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Ecology |
Department of Biology, Queen's University, Kingston, Ontario, Canada, K7L 3N6
Received for publication March 14, 2002. Accepted for publication October 3, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: allometry • development • functional response • light gradient • meristem allocation • nutrient gradient
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), and because competitive interactions tend to be asymmetric (Ellison and Rabinowitz, 1989
; Geber, 1989
; Thomas and Weiner, 1990
; Weiner, 1990
), large plants tend to be better competitors than small plants. Plant size is a product of factors including age (old plants tend to be larger than young plants) and resource availability (plants in resource-rich habitats tend to be larger than plants in resource-poor habitats). Thus, traits that are differentially expressed through development or across resource gradients should be related to size.
Some plant traits show consistent patterns of size dependence (i.e., allometry). For example, allometry in reproductive effort (the relative allocation to reproductive vs. vegetative structures) has been noted in many plant populations (Waite and Hutchings, 1982
; Samson and Werk, 1986
; Klinkhammer and DeJong, 1987
; Ohlson, 1988
; Hartnett, 1990
; Thompson et al., 1995
; Mendez and Obeso, 1993
; Reekie, 1998
; Sugiyama and Bazzaz, 1998
; Cheplick, 2001
). While allometry in reproductive effort across environments does not demonstrate adaptive responses to environmental variability (e.g., Weiner, 1988
; Shipley and Dion, 1992
), examining patterns of allometry can be an effective method for assessing the change in relative allocation traits across environments (Müller et al., 2000
; Bonser and Aarssen, 2001
). Considering allometric relationships for both reproductive and vegetative traits could reveal functional responses to resource availability. Plants displaying allometric relationships between certain traits across environments should have higher relative fitness than plants lacking these allometric relationships. For example, vegetative traits such as branching intensity may also show size-correlated responses to resource availability. Differences in branching allocation through development across environments can produce a range of plant sizes as high allocation to branching increases the potential production of new plant modules (Watson and Casper, 1988
; Geber, 1990
; Fagerström, 1992
; Bonser and Aarssen, 1996
; Watson et al., 1997
; Huber and During, 2001
), and allocation to branching tends to be greater in more favorable resource environments (e.g., Donohue et al., 2000
; Bonser and Aarssen, 2001
; Huhta et al., 2001
). In this study, we interpret size-correlated patterns of response to variation in resource availability in terms of meristem allocation.
Strategies of meristem allocation
Different patterns of plant form and function are fundamentally a product of patterns of meristem allocation (Harper, 1987
). Meristem allocation to vegetative and reproductive fates determines the schedule of growth and reproduction, respectively. All of the aboveground meristems on a plant are derived from apical meristem activity. New meristems are added in the axils of leaves and it is generally the case for flowering plants that each leaf axil subtends one meristem (or the product of its development) (Bell, 1991
). These axillary meristems have one of three fates: a reproductive (R) meristem forms a flower or inflorescence; an inactive (I) meristem is quiescent or undeveloped because of meristem suppression (by apical dominance), senescence, or death; and a growth (G) meristem becomes an apical meristem forming a vegetative branch. A growth meristem, however, produces vegetative biomass and new meristems and then becomes either inactive or reproductive. A G meristem, therefore, has two fates separated in time: G then I or G then R. Hence, there are two populations of both I and R meristems: those which that are apical (ap), at the terminal ends of shoots, and those that are axillary (ax), in the axils of leaves. At any one stage in development, the number of reproductive meristem fates is equal to the number of apical (Rap) plus axillary (Rax) flowers or inflorescences. The number of inactive meristem fates is equivalent to the number of axillary meristems (Iax) not committed to growth or reproduction plus the number of apical meristems (Iap) that fail to become reproductive and become inactive at final development. The number of growth meristem fates is equivalent to the number of times between seedling emergence and the current developmental stage that axillary meristems were committed to branching or growth (Gax). The total number of meristem fates is equal to the sum of each of the meristem fates on both the apical and axillary positions. In this study, we examine meristem allocation as an alternative to biomass allocation (see Bonser and Aarssen, 1996
). Plant responses to resource availability through continuous adjustments in vegetative and reproductive allocation can be followed by examining patterns of meristem allocation through development.
The aboveground meristems on a plant form a population of functional units with a particular distribution among the different fates at any one point in the life of a plant. Three life history traits can be defined by the relative allocation to each meristem fate. Reproductive effort (RE) is a measure of the proportion of the total number of reproductive meristem fates vs. those that could have but did not or have not yet become reproductive, i.e.,
 | (1) |
Apical dominance (AD) is a measure of the suppression of axillary meristem development and is defined as the proportion of axillary meristems that remain inactive vs. those that are committed to growth or reproduction, i.e.,
 | (2) |
Branching intensity (BI) is defined as the number of axillary meristems producing a new shoot vs. those that could have produced a new shoot but did not, i.e.,
 | (3) |
Correlation or regression analysis between reproductive and vegetative allocation has been an effective method of examining size-dependent reproductive effort (e.g., Samson and Werk, 1986
). Similarly, relative allocation to reproductive effort, apical dominance, and branching intensity can be examined using the relationship between the numerator and the denominator for each of the above equations.
Light and nutrient availability should be important selective forces in plant populations (Tilman, 1988
; Bonser and Aarssen, 1996
). We grew four species of annual plant on gradients of nutrient and light availability to examine the relationship between meristem fates across a range of plant sizes. We conducted this study on two resource gradients to extend our understanding of the generality of meristem responses to variable resource availability. We addressed the following questions: (1) Are there significant allometric relationships between meristem allocation and size within a species where age is held constant and size differences are due to variable resource availability, i.e., do plants respond to resource availability by altering meristem allocation? In annual plants, branching will maximize the number of reproductive meristems at the end of the growing season (Duffy et al., 1999
). Allocation to growth meristems (branching intensity) should be relatively high in favorable resource environments. Conversely, allocation to inactive meristems (apical dominance) should be relatively high in unfavorable resource environments. This response in apical dominance should be particularly important in light-limiting environments as plants shaded by neighbors may undergo a shade avoidance response to maximize their ability to compete for light (e.g., Schmitt and Wulff, 1993
). Differences in the response between light and nutrient gradients within each species should demonstrate that the type and/or strength of response are resource dependent. (2) How are patterns of response expressed throughout development? The examination of patterns of responses throughout development can lead to the identification of developmental stages in which plants have a greater capacity to respond to resource availability or stages where response in meristem allocation is important for attaining an optimal growth form. In annual plants, available resources and meristems should be committed to reproduction at the end of the growing season. Thus, fitness increases gained through strong apical dominance or branching intensity should diminish towards final development (Bonser and Aarssen, 2001
). We predict that responses in meristem allocation should occur at a relatively early age. (3) Are there significant differences in the pattern or strength of response across species? Differences in species ability to respond to resource availability have important implications in determining the range of habitats different species can occupy.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Seeds of Arabidopsis (Kil genotype) were obtained from the Arabidopsis information service (Frankfurt, Germany) and were originally collected from natural populations. Brassica (wild genotype) was obtained as seed from the Crucifer Genetics Cooperative (Madison, Wisconsin, USA). Seeds of Arenaria and Chaenorrhinum were collected from natural populations in the vicinity of Kingston, Ontario, Canada (44°20' N, 76°30' W). For these species, seeds were collected from a small number of plants (less than 25) growing in large populations. Seeds of each species were germinated on moist filter paper in sterilized petri dishes. Chaenorrhinum seeds required scarification prior to germination. Individual plants were transplanted individually into pots (10 cm diameter, 400 cm3 volume) containing sterilized potting mix (Fison's Sunshine Basic Mix #2, Bellevue, Washington, USA) at the emergence of both cotyledons. Individuals of each species were grown separately on nutrient and light gradients in order to produce a range of plant sizes.
Nutrient gradient
Pots were placed in a growth chamber under 16-h light (250 µmol · m–2 · <011>s–1, 23.0°C, 75% relative humidity) and 8-h dark (21°C, 75% relative humidity) and were randomly repositioned within the chamber once per week. Ten replicate plants of each species were assigned to each of six nutrient treatments, 0.00, 0.25, 0.50, 1.00, 2.00 and 4.00 g/L of a 20 : 20 :20/N : P : K fertilizer. For each nutrient level, 25 mL were applied per pot at weekly intervals until the end of the experiment. Plants were watered with reverse osmosis (RO) water evenly as required to ensure the soil did not become dry.
Light gradient
Nine replicates of each species were subjected to one of five shading treatments plus a control. Each pot was placed in a growth chamber under 16-h light (250 µmol · m–2 · <011>s–1, 23.0°C, 75% relative humidity) and 8-h dark (21°C, 75% relative humidity). Morphological responses to shading have been demonstrated to be due to changes in the spectral quality of light (e.g., Schmitt and Wulff, 1993
). We simulated the effect of light being filtered through a canopy of competing plants by adding a layer of green plastic film (Lee Filters, Andover, UK; number 121 Lee green) for each of the shading treatments. The filter reduced the photosynthetic photon flux density (PPFD) by 35% and produced a red : far red ratio of 0.20. The change in the spectral quality of light caused by the green filters approximates the change resulting from light filtered through leaves of competing plants. Further shading was achieved through the addition of 1–4 layers of a single sheet of neutral density shade cloth (Plant Products, Brampton, Ontario, Canada), which reduced PPFD by 33% with each successive layer without further changing the spectral quality of the light. The control treatment consisted of a single layer of clear plastic film (Lee Filters; Number 130 Clear), which reduced light transmittance by 5%, but had no effect on the spectral quality of the light. This treatment was included to control for any unintended effects (e.g., changes in humidity, air flow, etc.) of filtering light through a solid sheet of plastic. Light quality and quantity of each treatment are summarized in Table 1.
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Plants were grown to final developmental stage (i.e., when seed set was completed and when there was no further change in the number or allocation of meristems). Meristem allocation was recorded for all individuals of each species and resource treatment at regular intervals throughout development and at final developmental stage. Recording meristem allocation throughout development did not have any apparent unintended effects on the plants. There were often several weeks between the last measure of meristem allocation during development and the measure of meristem allocation at final development. Meristem allocation was difficult to assess late in life due to the large size of many plants. In addition, few changes in meristem allocation were noted in the last few weeks of each experiment, as many plants were only setting seeds at this time.
Data analyses
Meristems from each individual across the six light and nutrient treatments for each species at each age were used to test for significant allometry in meristem allocation. For a given genotype at a given age, the total number of reproductive (R) and vegetative (I + G) meristems in each plant was used to test for an allometric relationship between reproductive effort and plant size across individuals that differ in size due to primarily resource treatment (Eq. 1). Total number of vegetative meristems (I + G) was used as a measure of plant size rather than total number of meristems (R + I + G) since the latter can result in spurious correlations that may obscure the relationship between reproductive and vegetative components (e.g., Samson and Werk, 1986
; Klinkhammer et al., 1990
). Similarly, the relationship between axillary I and R + G meristems (Eq. 2) can be used to test for allometry of apical dominance, and the relationship between axillary G and R + I meristems (Eq. 3) can be used to test for allometry of branching intensity. We tested for allometric relationships between meristem fates and size for all individuals for each species at a given time (static allometry). We do not test for allometric relationships due to changes in relative meristem allocation within individuals throughout development (dynamic allometry).
Reduced major axis (model II) regression was used to test for an allometric relationship between meristem fates across a range of plant sizes. Model II regression is appropriate where there is error in both the x and y variables of the regression model (Sokal and Rolf, 1995
). A significant allometric relationship is indicated where the slope (scaling exponent) of the relationship between the logarithm of one meristem fate and the logarithm of the sum of the two other meristem fates differs from isometry (Niklas, 1994
). An isometric relationship between meristem fates indicates that relative meristem allocation is constant for all plant sizes. Use of the reduced major axis regression minimizes the chance of erroneous conclusions due to an artificially reduced slope of the model I regression (McArdle, 1988
; Henry and Aarssen, 1999
).
Allometry in meristem allocation is indicated where the slope of the above relationship between a given meristem fate and plant size differs from isometry. A significant difference of a reduced major axis slope (b) from a known slope (ß) can be performed on the test statistic (Clarke, 1980
; McArdle, 1988
):
 | (4) |
; McArdle, 1988
): 
.
The above equation was also used to assess significant differences in pairwise comparisons of the slopes of the allometric relationships within species growing on either a light or nutrient gradient. All pairwise comparisons were conducted at final developmental stage (a developmental stage easily defined and common to all individuals). Pairwise comparisons throughout development or across species were subjected to table-wide sequential Bonferroni correction to control for multiple comparisons (Rice, 1989
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Intraspecific differences in meristem response
Within species, the degree of response in meristem allocation to environmental conditions (i.e., the slope of the allometric relationship) tended not to differ significantly between plants grown on light gradients with those grown on nutrient gradients (Fig. 3). The response in reproductive effort and apical dominance were significantly different between light and nutrient gradients for both Arenaria and Chaennorhinum.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Increases in plant size increase the absolute number of meristems available to commit to each fate. The relationship between meristem fates across a range of plant sizes (in which size differences were due to differences in resource availability) was highly significant at all ages on both light and nutrient gradients. Larger plants tended to have a higher absolute number of reproductive (R), inactive (I), and growth (G) meristems. However, differences in the relative accumulation of a meristem fate in relation to the accumulation of other meristem fates across sizes result in allometric relationships between meristem fates. Thus, plants change their relative allocation to different meristem fates in response to resource availability. The allometric relationships between meristem fates were strikingly constant across the four species studied (Fig. 1). This indicates that there may be a common response in meristem allocation that may maximize fitness in a given environment in plants of very different body plans. The relatively high allocation to both growth (and reproductive) meristems in large plants (under high resource availability) and/or the relatively low allocation to these meristem fates in small plants (under low resource availability) may be products of natural selection acting on plants with a common annual life history. Plants with allometric relationships in branching intensity (BI) and apical dominance (AD) across these resource gradients should be relatively more fit than plants with isometric relationships in these traits.
Coefficients of allometry were significantly different between light and nutrient gradients for only two of a possible 12 contrasts (Fig. 3). This suggests that the type and magnitude of response is consistent across different resource environments. It is reasonable to assume that the response in apical dominance would be greater across the light gradient than the nutrient gradient, as light with low red : far red can promote a shade-avoidance response in plants (see Schmitt and Wulff, 1993
; Dudley and Schmitt, 1996
) and decrease the probability of axillary meristem development (e.g., Robin et al., 1994a
, b
). However, the response in apical dominance for Arenaria was significantly greater for plants grown on the nutrient gradient than for plants grown on the light gradient. There was no significant difference in the apical dominance response for the other three species. Short-lived annuals generally grow in open and disturbed habitats where competition for light should not be intense. Thus, the ability to respond to variable nutrient availability may be as important as the ability to respond to variable light availability. The consistency of the type and magnitude of response of meristem allocation on different resource gradients suggests that allometric relationships may be general responses in plants of different body plans to resource gradients.
While the patterns of response (slope greater than or less than isometry) were constant across species, we found significant differences in the magnitude of response across species on both light and nutrient gradients. Variability in the magnitude of response across species in branching intensity and apical dominance demonstrates that plants differ in their ability to respond to environmental conditions. This suggests that either selection continuously modifies the magnitude of response to environmental variability or that the magnitude of response is constrained in some species by architectural or body plan traits. Differences between species in these traits were not consistent between resource gradients. For plants growing on the light gradient, Arabidopsis had the strongest response in apical dominance and Chaenorrhinum had the weakest response. On the nutrient gradient, Chaenorrhinum had the strongest response in apical dominance and Brassica had the strongest response in branching intensity; Arenaria had the weakest response in both of these traits. The shift in relative strengths of response across species between gradients indicates that the magnitude of response is not entirely due to body-plan constraints. Further studies are required to explore how plants of contrasting body plans respond to selection for strong allometric responses to resource availability.
In general, allocation to both reproductive meristems (reproductive effort) and growth meristems (branching intensity) increased with increasing plant size, and allocation to inactive meristems (apical dominance) decreased with increasing plant size where increasing size was due to greater resource availability (Figs. 1 and 2). Meristem allocation to branching serves to maximize plant size, competitive ability, and the total number of meristems available to be committed to reproduction at final development. Significant allometry in the allocation in inactive meristems is due to plants expressing stronger apical dominance at smaller sizes. High apical dominance in low resource environments may be due to plants having insufficient resources to commit to branching or reproduction. For example, the ability to maximize seed production may be reduced if limited resources are spent on increased vegetative function or even increased flower production. Alternatively, plants allocating extensively to growth may have insufficient time for successful reproduction by the end of the growing season, as growth rates will be slow in resource-impoverished environments.
Interspecific differences in allometry of reproductive effort must be interpreted differently than interspecific differences in allometry of apical dominance or branching intensity. Significant differences in allometric coefficients between two species in reproductive meristem allocation demonstrate instances in which one species has relatively higher allocation to reproduction in low resource environments while the other species has relatively higher allocation to reproduction in high resource environments. These differences become especially important when the regression lines cross at intermediate sizes, as differences in allometry represent absolute differences in allocation to reproduction. For example, the regression lines cross for Anenaria and Chaenorrhinum for plants growing on the light gradient (Fig. 1a). Allocation to reproduction was greater for Arenaria in low resource environments and greater for Chaenhorinnum in high resource environments. Because reproductive meristems in both of these species form single fruits rather than inflorescences, differences in the number of reproductive meristems likely reflect differences in fitness between these species (Arenaria being more fit in the low light environments and Chaenorrhinum being more fit in high light environments). These changes in relative fitness across plant sizes could be an important mechanism for species coexistence across heterogeneous resource gradients.
Considering how traits are integrated within plants increases our understanding of how plants respond to environmental variables (e.g., Schlichting and Pigliucci, 1998
). A change in reproductive effort (RE), apical dominance (AD), or branching intensity (BI) can be correlated with the allocation to one or both other traits (Duffy et al., 1999
). For example, apical meristems are committed to reproduction by final development in three of the species examined here (Arenaria, Arabidopsis, Brassica). In favorable resource environments, increased allocation to new branches (growth meristems) will be positively correlated with reproductive meristems and negatively correlated with inactive meristems. Increasing reproductive effort with increasing plant size is a common relationship in plant populations. Reproductive allometry is potentially caused by a number of factors (Sugiyama and Bazzaz, 1998
), including a minimum size threshold required for reproduction (Weiner, 1988
). Our results are consistent with the hypothesis that adaptive responses in traits such as apical dominance and branching intensity can lead to allometry in reproductive effort. Results of a recent study demonstrate that allocation to reproductive meristems is not correlated with growth meristems in perennial species (Huber and During, 2001
). Future studies on species of different life histories on a number of other environmental gradients are needed to establish the degree to which these meristem allocation patterns are general strategies.
Allometry and development
In general, where significant responses of meristem allocation were detected, these responses were initiated relatively early in development. Capacity to alter patterns of meristem allocation early in development should be very important for both branching intensity and apical dominance. The potential increases in size and fitness (through a larger absolute number of meristems available to be allocated to reproduction) gained from greater commitment to growth meristems should be maximized earlier in development, as this maximizes the number of growing stems accumulating size and meristems early in development. Similarly, in resource-impoverished environments, suppressing axillary meristem activity relatively early in development through strong apical dominance could conserve limited resources in order to maximize lifetime reproductive success. In a previous study (Bonser and Aarssen, 2001
), we found the strength of meristem response tended to decrease toward final development in genotypes of Arabidopsis, demonstrating the decreasing value of later life responses in these meristem fates. While there were several cases where the strength of meristem response seemed to decline through development (e.g., Fig. 2a, b), these late-life allometric coefficients were not significantly different than earlier-life allometric coefficients.
Factors controlling plant growth and development (the pattern of meristem allocation) can be altered by the environment in a fairly consistent manner across species. While allocation to meristem fates may be guided by a set of rules in which the placement and timing of meristem activation (or inhibition) gives rise to species growth form, total meristem allocation is not a fixed property within a species. These results mirror the results found in plasticity through development in genotypes of Arabidopsis (Bonser and Aarssen, 2001
) and are consistent with the notion that the nature of the relationship between traits can change through development (Pigliucci et al., 1996
; Schlicting and Pigliucci, 1998
).
The differences in the range of meristem allocation found here may be due in part to differences in body plan (architecture) between each of the four species studied here. Floral meristem fates have been found to be contingent on architectural effects within individuals (Diggle, 1994
, 1997
). Similarly, differences in morphology between species significantly affect competitive interactions (Geber, 1989
). Further tests are required to examine the role of body plan on functional allocation to meristem fates in competitive interactions and the constraints that body plan sets on the evolution of adaptive strategies (Gould and Lewontin, 1979
). Studies on a range of species and a number of environmental gradients are required to further test the evolution of meristem allocation strategies and the evolution of the capacity to respond to environmental selection pressures.
Conclusions
We found significant allometry in meristem allocation for each of the four species examined in this study. Allocation to reproduction and growth meristems (reproductive effort and branching intensity, respectively) increases with increasing plant size (where differences in plant size are due to differences in light and nutrient availability). Allocation to inactive meristems (apical dominance) tends to decrease with increasing plant size. Since size variability was due to differences in light and nutrient availability, we suggest that allometry in meristem allocation demonstrates important functional responses of meristem allocation to resource availability. Species showed significant differences in the strength of meristem response to variable resource availability. These differences may be important in defining the range of habitats in which a species can exist and help to explain patterns of species competition and coexistence in habitats with variable resource availability.
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FOOTNOTES |
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2 Current address: Department of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, 2052, Australia
3 Author for reprint requests (aarssenl{at}biology.queensu.ca
)
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), Arenaria (
), Brassica (
), and Chaenorrhinum (
) at final developmental stage growing on a light gradient (a, b, c) and a nutrient gradient (d, e, f). Closed symbols represent the mean meristem allocation for the six resource treatments for each species. Each line represents the regression line plotted through the individuals (not the means) for each species. The leading diagonal line represents the slope (equal to one) of an isometric relationship. Solid lines represent slopes that are significantly different (P < 0.05) from isometry; dashed lines represent slopes that are not different from isometry. Regression lines coded with different letters are significantly different
) and nutrient (