Plasticity to light cues and resources in Arabidopsis thaliana: testing for adaptive value and costs

Plants shaded by neighbors or overhead foliage experience both a reduction in the ratio of red to far red light (R:FR), a specific cue perceived by phytochrome, and reduced photosynthetically active radiation (PAR), an essential resource. We tested the adaptive value of plasticity to crowding and to the cue and resource components of foliage shade in the annual plant Arabidopsis thaliana by exposing 36 inbred families from four natural populations to four experimental treatments: (1) high density, full sun; (2) low density, full sun; (3) low density, neutral shade; and (4) low density, low R:FR-simulated foliage shade. Genotypic selection analysis within each treatment revealed strong environmental differences in selection on plastic life-history traits. We used specific contrasts to measure plasticity to density and foliage shade, to partition responses to foliage shade into phytochrome-mediated responses to the R:FR cue and responses to PAR, and to test whether plasticity was adaptive (i.e., in the same direction as selection in each environment). Contrary to expectation, we found no evidence for adaptive plasticity to density. However, we observed both adaptive and maladaptive responses to foliage shade. In general, phytochrome-mediated plasticity to the R:FR cue of foliage shade was adaptive and counteracted maladaptive growth responses to reduced PAR. These results support the prediction that active developmental responses to environmental cues are more likely to be adaptive than are passive resource-mediated responses. Multiple regression analysis detected a few costs of adaptive plasticity and adaptive homeostasis, but such costs were infrequent and their expression depended on the environment. Thus, costs of plasticity may occasionally constrain the evolution of adaptive responses to foliage shade in Arabidopsis, but this constraint may differ among environments and is far from ubiquitous.     

Field-based insights to the evolution of specialization: plasticity and fitness across habitats in a specialist/generalist species pair

Factors promoting the evolution of specialists versus generalists have been little studied in ecological context. In a large-scale comparative field experiment, we studied genotypes from naturally evolved populations of a closely related generalist/specialist species pair (Polygonum persicaria and P. hydropiper), reciprocally transplanting replicates of multiple lines into open and partially shaded sites where the species naturally co-occur. We measured relative fitness, individual plasticity, herbivory, and genetic variance expressed in the contrasting light habitats at both low and high densities. Fitness data confirmed that the putative specialist out-performed the generalist in only one environment, the favorable full sun/low-density environment to which it is largely restricted in nature, while the generalist had higher lifetime reproduction in both canopy and dense neighbor shade. The generalist, P. persicaria, also expressed greater adaptive plasticity for biomass allocation and leaf size in shaded conditions than the specialist. We found no evidence that the ecological specialization of P. hydropiper reflects either genetically based fitness trade-offs or maintenance costs of plasticity, two types of genetic constraint often invoked to prevent the evolution of broadly adaptive genotypes. However, the patterns of fitness variance and herbivore damage revealed how release from herbivory in a new range can cause an introduced species to evolve as a specialist in that range, a surprising finding with important implications for invasion biology. Patterns of fitness variance between and within sites are also consistent with a possible role for the process of mutation accumulation (in this case, mutations affecting shade-expressed phenotypes) in the evolution and/or maintenance of specialization in P. hydropiper.     

Responses of carbon acquisition traits to irradiance and light quality in Mercurialis annua (Euphorbiaceae): evidence for weak integration of plastic responses

It is often suggested that traits will be integrated, either because of pleiotropy or because natural selection may favor suites of integrated traits. Plant responses to different environments can provide evidence of such integration. We grew Mercurialis annua plants in high-density stands in high irradiance, in neutral shade, and in high red to far-red (R:FR) shade, resulting in environments of high irradiance, low R:FR; low irradiance, low R:FR; and low irradiance, high R:FR. We measured gas exchange, leaf morphology, stem elongation, and biomass traits and tested the prediction that traits within each functional group would show higher trait integration, as evidenced by high correlations among traits within environments, higher correlations of trait plasticity, and lower plasticity of trait correlations. Overall, we found evidence of only moderate integration for some groups of traits. Functionally related groups of traits, or pairs of traits, could be strongly integrated by one criterion but weakly integrated by another of the criteria. Stem elongation traits, though often observed to be strongly integrated in other taxa, showed little evidence of integration. Internode traits exhibited a novel pattern of responses to low R:FR, with increased elongation of the hypocotyl, decreased elongation of the first internode, and no change in the second internode. We propose that these responses to light are more likely to be the result of natural selection than the consequence of constraints imposed by pleiotropy.     

Early developmental responses to seedling environment modulate later plasticity to light spectral quality

Correlations between developmentally plastic traits may constrain the joint evolution of traits. In plants, both seedling de-etiolation and shade avoidance elongation responses to crowding and foliage shade are mediated by partially overlapping developmental pathways, suggesting the possibility of pleiotropic constraints. To test for such constraints, we exposed inbred lines of Impatiens capensis to factorial combinations of leaf litter (which affects de-etiolation) and simulated foliage shade (which affects phytochrome-mediated shade avoidance). Increased elongation of hypocotyls caused by leaf litter phenotypically enhanced subsequent elongation of the first internode in response to low red:far red (R:FR). Trait expression was correlated across litter and shade conditions, suggesting that phenotypic effects of early plasticity on later plasticity may affect variation in elongation traits available to selection in different light environments.     

Differential responses of invasive Celastrus orbiculatus (Celastraceae) and native C. scandens to changes in light quality

When plants are subjected to leaf canopy shade in forest understories or from neighboring plants, they not only experience reduced light quantity, but light quality in lowered red : far red light (R : FR). Growth and other developmental responses of plants in reduced R : FR can vary and are not consistent across species. We compared how an invasive liana, Celastrus orbiculatus, and its closely related native congener, C. scandens, responded to changes in the R : FR under controlled, simulated understory conditions. We measured a suite of morphological and growth attributes under control, neutral shading, and low R : FR light treatments. Celastrus orbiculatus showed an increase in height, aboveground biomass, and total leaf mass in reduced R : FR treatments as compared to the neutral shade, while C. scandens had increased stem diameter, single leaf area, and leaf mass to stem mass ratio. These differences provide a mechanistic understanding of the ability of C. orbiculatus to increase height and actively forage for light resources in forest understories, while C. scandens appears unable to forage for light and instead depends upon a light gap forming. The plastic growth response of C. orbiculatus in shaded conditions points to its success in forested habitats where C. scandens is largely absent.     

Species-specific variation in the importance of the spectral quality gradient in canopies as a signal for photosynthetic resource partitioning

BACKGROUND AND AIMS: Plants adjust the distribution of photosynthetic capacity and chlorophyll to canopy density. The importance of the gradient in the red : far-red ratio (R : FR) relative to the irradiance gradient was studied for its perception with respect to this partitioning of photosynthetic resources. Whether the relative importance of these two signals varied between six species of different growth habit (Phaseolus vulgaris, Lysimachia vulgaris, Hedera helix, Ficus benjamina, Carex acutiformis and Brachypodium pinnatum) was investigated further. METHODS: Single leaves of plants were shaded in daylight by a spectrally neutral filter or a leaf. In another experiment, leaves were treated with supplemental FR. In most cases, treatment effects were evaluated after 2 weeks. KEY RESULTS: Nitrogen and photosynthetic capacity (Amax) per leaf area, parameters pertaining to between-leaf resource partitioning, were strongly reduced in neutral shade but not additionally by spectral leaf shade. Supplemental FR reduced these parameters also, except in Carex. Acceleration of induction of senescence was observed in spectral leaf shade in primary bean leaves. Amax per unit chlorophyll, a parameter pertaining to within-leaf resource partitioning, was reduced in neutral shade, but not in spectral leaf shade or supplemental FR. CONCLUSIONS: Signalling mechanisms associated with perception of the R : FR gradient in canopies were less important than those associated with the irradiance gradient for between-leaf and within-leaf partitioning of photosynthetic resources. The relative importance of the signals differed between species because Carex was the only species for which no indications were found for an involvement of the spectral gradient in perception of canopy density.     

Maternal and ambient environmental effects of light on germination in Plantago lanceolata: correlated responses to selection on leaf length

1. Seeds from artificial selection lines were exposed to different maternal and ambient conditions, simulating sunlight and vegetation shade.
2. Lines selected for longer leaves also produced larger seeds, indicating a positive genetic correlation between leaf length and seed size.
3. Light conditions during maturation had no large effect on seed size.
4. Seed germination was reduced by a low ratio of red to far-red light (R/FR ratio) in the ambient environment.
5. Seeds maturated under simulated vegetation shade germinated less readily and were more inhibited by a low ambient R/FR ratio than seeds maturated under full sunlight or R/FR-neutral shade. Thus, low R/FR-ratios in the maternal and ambient environment operated synergistically.
6. Large genotypic variation in the germination responses to both maternal and ambient light conditions was found among and within selection lines, indicating that such responses might have the potential to evolve in response to natural selection.
7. Artificial selection for leaf length had affected seed germination characteristics but correlated responses and thus genetic correlations largely depended on light conditions in the selective environment. Selection for longer leaves under a low R/FR ratio increased seed dormancy and plasticity of germination in response to the R/FR ratio. However, in the opposite selective environment selection for longer leaves reduced seed dormancy and plasticity to the R/FR ratio. It is argued that leaf length and seed germination characteristics are somehow linked by shared physiological mechanisms, which may facilitate concerted changes in shade avoidance responses.     

The adaptive evolution of plasticity: phytochrome-mediated shade avoidance responses

Many plants display a characteristic suite of developmental"shade avoidance" responses, such as stem elongation and acceleratedreproduction, to the low ratio of red to far-red wavelengths(R:FR) reflected or transmitted from green vegetation. ThisR:FR cue of crowding and vegetation shade is perceived by thephytochrome family of photoreceptors. Phytochrome-mediated responsesprovide an ideal system for investigating the adaptive evolutionof phenotypic plasticity in natural environments. The molecularand developmental mechanisms underlying shade avoidance responsesare well studied, and testable ecological hypotheses exist fortheir adaptive significance. Experimental manipulation of phenotypesdemonstrates that shade avoidance responses may be adaptive,resulting in phenotypes with high relative fitness in the environmentsthat induce those phenotypes. The adaptive value of shade avoidancedepends upon the competitive environment, resource availability,and the reliability of the R:FR cue for predicting the selectiveenvironment experienced by an induced phenotype. Comparativestudies and a reciprocal transplant experiment with Impatienscapensis provide evidence of adaptive divergence in shade avoidanceresponses between woodland and clearing habitats, which mayresult from population differences in the frequency of selectionon shade avoidance traits, as well as differences in the reliabilityof the R:FR cue. Recent rapid progress in elucidating phytochromesignaling pathways in the genetic model Arabidopsis thalianaand other species now provides the opportunity for studyinghow selection on shade avoidance traits in natural environmentsacts upon the molecular mechanisms underlying natural phenotypicvariation.     

Indirect consequences of artificial selection on plasticity to light quality in Arabidopsis thaliana

Covariation between light quality- and photoperiod-mediated phenotypic plasticity was investigated using Arabidopsis thaliana. Three episodes of artificial selection were imposed on an index that quantified the plastic response to reduced red to far-red ratios (R:FR), with higher index values indicating greater plasticity. Relative to control lines, two high plasticity (HP) lines showed 1.6- and 2.4-fold increases in the index; low plasticity (LP) lines showed 1.4- and 1.1-fold decreases. A factorial experiment combining high and low R:FR conditions with long and short photoperiods assessed indirect consequences of selection on plasticity. Despite divergent R:FR-mediated plasticities in HP vs. LP lines, all four lines showed increases in photoperiod-mediated responses and decreases in mean leaf number. Complex relationships among trait means, plasticities and underlying mechanisms caution against generalizing about the genetic architecture of plastic traits. Partially independent developmental and evolutionary responses to R:FR and photoperiod are somewhat unsurprising, given this species' cosmopolitan nature.     

Uncoupling light quality from light irradiance effects in Helianthus annuus shoots: putative roles for plant hormones in leaf and internode growth

An attempt has been made to uncouple the effects of the two primary components of shade light, a reduced red to far-red (R/FR) ratio and low photosynthetically active radiation (PAR), on the elongation of the youngest internode of sunflower (Helianthus annuus) seedlings. Maximal internode growth (length and biomass) was induced by a shade light having a reduced R/FR ratio (0.85) under the low PAR of 157 micromol m(-2) s(-1). Reducing the R/FR ratio under normal PAR (421 micromol m(-2) s(-1)) gave similar growth trends, albeit with a reduced magnitude of the response. Leaf area growth showed a rather different pattern, with maximal growth occurring at the higher (normal) PAR of 421 micromol m(-2) s(-1)), but with variable effects being seen with changes in light quality. Reducing the R/FR ratio (by enrichment with FR) gave significant increases in gibberellin A(1) (GA(1)) and indole-3-acetic acid (IAA) contents in both internodes and leaves. By contrast, a lower PAR irradiance had no significant effect on GA(1) and IAA levels in internodes or leaves, but did increase the levels of other GAs, including two precursors of GA(1). Interestingly, both leaf and internode hormone content (GAs, IAA) are positively and significantly correlated with growth of the internode, as are leaf levels of abscisic acid (ABA). However, changes in these three hormones bear little relationship to leaf growth. By implication, then, the leaf may be the major source of GAs and IAA, at least, for the rapidly elongating internode. Several other hormones were also assessed in leaves for plants grown under varying R/FR ratios and PARs. Leaf ethylene production was not influenced by changes in R/FR ratio, but was significantly reduced under the normal (higher) PAR, the irradiance treatment which increased leaf growth. Levels of the growth-active free base and riboside cytokinins were significantly increased in leaves under a reduced R/FR ratio, but only at the higher (normal) PAR irradiance; other light quality treatments evoked no significant changes. Taken in toto, these results indicate that both components of shade light can influence the levels of a wide range of endogenous hormones in internodes and leaves while evoking increased internode elongation and biomass accumulation. However, it is light quality changes (FR enrichment) which are most closely tied to increased hormone content, and especially with increased GA and IAA levels. Finally, the increases seen in internode and leaf GA content with a reduced R/FR ratio are consistent with FR enrichment inducing an overall increase in sunflower seedling GA biosynthesis.     

THE DEVELOPMENTAL RESPONSES OF PAPAYA LEAVES TO SIMULATED CANOPY SHADE

The developmental responses of plants to shade underneath foliage are influenced by reductions in irradiance and shifts in spectral quality (characterized by reductions in the quantum ratio of red to far-red wavelengths, R:FR). Previous research on the influence of shadelight on leaf development has neglected the reductions in R:FR characteristic of foliage shade, and these studies have almost certainly underestimated the extent and array of developmental responses to foliage shade. We have studied the effects of reduced irradiance and R:FR on the leaf development of papaya (Carica papaya L., Caricaceae). Using experimental shadehouses, replicates of plants grown in high light conditions (0.20 of sunlight and R:FR = 0.90) were compared to low light conditions (0.02 of sunlight) with either the spectral quality of sunlight (R:FR = 0.99) or of foliage shade (F:FR = 0.26). Although many characteristics, such as leaf thickness, specific leaf weight, stomatal density, palisade parenchyma cell shape, and the ratio of mesophyll air surface/leaf surface were affected by reductions in irradiance, reduced R:FR contributed to further changes. Some characters, such as reduced chlorophyll a/b ratios, reduced lobing, and greater internode length, were affected primarily by low R:FR. The reduced R:FR of foliage shade, presumably affecting phytochrome equilibrium, strongly influences the morphology and anatomy of papaya leaves.     

Canopy studies on ethylene-insensitive tobacco identify ethylene as a novel element in blue light and plant-plant signalling

Plants growing at high densities express shade avoidance traits as a response to the presence of neighbours. Enhanced shoot elongation is one of the best researched shade avoidance components and increases light capture in dense stands. We show here that also leaf movements, leading to a more vertical leaf orientation (hyponasty), may be crucial in the early phase of competition. The initiation of shade avoidance responses is classically attributed to the action of phytochrome photoreceptors that sense red:far-red (R:FR) ratios in light reflected by neighbours, but also other signals may be involved. It was recently shown that ethylene-insensitive, transgenic (Tetr) tobacco plants, which are insensitive to the gaseous plant hormone ethylene, have reduced shade avoidance responses to neighbours. Here, we report that this is not related to a reduced response to low R:FR ratio, but that Tetr tobacco plants are unresponsive to a reduced photon fluence rate of blue light, which normally suppresses growth inhibition in wild-type (WT) plants. In addition to these light signals, ethylene levels in the canopy atmosphere increased to concentrations that could induce shade avoidance responses in WT plants. Together, these data show that neighbour detection signals other than the R:FR ratio are more important than previously anticipated and argue for a particularly important role for ethylene in determining plant responses to neighbours.     

Temperature‐dependent shade avoidance involves the receptor‐like kinase ERECTA

Plants detect the presence of neighbouring vegetation by monitoring changes in the ratio of red (R) to far‐red (FR) wavelengths (R:FR) in ambient light. Reductions in R:FR are perceived by the phytochrome family of plant photoreceptors and initiate a suite of developmental responses termed the shade avoidance syndrome. These include increased elongation growth of stems and petioles, enabling plants to overtop competing vegetation. The majority of shade avoidance experiments are performed at standard laboratory growing temperatures (>20°C). In these conditions, elongation responses to low R:FR are often accompanied by reductions in leaf development and accumulation of plant biomass. Here we investigated shade avoidance responses at a cooler temperature (16°C). In these conditions, Arabidopsis thaliana displays considerable low R:FR‐mediated increases in leaf area, with reduced low R:FR‐mediated petiole elongation and leaf hyponasty responses. In Landsberg erecta, these strikingly different shade avoidance phenotypes are accompanied by increased leaf thickness, increased biomass and an altered metabolite profile. At 16°C, low R:FR treatment results in the accumulation of soluble sugars and metabolites associated with cold acclimation. Analyses of natural genetic variation in shade avoidance responses at 16°C have revealed a regulatory role for the receptor‐like kinase ERECTA.     

The regulation of cell wall extensibility during shade avoidance: a study using two contrasting ecotypes of Stellaria longipes

Shade avoidance in plants involves rapid shoot elongation to grow toward the light. Cell wall-modifying mechanisms are vital regulatory points for control of these elongation responses. Two protein families involved in cell wall modification are expansins and xyloglucan endotransglucosylase/hydrolases. We used an alpine and a prairie ecotype of Stellaria longipes differing in their response to shade to study the regulation of cell wall extensibility in response to low red to far-red ratio (R/FR), an early neighbor detection signal, and dense canopy shade (green shade: low R/FR, blue, and total light intensity). Alpine plants were nonresponsive to low R/FR, while prairie plants elongated rapidly. These responses reflect adaptation to the dense vegetation of the prairie habitat, unlike the alpine plants, which almost never encounter shade. Under green shade, both ecotypes rapidly elongate, showing that alpine plants can react only to a deep shade treatment. Xyloglucan endotransglucosylase/hydrolase activity was strongly regulated by green shade and low blue light conditions but not by low R/FR. Expansin activity, expressed as acid-induced extension, correlated with growth responses to all light changes. Expansin genes cloned from the internodes of the two ecotypes showed differential regulation in response to the light manipulations. This regulation was ecotype and light signal specific and correlated with the growth responses. Our results imply that elongation responses to shade require the regulation of cell wall extensibility via the control of expansin gene expression. Ecotypic differences demonstrate how responses to environmental stimuli are differently regulated to survive a particular habitat.     

Is photomorphogenic shade avoidance adaptive? Perspectives from population biology

Photomorphogenic shade avoidance responses provide an ideal model system for integrating genetic, physiological and population biology approaches to the study of adaptive plasticity. The adaptive plasticity hypothesis predicts that shade avoidance phenotypes induced by low ratios of red to far-red light (R:FR) will have high relative fitness in dense stands, but will suffer a fitness disadvantage at low density. Experiments with transgenic and mutant plants in which photomorphogenic genes are disabled, as well as phenotype manipulation by means of altered R:FR, strongly support the shade avoidance hypothesis. The observation of photomorphogenic ecotypes in different selective environments also suggests that the shade avoidance response has undergone adaptive evolution. Quantitative genetic variation in R:FR sensitivity has been detected in wild populations, indicating that the evolutionary potential exists for response to natural selection. However, evolutionary response may be constrained by genetic correlations among developmentally linked traits. Therefore it cannot be assumed that an observed suite of photomorphogenic responses represents an adaptive optimum for every trait.     

Testing adaptive plasticity to UV: costs and benefits of stem elongation and light-induced phenolics

On exposure to ultraviolet radiation (UV), many plant species both reduce stem elongation and increase production of phenolic compounds that absorb in the UV region of the spectrum. To demonstrate that such developmental plasticity to UV is adaptive, it is necessary to show that the induced phenotype is both beneficial in inductive environments and maladaptive in non-inductive environments. We measured selection on stem elongation and phenolic content of seedlings of Impatiens capensis transplanted into ambient-UV and UV-removal treatments. We extended the range of phenotypes expressed, and thus the opportunity for selection in each UV treatment, by pretreating seedlings with either a low ratio of red:far-red wavelengths (R:FR), which induced stem elongation and reduced phenolic concentrations, or high R:FR, which had the opposite effect on these two phenotypic traits. Reduced stem length relative to biomass was advantageous for elongated plants under ambient UV, whereas increased elongation was favored in the UV-removal treatment. Selection favored an increase in the level of phenolics induced by UV in the ambient-UV treatment, but a decrease in phenolics in the absence of UV. These results are consistent with the hypotheses that reduced elongation and increased phenolic concentrations serve a UV-protective function and provide the first explicit demonstration in a wild species that plasticity of these traits to UV is adaptive. The observed cost to phenolics in the absence of UV may explain why many species plastically upregulate phenolic production when exposed to UV, rather than evolve constitutively high levels of these compounds. Finally, pretreatment with low R:FR simulating foliar shade did not exacerbate the fitness impact of UV exposure when plants had several weeks to acclimate to UV. This observation suggests that the evolution of adaptive shade avoidance responses to low R:FR in crowded stands will not be constrained by increased sensitivity to UV in elongated plants when they overtop their neighbors.     

Variation in growth form in relation to spectral light quality (red/far-red ratio) in Plantago lanceolata L. in sun and shade populations

Plants from a sun and shade population were grown in two environments differing in the ratio of red to far-red light (R/FR ratio). A low R/FR ratio, simulating vegetation shade, promoted the formation of long, upright-growing leaves and allocation towards shoot growth, whereas a high R/FR ratio had the opposite effects. The increase in plant height under the low R/FR ratio was accompanied by a reduction in the number of leaves. Population differences in growth form resembled the differences between plants grown in different light environments: plants from the shade population had rosettes with long erect leaves, whereas plants from the sun population formed prostrate rosettes with short leaves. Plants from the shade population were more responsive to the R/FR ratio than plants from the sun population: the increases in leaf length, plant height, and leaf area ratio under a low R/FR ratio were larger in the shade population. However, differences in plasticity were small compared to the population difference in growth form itself. We argue that plants do not respond optimally to shading and that developmental constraints might have limited the evolution of an optimal response. Received: 8 December 1996 / Accepted: 31 March 1997     

Topology of a maize field: distinguishing the influence of end-of-day far-red light and shade avoidance syndrome on plant height

Correlations were established between plant height and Cartesian position in a field of diverse maize (Zea mays) germplasm. The influence of the shade avoidance syndrome (SAS), a series of responses to lower photosynthetically active radiation (PAR) and red to far-red light ratio (R:FR) at high planting density, was detected by a steep increase of plant height from the edge to interior rows of the field. In addition, a gradual increase in height was observed across the field from east to west. We attribute this result to a R:FR gradient caused by sunlight laterally penetrating the stand at dusk. Furthermore, we hypothesize that the increased height of west-positioned plants may be analogous to responses induced by end-of-day FR (EOD-FR) treatments used by photobiologists to induce SAS in controlled environments. While preliminary, these results nevertheless suggest that a plant's position in a field will influence the impact of daily fluctuations in PAR and R:FR in modulating plant height and, potentially, other agronomically relevant traits.     

A comparison of phenotypic plasticity in the native dandelion Taraxacum ceratophorum and its invasive congener T. officinale

We compared plastic responses to variation in the light environment for sympatric populations of native and exotic dandelion species, Taraxacum ceratophorum and Taraxacum officinale. Plasticity in leaf size, inflorescence height, reproductive phenology and dispersal-related traits were measured under experimentally altered light quality (red : far-red light ratio, R : FR) and light intensity (photosynthetically active radiation, PAR). To test whether differences in means and reaction norms of dispersal-related traits between species affected colonization potential, we created seed-dispersal models based on seed-fall rate and release height. Differences in plasticity between species were not systematic, but varied in direction and magnitude among traits. Taraxacum officinale produced larger leaves that exhibited greater plasticity in size under variable light intensity than T. ceratophorum. Plasticity in scape length at flowering occurred in relation to R : FR ratio in both species, but tended to be greater in T. ceratophorum. Seed-bearing scapes of T. officinale were taller and more canalized in height across light regimes than scapes of T. ceratophorum. Seeds of T. officinale were smaller than seeds of T. ceratophorum. Models predict greater dispersal in T. officinale within open and vegetated habitats. In contrast to the idea that plasticity promotes invasiveness, results suggest that the lack of plasticity in dispersal-related traits enhances the colonization potential of T. officinale.