对生境中异质性资源的有效利用。克隆植物劳动分工的生态学效应在于：提高对局部资源的摄食效率、克服局域资源限制、实现生物量的增益与适合度的提升,上述效应的机理可以用经济学边际成本分析和规模报酬规律来解释。同时,劳动分工还能提高种间竞争力、增强觅食有效性、减弱种内自疏,但同时,克隆植物在不稳定环境下的劳动分工效应也会增加生存风险。随着现代生物学研究手段的不断应用,有关克隆植物劳动分工的研究将会得到更加深入的发展。     

Clonal plant <Emphasis Type="Italic">Duchesnea indica</Emphasis> Focke forms an effective survival strategy in different degrees of Pb-contaminated environments

Clonal plants in heterogeneous environments can benefit from habitat selection, giving them the ability to utilize patchily distributed resources efficiently. However, most research is conducted in a resource heterogeneous environment, and the study of heterogeneous environment of non-resource material (copper, cadmium, lead, and so on) is limited. Research into clonal plant growth under heterogeneous toxic conditions could contribute to our understanding of the strategy of the selective establishment pattern. Thus, we examined clonal growth in a heterogeneous lead environment to enhance understanding of habitat selection strategies. The growth indices (stolon-length, ramet number, biomass, and lead-concentrations) of Duchesnea indica were examined under three levels (low, moderate, and high were represented by 0, 50 and 100%) of lead contamination and two degrees of heterogeneity (low and high heterogeneity under moderate contamination) environments in a glasshouse study. Habitat selection strategy was analyzed by clone growth pattern, labor division, and risk-spreading. The clones under the moderate contamination level, especially with high heterogeneity, demonstrated the optimal growth. They expanded their growth-pattern to escape the toxic environment, and rooted more ramets in the unpolluted patches, allocating more aboveground biomass to these areas. Moreover, parent ramets transported their lead in the soil to the offspring. The offspring spread the toxic risk by accumulating lead in their roots and producing more ramets. Optimal growth of clonal plants occurred in environments moderately contaminated with lead, especially under higher heterogeneity, which performed effective survival strategy by expensive growth architecture, aboveground biomass increase and risk-spreading.     

克隆整合提高淹水胁迫下狗牙根根部的活性氧清除能力

虽然国内外已开展大量关于克隆整合影响植物抗逆生理的研究,但迄今未见克隆整合是否会影响逆境下不同分株清除活性氧过程的报道。以河岸带适生克隆植物狗牙根(Cynodon dactylon)为例,研究克隆植物的抗氧化生理响应,检测了狗牙根在先端淹水/不淹水、先端与基端匍匐茎连接/切断两个因素的交互作用下的根部主要抗氧化酶:超氧化岐化酶(Superoxide dismutase, SOD)、抗坏血酸过氧化物酶(Ascorbate peroxidase, APX)、过氧化氢酶(Catalase, CAT)的活力以及生物量的变化。结果显示,淹水环境中狗牙根先端的生物量和根部SOD酶活力在匍匐茎连接处理下显著高于切断处理组,同一处理的生物量以及根部APX、CAT酶活力总体上表现出不同程度的提高趋势;与受淹先端连接的基端分株根部抗氧化酶活力均低于切断处理组,且SOD和CAT受连接处理影响显著;淹水和切断处理显著降低先端分株的生物量,但对基端和克隆片段影响不明显。这表明淹水胁迫下克隆整合提高了其根部活性氧清除能力,显著改善了先端分株的表现。     

青藏高原东缘过路黄在资源交互斑块性生境中的克隆内资源共享

在青藏高原和四川盆地过渡带,分别于618m和1800m两个海拔高度上研究匍匐茎克隆植物过路黄(Lysimachia christinae)在资源交互斑块性生境中的克隆内资源共享及其对生长的影响.结果显示, 在海拔1800m处,与资源的空间同质性处理(Ⅰ) 和(Ⅱ)相比, 资源的空间异质性处理(Ⅲ)和(Ⅳ)下过路黄整个克隆片段的生物量和分株数均获得显著增加;在海拔618m处,与资源的空间同质性处理(Ⅰ) 和(Ⅱ)相比,资源的空间异质性处理(Ⅲ)和(Ⅳ)下过路黄整个克隆片段生物量显著增加.在海拔618m和1800m处,生长在低光高养条件下的远端分株, 若与高光低养的近端分株相连, 相比连接到低光高养的近端分株, 它们分配更多的生物量到地下部分;在海拔1800m处,生长在高光低养条件下的远端分株, 若与低光高养的近端分株相连, 相比连接到高光低养的近端分株, 它们分配更多的生物量到地上部分.在海拔618m和1800m处,生长在高光低养条件下的近端分株, 若与低光高养的远端分株相连, 相比连接到高光低养的远端分株, 它们分配更多的生物量到地上部分.处于资源交互斑块性生境中的过路黄发生了克隆内分工,依靠相连分株间的功能分化, 克隆植物能有效的利用异质性分布的资源, 缓解资源交互斑块性分布对克隆植物生长的不利影响.通过间隔子(匍匐茎或根状茎),相连分株间能够相互传递和共享由不同分株获得的资源,这种资源共享能够提高克隆植物在异质性生境中的存活与生长.同时,方差分析显示环境异质性和海拔的交互作用显著影响克隆片段的生物量和分株数.相比于海拔618m,在海拔1800m处克隆内资源共享对克隆植物生长表现的影响更大.     

Spatial division of labour of Schoenoplectus americanus

If connected ramets are growing in heterogeneous environments, Division of Labour (DoL) among ramets potentially will result in more efficient sharing of resources and an overall benefit to the plants. As a result of DoL, connected ramets growing in a heterogeneous environment might achieve more biomass than ramets growing in a homogeneous environment. DoL has been demonstrated to occur in a few clonal plant species, although most studies simply focussed on biomass allocation, not on actual resource capturing such as water and nutrient consumption. The model system for our study is one in which two connected ramet groups of Schoenoplectus americanus were placed into contrasting environments. In one treatment, the connected ramets grew in heterogeneous environments and in the other treatment, the connected ramets grew in the same (i.e. homogeneous) environment. We manipulated two variables (light and salinity) in the experiment. We hypothesized that ramets growing in a shaded and fresh water condition in a heterogeneous environment would use more water than ramets growing in a similar condition but in a homogeneous environment. We further hypothesized that ramets growing in a light and saline condition in a heterogeneous environment would assimilate less water than ramets growing in a similar condition but in a homogeneous environment. These hypotheses are based on the assumption that ramets in a heterogeneous environment would translocate water from ramets growing in a shaded and fresh water condition to ramets growing in a light and saline water condition. We also hypothesized that ramets growing in heterogeneous environments achieve larger biomass than ramets in homogeneous environments. Ramets grown in light and saline conditions in heterogeneous environments allocated more biomass to aboveground parts, had taller shoots, larger Specific Green (leaf) Area and consumed less water than ramets grown in similar conditions but in a homogeneous environment. Results confirm the hypothesis that connected ramets in heterogeneous environments are specialised to capture locally abundant resources and share these with connected ramets growing in other habitats. The result of DoL is that the entire connected ramet system benefits and achieves higher biomass.     

Differential influence of clonal integration on morphological and growth responses to light in two invasive herbs

Background and aims

In contrast to seeds, high sensitivity of vegetative fragments to unfavourable environments may limit the expansion of clonal invasive plants. However, clonal integration promotes the establishment of propagules in less suitable habitats and may facilitate the expansion of clonal invaders into intact native communities. Here, we examine the influence of clonal integration on the morphology and growth of ramets in two invasive plants, Alternanthera philoxeroides and Phyla canescens, under varying light conditions.

Methods

In a greenhouse experiment, branches, connected ramets and severed ramets of the same mother plant were exposed under full sun and 85% shade and their morphological and growth responses were assessed.

Key results

The influence of clonal integration on the light reaction norm (connection×light interaction) of daughter ramets was species-specific. For A. philoxeroides, clonal integration evened out the light response (total biomass, leaf mass per area, and stem number, diameter and length) displayed in severed ramets, but these connection×light interactions were largely absent for P. canescens. Nevertheless, for both species, clonal integration overwhelmed light effect in promoting the growth of juvenile ramets during early development. Also, vertical growth, as an apparent shade acclimation response, was more prevalent in severed ramets than in connected ramets. Finally, unrooted branches displayed smaller organ size and slower growth than connected ramets, but the pattern of light reaction was similar, suggesting mother plants invest in daughter ramets prior to their own branches.

Conclusions

Clonal integration modifies light reaction norms of morphological and growth traits in a species-specific manner for A. philoxeroides and P. canescens, but it improves the establishment of juvenile ramets of both species in light-limiting environments by promoting their growth during early development. This factor may be partially responsible for their ability to successfully colonize native plant communities.     

Arbuscular mycorrhizal fungi reduce effects of physiological integration in Trifolium repens

Background and Aims

One of the special properties of clonal plants is the capacity for physiological integration, which can increase plant performance through mechanisms such as resource sharing and co-ordinated phenotypic plasticity when plants grow in microsites with contrasting resource availabilities. However, many clonal plants are colonized by arbuscular mycorrhizal fungi (AMF). Since AMF are likely to reduce contrasts in effective resource levels, they could also reduce these effects of clonal integration on plasticity and performance in heterogeneous environments.

Methods

To test this hypothesis, pairs of connected and disconnected ramets of the stoloniferous herb Trifolium repens were grown. One ramet in a pair was given high light and low nutrients while the other ramet was given high nutrients and low light. The pairs were inoculated with zero, one or five species of AMF.

Key Results

Pairs of ramets grown without AMF developed division of labour and benefited from resource sharing, as indicated by effects of connection on allocation to roots, accumulation of mass, and ramet production. Inoculation with five species of AMF significantly reduced these effects of connection, both by inhibiting them in ramets given high nutrients and inducing them in ramets given high light. Inoculation with one species of AMF also reduced some effects of connection, but generally to a lesser degree.

Conclusions

The results show that AMF can significantly modify the effects of clonal integration on the plasticity and performance of clonal plants in heterogeneous environments. In particular, AMF may partly replace the effects and benefits of clonal integration in low-nutrient habitats, possibly more so where species richness of AMF is high. This provides the first test of interaction between colonization by AMF and physiological integration in a clonal plant, and a new example of how biotic and abiotic factors could interact to determine the ecological importance of clonal growth.Key words: Arbuscular mycorrhizal fungi, biomass allocation, clonal plant, division of labour, environmental heterogeneity, light availability, nutrients, white clover     

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.     

Influence of light and nitrate assimilation on the growth strategy in clonal weed <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>

We studied as to how the inter-connected modular architecture of clonal Eichhornia crassipes allows nutrient to transfer from established ramets to developing ramets, and nitrate translocation within clonal system and how such a strategy may play an important role in successful establishment and expansion of this clonal plant. Using this stoloniferous E. crassipes as a model, we studied the effects of light and nitrate availability on growth and nitrate assimilation in inter-connected parent and offspring ramets. Our results showed that increase in light and nitrate availability significantly increased growth rate of the whole clonal fragments and reproduction of offspring ramets in E. crassipes. In addition, increases in nitrate reductase (NR) activity and glutamine synthetase (GS) activity were observed in both parent and offspring ramets with increase in light density and nitrate supply. We also found that nitrate translocation is greater in offspring ramets than in parent ramets under abundant light and nitrate environment in this fast-growing clonal plant. Consequently, majority of nitrate assimilation in offspring ramets is beneficial to the growth of whole clonal system, as indicated by a close correlation between nitrate assimilation in offspring ramets and RGR of whole clonal fragments. We strongly contend that nitrate translocation and assimilation within clone system is important for efficient utilization of nitrogen in alien clonal plant E. crassipes during establishment and expansion, and thus for increase in its invasiveness in natural water columns.