Potential and limitations of current concepts regarding the response of clonal plants to environmental heterogeneity

Plant ecologists have spent considerable effort investigating the physiological mechanisms and ecological consequences of clonal growth in plants. One line of research is concerned with the response of clonal plants to environmental heterogeneity. Several concepts and hypotheses have been formulated so far, suggesting that intra-clonal resource translocation, morphological plasticity on different organizational levels (e.g. leaves, ramets, fragments), and other features of clonal plants may represent potentially adaptive traits enabling stoloniferous and rhizomatous species to cope better with habitat patchiness. Although each of these concepts contributes substantially to our understanding of the ecology of clonal species, it is difficult to combine them into a consistent theoretical framework. This apparent lack of conceptual coherence seems partly be caused by an uncritical use of the term habitat heterogeneity. Researchers have not always acknowledged the fact that heterogeneity may refer to a number of fundamentally different aspects of environmental variability (i.e. scale, contrast, predictability, temporal vs. spatial heterogeneity), and that each of these aspects may, on one hand, allow for the evolution of specific plant responses to heterogeneity and, on the other, severely constrain the viability of potentially adaptive traits. Since adaptive responses are operational only in a narrow range of conditions (delimited by external environmental conditions and constraints internal to plants) it seems imperative to clearly define the context and the limits within which concepts regarding clonal plants' responses to heterogeneity are valid. In this paper an attempt is made to review a number of these concepts and to try and identify the necessary conditions for them to be operational. Special attention is paid (1) to different aspects of environmental heterogeneity and how they may affect clonal plants, and (2) to possible constraints (e.g. sectoriality, perception of environmental signals, morphological plasticity) on plant responses to patchiness.     

Effects of Spatial Scale of Soil Heterogeneity on the Growth of a Clonal Plant Producing Both Spreading and Clumping Ramets

Soil nutrients are commonly heterogeneously distributed at different spatial scales. Although numerous studies have tested the effects of soil nutrient heterogeneity on growth of clonal plants producing either spreading ramets or clumping ramets, no study has examined the effects on the growth of clonal plants producing both spreading and clumping ramets and how spatial scale affects such effects. To test these effects, clones of Buchloe dactyloides, a stoloniferous clonal plant that produces both clumping and spreading ramets, were grown in six heterogeneous environments with different patch sizes and one homogeneous environment containing the same quantity of nutrients. Total biomass, total number of ramets, number of clumping ramets, number of spreading ramets, spacer length, or root:shoot ratio of the whole plants did not differ significantly among the seven treatments. However, at the patch level there were significant effects of patch size by nutrient level on biomass, number of ramets, number of spreading ramets, and number of clumping ramets, and these four variables were significantly larger in the nutrient-rich patches than in the nutrient-poor patches in the heterogeneous treatment with the largest patch size, but not in the other five heterogeneous treatments with smaller patch sizes. Neither nutrient level nor patch size significantly affected spacer length or root:shoot ratio. Based on our results, we propose that B. dactyloides can efficiently exploit nutrient-rich patches by a plastic response of clumping ramets and spreading ramets at larger spatial scales of soil heterogeneity but not at smaller ones.     

Small-scale heterogeneity in soil quality influences photosynthetic efficiency and habitat selection in a clonal plant

BACKGROUND AND AIMS: In clonal plants, internode connections allow translocation of photosynthates, water, nutrients and other substances among ramets. Clonal plants form large systems that are likely to experience small-scale spatial heterogeneity. Physiological and morphological responses of Fragaria vesca to small-scale heterogeneity in soil quality were investigated, together with how such heterogeneity influences the placement of ramets. As a result of their own activities plants may modify the suitability of their habitats over time. However, most experiments on habitat selection by clonal plants have not generally considered time as an important variable. In the present study, how the foraging behaviour of clonal plants may change over time was also investigated. METHODS: In a complex of environments with different heterogeneity, plant performance was determined in terms of biomass, ramet production and photosynthetic activity. To identify habitat selection, the number of ramets produced and patch where they rooted were monitored. KEY RESULTS: Parent ramets in heterogeneous environments showed significantly higher maximum and effective quantum yields of photosystem II than parents in homogeneous environments. Parents in heterogeneous environments also showed significantly higher investment in photosynthetic biomass and stolon/total biomass, produced longer stolons, and had higher mean leaf size than parents in homogeneous environments. Total biomass and number of offspring ramets were similar in both environments. However, plants in homogeneous environments showed random allocation of offspring ramets to surrounding patches, whereas plants in heterogeneous environments showed preferential allocation of offspring to higher-quality patches. CONCLUSIONS: The results suggest that F. vesca employs physiological and morphological strategies to enable efficient resource foraging in heterogeneous environments and demonstrate the benefits of physiological integration in terms of photosynthetic efficiency. The findings indicate that short-term responses cannot be directly extrapolated to the longer term principally because preferential colonization of high-quality patches means that these patches eventually show reduced quality. This highlights the importance of considering the time factor in experiments examining responses of clonal plants to heterogeneity.     

根状茎型植物扁秆荆三棱对土壤养分异质性尺度和对比度的生长响应

土壤养分的空间异质性在自然界普遍存在, 而克隆植物被认为能很好地适应和利用土壤养分异质性。尽管尺度和对比度是异质性的两个重要属性, 但有关土壤养分异质性的尺度和对比度及其交互作用对克隆植物生长和分株分布格局影响的研究仍比较缺乏。在一个温室实验中, 根状茎型草本克隆植物扁秆荆三棱(Bolboschoenus planiculmis) (异名扁秆藨草(Scirpus planiculumis))被种植在由高养分斑块和低养分斑块组成的异质性环境中。实验为两种尺度处理(大斑块和小斑块)和两种对比度处理(高对比度和低对比度)交叉组成的4种处理组合。在每个处理中, 高养分和低养分斑块的总面积相同; 在所有4种处理中, 土壤养分的总量也完全相同。无论在整个克隆(植株)水平, 还是在斑块水平, 尺度、对比度及其交互作用对扁秆荆三棱的生物量、分株数、根状茎长和块茎数的影响均不显著。然而, 在斑块水平, 扁秆荆三棱在高养分斑块中的生物量、分株数、根状茎长和块茎数均显著高于低养分斑块, 而在高养分斑块中相邻分株间的距离(间隔物长)小于低养分斑块, 并且这种效应均不依赖于斑块尺度的大小和对比度的高低。因此, 在土壤养分异质性环境中, 扁秆荆三棱可以通过缩短间隔物长, 并可能通过提高根状茎的分枝强度, 把较多的分株和潜在分株放置在养分条件好的斑块中。这种响应格局体现出克隆植物的觅食行为, 有利于整个克隆对异质性资源的吸收和利用。然而, 该实验中的尺度和对比度对扁秆荆三棱分株的放置格局均没有显著效应。作者推测, 在一个更大的斑块尺度和(或)对比度范围内, 扁秆荆三棱对土壤养分异质性的响应可能不同。因此, 下一步的研究应涉及更广泛的尺度和对比度。     

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.     

A simulation study of the effects of architectural constraints and resource translocation on population structure and competition in clonal plants

(1) Spatially explicit simulation of clonal plant growth is used to determine how ramet-level traits affect ramet density, spatial pattern of ramets and competitive ability of a clonal plant. The simulation model used combines elements of (i) an individual-based model of plant interactions, (ii) an architectural model of clonal plant growth, and (iii) a model of resource translocation within a set of physiologically integrated plant individuals. (2) The effects of two groups of parameters were studied: growth and resource acquisition parameters (resource accumulation, density-dependence of resource accumulation, resource translocation between ramets) and architectural rules (branching angle and probability of branching, internode length). The model was parameterised by values approximating those of clonally growing grasses as closely as possible. The basic parameter values were chosen from a short-turf grassland. Sensitivity analysis was carried out on relevant parameters around three basic points in the parameter space. Both single-species and two-species systems were studied. (3) It is shown that increasing resource acquisition and growth parameters increase ramet density, genet number and competitive ability. Translocation parameters and architectural parameters modify the effects of resource acquisition and growth, but their effect in single-species stands was smaller. (4) The simulations of species with fixed ramet sizes showed that ramet density in single-species stands cannot be used for predicting competitive ability. Increase in resource acquisition and growth parameters was correlated with an increase in equilibrium ramet density and competitive ability. Increasing branching angle, branching probability or internode length lead to an increased competitive ability, but did not affect equilibrium ramet density. Change of architectural parameters could therefore affect competitive ability independently of their effect on the final ramet density. (5) Spatial pattern both in single-species and two-species stands was also highly parameter-dependent. Changes in architectural parameters and in translocation usually lead to pronounced change in the spatial pattern; change in growth and resource acquisition parameters generally had little effect on spatial pattern.     

Foraging responses of clonal plants to multi-patch environmental heterogeneity: spatial preference and temporal reversibility

Background and aims

Plant root placement is highly plastic in order to acquire patchily distributed nutrients and to ensure their survival, growth and reproduction. Considering the spatial extension of clonal organs, we selected two clonal plants (Leymus chinensis (Trin.) Tzvel. and Hierochloe glabra Trin.) to determine the spatio-temporal effects of environmental heterogeneity on belowground organs and newly-born ramets.

Methods

Small-scale and multi-patch heterogeneous environments were manipulated by creating four patches filled with different types of soil in a same pot. The four patches were composed of sandy soil, sandy loam, loam soil and humus soil, respectively. Ramet number, bud number, mean spacer length, rhizome length, and biomass allocation within each patch were measured to identify plant foraging responses.

Results

The preferential patch of L. chinensis was humus soil patch which was the highest in nutrient availability, whereas H. glabra preferred to place ramets in sandy loam and loam soil patches. When growing in homogeneous environments, both species randomly rooted their offspring ramets in the four compartments. In heterogeneous environments, foraging responses were detected in ramet placement, aboveground biomass and total rhizome length. However, there were no differences in bud number or belowground biomass among four types of patches in heterogeneous environments, which might suggest that there would be no inter-patch differences in seedling establishment in the next year.

Conclusions

Plants show selective allocation of offspring ramets to preferential patches in the presence of multi-patch environmental heterogeneity. Responses of H. glabra to multi-patch heterogeneity were faster than those of L. chinensis, demonstrating that the foraging patterns are species-specific. Clonal plants can rapidly respond to environmental heterogeneity, whereas foraging responses are potentially reversible over a longer temporal scale.     

Clonal Growth in Plants in Relation to Resource Heterogeneity:Foraging Behavior

In nature, essential resources for organisms, such as food for animals and light, water and nutrients for plants, are usually heterogeneously distributed, even at very small scale. As a result, all organisms, particularly plants mostly sessile, have a difficulty in acquiring essential resources from their environments. Animals express various types of foraging behavior to capture heterogeneously distributed essential foods. Clonal growth ( a vegetative reproductive process where by more than one individual of identical genetic composition is formed ) provides clonal plant not only with many "mouths" at different spatial positions, but also with a large spacial movability. As a clonal plant grows in environments characterized by a small-scale resource heterogeneity, its inter ramet connection permits a resource-sharing among the connected tamers. In addition, it may also allow certain ramets to respond locally and non-locally to resousce heterogeneity. This may lead to a division of labor among the connected ramets and a selective placement of ramets in favorable micro-habitats. Together these may enhance exploitation of resource heterogeneity by clonal plants, and in turn greatly contribute to maintenance or improvement of fitness. Such a behavior of clonal plants, expressed in heterogeneous environments, is to a large extent comparable to that of animals. Therefore, it has been considered as foraging behavior in clonal plants. More recently, it has been observed that phenotypic plasticity of clonal plants, which is relevant to foraging behavior, varies among species, types of genet architecture as well as among types of plants habitats. Foraging in clonal plants and its diversity have been receiving increasingly intensive investigations.     

Habitat heterogeneity and niche structure of trees in two tropical rain forests

Dispersal-assembly theories of species coexistence posit that environmental factors play no role in explaining community diversity and structure. Dispersal-assembly theories shed light on some aspects of community structure such as species-area and species-abundance relationships. However, species environmental associations also affect these measures of community structure. Measurements of species niche breadth and overlap address this influence. Using a new continuous measure of niche and a dispersal-assembly null model that maintains species niche breadth and aggregation, we tested two hypotheses assessing the effects of habitat heterogeneity on the ability of dispersal-assembly theories to explain community niche structure. We found that in both homogenous and heterogeneous environments dispersal-assembly theories cannot fully explain observed niche structure. The performance of the dispersal-assembly null models was particularly poor in heterogeneous environments. These results indicate that non-dispersal based mechanisms are in part responsible for observed community structure and measures of community structure which include species environmental associations should be used to test theories of species diversity.     

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 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.     

Clonal plants as cooperative systems: Benefits in heterogeneous environments

All natural environments are spatially and temporally heterogeneous. Consequently, their ability to provide essential resources for the growth of plants is variable. Modular plant species produce repeated basic structures which, in the case of clonal species, are called ramets. Ramets belonging to the same clone are distributed throughout the environment in space and time, and therefore they may be located in sites which differ in resource-providing quality. The connections between ramets may allow resources to be shared, enabling the clone to behave as a cooperative system. As a result of such physiological integration, ramets can survive in conditions where there is lethal shortage of a resource because they are connected to, and supported by, ramets located in conditions where there is ample supply of the same resource. Physiological integration between connected ramets presents opportunities for heterogeneous environments to be exploited to an extent that is only just becoming apparent. As heterogeneity is ubiquitous in natural environments, it may be expected that plants, as relatively immobile organisms, will have evolved the capacity to cope with it by making appropriate localized morphological and/or physiological plastic responses. Recent studies suggest that such responses not only enable clonal species to cope with environmental heterogeneity, but that under some circumstances they can benefit more from environments which are heterogeneous rather than homogeneous, even when both types of environment contain the same amount of resources. Studies on Glechoma hederacea (Lamiaceae) that illustrate this phenomenon are described.     

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.     

CLO-PLA2 – a database of clonal plants in central Europe

CLO-PLA2' (CLOnal PLAnts, version 2) is a database on architectural aspects of clonal growth in vascular plants of central Europe. The database includes 2749 species, characterised by 25 variables, either directly or indirectly related to clonal growth. The total number of items in the database is over 12750. The structure of the database is described and the variables used to characterise clonal growth of individual species are listed. Two examples of database utilisation are given. The first concerns the relationship between habitat niche width and the mode of clonal growth. Turf graminoids, species with long-lived rhizomes either short to long and formed below-ground, or short and formed above-ground, and short-lived rhizomes formed above-ground, are over-represented among the species with very broad niches and under-represented among the species with narrow niches. In contrast, species multiplying by plant fragments are missing among the species with the broadest niches. The second example explores how individual clonal growth modes are combined in individual species. About 21% of species of clonal plants have more than one mode of clonal growth. Some combinations are over-represented in certain families and environments. The application of phylogenetic independent contrasts (PIC) showed that both phylogenetic constraints and adaptations to particular environmental conditions play important roles in determining the observed pattern.     

Clonal architecture in marine macroalgae: ecological and evolutionary perspectives

The study of the ecological and evolutionary consequences of clonal growth in vascular plants has been widely addressed; however, marine macroalgae, which are interesting modular organisms that combine simple morphologies and complex life cycles, have been almost ignored. This paper presents a review and analysis of the ecological and evolutionary consequences of clonality in marine macroalgae, including three main subjects: (1) modular construction (modules and ramets); (2) life cycle and evolutionary perspectives, and (3) ecological perspectives of clonality in marine macroalge. The biological emergent attributes of clonality are present in marine macroalgae e.g. high longevity of the genet by the continual renewal of modules, and variable morphological plasticity of ramets and modules in relation to environmental conditions. However, experimental work is still needed to solve questions such as the effect of crowding on survival rates and use of resources, as well as its effect on sexual or asexual patterns of reproduction. I expect that the study of the evolutionary consequences of the combined presence of alternation of generations and clonal growth in marine macroalgae will make important contributions to clonal plant theory.     

Clonal integration affects allocation in the perennial herb <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> in N-limited homogeneous environments

Most work on clonal growth in plants has focused on the advantages of clonality in heterogeneous habitats. We hypothesized (1) that physiological integration of connected ramets within a clone can also increase plant performance in homogeneous environments, (2) that this effect depends on whether ramets differ in ability to take up resources, and (3) that only ramets with relatively low uptake ability benefit. We tested these hypotheses using the perennial amphibious herb Alternanthera philoxeroides. We grew clonal fragments and varied numbers of rooted versus unrooted ramets, connection between the apical and basal parts of fragments, and availability of nitrogen. Patterns of final size and mass of fragments did not support these hypotheses. By some measures, severance did reduce the growth of more apical ramets and increase the growth of less apical ones, consistent with net apical transfer of resources. Rooting of individual ramets strongly influenced their growth: second and third most apical ramets each grew most when they were the most apical rooted ramet, and this pattern was more pronounced under higher nitrogen levels. This adds to the evidence that signalling between ramets is an important aspect of clonal integration. Overall, the results indicate that physiological integration between ramets within clones in homogeneous environments can alter the allocation of resources between connected ramets even when it does not affect the total growth of clonal fragments.     

Nutrient addition does not increase the benefits of clonal integration in an invasive plant spreading from open patches into plant communities

• One benefit of clonal integration is that resource translocation between connected ramets enhances the growth of the ramets grown under stressful conditions, but whether such resource translocation reduces the performance of the ramets grown under favourable conditions has not produced consistent results. In this study, we tested the hypothesis that resource translocation to recipient ramets may reduce the performance of donor ramets when resources are limiting but not when resources are abundant.
• We grew Mikania micrantha stolon fragments (each consisting of two ramets, either connected or not connected) under spatially heterogeneous competition conditions such that the developmentally younger, distal ramets were grown in competition with a plant community and the developmentally older, proximal ramets were grown without competition. For half of the stolon fragments, slow‐release fertiliser pellets were applied to both the distal and proximal ramets.
• Under both the low and increased soil nutrient conditions, the biomass, leaf number and stolon length of the distal ramets were higher, and those of the proximal ramets were lower when the stolon internode was intact than when it was severed. For the whole clone, the biomass, leaf number and stolon length did not differ between the two connection treatments. Connection did not change the biomass of the plant communities competing with distal ramets of M. micrantha.
• Although clonal integration may promote the invasion of M. micrantha into plant communities, resource translocation to recipient ramets of M. micrantha will induce a cost to the donor ramets, even when resources are relatively abundant.

     

Partial mechanical stimulation facilitates the growth of the rhizomatous plant Leymus secalinus: modulation by clonal integration

Background and Aims

Mechanical stimulation (MS) often induces plants to undergo thigmomorphogenesis and to synthesize an array of signalling substances. In clonal plants, connected ramets often share resources and hormones. However, little is known about whether and how clonal integration influences the ability of clonal plants to withstand MS. We hypothesized that the effects of MS may be modulated by clonal integration.

Methods

We conducted an experiment in which ramet pairs of Leymus secalinus were subjected to three treatments: (1) connected ramet pairs under a homogeneous condition [i.e. the proximal (relatively old) and distal (relatively young) ramets were not mechanically stressed]; (2) connected ramet pairs under a heterogeneous condition (i.e. the proximal ramet was mechanically stressed but the distal ramet was not); and (3) disconnected ramet pairs under the same condition as in treatment 2. At the end of the experiment, we harvested all plants and determined their biomass and allocation.

Key Results

Clonal integration had no significant influence on measured traits of distal L. secalinus ramets without MS. However, under MS, plants with distal ramets that were connected to a mother ramet produced more total plant biomass, below-ground biomass, ramets and total rhizome length than those that were not connected. Partial MS exerted local effects on stimulated ramets and remote effects on connected unstimulated ramets. Partial MS increased total biomass, root/shoot ratio, number of ramets and total rhizome length of stimulated proximal ramets, and increased total biomass, root weight ratio, number of ramets and total rhizome length of connected unstimulated ramets due to clonal integration.

Conclusions

These findings suggest that thigmomorphogenesis may protect plants from the stresses caused by high winds or trampling and that thigmomorphogenesis can be strongly modulated by the degree of clonal integration.     

Autotetraploid tangor plant regeneration from in vitro Citrus somatic embryogenic callus treated with colchicine

In vitro chromosome doubling of embryogenic callus lines of the Citrus cultivars Umatilla and Dweet tangors (Citrus reticulata Blanco×C. sinensis [L.] Osb.), Caffin clementine (C. clementina Hort. ex Tan.) and Wheeny grapefruit (C. paradisi Macf) was carried out in the presence of either 0.05 or 0.1% colchicine, or 0.01, 0.05 or 0.1% oryzalin. Embryogenic callus development was partly suppressed in the presence of colchicine, and completely suppressed by oryzalin at all concentrations tested. No plants were regenerated from any of the oryzalin treatments. Ploidy level of plants regenerated from the colchicine treatments was determined using flow cytometry and chromosome squashes. Three desirable non-chimeric, autotetraploid plants of the mono-embryonic cultivar Umatilla were produced using 0.05% colchicine and one from 0.1% colchicine. One mixoploid Dweet plant was produced using 0.1% colchicine.