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Keywords:

  • adaptive plasticity;
  • density responses;
  • Picea omorika;
  • plasticity costs;
  • shade avoidance syndrome;
  • trait correlations

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

The adaptiveness of shade avoidance responses to density was studied in Picea omorika seedlings raised in a growth-room. Siblings of a synthetic population comprising 117 families from six natural populations were exposed to contrasting density conditions in order to score variation in phenotypic expression of several epicotyl and bud traits included in the shade avoidance syndrome. As predicted for the adaptive plasticity to foliage shade, epicotyl elongation traits tended toward higher, while axillary bud traits toward lower values in high-density vs. low-density conditions. Phenotypic selection analysis revealed that the elongated plants had greater relative fitness than the suppressed ones in both density treatments which could be ascribed to the effect of direct selection on epicotyl length. There was no evidence for plasticity costs associated with the expression of the shade avoidance phenotype either under low or under high density, with only a single exception. Estimates of variance component genetic correlations across densities were significantly different from unity for the majority of the seedling traits studied, indicating the existence of heritable variation within reaction norms of these traits. However, since all these correlations were positive in sign and large in magnitude, this conclusively means that the level of the additive genetic variation for plasticity in the shade-avoidance traits of P. omorika is rather low.

The struggle for existence among plants is, to a large extent, the struggle to grow in the face of competition from neighbours.’Jacob Weiner, 1995.

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Adaptive phenotypic plasticity, the ability of a single genotype to express different, functionally appropriate phenotypes under contrasting environmental conditions, has been considered as a major component of evolutionary change (Bradshaw, 1965; Sultan, 1987; Thomson, 1991; Schlichting & Pigliucci, 1998). Current quantitative genetic models of plasticity evolution predict that without any evolutionary constraints on the evolvability of plasticity, the shape of a reaction norm should evolve toward the optimal phenotypic value within each environment (Van Tienderen, 1991). Conversely, if additive genetic variation for plasticity is lacking or if there is a fitness deficit associated with plastic reactions, the evolution of reaction norms will be precluded from reaching their optimal shape. Instead, populations having a compromise mean phenotype in between the demands of the different habitats and the costs of plasticity are expected to evolve. Despite considerable theoretical and empirical interests in the evolution of phenotypic plasticity, the empirical studies that have explicitly tested the hypothesis that plasticity is adaptive are very scarce (Schmitt et al., 1995; Dudley & Schmitt, 1996; Kingsolver, 1996; Baldwin, 1998; Tucićet al., 1998), as are the experimental data assessing the costs and limits of plasticity (DeWitt, 1998; Scheiner & Berrigan, 1998; Tucićet al., 1998).

In plants, one of the classical examples of adaptive phenotypic plasticity is a suite of photomorphogenic responses to foliage shade generally termed the ‘shade avoidance’ syndrome (Smith & Whitelam, 1997). Because chlorophyll of green plants preferentially absorbs red light, solar radiation transmitted through or reflected from a leaf canopy exhibits a lower ratio of red to far-red photons (R:FR) than does full sunlight (Smith et al., 1990). These changes in relative amounts of red and far-red radiation are quantitatively correlated with the density and proximity of the surrounding vegetation (Ballaréet al., 1989). Plants are able to use low R:FR ratios of the ambient light as environmental cues of the presence of future competitors and to transduce these cues into an array of morphological responses mostly directed to enhancing a single function – acquisition of the radiant energy for photosynthesis (Ballaréet al., 1987, 1990; Smith et al., 1990; Smith & Whitelam, 1997). For example, plants exposed to a low R:FR radiation ratio, commonly found within dense communities, are usually taller (relative to dry biomass) and less laterally branched than those grown in isolation. This allometric shift in plant growth form is achieved through an increased shoot extension rate coupled with a strong apical dominance. Because light is an environmental resource that can be ‘pre-empted’ by larger individuals, the primary fitness advantage of becoming taller the fastest under crowded conditions would be in minimizing mutual shading by neighbours and maximizing the ability to deny light to proximal plants (Aarsen, 1995). Recent experiments with transgenic and mutant plants (disabled in phytochrome-mediated elongation responses to neighbours), and the ones using plants with manipulated phenotypes by means of altered R:FR (Schmitt et al., 1995; Dudley & Schmitt, 1996), strongly corroborated the evolutionary–ecological prediction that the shade avoidance phenotype evoked by neighbour-induced light depletion is indeed an adaptation, likely moulded by natural selection from competition for light (Dudley & Schmitt, 1996; Schmitt, 1997; Schwinning & Weiner, 1998).

In conifers, the photomorphogenic shade avoidance is a conspicuous phenotypic response to suboptimal light level. Longitudinally extended growth form of trees is commonly found in dense forest stands, but also at higher latitudes where individual plants receive light from a largely horizontal direction (Aarsen, 1995). Although the leaf canopies within a deep coniferous forest can significantly alter light spectral quality as radiation flux passes through the tree crown, empirical data on shade avoidance responses to R:FR radiation signals in these woody perennials are extremely scarce (Morgan et al., 1983; Fernbach & Mohr, 1990; Ritchie, 1997). Morgan et al. (1983) were the first to detect the phytochrome-mediated photomorphogenic responses at juvenile and postjuvenile developmental stages in radiata pine, Pinus radiata (D. Don.). In this species, stem elongation rate was negatively correlated with the R:FR light ratio to which the plants were exposed. Ritchie (1997) provided evidence that young Douglas fir (Pseudotsuga menziesii Mirb.) seedlings grown at high density were able to perceive the FR cues reflected from the foliage of the neighbouring plants and to transduce these radiation signals into photomorphogenic responses long before the canopy closure occurs. Such early shade avoidance reactions to the FR proximity cues are thought to be especially beneficial for light interception in primary successional forest understories, where light availability increases quickly with plant height, because it allows rapid placement of photosynthetically active shoot parts into more illuminated vegetation strata (Aarsen, 1995; Henry & Aarsen, 1997).

In this study, we examined the fitness consequences of photomorphogenic shade avoidance responses in the conifer species Picea omorika elicited by contrasting density conditions that prevailed during the first year of growth in a growth-room. P. omorika is a pioneer tree species which predominates as an early recruit in forest succession. Its natural regeneration occurs exclusively within disturbed and relatively open habitats such as forest clearings and vegetation gaps (Čolić, 1957, 1966). Since the light conditions in the gap-understories are extremely variable, both in space and in time (Bazzaz, 1996), P. omorika seedlings capable of expressing elongated shade avoidance phenotypes in response to neighbour proximity are expected to be selectively favoured in these habitats, providing that evolutionary constraints within the reaction norms of characteristic shade-avoidance traits are absent. To test this prediction, we addressed the following questions: (1) How does the conspecific density affect the performance of P. omorika seedlings. (2) Is the selective importance of seedling epicotyl traits across densities consistent with the functional arguments of the adaptive plasticity hypothesis for the shade avoidance responses in plants. (3) Does the phenotypic plasticity to ambient light conditions impose an energetic cost or a developmental limit to P. omorika individuals. (4) Is there sufficient genetic variation in reaction norms of shade avoidance traits for the evolution of optimal plasticity to ambient light conditions?

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Study species

The Serbian spruce, Picea omorika (Pančić) Purkyne, is an endemic conifer tree and Tertiary relict of the European flora. Range of distribution of this species is rather narrow (approx. 10.000 km2), occupying exclusively the middle and upper courses around the river Drina. The extant P. omorika populations are scarce and not abundant. In total, the species consists of about 10 natural populations, varying in size from large to small in Bosnia, and about 20, mostly smaller populations, in Serbia (Fukarek, 1951).

For this study we selected six natural P. omorika populations (see Table 1 for localities), each consisting of more than 30 individuals. These populations encompassed almost the full range of the species distribution. To avoid including individuals with common lineage, seed cones were sampled separately from at least 20 randomly chosen, non-adjacent trees within each population. Because P. omorika is a wind-pollinated outcrossing plant, it is likely that the ovules produced by an individual tree are fertilized by pollen from many different paternal plants. If so, a sample of the seed collected from a particular naturally pollinated tree will represent a maternal half-sib family.

Table 1.   Location and sample size (no. of maternal families) of six natural Picea omorika populations included in the study. Thumbnail image of

Experimental set-up

Seeds from all six populations were pooled into a synthetic population comprising 117 maternal families in order to enlarge the range of phenotypic variation available for selection. Prior to planting, 50 seeds from each family were spread over the wet filter paper (soaked with 2% fungicide Venturin–S 50; Župa, Kruševac, YU) in Petri dishes and kept in the dark (2 °C, 30 days) in order to synchronize germination. The incubated seeds were then exposed to room temperature where the germination began after 2–3 days. Sixteen offspring of each maternal family were planted into 300-mL pots in two densities: a single individual plant per pot (10 replicates) and three individuals per pot (two replicates). Pots were filled with a mixture of humus, peat and sand (1:1:1). The potted seedlings were transferred to a growth room where each pot was placed at a randomly chosen position on a shelf. The distance between the centres of neighbouring pots was approximately 9 cm. The ambient temperature in the growth-room was kept at 21/16 °C (light/dark) with a 16-h photoperiod. The light was provided by a set of four Philips TLD 36-W/33 fluorescent tubes. At the onset of the experiment, average photosynthetically active radiation (PAR) above the growing plants amounted 110 μmol m–2 s–1, while the R:FR ratio (quantum flux ratio between 665 and 735 nm) was 8.20, i.e. far above the normal, sunlight, ratio of 1.0. The plants were regularly bottom-watered and fertilized every 10 days with a 0.5% water-soluble fertilizer (Floravit, N : P : K 12 : 6 : 6). To minimize the position effects (genotype–environment correlation) the pots were rotated every 2 days.

On the 240th day after the germination, plants were harvested individually. Eleven seedling traits were recorded on each seedling. Table 2 gives the acronyms and a short description for each trait measured. Based upon the functional roles that distinct traits play within a plant organism (Berg, 1960; Armbruster et al., 1999), the recorded seedling traits of P. omorika were placed into three functional groups: epicotyl traits (involved in resources aquisition), bud traits (associated with resource allocation) and performance traits (fitness components). Except for total plant dry weight, all the readings were taken non-destructively on each individual seedling. To estimate total plant biomass, harvested seedlings were weighed after being oven-dried (72 h, 70 °C). Total plant dry weight was used as a measure of performance.

Table 2.   Morphological traits measured. Thumbnail image of

Statistical analyses

The trait means based on all data points in the sample were calculated separately for each density treatment using a GLM procedure in SAS (SAS, 1989). Significance of the differences in trait means between densities was tested by a standard Student’s t-test. Following Schlichting (1986), plasticity was measured as the absolute value of the difference in family mean phenotype between density environments.

Within each density, phenotypic correlations among trait pairs were calculated as the Pearson correlations of the individual data points. Genetic correlations within treatments were estimated as the Pearson product-moment correlations of the family means, rm (Via, 1984):

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where COVm(xy) is the covariance among the family means of the traits x and y, and Vm(x)(y) are the variances among the family means of traits x and y. The rm correlations are only an approximation of the additive genetic correlations. A fraction of the within-family (special environmental) (co)variance component is included in the (co)variance among family means:

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where n is the number of siblings per family. When maternal half-sib families (MHS) are used, the family mean correlations can be inflated by common maternal effects included into the phenotypic covariance between maternal sibships:

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where VA represents the additive genetic variance, VGm is the variance due to genetic maternal effects and VEc is the variance due to environmental maternal effects (Lynch & Walsh, 1998). The maternal effects are often difficult to isolate and, moreover, these effects are not decreased by increasing the number of individuals (n) per family. One advantage of using the family mean approach is that rm is a true product-moment correlation, which, unlike the variance component correlation, cannot exceed ±1.0; and its significance is tested using standard tables of critical values for the sample correlation coefficients. Analogously to the genetic correlations within densities, plasticity correlations were calculated as the Pearson correlations between the trait plasticity values (Schlichting, 1986). Confidence intervals and standard errors of all correlations based on bootstrap resampling were calculated to test the hypothesis that correlations were significantly different from 1 or – 1.

Variance component correlations across density environments were estimated using the assumptions of the SAS mixed-model factorial ANOVA (Fry, 1992; Fry et al., 1996). In this approach, the genetic correlation between a trait expressed in two density environments can be written as

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where VF,hl is the variance component due to the random family main effect computed as the covariance of family means across levels of the fixed factor – density environment, and VF,h and VF,l, are the variance components due to family effects obtained from separate ANOVAs within high- and low-density treatments (REML option of VARCOMP procedure), respectively. To test whether the correlation rg differs significantly from unity, the data from each density treatment were first standardized such that the variance components explained by the family effect were equal to one, and then were subjected to a mixed-model factorial ANOVA performed on the entire dataset. A significant family–density interaction in the ANOVA on the transformed data provides evidence that the corresponding across-environment genetic correlation is less than one.

To estimate the intensity of phenotypic selection acting on seedling traits, standardized selection gradients (Lande & Arnold, 1983) were computed by regressing relative fitness on all traits simultaneously, after standardizing each one to a mean of 0 and a variance of 1 (z-transformation; Sokal & Rohlf, 1981). Absolute fitness was transformed to a relative one by dividing each absolute value by the average absolute fitness of all plants in the synthetic population. Linear and quadratic terms in the regression were used to test for directional and stabilizing/disruptive selection, respectively.

The standardized directional selection gradients were calculated by a multiple regression analysis of relative fitness on the standardized trait values, while the stabilizing/disruptive selection gradients were obtained from a multiple regression analysis of the relative fitness on the standardized traits and their squares (PROC GLM in SAS). Selection gradients estimated in a multiple regression of relative fitness on the standardized trait values are, in fact, the coefficients of a partial regression which quantify (in units of phenotypic standard deviation) the effect of each trait on relative fitness, holding other traits fixed (Lande & Arnold, 1983).

To estimate the cost of plasticity, we implemented a multiple regression analysis as suggested by Scheiner & Berrigan (1998). This method is based on the following statistical model:

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where W is the absolute fitness for an individual in one of the environments, X represents the trait value in that environment and plX is the plasticity of the trait. Significant regression coefficients β1 and β2 measure direct selection on the trait value and account for the linear and non-linear component of selection, respectively. The regression coefficient β3 describes how the ability to be plastic affects fitness, once the direct effects are taken into account. A cost of plasticity appears as a significant negative regression coefficient for the plX term. This term encompasses both maintenance and production costs. Regression coefficients for the interaction terms, β4 and β5, measure additional production costs. A significant positive regression coefficient for the interaction term (X * plX or X2 * plX) would indicate that production costs are greater for more plastic genotypes. This regression is calculated for each density environment separately.

A sequential Bonferroni procedure was also applied to all P-values in order to correct for multiple comparisons (Rice, 1989).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Phenotypic responses to conspecific density

In this experiment, all of the P. omorika epicotyl traits, except for epicotyl length 30, were significantly affected by conspecific density (Table 3). During the first 20 weeks of growth, plants raised at low density exhibited a higher mean epicotyl length than those grown at high density. By contrast, relative epicotyl length (to seedling dry biomass) recorded at three successive censuses (10th, 20th and 30th week after germination) appeared to be greater and its rate of increase higher (approx. 16%, 14% and 24%, respectively) in seedlings grown at high density than in those grown at low density. Lateral bud number was significantly lower under high density, but its rate of appearance was similar between the two density treatments (Table 3). The early epicotyl elongation rate, RGR1 (interval between 10th and 20th week after germination), did not differ significantly between the two densities. As seedling age progressed, relative epicotyl elongation rate began accelerating in plants grown at high density, so that between the 20th and 30th week after germination, RGR2 was significantly greater at high compared to low density (Table 3). Interestingly, under low-density conditions, the majority of P. omorika families tended to converge on a single phenotypic value, contrary to high density, where the family means were more divergent (Fig. 1). Finally, the mean seedling dry biomass at harvest was significantly greater in plants raised under low-density conditions than in those grown under high-density conditions (Table 3). Although we did not measure the changes in spectral composition of ambient light to which the seedlings were exposed during the course of the experiment, the phenotypic responses observed were parallel with the results of empirical studies manipulating R:FR and suggested that plasticity of P. omorika seedlings to conspecific density might be phytochrome-mediated, as well.

Table 3.   Sample size (N), mean values (X) and standard errors (SE) of Picea omorika trait expressions in different density treatments. V% is the proportion of variance explained by family effect within densities. P denotes significance levels taken from a t-test of observed differences in trait means between densities. The P-values subjected to a sequential Bonferroni correction (α < 0.05) are given in bold type. Thumbnail image of
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Figure 1.  Reaction norm plots for epicotyl elongation rate between the 20th and 30th week after germination (RGR2, cm cm–1 week) in distinct Picea omorika families exposed to high- and low-density conditions. The endpoints of the lines represent the phenotypic means of each of the 117 families.

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Phenotypic selection and plasticity costs

It is generally assumed that the fitness consequences of the R:FR-induced shade avoidance responses in plants are density-dependent (Ballaréet al., 1987; Casal & Smith, 1989; Schmitt, 1997). According to the adaptive plasticity hypothesis for shade avoidance, elongated plants are expected to have greater relative fitness than suppressed plants at high density, but will suffer a fitness disadvantage at low density, and, specifically, selection for increased stem height will be greater at high relative to low density (Schmitt et al., 1995; Dudley & Schmitt, 1996).

To test whether the phenotypic variation in epicotyl elongation traits expressed by individual P. omorika families under contrasting density conditions represents adaptive plasticity, a multivariate selection analysis has been implemented to the data from each density treatment. In such a quantitative genetic approach, the adaptiveness of a particular trait is quantified in terms of selection coefficients. Because the environmental conditions determine a causal relationship between phenotype and fitness, the experimental estimates of selection coefficients are predicted to be near zero in environments where a trait is not expected to be adaptive (directly related to fitness), but significantly positive or negative in environments where it confers a fitness benefit (Dudley & Schmitt, 1996; Dudley, 1996).

Phenotypic selection analyses revealed that epicotyl responses exhibited by P. omorika seedlings to conspecific density confer fitness advantage under both density conditions. Elongated individuals were associated with a higher seedling performance (vegetative fitness) in both density treatments, as indicated by significantly positive linear selection gradients evaluated for the majority of the epicotyl length traits (Table 4A,B). However, contrary to the predictions of the adaptive shade avoidance hypothesis, the magnitude of directional selection on epicotyl elongation was even greater at low than at high density, suggesting that the adaptive plastic response of P. omorika seedlings to low density would be to increase epicotyl length, as well. In both density treatments, non-linear selection gradients were also positive for epicotyl elongation rates, RGR1 and RGR2, reflecting a monotonic relationship between fitness and these traits, rather than disruptive selection (Table 4C; Mitchell-Olds & Shaw, 1989).

Table 4.   Standardized linear (β′) and non-linear (γ′) selection gradients for epicotyl and bud traits (A), epicotyl elongation rates (B) and bud natality rates (C) in each density treatment. Bold type indicates effects significant at the P < 0.05 level after a sequential Bonferroni correction for multiple comparisons. Thumbnail image of

In contrast to the selection coefficients for epicotyl length traits, the magnitude of the selection coefficients associated with bud production was consistent with the phenotypic responses observed. All of the linear selection gradients estimated for plants exposed to low density, except for NB80, were found to be significantly positive and greater in strength than in plants grown at high density. Thus, the adaptive plastic response to low density in P. omorika would be to increase the number of the buds formed. Since the majority of selection gradients reported in this study were non-zero regardless of environmental circumstances, this implies that the seedling performance was at suboptimal level in both density environments. It should be noted, however, that these estimates were obtained from the growth-room raised plants and as such may not reflect accurately the pattern of phenotypic selection operating in natural habitats of the species.

Although in sessile organisms such as plants, a longitudinally elongated growth habit at high density may confer a fitness advantage in asymmetric competition for light, evolution toward the optimal shade-avoidance phenotype in various density environments could be constrained if there is an energetic cost to being plastic. This study found scant evidence for plasticity costs either at high- or at low-density conditions. At high density, none of the regression coefficients were significantly negative (Table 5), indicating that in P. omorika the production of shade avoidance phenotypes in response to neighbour proximity is not costly. Surprisingly, in seedlings grown at low density, epicotyl length 20 (EL20) did incur energetic costs (Table 5). Moreover, a significant positive interaction term obtained by the regression analysis indicates that the production cost will increase with the value of this trait.

Table 5.   Regression analysis testing for plasticity costs in Picea omorika seedling traits to conspecific density. Bold type indicates regression coefficients significant at the P > 0.05 level after a sequential Bonferroni correction for multiple comparisons. Thumbnail image of

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

Photomorphogenic responses to density

The phenotypic responses of P. omorika seedlings to conspecific density were consistent with functional arguments postulated in the adaptive plasticity hypothesis for shade avoidance: epicotyl length trait values tended to increase, while axillary bud production tended to decrease at high- relative to low-density conditions. Similar plastic responses to density have been observed in many other plant taxa (Weiner et al., 1990; Weiner & Thomas, 1992; Dudley & Schmitt, 1996; Donohue & Schmitt, 1999), and physiological mechanisms underlaying most density-induced developmental modifications are well explained (Ballaréet al., 1990; Schmitt et al., 1995; Dudley & Schmitt, 1996). Among the shade avoidance responses to crowding or vegetation shade, enhanced stem elongation rate at high density is hypothesized to be one of the classic examples of adaptive phenotypic plasticity (Schmitt et al., 1995; Dudley & Schmitt, 1996; Schmitt, 1997).

In the present study, the measurement of phenotypic selection in different density treatments provided evidence that the estimated values of selection coefficients for epicotyl elongation traits in P. omorika seedlings were inconsistent with the phenotypic responses observed. The strength of directional selection on epicotyl length was greater at low than at high density, suggesting that elongated seedlings confered a higher relative fitness than suppressed seedlings even at low density. The observed results opposed the predictions of the shade avoidance hypothesis that enhanced epicotyl elongation is an adaptive plastic response specific to higher densities. Because phenotypic plasticity to crowding or vegetation shade can be elicited either by light availability and/or by a specific environmental cue (the R:FR ratio perceived by phytochrome), these contrasting results might be ascribed to the changes in the radiation environment to which the growing seedlings were exposed. Recent analyses of the plant growth in even-aged populations revealed that plants begin responding to the presence of their neighbours by an increased rate of stem elongation well before mutual shading occurs (Ballaréet al., 1987, 1990). This early reaction to the neighbour proximity is triggered by FR radiation reflected from the foliage of adjacent plants (Ballaréet al., 1990). Field measurements of reflection signals from the artificial canopies of several plant species showed that, in general, the back-scattered radiation exhibited a substantial depletion in the visible (400–700 nm) and a large enrichment in the far-red (>700 nm) portion of the daylight spectrum, whereas its R:FR ratio increased gradually with the distance from the foliage canopy (Smith et al., 1990; Ritchie, 1997). Under light conditions approximating those prevailing in the present study, the radiation reflected from a cohort of 40-cm-tall P. omorika seedlings displayed an appreciable increase in R:FR (from 3.6 at 0 cm to 8.03 at 45 cm) out of its foliage canopy (B. Tucić, personal observation). Ballaréet al. (1987, 1990) argued that the FR radiation reflected from the leaves of adjacent plants plays a role of ‘an early warning signal’ for the presence of nearby future competitors. In even-aged stands of very low densities (leaf area index < 1), the R:FR ratio of light impinging laterally at the internode surface is found to be the principal environmental factor controlling plastic stem elongation (Ballaréet al., 1987, 1990). Recently, Pigliucci & Schmitt (1999) formulated the hypothesis that shade avoidance represents the default developmental state in plants under suboptimal light conditions, i.e. stem elongation will proceed until individuals experience R:FR ratios similar to those prevailing under full sunlight. This prediction conclusively means that the light-sensitive plasticity genes express their biological activity exclusively in high-R:FR/high-irradiance environments (Pigliucci & Schmitt, 1999). If the Pigliucci–Schmitt’s adaptive shade avoidance hypothesis is correct, then the phenotypic variation in epicotyl elongation traits of P. omorika at low density could be interpreted as an early shade avoidance response to the FR light scattered from neighbouring plants. This conclusion supports the relatively convergent reaction norms for epicotyl elongation rate (RGR2; Fig. 1) – a trait central to light exploitation – expressed by different P. omorika families in response to low-density conditions. Plastic convergence in response to low R:FR was also demonstrated in experiments with photomorphogenic mutants of Arabidopsis thaliana, where all genotypes – mutants affected in light perception and plastic wild-type plants – shared a similar shade avoidance phenotype (Pigliucci & Schmitt, 1999). Uniform phenotypic responses by diverse genotypes to a particular environmental stress are frequently detected in traits that contribute directly to functional adjustment to limiting environment conditions (Sultan, 1987; Sultan & Bazzaz, 1993). The establishment of P. omorika seedlings in their natural habitats occurs during the early stages of vegetation succession – usually during the second year after a forest fire (Čolić, 1966). Within the early successional understories, the variability in available light occurs at lower vegetation strata, where a slight difference in plant height often results in a large difference in light energy acquisition. In the face of such light conditions, the capacity of P. omorika seedlings to sense and respond to localized changes in the R:FR ratios by accelerating the rate of stem elongation might be indeed of principal importance for their success in the competition for radiant energy.

Correlation patterns among light-sensitive traits

Morphological plasticity to conspecific density in P. omorika seedlings involved concurrent responses of both epicotyl elongation and axillary bud traits. With the exception of few negative correlations, which encompassed bud natality rates (BRN1 and BRN2) in both density treatments, the majority of significant phenotypic correlations appeared to be positive (Table 6). The mean correlation values computed as the averages of all the bivariate phenotypic correlations within or between suites of related traits were relatively higher among the epicotyl length traits (EL10, EL20, EL30) and the bud number traits (NB80, NB120, NB160) than the correlations between these trait groups (e.g. mean r=0.494, 0.782 and 0.522 at low density vs. 0.507, 0.733 and 0.487 at high density, among epicotyl, among bud, and between epicotyl and bud traits, respectively).

Table 6.   Phenotypic correlations between trait pairs in low- (above diagonal) and high- (below diagonal) density conditions. Bold type indicates correlations significant at the P < 0.05 level after a sequential Bonferroni correction for multiple comparisons. Thumbnail image of

Apart from three negative correlations evaluated under high-density conditions which involved RBN2, most of the statistically significant genetic correlations appeared also to be positive, independent of density treatment (Table 7). At low density, the mean genetic correlations for the suites of epicotyl length and bud number traits were greater than the mean correlation computed between all pairs of these traits (0.619, 0.838 and 0.555, respectively). A similar trend was observed under high-density conditions (mean r=0.515, 0.747, and 0.435 within epicotyl, within bud, and between epicotyl and bud traits, respectively).

Table 7.   Genetic (family mean) correlations between pairs of traits in low- (above diagonal) and high- (below diagonal) density conditions, and variance component genetic correlations between the same trait expressed in low and high density (on the diagonal, underlined). Bold type indicates correlations significant at the P < 0.05 level after a sequential Bonferroni correction for multiple comparisons. Thumbnail image of

Plastic responses for the majority of seedling traits were also genetically correlated with each other (Table 8). The degree of plasticity integration was the highest among the bud number traits (mean plastic correlation r=0.630) regardless of seedling age, while the plastic responses of epicotyl length traits were stronger at later stages of seedling development, i.e. between 20th and 30th week after germination (plastic correlation r=0.608 between EL20 and EL30; Table 8). Plastic correlations between epicotyl length traits and bud number traits were lower than the correlations within each of these groups, with a mean value of r=0.431. Interestingly, plastic responses of epicotyl growth rates (RGR1 and RGE2) were not genetically correlated (Table 8).

Table 8.   Plastic correlations between trait pairs, and correlations between the family mean in a focal environment (low density or high density) and the level of plasticity across density environments. Bold type indicates correlations significant at the P < 0.05 level after a sequential Bonferroni correction for multiple comparisons. Thumbnail image of

Correlation patterns among P. omorika seedling traits presented in this study are consistent with the prediction that traits whose phenotypic responses are induced by a common environmental factor should be more phenotypically integrated than those that do not share such sensitivity (Berg, 1960; Armbruster et al., 1999; Donohue & Schmitt, 1999). Morphological responses to density or vegetation shade may be elicited either by resource availability or by the R:FR environmental cue perceived by phytochrome. These alternative causes of phenotypic plasticity may affect differently the amount of plasticity expressed by each of the light-sensitive traits, as well as the degree of trait integration within and across density environments. Traits whose plasticities are strongly phytochrome mediated in response to the R:FR cue are expected to exhibit similar plasticity and to be tightly correlated themselves, regardless of the density environment (Donohue & Schmitt, 1999). Moreover, in many situations, traits with plasticity to density mediated by a common environmental signal also share a common genetic mechanism for the response within density environments. For example, Donohue & Schmitt (1999) have observed in the annual plant Impatiens capensis that stem elongation responses (hypocotyl, first and second internode length and final height) were phytochrome mediated and tightly genetically coupled regardless of density. Conversely, meristem and allocation traits (number of buds, nodes, primary flowers and branches) were less strongly induced by the R:FR cue perceived by the phytochromes and their genetic correlation differed across densities. A similar pattern was revealed for length and non-length traits in Plantago lanceolata, a rosette plant that usually changes a whole suite of traits in accordance with density conditions prevailing within its natural habitats (Van der Toorn & Van Tienderen, 1992). In P. lanceolata, length-related traits tended to be positively correlated regardless of whether the variation resulted from the action of genetic or environmental factors, while their patterns of association with non-length-related traits appeared to be strongly environment-dependent (Van Tienderen & Van Hinsberg, 1996).

Because natural selection acts on the whole phenotype, not independently on individual traits, present patterns of genetic correlations in a population can suggest not only how past selection might have operated, but also indicate how selection acting on one trait will affect the evolutionary trajectory of the others. In P. omorika, positive genetic correlations among shade avoidance traits may cause some of the traits to evolve away from their optimal values, even if the whole seedling phenotype is undergoing adaptive evolution. Obviously, in order for adaptive plasticity to evolve, genetic variation for plasticity within populations must be available (Via & Lande, 1985). In this study, variance component genetic correlations between trait expressions in different densities were used as a measure of the additive genetic variation for phenotypic plasticity (Fry et al., 1996). According to Via & Lande (1985), any across-environment genetic correlation which is less than unity may be taken as evidence for the presence of genetic variation for phenotypic plasticity in that trait. Estimates of genetic correlations across densities were significantly different from one for the majority of the seedling traits studied (Table 7), indicating the existence of heritable variation within reaction norms of these traits. However, since all the across-environment correlations were positive in sign and high in magnitude, this conclusively means that the level of the additive genetic variation for plasticity in the shade-avoidance traits of P. omorika is rather low.

In this study, a correlation analysis was used to test the developmental range hypothesis which predicts that the most extreme values of a trait (family mean) will be produced by the least plastic genotypes (DeWitt, 1998). A negative correlation between the family means within one of the environments and the degree of plasticity across environments would support the view that plasticity limits developmental range. In P. omorika, correlations between the family means and the level of plasticity were strongly density-dependent for every pair of the seedling traits analysed. At low density, plasticity and trait means were significantly correlated for all but three of the seedling traits, EL30, RGR1 and RGR2 (Table 8), indicating that the fastest growing genotypes and those having the longest epicotyls did not neccessarily display the highest plasticity, at least in this stage of P. omorika development. Under high density, however, the significant correlation between trait mean and plasticity were detected for RBN1, RBN2 and RGR2 (Table 8). These results strongly suggest that the degree of plasticity to shading and the developmental range of shade avoidance traits in P. omorika seedlings are not traded-off in any of the density environments.

Costs of penotypic plasticity

Although the benefits of adaptive phenotypic plasticity are obvious, the evolution of optimal reaction norms in response to heterogeneous selection may be constrained if the ability to be plastic entails some costs for an organism (Van Tienderen, 1991; DeWitt et al., 1998; DeWitt, 1998; Scheiner & Berrigan, 1998). Recent empirical data concerning this issue are very scarce and suggest that plasticity costs are small and hardly detectable (Dudley & Schmitt, 1996; DeWitt, 1998; Scheiner & Berrigan, 1998; Tucićet al., 1998). In the present study, a significant negative regression coefficient associated with the plasticity term was found only for one of the shade avoidance traits, epicotyl length 20 (EL20), in P. omorika seedlings exposed to low density. The regression coefficient for the interaction term between the value of EL20 and its plasticity was also significant, but positive. Following current distinctions among various costs of plasticity (DeWitt, 1998; Scheiner & Berrigan, 1998), the observed results could be taken as an indication for the presence of both maintenance costs (i.e. the energetic and material costs of maintaining sensory and regulatory mechanisms which produce plasticity) and additional production costs (i.e. the cost of producing an inducible phenotype) of phenotypic plasticity. Dudley & Schmitt (1996) also revealed an intrinsic cost of stem height in Impatiens capensis plants grown in a low-density environment, independent of selection on morphology. The reduced fitness of elongated relative to non-elongated phenotypes in low density was attributed to the opportunity costs of suppressed branching in elongated individuals (Geber, 1990). In P. omorika, the cost of epicotyl elongation in low density could be related to the cost of biomass allocation to stem tissue (Givnish, 1982; Casal & Schmitt, 1989) and decreased mechanical strength of elongated plants (Niklas, 1992). With respect to other shade avoidance traits of P. omorika, no evidence for either production or maintenance costs of plasticity was observed. Our inability to reveal costs was not due to a lack of genetic variation in trait means and/or reaction norms. Probably, allelic variants conferring costly plasticity in this species are selectively replaced by alleles promoting the plasticity that is less expensive or without costs, as has been already suggested (see DeWitt et al., 1998; DeWitt, 1998).

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References

We are grateful to Dr Susan J. Mazer and an anonymous reviewer for making numerous valuable comments on a previous version of the manuscript.

References

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  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
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