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.     

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.     

Differential response to shading in orthotropic and plagiotropic shoots of the clonal herb Glechoma hirsuta

Previous studies have shown that internodes and petioles of closely related erect and stoloniferous species show marked differences in their response to shading. Vertical structures show strong elongation responses while horizontal structures show significantly smaller elongation responses. This paper reports an experiment designed to test whether internodes and petioles on orthotropic (vertically oriented) and plagiotropic (horizontally oriented) shoots produced by the same plant, also show such differential responses. The study species, Glechoma hirsuta, produces plagiotropic shoots in its vegetative stage and orthotropic shoots during its generative stage. Shoots of G. hirsuta were grown either in full daylight or under simulated canopy shade. Internode and petiole elongation, biomass investment patterns and other growth-related parameters were measured on plants in each light treatment. In orthotropic shoots the length of internodes responded more strongly to shading than the length of petioles, while the opposite was true for plagiotropic shoots, confirming the hypothesis that vertical spacers have higher degrees of shade-induced plasticity than horizontally oriented spacers. Growth and development of horizontally oriented shoots was reduced by shading, whereas that of vertically oriented shoots was unaffected by light treatment. The results are discussed in terms of the differences in function of vertically and horizontally oriented spacers, and the probable benefits of plasticity in enhancing essential activities including photosynthesis, habitat exploration and seed dispersal. Received: 23 December 1996 / Accepted: 19 July 1997     

Field evidence of plastic growth responses to habitat heterogeneity in the clonal herbRanunculus repens

Stolon internode lengths were measured on plants of the clonal herbRanunculus repens growing in a hay meadow which was subject to disturbance by mole (Talpa europaea) activity. Within the site three habitat types were recognized: closed grassland, the open ground of fresh molehills and the grass-molehill boundary. The lengths of stolon internodes ofR. repens differed significantly in each of the three habitats. The shortest internodes occurred on stolons on the open molchills. The longest occurred in the closed grassland habitat. The type of habitat in which parent ramets were rooted did not significantly influence the length of internodes on their daughter stolons. The length of a stolon internode was determined by its immediate surrounding habitat type. Consecutive internode lengths on a given stolon showed considerable plasticity, shortening significantly as stolons spread onto molehills from surrounding habitats, and increasing significantly as stolons advanced from a molehill into the surrounding closed grassland habitat. These results are consistent with the proposition that under favorable conditions (on the molehills, where resources are expected to be more abundant, and competition absent) internode lengths shorten and the plant forages intensively, whereas under conditions of low resource availability (in the closed grassland, where competition occurs) internode lengths increase, allowingR. repens to forage extensively. Such morphological plasticity may promote more efficient exploitation of resource-rich sites and more rapid vacation of resource-poor sites.     

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.     

Plasticity in R/S ratio, morphology and fitness-related traits in response to reciprocal patchiness of light and nutrients in the stoloniferous herb, Glechoma longituba L

Clonal fragments of the stoloniferous herb Glechoma longituba were subjected to a complementary patchiness of light and soil nutrients including two spatially homogeneous treatments (SR–SR and IP–IP) and two spatially heterogeneous treatments (IP–SR and SR–IP). SR and IP indicate patches (shaded, rich) with low light intensity (shaded, S), high nutrient availability (rich, R) and patches (illuminated, poor) with high light intensity (illuminated, I) and low nutrient availability (poor, P), respectively. Plasticity of the species in root–shoot ratio, fitness-related traits (biomass, number of ramets and dry weight per ramet) and clonal morphological traits (length and specific length of stolon internodes, area and specific area of laminae, length and specific length of petioles) were experimentally examined. The aim is to understand adaptation of G. longituba to the environment with reciprocal patches of light and soil nutrients by plasticities both in root–shoot ratio and in (clonal) morphology. Our experiment revealed performance of the clonal fragments growing from patches with high light intensity and low soil nutrient availability into the adjacent opposite patches was increased in terms of the fitness-related characters. R/S ratio and clonal morphology were plastic. Meanwhile, the capture of light resource from the light-rich patches was enhanced while the capture of soil nutrients from either the nutrient-rich or the nutrient-poor patches was not. Analysis of cost and benefit disclosed positive effects of clonal integration on biomass production of ramets in the patches with low light intensity and high soil nutrient availability. These results suggest an existence of reciprocal translocation of assimilates and nutrients between the interconnected ramets. The reinforced performance of the clonal fragments seems to be related with specialization of clonal morphology in the species.     

Shade avoidance in the clonal herb Trifolium fragiferum/: a field study with experimentally manipulated vegetation height

In herbaceous canopies light availability can show high degrees of spatial variability in a vertical and also in a horizontal direction. Stoloniferous plants are hence likely to encounter differences in light availability during their ontogenetic development. Different mechanisms, such as petiole elongation, plasticity in internode length and branching, and an enhanced allocation to sexual reproduction have been suggested to represent viable shade-avoidance mechanisms for clonal plants.In a field experiment we tested the response of the stoloniferous herb Trifolium fragiferum L. to experimentally manipulated vegetation heights. Naturally occurring clonal fragments were exposed to four different vegetation heights ranging from 0 cm (high light availability created by clipping the surrounding natural vegetation at ground level) to 20 cm (natural shading in closed canopy). The growth and development of individual clones was followed for two months. At the end of the experiment above-ground plant parts were harvested. Growth-related and morphological parameters (e.g., petiole and internode length) as well as patterns of meristem utilization (i.e., flowering, branching) were recorded.Neither primary stolon growth and biomass accumulation nor branching and flowering were significantly affected by treatments. However, increased vegetation height resulted in a reduced number of secondary ramets and also had strong positive effects on petiole length, leading to marked changes in the architecture of plants growing in canopies of different heights. In addition, the average weight of individual ramets on the primary stolon was markedly higher in plants exposed to taller vegetation as compared to shorter vegetation.The results of this study suggest the occurrence of a trade-off between clonal expansion (i.e., secondary ramet production) and the average size of clonal offspring. If grown under higher vegetation plants invested more into the size of individual ramets, especially into elongating petioles, and less into the growth and development of lateral branches. Placing leaf laminae higher up in the canopy results in an enhanced light interception which has apparently buffered negative effects of increased vegetation height on whole-clone biomass. Plants grown under shorter vegetation invested more into lateral spread by producing more, but smaller ramets.     

Stochastic simulation of clonal growth in the tall goldenrod,Solidago altissima

Summary As clonal plants grow they move through space. The movement patterns that result can be complex and difficult to interpret without the aid of models. We developed a stochastic simulation model of clonal growth in the tall goldenrod, Solidago altissima. Our model was calibrated with field data on the clonal expansion of both seedlings and established clones, and model assumptions were verified by statistical analyses.When simulations were based on empirical distributions with long rhizome lengths, there was greater dispersal, less leaf overlap, and less spatial aggregation than when simulations were based on distributions with comparatively short rhizome lengths. For the field data that we utilized, variation in rhizome lengths had a greater effect than variation for either branching angles or rhizome initiation points (see text). We also found that observed patterns of clonal growth in S. altissima did not cause the formation of fairy rings. However, simulations with an artificial distribution of branching angles demonstrate that fairy rings can result solely from a plant's clonal morphology.Stochastic simulation models that incorporated variation in rhizome lengths, branching angles, and rhizome initiation points produced greater dispersal and less leaf overlap than deterministic models. Thus, variation for clonal growth parameters may increase the efficiency of substrate exploration by increasing the area covered and by decreasing the potential for intraclonal competition. We also demonstrated that ramet displacements were slightly, but consistently lower in stochastic simulation models than in random-walk models. This difference was due to the incorporation of details on rhizome bud initiation into stochastic simulation models, but not random-walk models. We discuss the advantages and disadvantages of deterministic, stochastic simulation, and random-walk models of clonal growth.     

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.     

Leaf shape variation and herbivore consumption and performance: a case study with Ipomoea hederacea and three generalists

The effect of leaf shape variation on plant-herbivore interactions has primarily been studied from the perspective of host seeking behavior. Yet for leaf shape to affect plant-herbivore coevolution, there must be reciprocal effects of leaf shape variation on herbivore consumption and performance. We investigated whether alternative leaf morphs affected the performance of three generalist insect herbivores by taking advantage of a genetic polymorphism and developmental plasticity in leaf shape in the Ivyleaf morning glory, Ipomoea hederacea. Across four experiments, we found variable support for an effect of leaf shape genotype on insects. For cabbage loopers (Trichoplusia ni) and corn earworms (Helicoverpa zea) we found opposing, non-significant trends: T. ni gained more biomass on lobed genotypes, while H. zea gained more biomass on heart-shaped genotypes. For army beetworms (Spodoptera exigua), the effects of leaf shape genotype differed depending on the age of the plants and photoperiod of growing conditions. Caterpillars feeding on tissue from older plants (95 days) grown under long day photoperiods had significantly greater consumption, dry biomass, and digestive efficiency on lobed genotypes. In contrast, there were no significant differences between heart-shaped and lobed genotypes for caterpillars feeding on tissue from younger plants (50 days) grown under short day photoperiods. For plants grown under short days, we found that S. exigua consumed significantly less leaf area when feeding on mature leaves than juvenile leaves, regardless of leaf shape genotype. Taken together, our results suggest that the effects of leaf shape variation on insect performance are likely to vary between insect species, growth conditions of the plant, and the developmental stage and age of leaves sampled. Handling editor: May Berenbaum.     

Directional growth of a clonal bromeliad species in response to spatial habitat heterogeneity

Habitat selection by directional growth of plants has previously been investigated but field evidence for this phenomenon is extremely scarce. In this study we demonstrate directional clonal growth in Aechmea nudicaulis, a monocarpic, perennial bromeliad native to spatially heterogeneous sandy coastal plains (restinga) in Brazil. This habitat is characterized by a matrix of bare sand with interspersed vegetation islands. Due to very high soil surface temperatures and other stress factors such as drought, A. nudicauliscan only germinate inside vegetation islands. Nevertheless, this species is very common on bare sand. In this study we tested the hypothesis that clonal fragments occurring at the border and inside vegetation islands show habitat selection by growing preferentially towards the bare sand habitat (i.e. away from the center of vegetation islands).We randomly chose 116 clonal fragments in two distinct micro-environments (inside vegetation islands, and in the border area between bare sand and vegetation islands) in the natural habitat of A.nudicaulisand measured their growth direction in relation to the island center. We measured the growth directions of entire clonal fragments (defined as the line that connects the oldest and the youngest ramets of a clonal fragment) as well as the growth direction of the youngest internode on each fragment (the growth direction of the youngest ramet in relation to its parent ramet). We used Monte Carlo simulations to test for deviations from randomness in the growth direction of clonal fragments and individual internodes. The clonal fragments of A.nudicaulis showed a significant tendency to grow away from the center of vegetation islands. In other words, the main growth direction of clonal fragments growing inside vegetation islands or at the border between bare sand and vegetation islands was preferentially directed towards bare sand environments. Individual internodes at the border of vegetation islands also exhibited this tendency to grow towards the outside of vegetation islands, but internodes growing inside vegetation islands did not show directional growth. These results provide the first field evidence for habitat selection through directional growth of a clonal plant species.Co-ordinationg editor: J. Tuomi     

Sex expression,female frequency,and reproductive output in a gynodioecious clonal herb, <Emphasis Type="Italic">Glechoma longituba</Emphasis> (Lamiaceae)

We investigated the sex morphs and frequency of females in 24 natural populations of the clonal, gynodioecious species Glechoma longituba. In addition to the presence of hermaphrodites and females, there were some individuals with intermediate sex expression that could not be categorized as either morph. The majority of populations contained a predominance of hermaphrodites, although female frequency varied from 0 to 100%. The sexual expression of individual ramets did not change over three consecutive years or when ramets were transplanted to different populations, but it did change over the course of a flowering season. Seeds from females weighed more than those from hermaphrodites. The seed set of hermaphrodites was relatively constant across populations; the seed set of females was higher than that of hermaphrodites in most populations, but lower in populations in which females occurred at a very high frequency because of pollen limitation. This led to a negative correlation being detected between the seed set of females and the frequency of females in a population. We postulated that the adjustment of sex expression within seasons could be an adaptive mechanism of reproductive assurance in this system.     

Environmental heterogeneity,fungal parasitism and the demography of the grass Stipa leucotricha

This study investigated the demographic consequences of fungal infection of a perennial grass, Stipa leucotricha. The rate of parasitism of this grass by the host-specific, systemic fungus Atkinsonella texensis varies over short distances. Infection was frequent (57% of plants) in mottes (clusters of woody plants) but rarer in adjacent open grasslands (9%). To test the hypothesis that the relative performance of infected and uninfected plants differed in the two habitats, infected and uninfected genotypes were collected from mottes in a central Texas population, propagated in the green-house and then transplanted into the same site in replicate plots within mottes, at the edges of mottes, and in open grassy areas. Demographic data were recorded for 30 months over three growing seasons. Plants were observed to lose and gain infection. Infection had no significant effect on plant survival, tiller number or dry mass although infected plants tended to be larger. Uninfected plants had a significantly higher probability of setting seed but there were no differences in seed production by reproductive plants. There were significant effects of planting environment on all of these measures. Motte edges were most favorable for S. leucotricha transplants while motte interiors were least favorable and open areas were intermediate. There was no evidence of habitat x infection interactions; therefore the fungal infection had similar effects in different habitats. The high frequency of infection in motte habitats is best explained by more efficient contagious spread there. The favorability of motte edges for plant growth is substantially offset by higher infection rates at the edges of mottes.     

Density-dependent growth responses in two clonal herbs: regulation of shoot density

Summary It has been shown in clonal perennial herbs that shoot natality decreases, and shoot mortality increases, in stands of increasing density. In a two-year garden experiment, we have tested Hutchings' (1979) hypothesis that these responses are the result of physiological integration, i.e. the exchange of resources and growth substances between shoots of a single clone. Dense monocultures of two rhizomatous graminoids, Brachypodium pinnatum and Carex flacca, were created that differed more than 10-fold in the density of clones (genets), but that had similar densities of shoots. A more effective shoot density control was expected in stands with the smaller clone densities (larger clones) due to more extensive clonal connections. Shoot turnover was evaluated by counting living and dead shoots at different times. In the summer of the second year, when shoot densities and stand structure were similar between treatments, shoot natality (the number of shoots born per plot) and shoot mortality (the number of shoots that died per plot) were usually unrelated to clone density in either species. If there was a significant treatment effect, it could be attributed to (small) differences in shoot density. Over the whole range of shoot densities, natality was negatively density-dependent. The number of shoots that died in a given growth period was proportional to the number of shoots present, suggesting that mortality rates were density independent. In Carex, however, there were some indications that mortality rate increased with increasing density. Our study confirms that clonal herbaceous species can effectively prevent an overproduction of shoots, but in contrast to Hutchings' (1979) propositions, we found no evidence that physiological integration may be the responsible mechanism. An alternative explanation for the observed patterns is proposed.     

Effects of light quality and quantity on growth of the clonal plant Eichhornia crassipes

Summary Plant canopy shade reduces photosynthetic photon flux density (PPFD) and ratio of red to far-red light (z). Both effects can cause plants to increase potential for light acquisition through vertical growth and leaf area expansion. Clonal plants such as Eichhornia crassipes might alternatively increase light interception via horizontal growth of stolons or rhizomes and placement of new ramets in less shaded microsites. Effect of simulated canopy shade and component effects of PPFD and z were tested by filtering or adding light uniformly, to a whole group of connected ramets, or locally, to individual ramets within a group. In uniform treatments, low PPFD reduced total growth but low z did not. Low PPFD and low z independently reduced stolon and ramet production and caused etiolation of petioles; effect of low PPFD plus low z on ramet production was greater than that of either factor alone. Lateral clonal growth thus did not seem to be a response to uniform shading; instead, uniformly low PPFD or low z increased partitioning to established ramets. Low z changed partitioning without changing total growth. In local treatments, reduction of growth of individual ramets due to low PPFD and inhibition of new ramet production attributable to spectral composition of light were mitigated when connected ramets were unshaded; plants may respond differently to patchy than to uniform shade.     

Effects of simulated root herbivory and fertilizer application on growth and biomass allocation in the clonal perennialSolidago canadensis

Summary Compensatory growth in response to simulated belowground herbivory was studied in the old-field clonal perennialSolidago canadensis. We grew rootpruned plants and plants with intact root systems in soil with or without fertilizer. For individual current shoots (aerial shoot with rhizome and roots) and for whole clones the following predictions were tested: a) root removal is compensated by increased root growth, b) fertilizer application leads to increased allocation to aboveground plant organs and increased leaf turnover, c) effects of fertilizer application are reduced in rootpruned plants. When most roots (90%) were removed current shoots quickly restored equilibrium between above-and belowground parts by compensatory belowground growth whereas the whole clone responded with reduced aboveground growth. This suggests that parts of a clone which are shared by actively growing shoots act as a buffer that can be used as source of material for compensatory growth in response to herbivory. Current shoots increased aboveground mass and whole clones reduced belowground mass in response to fertilizer application, both leading to increased allocation to aboverground parts. Also with fertilizer application both root-pruned and not root-pruned plants increased leaf and shoot turnover. Unfertilized plants, whether rootpruned or not, showed practically no aboveground growth and very little leaf and shoot turnover. Effects of root removal were as severe or more severe under conditions of high as under conditions of low nutrients, suggesting that negative effects of belowground herbivory are not ameliorated by abundant nutrients. Root removal may negate some effects of fertilizer application on the growth of current shoots and whole clones.     

Effects of moisture availability on clonal growth in bamboo Pleioblastus maculata

The effects of moisture availability on clonal growth and biomass investment in the bamboo Pleioblastus maculata were investigated over a four-year period by transplanting Pleioblastus maculata clones into soils with different levels of moisture availability in the field. The results showed that: (1) The higher the moisture availability, the greater the total biomass of P. maculata clones. Although fewer culms are produced at the higher moisture levels, mean tiller biomass is greater. (2) Under different levels of moisture availability, obvious differences in the total rhizome length (p < 0.01), spacer length (p < 0.05) and the sizes of bamboo culms (height, p < 0.01; diameter, p < 0.01) were observed. Thus, the higher the moisture availability, the shorter the rhizomes and the larger ramets. (3) In microhabitats with low moisture availability, bamboo allocated more biomass to underground organs, which promotes elongation of rhizomes and increases root production, thereby helping to capture underground resources essential to growth. In microhabitats of high moisture availability, the biomass is primarily allocated to the aboveground growth of ramets. (4) We suggest that soil moisture availability effects the foraging strategies of bamboo, that bamboo plants growing with low moisture availability produce longer rhizomes (that is, more, although shorter, spacers) with more biomass allocation than plants in high moisture and have a better ability to forage to increase the probability of locating adequate moisture patches. Also, longer length distance between shoots (that is, longer spacers) in high soil moisture than in low is adapted to avoid intense competition from faster growing aboveground growth in high moisture patches.     

Within-season adjustment of sex expression in females and hermaphrodites of the clonal gynodioecious herb <Emphasis Type="Italic">Glechoma longituba</Emphasis> (Lamiaceae)

The within-season modification or adjustment of sex expression was studied in the gynodioecious herb Glechoma longituba. The results from seven natural populations indicated that the seed sets of hermaphrodite ramets were relatively stable compared to those of female ramets. However, a negative correlation was detected between the seed set of female ramets and their frequency of occurrence in a population. Moreover, a negative correlation was also detected between the degree of gender modification in the late flowering period and the seed set in female ramets. Controlled experiments on both wild and cultivated plants indicated that the resource adjustment, presumably caused by flower removal or the hand pollination of flowers, in the early-middle flowering season could induce a greater degree of gender modification in later flowers from female to hermaphrodite and vice versa. However, the ranges of degree of gender modification were different between the two sex morphs, and the variation in females was greater than in hermaphrodites. This phenomenon could not be adequately explained based on the sequential adjustment of investment hypothesis which has been invoked in previous studies. We propose that the results of this study may provide a variation to the model of sequential adjustment of investment in serial flowering annual gynodioecious plants, including Glechoma longituba. We postulate that the variation of sex expression might be beneficial for the female in order to avoid pollen limitation and, further, to obtain a selective advantage.     

Response of physiological integration in Trifolium repens to heterogeneity of UV-B radiation

Physiological integration has been documented in many clonal plants growing under resource heterogeneity. Little is still known about the response of physiological integration to heterogeneous ultraviolet-B radiation. In this paper, the changes in intensity of physiological integration and of physiological parameters under homogeneous and heterogeneous ultraviolet-B radiation (280-315 nm) were measured in order to test the hypothesis that in addition to resource integration a defensive integration in Trifolium repens might exist as well. For this purpose, homogeneous and heterogeneous ultraviolet-B radiation was applied to pairs of connected and severed ramets of the stoloniferous herb Trifolium repens. Changes in intensity of water and nutrient integration were followed with acid fuchsin dye and ¹⁵N-isotope labeling of the xylem water transport. In order to assess the patterns of physiological integration contents of chlorophyll, ultraviolet-B absorbing compounds, soluble sugar and protein were determined and activities of superoxide dismutase (SOD) and peroxidase (POD) measured. When ramets were connected and exposed to heterogeneous UV-B radiation, the velocity of water transportation from the UV-B treated ramet to its connected sister ramet was markedly lower and the percentage of ¹⁵N left in labelled ramets that suffered from enhanced UV-B radiation was higher and their transfer to unlabelled ramets lower. In comparison with clones under homogeneous ultraviolet-B radiation, the content of chlorophyll, ultraviolet-B absorbing compounds, soluble sugar and activities of SOD and POD increased notably if ultraviolet-B stressed ramets were connected to untreated ramets. Chlorophyll and UV-B absorbing compounds were shared between connected ramets under heterogeneous UV-B radiation. This indicated that physiological connection improved the performance of whole clonal plants under heterogeneous ultraviolet-B radiation. The intensity of physiological integration of T. repens for resources decreased under heterogeneous ultraviolet-B radiation in favor of the stressed ramets. Ultraviolet-B stressed ramets benefited from unstressed ramets by physiological integration, supporting the hypothesis that clonal plants are able to optimize the efficiency of their resistance maintaining their presence also in less favorable sites. The results could be helpful for further understanding of the function of heterogeneous UV-B radiation on growth regulation and microevolution in clonal plants.