Plant High

Plant Ecology ***: ** ***, ****.

**** ****** ******** **********. ******* in the Netherlands.

Effects of clonal integration on plant plasticity in Fragaria chiloensis

Peter Alpert

Department of Biology, University of Massachusetts, Amherst, MA 01003-5810, USA

(e-mail: *******@***.*****.***)

Key words: Light, Nitrogen, Phenotypic plasticity, Physiological integration

Abstract

The ability of clonal plants to transport substances between ramets located in different microsites also allows them

to modify the plastic responses of individual ramets to local environmental conditions. By equalising concentrations

of substances between ramets, physiological integration might decrease responses to local conditions. However,

integration has also been observed to increase plasticity and induce novel plastic responses in ramets. To ask

how integration modi es plant plasticity in the clonal herb, Fragaria chiloensis, ramets were given either low

light and high nitrogen or high light and low nitrogen, simulating a pattern of resource patchiness in their native

habitat. Ramets in contrasting light/nitrogen treatments were either connected or single. Effects of light/nitrogen

and connection were measured at three levels of morphological organisation, the organ, the ramet, and the clonal

fragment. Connection between ramets reduced or had no effect on plastic responses in leaf size at the level of

the plant organ. This suggested that integration dampened certain plastic responses. Connection induced a new

plastic response at the level of the clonal fragment, an increase in allocation to vegetative reproduction in patches

of low light and high nitrogen. It is concluded that clonal integration can have different effects on plant plasticity

at different levels of plant organisation. It appears that, at least in this species, integration can increase plasticity at

the level of the clonal fragment and concentrate vegetative reproduction in particular microsite types.

Introduction The ecological patterns and physiological mechanisms

that may explain these effects of clonal integration on

In plants, phenotypic plasticity (i.e., environmentally plant plasticity remain largely unknown (Hutchings &

induced variation in growth or development; Brad- Price 1993; Hutchings & Wijesinghe 1997). Of partic-

shaw 1965; Scheiner 1993) is often modi ed by phys- ular interest is whether integration modi es plasticity

iological integration between modules (de Kroon & in ways that increase plant tness.

Hutchings 1995; Sachs & Hassidim 1996; Hutchings Perhaps the most obvious prediction concerning

1997; Sachs & Novoplansky 1997). The transport of clonal integration and plant plasticity is that inte-

substances between organs and branches in non-clonal gration will reduce plasticity. Clonal integration has

plants, and between ramets in clonal plants, frequently generally been observed to tend to equalise concen-

modi es the plastic responses of individual modules trations of substances among ramets (Marshall 1990;

to the conditions that they experience directly. In dif- Hutchings & Wijesinghe 1997). This should cause

ferent cases, clonal integration has been observed to connected ramets to experience more similar inter-

reduce, enhance, or induce novel plastic responses at nal conditions and could even out their responses to

various levels of morphological organisation: organs, different microsites. Several studies have con rmed

ramets, and clonal fragments (groups of connected this prediction in particular species. For example,

ramets). In a given species, integration may affect Dong (1995) showed that connection between ram-

some plastic responses but not others, and modify ets reduced the plastic responses of individual leaves

plasticity at one level of morphological organisation and ramets to different light levels in a species of

but not at another (Turkington et al. 1991; Dong 1995). Lamiastrum. Physiological integration may also be re-

100

sponsible for reduced self-thinning and size hierarchy garia chiloensis was collected from a natural popu-

in clonal plants (e.g., Hutchings & Bradbury 1986), al- lation on coastal sand dunes at Franklin Point, A o

though this phenomenon has been called into question Nuevo State Reserve, California, and propagated veg-

(de Kroon & Kalliola 1995). etatively in the Morrill Greenhouses at the University

However, clonal integration can also enhance plas- of Massachusetts in Amherst. Pairs of adjacent, con-

ticity and induce new plastic responses (Hutchings & nected, sibling ramets produced in the greenhouse

Price 1993; Dong 1995; Hutchings 1997). A well- were potted in acid-washed sand, with each ramet in

documented example of enhanced plasticity in certain a separate pot.

species is plasticity of internode length in stolons and Three factors light/nitrogen, connection, and

rhizomes (de Kroon & Hutchings 1995). In a Hydro- age were crossed in a full factorial design, with

cotyle species, clones produced longer rhizome intern- two levels per factor (Figure 1). To test the effect of

odes in low light only when ramets in low light were light/nitrogen, one of the ramets in each pair was given

connected to ramets in high light (Evans & Cain 1995). a high level of light (100% of ambient) and a low

In Eichhornia crassipes (C. Martius) Solms-Laubach level of nitrogen (3 mg N/L modi ed Hoagland s so-

(nomenclature follows Hickman 1993), ramets in high lution (Friedman & Alpert 1991)); the other was given

light produced longer stolon internodes only when a low level of light (20% of ambient, using neutral

connected to ramets given low light with a low red:far shade cloth) and a higher, but still moderate level of

red ratio (M thy et al. 1990). nitrogen (10 mg N/L modi ed Hoagland s solution).

A well-documented example of a novel plastic Where useful for brevity, light/nitrogen is sometimes

response induced by clonal integration is a reversal referred to simply as light . Effects of light/nitrogen

in patterns of root:shoot allocation that appears to treatments could of course be due to light, to nitrogen,

produce a functional division of labour between con- or to both. These light/nitrogen treatments were cho-

nected ramets (Stuefer et al. 1996; Alpert & Stuefer sen to simulate a natural resource pattern. At Franklin

1997). Typically, plants allocate a relatively high pro- Point, nitrogen- xing shrubs create a negative correla-

portion of biomass to roots when water or nutrients tion between light and nitrogen availability (Alpert &

limit growth more than light does. In at least eight Mooney 1996).

species of clonal plants, ramets limited by water or nu- Each ramet was given 100 ml of modi ed

trients allocate a relatively low proportion of biomass Hoagland s solution every 10 12 days. This amount

to roots if they are connected to ramets with abundant was chosen so that solution drained freely from the

water and nutrients. bottom of the pots after addition, helping to ush the

The objective of this study was to test for ad- soil and avoid nutrient accumulation over time. Occa-

ditional ways in which clonal integration can mod- sionally, the soil in one or more pots would become

ify phenotypic plasticity under realistic environmental dry on the surface between additions of nutrient solu-

conditions at different levels of morphological organi- tion. When this occurred, all plants were watered with

sation, using the stoloniferous herb Fragaria chiloen- 100 ml of distilled, deionized water. This was done to

sis (L.) Duchesne. Integration has already been shown avoid water stress to the plants and to help keep water

to modify root:shoot ratio but not stolon internode from being a signi cant factor in treatments. It may

length in this species (Alpert & Mooney 1986; Fried- also have slightly reduced differences between nutri-

man & Alpert 1991). Additional modi cations were ent levels. Results may thus understate plant responses

sought in leaf architecture and in allocation of mass to nutrient levels.

within and between ramets. Ramets were subjected to To test the effect of connection, and by inference

different levels of light and nitrogen availability. The of integration, the stolon connecting the potted ram-

responses of connected and single ramets were com- ets in a pair was severed in half of the pairs in each

pared in organs (individual leaves), ramets, and clonal light/nitrogen treatment. The stolon was left intact in

fragments. the other pairs. The two connection treatments are

referred to as single and connected. To test for any

effect of relative ramet age, the older ramet in a pair

Methods was given the low light/high nitrogen treatment in half

of the pairs in each connection treatment, and the

Methods were generally similar to those used by younger ramet was given the low light/high nitrogen

Friedman and Alpert (1991). A single clone of Fra- treatment in the other pairs.

101

Figure 1. Experimental scheme, showing crossed light/nitrogen and connection treatments and the de nition of rooted ramets and offspring.

Age treatments are not shown; see text.

Table 1. Signi cance (three-way, xed factors ANOVA: P ) of effects of light/nitrogen, connection, and relative ramet age on leaf form and

biomass of Fragaria chiloensis. Values 0.02 are in bold. Values for three-way interactions were >0.19.

Effect Maximum Maximum Speci c leaf area Speci c leaf Mass of Mass of Total

blade length petiole length of rooted ramet area of offspring rooted ramet offspring mass

(text only) (text only) (Figure 2) (Figure 2) (Figure 4) (Figure 4) (Figure 4)

0.2

Light/N 0.14 0.16

>0.2 >0.2 >0.2

Connection 0.18 0.02 0.001 0.09

Light/N connection >0.2 >0.2 0.2 >0.2 >0.2 >0.2 >0.2 >0.2

Age 0.19

Age light/N >0.2 >0.2 >0.2 >0.2 >0.2

0.14 0.10

Age connection >0.2 >0.2 >0.2 >0.2 >0.2 0.02 0.004

Twenty replicates of each treatment were arranged harvested and separated into rooted ramet and off-

in a random block design on a single bench in the spring. Leaves were detached, the length of the termi-

centre of the greenhouse. The experiment was run for nal lea et was measured, and leaf area was measured

six months. During the experiment, many of the pot- with a leaf area meter (LiCor, Inc.). All plant parts

were then dried to constant mass at 60 C and weighed

ted ramets produced new stolons and offspring ramets

(Figure 1). These are referred to as offspring; the orig- for total dry mass of rooted ramet and offspring.

inal potted ramets are called rooted ramets. Offspring Regressions of length on area of the leaves har-

were given the same light treatment as their parent, left vested at the end of the experiment were calculated

connected to the parent, and not allowed to root. This for each light/nitrogen treatment using the model,

log(area) = a +b(log[length]). These regressions were

was also intended to simulate natural conditions. Most

growth of new stolons takes place during the summer used to estimate leaf areas from leaf lengths measured

dry season at Franklin Point, and ramets will not root during treatment. A regression based on leaves of ex-

in dry sand. tra ramets harvested at the start of the experiment was

Leaf areas of ramets at the start of treatment and used to estimate leaf areas at the start of treatment. For

all regressions, R 2 exceeded 0.9. Net change in the

after one and three months of treatment were esti-

mated non-destructively from the length of the termi- leaf area of a ramet was calculated as the difference

nal lea et of each non-senescent leaf of each ramet between its estimated leaf areas at different times.

(see below). At the end of the experiment, plants were

102

Table 2. Signi cance (repeated measures, two-way ANOVA: P ) of the

effects of light/nitrogen and connection over time on the total leaf area

and rate of net change in leaf area of ramets of Fragaria chiloensis. Values

0.02 are in bold.

Effect Leaf area Net change in leaf area

0.2

Connection 0.08

Time light/N 0.2 0.14

Light/N connection >0.2 >0.2

Time light/N connection >0.2 >0.2

Statistical tests were performed with SYSTAT

(Wilkinson 1990). Proportional data were transformed

to the arcsine of the square root before analysis.

ANOVA models are described along with their results

in Tables 1 and 2.

Results

At the level of the individual leaf, low light/high ni-

trogen induced three typical plastic responses to low

Figure 2. Speci c leaf area (cm2 area/g dry mass of leaf blades)

light: longer petioles, larger blades, and higher speci c

of ramets of Fragaria chiloensis in different light/nitrogen and con-

leaf area (area per unit of leaf dry mass). All three re- nection treatments. (a) offspring. (b) rooted ramets. Mean SE. See

sponses were highly statistically signi cant (Table 1: Table 1 for tests of signi cance.

effect of light/nitrogen). Connection did not signi -

cantly modify plasticity in petiole or blade length but

signi cant (Table 2). Connection did not signi cantly

did affect plasticity in speci c leaf area of leaves on

modify these effects (Table 2: interactive effects of

offspring (Table 1: effect of light/nitrogen connec-

connection). However, there was a nominal trend over

tion). Connection decreased the difference in speci c

time towards an increased response in net change in

leaf area of leaves grown in high and low light by

leaf area to light/nitrogen in connected ramets (Fig-

about 75% in offspring and about 50% in rooted ram-

ure 3b). During months 3 6, ramets lost more leaf area

ets (Figure 2). Connection thus reduced or had no

in high light and gained more area in low light if they

effect on the plastic responses observed at the level of

were connected than if they were single.

the plant organ.

At the level of the clonal fragment, connection

At the level of the ramet, plants given low

induced a suite of plastic responses to light/nitrogen

light/high nitrogen showed another typical plastic re-

that reversed a plastic response seen in single ram-

sponse to low light, an increase in total leaf area.

ets, increased the difference in growth between ram-

Over time, ramets in low light/high nitrogen devel-

ets, and concentrated vegetative reproduction in low

oped greater standing leaf areas and more positive

light/high nitrogen patches by shifting the alloca-

rates of net change in leaf area than ramets given high

tion of biomass from rooted ramets and offspring in

light/low nitrogen (Figure 3). Ramets in high light

high light/low nitrogen to offspring to low light/high

showed no net increase in leaf area after the rst month

nitrogen. In fragments with single ramets, the bio-

of treatment; the addition of leaf area on new leaves

masses of rooted ramets and offspring were lower in

was approximately balanced by the loss of leaf area on

low light/high nitrogen than in high light/low nitro-

leaves that died. The effects of light/nitrogen and time

gen (Figure 4a). Connection reversed this, so that

on leaf area and net change in leaf area were highly

103

Figure 3. Total leaf area of ramets of Fragaria chiloensis in differ- Figure 4. Mean nal biomass of ramets of Fragaria chiloensis

ent light/nitrogen and connection treatments: low light/high nitrogen in different light/nitrogen and connection treatments. (a) across

thick line over bars; high light/low nitrogen no line over bars); c age treatments. (b) within age treatments: O older ramet; Y

connected; s single. (a) mean leaf area of rooted ramets and off- younger ramet. I-bars show SE for total (upward) and components

spring after 1, 3, or 6 months of treatment; I-bars show SE for total (downward). See Table 1 for tests of signi cance.

(upward) and components (downward). (b) mean rate of net change

in total leaf area SE. See Tables 1 and 2 for tests of signi cance.

older ramet was given high light/low nitrogen and

the younger ramet was given low light/high nitrogen

the biomasses of rooted ramets and offspring were

(Figure 4b). In high light/low nitrogen, connected

higher in low light/high nitrogen. Connection also

ramets had about 50% less biomass than single ramets

doubled the absolute difference between offspring

when ramets were older, compared to about 20% less

masses in the two light/nitrogen treatments, from a

when ramets were younger. In low light/high nitrogen,

two-fold difference in fragments with single ramets

connected ramets had about 40% more biomass than

to a four-fold difference in fragments with connected

single ramets when ramets were younger, compared

ramets. As a result, fragments with connected ramets

to about 20% when ramets were older. Interactions

had about 80% of their biomass of offspring in low

between effects of age and connection were signi cant

light/high nitrogen, compared to 30% in fragments

for both biomass of offspring and total mass of rooted

with single ramets. Interactions between connection

ramet and offspring combined (Table 1).

and light/nitrogen treatment were highly signi cant

The overall growth of fragments, measured by nal

for mass of rooted ramets, mass of offspring, and their

biomass of all ramets in a fragment combined, was

total (Table 1). Connection thus altered and increased

not affected by connection. Fragments accumulated

plasticity at the level of the clonal fragment.

slightly less mean total biomass when rooted ramets

The relative age of the rooted ramets in the dif-

were connected (5.06 g) than when they were sin-

ferent light/nitrogen treatments affected the degree to

gle (5.46 g). However, the effect of connection on

which connection modi ed plasticity at the fragment

level. Effects of connection were stronger when the

104

the total mass across light/nitrogen treatments was not by Alpert & Mooney (1996) at the site where the

signi cant (Table 1: effect of connection). Fragaria was collected showed that ramets growing

Connection did decrease the amount of biomass under the common nitrogen- xing shrub, Lupinus ar-

that fragments accumulated in their rooted ramets boreus Sims, where light strongly limits their growth,

and may have increased the amount of biomass that could increase their light interception by about 50%

fragments accumulated in offspring. The net nega- by producing longer petioles. Reducing plastic re-

tive effect of connection on the biomass of rooted sponses that tend to increase light capture, such as

ramets was due to the low biomass of rooted ramets increase in high speci c leaf area, should reduce plant

in high light/low nitrogen in the connected treatment performance in this microhabitat. This is the same mi-

(Figure 4a). The possible net positive effect of con- crohabitat after which light and nitrogen levels in the

nection on vegetative reproduction was due to the low light/high nitrogen treatment were modeled. The

high biomass of offspring in low light/high nitrogen reduction of leaf plasticity by clonal integration seen

in the connected treatment (Figure 4a). The effect of in the experiment should therefore reduce plant perfor-

connection on mass of rooted ramets was highly sig- mance under lupines in nature and may be a negative

ni cant, while the effect on mass of offspring was only consequence of integration rather than an advantage.

marginally signi cant (Table 1: effect of connection). In contrast to its effects on the plasticity of indi-

vidual leaves, clonal integration increased plasticity at

Summary of results the level of the clonal fragment in Fragaria chiloensis.

Integration caused growth and allocation to become

In the absence of connection between ramets, Fra-

less, not more, equal among connected ramets and

garia chiloensis showed at least ve responses to low

concentrated vegetative reproduction in experimental

light/high nitrogen (Table 3). Connection, and by in-

microsites where light was relatively low and nitrogen

ference clonal integration, modi ed these responses in

was relatively high. The mechanism of this response is

two main ways. At the level of the plant organ, clonal

unknown, but it is likely that nitrogen rather than light

integration decreased or did not signi cantly affect

is the primary resource involved, for three reasons.

plastic responses observed in leaves. At the level of

First, the response was greater when the rooted ramet

the clonal fragment, integration reversed and increased

in low light/high nitrogen was younger than the ramet

plastic responses. The main effect of integration at the

in high light/low nitrogen. In many clonal plants (Mar-

fragment level was to shift the allocation of biomass

shall 1990), including Fragaria chiloensis (Alpert &

within fragments from ramets in high light/low nitro-

Mooney 1986, Alpert 1996), nitrogen is transported

gen to offspring in low light/high nitrogen. This con-

mainly from older to younger ramets, whereas carbon

centrated vegetative reproduction in a low light/high

transport is less constrained by direction. Second, ni-

nitrogen patch.

trogen import from other ramets appears speci cally

to promote growth of new stolons and offspring ram-

ets in Fragaria (Alpert 1991, 1996). Third, there is

Discussion

some evidence that ramets of Fragaria given high ni-

trogen will import carbohydrates from ramets given

In Fragaria chiloensis, integration either failed

low nitrogen, even when light levels are uniform, and

to change or reduced the plastic responses to

that this can shift the allocation of biomass within

light/nitrogen observed at the level of the plant organ,

fragments to ramets in more fertile microsites (Alpert

at least in leaves. This could be common in clonal

1996).

plants. Similar results have been reported in at least

Whether the ability to allocate more biomass to

three other clonal species (Turkington et al. 1991;

vegetative reproduction in low light/high nitrogen

Dong 1995), and there seem to be few, if any, reported

patches increases plant performance in Fragaria is un-

cases in which clonal integration increases plasticity

certain. Two reasonable measures of the performance

in individual leaves.

of clones are total growth and reproduction. In this

The observed reduction of plasticity in speci c leaf

experiment, connection and its accompanying effects

area by clonal integration in Fragaria is probably not

on allocation of biomass failed to increase the total

an adaptive response. The plastic responses in leaf

growth of fragments, as measured by accumulation of

form seen in the single ramets of Fragaria exposed to

total biomass; and only marginally increased their re-

low light and high nitrogen (Table 3) seem likely to in-

production, as measured by accumulation of biomass

crease light capture. For example, light measurements

105

Table 3. Summary of plastic responses to low light/high nitrogen observed in Fragaria chiloensis, and their

modi cations by clonal integration. Responses and modi cations likely to increase plant performance are italicized.

Level Response without clonal integration Modi cation by clonal integration

Organ increase in speci c leaf area decrease in response

increase in petiole length none

increase in blade length none

Ramet increase in total leaf area nominal increase in response

Clonal fragment decrease in biomass reversal of response, greater when ramet is younger

(no signi cant response in allocation to new response: increase in allocation to offspring,

offspring) greater when ramet is younger

in offspring. Nor is it clear why it should be more ad- Beckwith and Monika Johnson for greenhouse man-

vantageous to concentrate vegetative reproduction in agement; and Christopher Marshall and Elizabeth

low light/high nitrogen patches than in high light/low Price for organising the 1997 clonal plant biology

nitrogen patches or than to reproduce in both types of workshop. Research was supported by U.S. National

patch equally. Science Foundation Grant IBN-9507497.

Overall, these results con rm the general observa-

tion that clonal integration can both decrease and in-

crease plant plasticity (Hutchings & Price 1993). They References

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