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Ecology |
Department of Biology, College of Staten Island, City University of New York, Staten Island, New York 10314 USA
Received for publication July 12, 2005. Accepted for publication January 9, 2006.
ABSTRACT
The invasive grass Microstegium vimineum grows in low light beneath the canopy of eastern forests in North America by reiteration of modules (phytomers) along a tiller. Basal phytomers are vegetative; terminal phytomers produce a raceme of chasmogamous (CH) spikelets plus an axillary raceme of cleistogamous (CL) spikelets. Additional subterminal phytomers with CL racemes mature basipetally. Allocation to culms, leaves, and CH and CL within phytomers was examined in relation to light conditions for a population in New Jersey, USA. Plants were reared in a greenhouse from seed families of parents in deep shade (2–8% full sun) or sunny, edge habitats. Primary tillers were subdivided into phytomers, dried, and weighed. Tillers from field habitats were similarly treated. For vegetative and subterminal phytomers, allocation to leaves and CH was greatest for the shady habitat. CL allocation decreased from terminal to reproductively immature subterminal phytomers. CH and CL mass was positively correlated with leaf mass, suggesting that reproduction is determined by available photosynthate. CH mass showed a genetic correlation with leaf mass. Developmental plasticity in modular allocation allows Microstegium to maximize fitness when conditions are favorable (e.g., high light along forest edges) by continual maturation of CL caryopses on axillary racemes.
Key Words: annual grass • chasmogamy • cleistogamy • invasive species • Microstegium vimineum • phytomers • Poaceae
The grass plant exemplifies the modular construction typical of many plant species (White, 1979
; Vuorisalo and Mutikainen, 1999
). Repeating units or modules are assembled along the vertical axis that comprises a tiller (Briske, 1991
; Nelson, 1996
). In grasses, each module is a phytomer, defined as a node with its lateral, axillary bud, the leaf (sheath and blade) attached to the node, plus the internode above, enveloped by the sheath (Clark and Fisher, 1987
). The internode is the culm (stem), which elongates by means of an intercalary meristem. Axillary buds may be dormant, but have the potential to produce new phytomers along a lateral tiller. When flowering occurs, apical meristems of primary and lateral tillers differentiate to form the inflorescence (Nelson, 1996
).
Much variation in morphology can be found among the phytomers of a tiller due to developmental and position-dependent effects (Cheplick, 2005a
). The phenotype can be adjusted through plastic responses in the number, position, and type of its component modules to environmental conditions (Sachs, 1999
; Vuorisalo and Mutikainen, 1999
). Recognition of modular structure has been especially important to investigations of physiological integration and autonomy in clonal plants (Vuorisalo and Hutchings, 1996
; Marshall and Price, 1997
). The modular nature of clonal growth contributes to the invasiveness of some plants worldwide (Py
ek, 1997
; Myers and Bazely, 2003
). Although modularity in annual species has not been widely investigated compared to clonal species, reiteration of phytomers that can vary in type or number typifies annual grasses, some of which are globally significant invasive weeds.
Modularity is critical to the evolution of development and life history (West-Eberhard, 2003
). For plants, modular growth allows temporal adjustments to the phenotype as new parts are added during development (Preston and Ackerly, 2004
) and is one way to change the allocation of resources or biomass to different functions such as growth or reproduction (Bonser and Aarssen, 2003
). The principle of biomass allocation has been central to many studies of life history evolution in plants (Bazzaz, 1997
; Bazzaz et al., 2000
; Niklas and Enquist, 2002
; Weiner, 2004
). Specifically, reproductive allocation has been used to characterize species-specific life histories and variation in relation to environmental conditions or experimental treatments (Poorter and Nagel, 2000
; Cheplick, 2005a
). Allocation patterns depict a complex set of growth processes, which collectively produce the integrated plant. A modular approach can elucidate the putative adaptive advantage of variation in allocation to vegetative or reproductive functions among reiterated components (Sachs, 1999
; Vuorisalo and Mutikainen, 1999
; Obeso, 2004
; de Kroon et al., 2005
). Allocational decisions between vegetative and reproductive functions may be especially important to annuals because relative fitness, assessed by seed production, can be correlated with reproductive allocation, as shown by Cheplick (2005a)
for the annual grass Triplasis purpurea (Walt.) Chapm.
This research examines allocation to vegetative and reproductive structures within phytomers of sampled tillers from Microstegium vimineum (Trin.) A. Camus, a highly invasive annual grass. Modular allocation is explored in relation to habitat light conditions because this shade-tolerant species can be abundant in low light beneath the canopy of eastern North American forests (Winter et al., 1982
; Horton and Neufeld, 1998
) and also along sunny forest edges (Gibson et al., 2002
; Cheplick, 2005b
). Given the importance of genetic variation to the ecological success of invasive plants (Sakai et al., 2001
), one objective was to determine whether or not there was quantitative genetic (among-family) variation or intrapopulation divergence in modular allocation patterns to vegetative and reproductive organs between shady or sunny habitats.
When it flowers, M. vimineum produces open chasmogamous (CH) spikelets on a terminal raceme and leaf sheath-enclosed axillary racemes with cleistogamous (CL) spikelets (Cheplick, 2005b
). The importance of the two reproductive modes to the population ecology of M. vimineum in different habitats is not known. In grasses, relative allocation to CH and CL is influenced by ontogeny and environment (Campbell et al., 1983
; Cheplick, 2006
). Suboptimal conditions are predicted to favor a higher allocation to CL reproduction, due to the reduced cost of CL flowers (Campbell et al., 1983
; Schoen, 1984
). Thus, a second objective was to determine the pattern of allocation to CH and CL within reproductive phytomers for tillers from shady and sunny habitats. Allocation to CL was predicted to be greater under low light, while allocation to CH was predicted to be more opportunistic, increasing under high light.
Both CH and CL allocation (expressed per vegetative dry mass of entire tillers) in M. vimineum were lowest in shade (Cheplick, 2005b
). Also, allocation to leaves was greatest in shade-collected tillers and in tillers of plants reared from shade-collected seeds in a high-light greenhouse. This suggested population differentiation in whole-tiller allocation in relation to the light environment and that low light may limit allocation to reproduction (Cheplick, 2005b
). The third objective here was to determine the relationship of CH and CL reproductive mass to leaf mass. Due to the greater expense of producing CH spikelets (Campbell et al., 1983
; Schoen, 1984
), CH reproduction was predicted to be more tightly correlated with leaf mass, especially under low light, compared to CL reproduction.
MATERIALS AND METHODS
The species
Microstegium vimineum (Japanese stiltgrass) is an annual grass of Asiatic origin that has become an invasive colonizer of fertile, mesic woodlands in the eastern United States (Hunt and Zaremba, 1992
; Gibson et al., 2002
). Because it is an annual, seed production is critical to long-term persistence of M. vimineum at a site.
Open-pollinated chasmogamous (CH) spikelets mature on an emergent, terminal raceme in early autumn. At the same time, axillary, sheath-enclosed racemes with self-fertilizing cleistogamous (CL) spikelets can be found in the upper phytomers along a reproductive tiller. For modular analysis, this means there are three distinct types of phytomer in a reproductive tiller (Fig. 1). (1) Vegetative phytomers are the earliest ones produced. Each has a leaf, whose sheath envelops a culm segment. On large tillers there are often lateral (branch) tillers produced from axillary buds on the lowest vegetative phytomers. (2) The subterminal phytomers are reproductive and mature axillary racemes with CL spikelets in a basipetal manner. The uppermost subterminal phytomers mature first; each phytomer progressively further back from the terminal phytomer contains developmentally younger CL racemes (Cheplick, 2005a
). Typically there are 2–7 subterminal phytomers per reproductive tiller (Cheplick, 2005b
). (3) The terminal phytomer is the most complex (Fig. 1a). It contains an axillary, sheath-enclosed CL raceme plus an emergent terminal culm that bears a CH raceme.
|
). Canopy trees include Acer rubrum L., Carya ovata (Mill) K. Koch, Liquidambar styraciflua L., and Quercus palustris Muenchh. (all nomenclature follows U.S. Department of Agriculture, 2004
). Besides Microstegium, other understory herbs include Allium vineale L., Glyceria striata (Lam.) Hitchc., Impatiens capensis Meerb., Symplocarpus foetidus (L.) Salisb. ex Nutt., and Toxicodendron radicans (L.) Kuntze. Note that many of these species are indicative of moist lowland forests of the inner coastal plain in central New Jersey (Robichaud and Buell, 1983
).
Although low light can be a limitation to invasion of the forest understory (Luken, 2003
; Cole and Weltzin, 2005
), M. vimineum is very abundant at the site, where light levels under the canopy in summer are only 2–8% of full sun (Cheplick, 2005b
). However, sunflecks are available under the canopy; furthermore, light availability can be much greater along the forest edge, where M. vimineum also occurs.
Tiller sampling and processing
For quantification of modular allocation, mature (reproductive) tillers were sampled directly from deep shade under the canopy (N = 20) and along the sunny edge of the forest (N = 15) in early October 2002. A single tiller was collected every 2 m along the north-facing forest edge where full sun occurs for 3–5 h/d during the growing season. Tillers were also collected every 2 m along a transect beneath the canopy, perpendicular to the edge. Before a tiller was collected, a coin envelope was placed over the terminal raceme to avoid loss of CH seeds. Tillers from sunny and shady habitats had a dry mass of 342.9 ± 30.0 mg (0 ± SE) and 156.2 ± 18.8 mg, respectively.
Tillers were also sampled from large, greenhouse-reared plants used in another study (Cheplick, 2005b
). This previous study was designed to investigate potential intrapopulation differentiation and genetic variation in CH and CL reproduction among families from sunny vs. shady habitats. Seeds had been collected from 10 plants, each separated from the next by 20 m, along edge and forest transects on 5 October 2001. All seeds from one plant constitute a maternal family or sibship. Seeds were subjected to cold, moist stratification over winter and six seedlings/family were planted following germination in April 2002 (additional details in Cheplick, 2005b
). Square pots (8 x 8 x 7.4 cm depth) containing a 1 : 1 mix of fine vermiculite and topsoil were used to rear the plants to maturity over summer. Plants were fertilized on two occasions and grew in full sun (Cheplick, 2005b
); thus, these tillers were collected (in September 2002) from plants in an environment where sunlight and soil nutrients were not limiting. This is reflected in the mean (±SE) dry mass of greenhouse tillers, which was 881.6 ± 33.5 mg and 962.4 ± 32.5 mg for plants from seeds originally collected in sunny and shady habitats, respectively. Due to some mortality, final sample size for plants from sunny and shady habitats was 55 and 47, respectively (4–6 plants per family).
Both field and greenhouse tillers were processed in the same way. First, the vegetative and reproductive phytomers were counted. Then, all phytomers were separated with a razor blade and placed individually into open vials. After drying to constant mass at 60°C for 48 h, the mass of the leaf and culm (all phytomers), CL spikelets and caryopses (subterminal and terminal phytomers), and CH spikelets and caryopses (terminal phytomer) was determined. Note that CL and CH racemes bore spikelets with mature caryopses as well as some immature spikelets (especially on lower subterminal CL racemes).
Analysis
For any one modular component, allocation was defined as the dry mass of the component (leaf, culm, CL or CH reproductive structures) divided by the dry mass of the summed components for that module (i.e., phytomer). Because many plants did not manage to mature reproductive 5th and 6th subterminal phytomers, thereby reducing the available sample size substantially, only the first four subterminal phytomers were statistically analyzed. Vegetative phytomers, which consist only of a culm and leaf (Fig. 1c) were summed and averaged for each tiller.
The numbers of vegetative and reproductive phytomers per tiller, and the modular allocation variables for each phytomer type, were analyzed by one-way analysis of variance (ANOVA) for the field-collected tillers. The primary factor (fixed) was the habitat from which tillers were collected (shady or sunny). The same variables were analyzed by mixed model, nested ANOVA for tillers from the greenhouse experiment. Habitat of origin (shady or sunny) was the fixed factor, and family nested within habitat of origin was the random factor. The type III mean square for habitat was tested over the family (habitat) mean square to provide F values. The family (habitat) mean square was tested over the error mean square. For all analyses, Proc GLM of the Statistical Analysis System (version 8.2, SAS Institute, Cary, North Carolina, USA) was employed. To conform to ANOVA assumptions, for field-collected and greenhouse tillers the numbers of vegetative and reproductive phytomers per tiller were square-root transformed, and modular allocation proportions were arcsine, square-root transformed, prior to analysis.
To adjust for the number of simultaneous ANOVAs performed for the six phytomer categories (Tables 1 and 2), a sequential Bonferroni procedure was used to determine the acceptable level for statistical significance (Rice, 1989
; Cabin and Mitchell, 2000
). Failure to use the Bonferroni procedure when performing multiple ANOVAs would inflate type I error rates (Cabin and Mitchell, 2000
; García, 2004
).
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), scaling coefficients (
RMA ± SE) from an RMA regression are also indicated. In addition, to explore the possibility of genetic correlation between leaf mass and reproductive mass, the regressions were also performed on the family means for tillers of greenhouse plants. RESULTS
Field-collected tillers
Mean (±SE for all) numbers of vegetative phytomers for tillers from the shady and sunny habitats were 10.4 ± 0.5 and 5.7 ± 0.4, respectively (F1,33 = 52.10, P < 0.0001). The number of reproductive phytomers did not differ between shady (5.4 ± 0.3) and sunny (4.7 ± 0.3) habitats (F1,33 = 3.22, P = 0.0818).
There were many significant differences in modular allocation for tillers from the two habitats. Allocation to culms was significantly greater in sun-collected tillers for all phytomers except vegetative ones (Table 1; Fig. 2). The difference in allocation to culms was particularly striking for the terminal culm that bears the CH spikelets and seeds: 0.9 ± 0.1% in plants from the shady habitat, compared to 14.1 ± 1.2% in plants from the sunny habitat. In contrast to culm allocation, allocation to leaves was significantly greater in shade-collected tillers for all subterminal phytomers (Fig. 2).
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For shade-collected tillers, the mass of CL and CH spikelets and caryopses were positively correlated with tiller leaf mass (CL mass = –0.47 [leaf mass]0.73, RMA = 1.35 ± 0.27; r2 = 0.29, P < 0.05; CH mass = –0.51 [leaf mass]0.72, RMA = 1.10 ± 0.19; r2 = 0.43, P < 0.01; Fig. 3a). CL mass was also positively correlated with leaf mass in sun-collected tillers (CL mass = –0.06 [leaf mass]0.77, RMA = 1.28 ± 0.28; r2 = 0.37, P = 0.02; Fig. 3b); however, CH mass was not related to leaf mass (r2 = 0.03, P = 0.52).
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There were few significant differences among families (within habitat) in modular allocation; however, greenhouse tillers of plants from the two habitats differed in allocation for all types of phytomers (Table 2). Although all plants grew in a common greenhouse environment where light was never limiting, allocation to leaves was significantly greater in plants from the shady habitat for vegetative and subterminal phytomers (Fig. 4). For example, in vegetative phytomers, allocation to leaves was 31.0 ± 0.4% in plants from the shady habitat, compared to 26.1 ± 0.4% in plants from the sunny habitat. In contrast, allocation to culms was significantly greater in plants from the sunny habitat for the terminal and first three subterminal phytomers (Table 2; Fig. 4). This difference in allocation to culms was especially pronounced for the terminal culm that bears the CH spikelets and caryopses: 18.0 ± 1.5% in plants from the shady habitat, compared to 31.3 ± 1.0% in plants from the sunny habitat.
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For greenhouse tillers of plants reared from seeds from the shady habitat, the mass of CL and CH spikelets and caryopses were positively correlated with tiller leaf mass (CL mass = 0.05 [leaf mass]0.58, RMA = 1.47 ± 0.20; r2 = 0.15, P < 0.01; CH mass = –0.08 [leaf mass]0.52, RMA = 1.73 ± 0.25; r2 = 0.09, P = 0.04; Fig. 5a). In plants reared from seeds from the sunny habitat, CH mass was positively correlated with tiller leaf mass (CH mass = –0.06 [leaf mass]0.53, RMA = 0.99 ± 0.11; r2 = 0.29, P < 0.0001). In contrast, CL mass was not related to leaf mass (r2 < 0.01, P = 0.52; Fig. 5b).
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RMA = 1.03 ± 0.21; r2 = 0.28, P = 0.02). DISCUSSION
Intrapopulation divergence in modular allocation
As an invasive colonizer Microstegium vimineum can grow along sunny, edge habitats as well as in shade beneath the forest canopy (Winter et al., 1982
; Horton and Neufeld, 1998
; Gibson et al., 2002
; Cheplick, 2005b
). Such shade-tolerance is unusual for an annual weed (Sutherland, 2004
). Light-induced plasticity may explain some of the phenotypic differences between plants in the two light habitats (Claridge and Franklin, 2002
; Sultan, 2003
). Increased allocation to leaves in most phytomers under low light (Fig. 2) is likely to maximize photosynthetic production (Claridge and Franklin, 2002
; Cheplick, 2005b
). Also, a greater number of vegetative phytomers will support a proportionately more leafy shoot when light availability is low. Sexual reproduction by both chasmogamy (CH) and cleistogamy (CL) is positively correlated with leaf mass in tillers from shady habitats (Fig. 3), but vegetative modules could be less costly to produce than reproductive modules when resources are limited (Pyke, 1989
). However, absolute leaf mass was greater under high light (Fig. 3); increased allocation to culms (Fig. 2) may be a structural cost necessary to support a greater weight of leaves.
Although phenotypic plasticity can explain allocational differences between field-collected tillers from shady and sunny habitats, many of the differences were maintained when plants were reared in a greenhouse under high light (Table 2, Fig. 4). Allocation to leaves was greater, while allocation to culms was lower, for most phytomers in tillers from greenhouse plants reared from the seeds of parents in the shady habitat. This suggests adaptive divergence within this Microstegium population in modular allocation to leaves and culms due to light or other unmeasured variables that vary between the two habitats, supporting Cheplick's (2005b) results for whole-tiller biomass partitioning.
Microstegium families within each habitat mostly did not vary significantly for most modular allocation traits (Table 2) and whole-tiller traits as well (Cheplick, 2005b
). It is presently not known if developmental plasticity or genotype by environment interactions occur in this inbred annual and impacts its evolutionary potential. However, a self-fertilizing breeding system has been found to be associated with a variety of widespread annual weeds (Price and Jain, 1981
; Sutherland, 2004
).
Plants reared from seed families of parents within the sunny and shady habitats maintained differences in allocation to CH and CL in the upper phytomers when grown in a sunny greenhouse (Table 2, Fig. 4). This suggests some adaptive value to preferential allocation to outcrossed, CH spikelets relative to selfing, CL spikelets under low light, but at present it is not known what this benefit might be. Besides light, it is possible that other unmeasured features of the two habitats were more relevant to the intrapopulation divergence found here. However, such differences were not visually apparent in the field as the two habitats were underlain by the same soil, and few other potential competitors were present. Also, the transects from which plants were sampled within the habitats were separated by only 50–100 m (Cheplick, 2005b
).
When comparisons were made across family means, CH (but not CL) reproductive mass was positively correlated with tiller leaf mass, suggesting a genetic correlation. Thus, selection in a shady habitat for increased leaf mass, which showed significant variation among families, may result in greater CH reproduction regardless of whether or not CH is more advantageous than CL under those conditions.
Adaptive benefits of modularity
Modular organization is a "universal property of living things" (West-Eberhard, 2003
, p. 56) that has important consequences for development and the evolution of plant life histories (Vuorisalo and Mutikainen, 1999
; Preston and Ackerly, 2004
). During development, serial adjustment of the phenotype can occur as different modules with specific functions are delineated. Increases in modularity can provide the opportunity for division of labor among morphologically discrete components (West-Eberhard, 2003
). Adaptive modular plasticity can involve the partitioning of fixed resources among structures (Preston and Ackerly, 2004
), presumably in a way that improves individual reproductive fitness (Pedersen and Tuomi, 1995
).
The organization of phytomers along the tillers of Microstegium, and the patterns of allocation within them, vary considerably with ontogeny and position. Following seed germination, the first phytomers are completely vegetative and lateral branch tillers later arise from lower nodes, especially under sunny conditions (Cheplick, 2005b
). Like the primary tiller, lateral tillers elongate as more vegetative phytomers are added during development. As in other annuals, gradual accumulation of vegetative nodes eventually terminates in the production of one or more reproductive nodes (Sachs, 1999
). The increased branching that is possible under high light represents developmental plasticity that permits adjustment to variable conditions (Novoplansky et al., 1994
; Cheplick, 2002
). This can increase the number of caryopses produced by Microstegium because many lateral tillers also produce reproductive phytomers as they mature.
Modularity in an annual plant might allow reproductive investment to be modified in a way that enhances seed production. For example, in some plant species the relative number of seeds produced increases with greater reproductive allocation (Cheplick, 2005a
). Along a Microstegium tiller, production of the terminal phytomer with an elongate raceme of CH spikelets ends the growth of that tiller. For the terminal phytomer, allocation to CH is relatively high (Figs. 2, 4) and may represent a costly investment (Cheplick, 2005a
). With the exception of sun-collected tillers, CH mass was positively correlated with the total mass of leaves on a tiller (Figs. 3, 5), suggesting that CH reproduction is determined by available photosynthate. Although the terminal phytomer is a determinate module, subterminal phytomers (Fig. 1b) continue to mature axillary CL racemes within the leaf sheath for as long as the growing season permits. Thus, maturation of caryopses in CL spikelets represents a fail-safe method of reproduction in the event of terminal raceme loss or pollination failure in outcrossed CH spikelets. In addition, continued production of caryopses in CL racemes should improve individual fitness. The positive correlation of CL reproductive mass with total dry mass of leaves on both shade- and sun-collected tillers (Fig. 3) suggests that CL reproduction, like CH reproduction, is also determined by available photosynthate.
The hypothesis that CH reproduction would be opportunistic and increase under improved conditions (because CH spikelets are more expensive than CL spikelets; Schoen, 1984
) and that CL reproduction would be favored in resource-limited conditions (Campbell et al., 1983
) was not supported by the allocation data. The ratio of CH to CL was actually lowest for large plants from a sunny environment; this was partly an allometric effect, as larger tillers allocated proportionately less to CH (see Cheplick, 2006
for further discussion). In contrast to the tested hypothesis, allocation to CH spikelets and caryopses was greatest for small tillers in a shady habitat. Rather than only providing reproductive assurance under suboptimal conditions, the basipetal maturation of axillary CL racemes is a mechanism at the modular level (i.e., in subterminal phytomers) that adds on to the fitness from caryopses matured on co-occurring CH racemes. This developmental plasticity in modular allocation provides the opportunity for Microstegium to maximize evolutionary fitness when environmental conditions are favorable.
Conclusion
For invasive colonizers like Microstegium, a breeding system that is predominantly selfing coupled to an annual life history may be an especially favorable combination (Price and Jain, 1981
; Sutherland, 2004
). Campbell et al. (1983
, p. 429) remarked that about 60% of grass species with CL "are colonizers of disturbed or early-successional habitats." The ability of Microstegium to adjust modular allocation to leaves, culms, and CH and CL reproduction in relation to light conditions experienced by distinct phytomers during development maximizes its reproductive fitness in disturbed forests of eastern North America. The maturation of additional caryopses in CL spikelets on axillary racemes when conditions are favorable, such as along the sunny edges so common in fragmented forests, probably contributes to its continued spread as an invasive species.
FOOTNOTES
1 The author thanks K. A. Preston and two anonymous reviewers for valuable comments on an early version of the manuscript. 
2 Author for correspondence (cheplick{at}mail.csi.cuny.edu
)
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