Annals of Botany 86: 739±744, 2000doi:10.1006/anbo.2000.1233, available online at http://www.idealibrary.com on

Stem Elongation Response to Neighbour Shade in Sprawling and UprightPolygonum Species

BEVERLY COLLINS* and GARY WEIN

Savannah River Ecology Laboratory, P. O. Drawer E, Aiken, South Carolina 29802, USA

Received: 27 March 2000 Returned for revision: 2 May 2000 Accepted: 13 June 2000 Published electronically: 10 August 2000

tolerant or

0305-7364/0

* For corre

Shading by neighbouring plants, which reduces energy for photosynthesis and lowers the ratio of red : far red light,can trigger a stem elongation or `overtopping' response in herbaceous plants. We compared the stem elongationresponse of two Polygonum species in a greenhouse experiment. P. sagittatum, a sprawling, vine-like annual, andP. hydropiperoides, an upright perennial, were grown from seeds at three levels of neighbour shade produced bycrowding a cohort of real neighbour plants or adult-sized fake neighbour plants that provided shade and reduced thered : far-red ratio. We hypothesized that the annual would show a more pronounced elongation response todeveloping or adult neighbour shade because vine-like plants are less mechanically constrained to remain upright andself-supporting. Internodes on stems of both P. sagittatum and P. hydropiperoides increased in length as the amountof shading by real or fake neighbours increased. P. sagittatum climbed on adjacent plants, and had longer stems withmore nodes than those of P. hydropiperoides. Although both P. sagittatum and P. hydropiperoides tended to elongatewith crowding, the greater elongation response to both real and fake neighbours shown by the sprawling annualre¯ects its ability to extend upward into a canopy beyond self-supporting height. Self-supporting P. hydropiperoidescan extend upward with, or overtop, cohort neighbours, but might less readily elongate into an extant canopy. Indense stands, P. sagittatum can become structurally dependent on close P. hydropiperoides neighbours. Our resultssuggest that the elongation response of P. sagittatum to neighbours can contribute to structural dependence and couldfacilitate coexistence of these species. # 2000 Annals of Botany Company

Key words: Neighbour e�ects, light variation, red : far-red, plant growth strategy, Polygonum sagittatum, Polygonumhydropiperoides, smartweeds.

INTRODUCTION

Shading by neighbouring species reduces energy forphotosynthesis and lowers the ratio of red : far red light(Me thy et al., 1990; Aphalo and Ballare , 1995). In greenplants, phytochromes can mediate a shade avoidanceor `overtopping' response to neighbours. This responseincludes stem elongation (Ballare et al., 1987; Smith, 1990;Aphalo and Ballare , 1995; Ballare and Scopel, 1997),decreased branching and tillering (Aphalo and Ballare ,1995; Ballare and Scopel, 1997), increased apical dom-inance (Aphalo and Ballare , 1995), and decreased rootgrowth (Fitter and Ashmore, 1974).

The stem elongation response to neighbour shade variesamong species and can be an adaptive phenotypic response(Schmitt and Wul�, 1993; Schmitt et al., 1995; Maliakalet al., 1999). Crowded plants of some species are suppressed(Collins and Wein, 1993); others grow away from neigh-bours (Novoplansky et al., 1990). Crowded plants of somesalt marsh species grow taller and thinner and becomestructurally dependent on neighbours, which can facilitatecoexistence (Harley and Bertness, 1996). Plants of edges,margins, and other habitats that exhibit variable, ¯uctuat-ing, or unpredictable light environments can be broadly

able to adjust their stem growth strategy to light

0/100739+06 $35.00/00

spondence. Fax 803-725-3309, e-mail collins@srel.edu

variation (Smith, 1982; Veres and Pickett, 1982; Matlack,1993; Bazzaz and Wayne, 1994; Niinemets, 1998).

Plant morphology and growth form can in¯uence stemelongation response. Elongation to the point of structuralweakness has been shown in response to neighbour shade orcrowding in plants as architecturally di�erent as clonalrushes and grasses, succulent herbs, shrubs, and trees(Holbrook and Putz, 1989; Harley and Bertness, 1996).Sprawling and vine-like plants might represent one end ofthe elongation response spectrum. Such species are expectedto have fewer mechanical constraints on internode elonga-tion than species that maintain self-support. Neighbours cancue elongation into the canopy, and vines typically extendbeyond maximum self-supporting height when supportedon another plant (Gartner, 1991; Den Dubbelden andOosterbeek, 1995). Some vines have shown internodeelongation in response to shade or low red : far red light(r : fr). Speci®cally, three tropical leguminous vines hadgreater internode length when subjected to shade or shadeplus lowered r : fr (Lee, 1988). To our knowledge, however,no research has compared the elongation response betweensprawling, or vine-like, plants and other growth forms.

The objective of our research was to determine whetherdi�erences in elongation response to neighbour shadebetween two Polygonum species, the sprawling annualP. sagittatum L. and the upright perennial P. hydropiper-oides Michaux., agree with expectations based on observed

growth form di�erences and species interactions in the ®eld.

# 2000 Annals of Botany Company

m Stem Elongation Response

P. sagittatum and P. hydropiperoides coexist along lakeshoremargins at interplant distances so close that P. sagittatumoften sprawls over, and is supported by, P. hydropiperoides.We hypothesized that the more vine-like annual would havea greater elongation response to neighbour shade. We testedthe hypothesis in a greenhouse experiment by varyingneighbour shade through crowding. Neighbours were eitherreal plants of both species or `fake' plants, constructed ofwire and plastic. The real neighbours were planted as seedsalong with target individuals to examine target plantelongation response during canopy development. The fakeplant neighbours were the height of mature P. hydro-piperoides. They allowed us to vary canopy crowdinguniformly and to examine target plant elongation responseto an established neighbourhood with canopy shade and

740 Collins and WeinÐPolygonu

reduced r : fr.

measured.

FIG. 1. Layout of one set of conetainers to test neighbour crowdinge�ects on Polygonium stem growth. kl, d in rows 1±3 are targetP. sagittatum and P. hydropiperoides; s, fake or real neighbours.Species alternated within the target and real neighbour rows.Conetainer circles are enlarged; interconetainer distances are to scale.

MATERIALS AND METHODS

P. sagittatum and P. hydropiperoides seeds were collectedin October 1994 from a 30 m length of the shoreline ofL-Lake, a reservoir on the Savannah River Site near Aiken,South Carolina, USA. Seeds were strati®ed for 90 d at 48C.After strati®cation, seeds of each species were randomlyassigned to a set of conetainers2 (1.5 cm2 � 16 cm; Stueweand Sons, Inc., Corvalis, Oregon, USA) in an unshadedgreenhouse with no supplemental lighting. Conetainerswere arranged in ®ve rows, with 14 plants (seven of eachspecies in alternate conetainers) in each target row (Fig. 1).Neighbours were arranged to provide di�erent levels ofcrowding (shade) among the target rows. The outermostrow (3) of target plants had neighbours only on one side;this arrangement provided potential support of the sprawl-ing annual, but also allowed plants to escape neighbourshade by directional growth toward the open side.

Neighbour treatments were either real plants of eachspecies established from seeds and planted so that individ-uals of the two species alternated within a row, or `fake'plants constructed of green plastic-coated wire stems andcoloured cellophane (Fisher Scienti®c, S52570) leaves withtwo 1 cm � 3 cm `leaves' at each of ten nodes. Seeds of thereal neighbours were planted along with those of targetplants to establish a cohort of targets and neighbours. Thefake plants were 60 cm tall, the average height of matureP. hydropiperoides. They provided an established canopywith shade and reduced r : fr light. We used fake, rather thanreal, mature plants for two reasons: ®rst, these Polygonumspp. can show day-length dependent growth and ¯owering,so that it is not possible to grow neighbours that would bemature when target seeds were planted and that wouldpersist until the end of the experiment. Second, the fakeplants allowed us to establish more uniform neighbourcrowding treatments by reducing plant-to-plant variation incanopy shade due to individual growth of real plants.

Red(600±700 nm) : far red(700±800 nm) (Smith and Whitelam,1990) was measured with a spectroradiometer (Li-Cor18002 Lincoln, Nebraska, USA) equipped with a ®breoptic probe, in 4 mm band width increments. R : frdecreased from 1.12 in full greenhouse sunlight to 1.04

beneath the most dense canopy of fake plants. Full ®lm

cover changed the spectral distribution of transmitted lightonly between 600 and 800 nm, but reduced total photo-synthetically active radiation (PAR, 400±700 nm) to 57%full light. In natural Polygonum stands along the shorelineof L-Lake, r : fr ranged from 1.41 in full sun to 1.14 beneatha light canopy and 0.66 beneath a heavy canopy. PAR wasreduced to 10% of full sunlight beneath heavy Polygonumcanopy.

There were three sets of each neighbour type. This designyielded three replicate sets � two neighbour types (real,fake) � three shade levels per set (rows 1, 2, 3) � sevenplants per species per target row � 126 target plants of eachspecies. Each set was rotated 908 on the greenhouse benchbiweekly to prevent directional greenhouse shade e�ects ontarget plant elongation. Although they did intertwine witheach other during growth, target plants did not grow intoadjacent rows or lean on either real or fake neighbours asexpected. Target plants were harvested after 34 weeks at theonset of seed set in late September; internode length, stemlength, and individual leaf area were measured using anAgvision2 imaging system (Decagon Devices, Inc., Pull-man, Washington, USA) calibrated to 1 mm length or1 mm2 area. This computer-aided imaging system consistsof a videocamera suspended over a light box. Capturedtwo-dimensional plant images were counted, and length ofstems and internodes, and individual and total leaf area

Data analyses

All analyses were carried out using Statistical AnalysisSystem (SAS) v. 6.12. Analysis of variance (ANOVA, procGLM) was used to compare mean stem length, internodelength, leaf area, and nodes per stem between species,among row types (1, inner row; 2, middle two rows; 3, outerrows), and between real and fake plant neighbour treat-

ments. Treatments were considered ®xed e�ects. A split-

m

strip-plot design was used, with neighbour ( fake vs. real)treatment applied to the whole plot (tray); rows (1, 2, and 3)considered to be strips within trays; and species plantedwithin rows. Only species � row and species � neighbourinteractions were tested because the neighbour � rowinteraction was part of the experimental design and wasnot of biological interest. The three-way interaction(species � row � neighbour) was not signi®cant for anymeasure and was deleted from the analysis. Lengthmeasures were log10-transformed and count data werearcsine-square root-transformed if needed to conform tonormal distributions. Di�erences among means withine�ects were compared a posteriori by least signi®cantdi�erence (LSD); in the text, means within a treatmentwith the same superscript do not di�er signi®cantly.Regression (proc REG) of internode length on nodeposition was used to examine the stem elongation responseof each species to real or fake neighbours. Regression lineslopes were compared between treatments and betweenspecies at the 0.95% comparison level by the T0 method(Sokal and Rohlf, 1981). Slopes di�er if the comparison

Collins and WeinÐPolygonu

intervals do not overlap.

RESULTS

The sprawling annual P. sagittatum had longer stems withmore nodes and less leaf area per node than its perennial,more upright congener, P. hydropiperoides (Table 1, Fig. 2).Signi®cant species � neighbour type interactions in stem

length and internode length re¯ected the fact that the

TABLE 1. Signi®cance levels from ANOVAs that tested distem length (cm), node number, leaf area per node (cm), an

row pos

Degrees of freedom Sums of squares

Stem lengthSpecies 1 88 766Neighbour type 1 269Row 2 40 265Species � neighbour 1 14 222Species � row 2 8126

Number of nodesSpecies 1 651.01Neighbour type 1 15.11Row 2 37.24Species � neighbour 1 3.04Species � row 2 0.91

Leaf area/nodeSpecies 1 609.84Neighbour type 1 0.02Row 2 4.78Species � neighbour 1 5.74Species � row 2 8.69

Internode lengthSpecies 1 82Neighbour type 1 81Row 2 2476Species � neighbour 1 1958Species � row 2 414

elongation response to real vs. fake neighbours di�eredbetween species. P. hydropiperoides was taller and hadlonger internodes when grown with real compared to fakeneighbours; P. sagittatum was taller and had longerinternodes with fake compared to real neighbours(Table 1, Fig. 2).

The elongation response of both species was in¯uencedby the degree of crowding. In both species and with realand fake neighbours, stem and internode length tended tograde from long to short from the inner to outer rows.Mean stem length (cm) was 82.6a, 77.8a and 64.1b and meaninternode length (cm) was 11.4a, 9.6ab and 7.6b in rows 1, 2and 3, respectively (Table 1, signi®cant row e�ect; Fig. 2).A signi®cant species � row interaction re¯ects morepronounced and consistent gradation in P. sagittatumcompared to P. hydropiperoides (Table 1, Fig. 2).

Regression of internode length against relative nodeposition from the base of the stem was used to examine thestem elongation response of each species to neighbour typein rows 1 and 2, with data pooled over both rows and thethree trays in each neighbour type treatment. Row 3 plantswere omitted from the regression because they were notfully surrounded by neighbours. In both species with bothreal and fake neighbours, internode length showed asigni®cant negative relationship to node position; stemsgrew more compact along their length (Fig. 3A). Thisresponse was more pronounced in the sprawling annual,and both species tended to have steeper slopes with realneighbours than with fake neighbours (Fig. 3B, non-

Stem Elongation Response 741

overlapping slope comparison intervals).

�erences in mean P. sagittatum and P. hydropiperoidesd internode length (cm) response to neighbour identity andition

Mean square F P

88 766 27.42 0.0001269 0.08 0.77

20 133 6.22 0.00214 222 4.39 0.044063 1.25 0.29

651.01 71.41 0.000115.11 1.66 0.2037.24 2.04 0.133.04 0.33 0.560.45 0.05 0.95

609.84 70.88 0.00010.02 0.00 0.962.39 0.28 0.765.74 0.67 0.424.35 0.51 0.61

82 1.78 0.1881 1.75 0.19

1273 26.85 0.00011958 4.39 0.0001414 4.49 0.01

P. hydropiperoides P. sagittatumFake RealFake Real

r1 r2 r3 r1 r2 r3 r1 r2 r3 r1 r2 r3

Ste

m le

ngt

h (

cm)

Nu

mbe

r of

nod

esL

eaf

area

/nod

eIn

tern

ode

len

gth

(cm

)

140

120

100

80

60

40

20

14

12

10

8

614

12

10

8

6

4

2

14

12

10

8

6

FIG. 2. Means (lines show + 1 s.d.) of stem length (cm), number ofnodes, leaf area (cm2) per node, and internode length (cm) ofP. hydropiperoides and P. sagittatum at three crowding levels (rows 1,

2, 3) with fake or real neighbour plants.

Node position

10.80.60.40.20

150

100

50

0

150

Inte

rnod

e le

ngt

h (

mm

)

150

100

50

0

100

50

0

100

50

0

150

−100

Slo

pe w

ith

95%

com

pari

son

inte

rval

−90

−80

−70

−60

−30

−40

−50

Fake Real Fake Real

P. sagittatum P. hydropiperoides

P. sagittatum, RealInl = 73.61 − 81.96 NpR2 = 0.67P < 0.0001

P. hydropiperoides, FakeInl = 35.29 − 34.58 NpR2 = 0.51P < 0.0001

P. hydropiperoides, RealInl = 44.89 − 44.19 NpR2 = 0.58P < 0.0001

P. saggittatum, FakeInl = 63.1 − 67.92 NpR2 = 0.60P < 0.0001

A

B

FIG. 3. Linear regression [A, including the regression equation,goodness of ®t (R2), and signi®cance (P)] and regression slopes(B, with 95% comparison interval) of internode length (Inl) on nodeposition (Np) of P. sagittatum and P. hydropiperoides with real or fakeneighbours. Node position is relative from stem base (0) to apex (1).Data were pooled over the two inner rows and over the three replicate

742 Collins and WeinÐPolygonum Stem Elongation Response

DISCUSSION

When surrounded by a cohort of real neighbours or byadult-sized fake plants that increased shading and loweredthe r : fr ratio in the greenhouse, the sprawling annual,P. sagittatum, had longer stems with more nodes and lessleaf area than its upright perennial congener P. hydro-piperoides. Average stem and internode lengths of bothspecies graded from long to short as crowding by real orfake neighbours decreased. However, the response wasmore pronounced in P. sagittatum. The species also hadopposite responses to real vs. fake neighbours. Stem andinternode lengths of P. sagittatum were longer whensurrounded by full-sized fake plants compared to a cohortof real plants. P. hydropiperoides was taller and had longerinternodes with real than fake neighbours. Both speciesgrew more compact along their length because internodelength decreased from the stem base to top; this responsewas more pronounced in the annual and with realcompared to fake neighbours.

Although our research did not test a full suite of potentialneighbour e�ects (e.g. temperature, mechanical e�ects, andneutral shade were not tested individually), the results

from P. hydropiperoides and P. sagittatum indicate that

neighbour shade and lowered r : fr do promote stemextension in both species as in other herbs (Ballare et al.,1987, 1991; Smith, 1990; Ballare , 1994; Aphalo and Ballare ,1995; Ballare and Scopel, 1997). Our results also support

sets of each neighbour type.

research with other herbs that found a positive relationship

m

between stem elongation and plant density or neighbourproximity (Ballare et al., 1991; Dudley and Schmitt, 1996;Harley and Bertness, 1996).

Di�erences between P. hydropiperoides and P. sagittatumin elongation response to a cohort of real neighbours vs. `fullsize' fake neighbours possibly re¯ect di�erent elongationresponse strategies to neighbour shade. The fact that plantsof P. sagittatum grew taller and showed a more pronouncedoverall elongation response with fake than real neighbourssupports the hypothesis that sprawling or vine-like speciesshow a greater tendency to elongate into an establishedcanopy (Den Dubbelden and Oosterbeek, 1995; Harley andBertness, 1996) than do upright, consistently self-supportingspecies such as P. hydropiperoides. The latter would extendupward with, or overtop, cohort neighbours, but might lessreadily elongate upward into shade.

Response to neighbour shade has been examined in otherPolygonum species. Two annual species, prostrate andprofusely branching P. douglassi and more uprightP. arenastrum, developed size hierarchies as densityincreased (Geber, 1989). Relative branch length andbranching were negatively related to density; plants athigh density were smaller and less branched (Geber, 1989).The colonizing annual P. pensylvanicum became taller andthinner with crowding (Weiner and Thomas, 1992; Thomasand Bazzaz, 1993). When grown with other colonizing forbsand a grass, P. pensylvanicum was overtopped early, butcould escape neighbour shade to eventually overtop onespecies, Datura stramonium (Tremmel and Bazzaz, 1993).Collectively, Polygonum species shows a stem elongationresponse to neighbours that may confer performance orcompetitive advantage, or may facilitate coexistence(Geber, 1989; Weiner and Thomas, 1992; Thomas andBazzaz, 1993; Tremmel and Bazzaz, 1993; this research).However, elongation responses di�er among species andwithin populations (Geber, 1989; this research).

The results of our research have implications forobserved interactions between these Polygonum species,which coexist on lake shore margins at neighbour distancesso close that P. sagittatum is supported by P. hydro-piperoides (pers. obs.). Bertness and colleagues (Bertnessand Callaway, 1994; Bertness and Hacker, 1994; Harley andBertness, 1996) have suggested that positive (�, � or �, 0)species interactions, such as structural dependence orinterdependence, reduce competition or facilitate coexis-tence in stressed or crowded environments. Our resultssuggest that its sprawling growth form allows a pronouncedP. sagittatum stem elongation response to neighbour shade,which contributes to its structural dependence onP. hydropiperoides and may ultimately facilitate coexistence.Further research is needed to explicitly test the relationshipbetween neighbour proximity, structural dependence, and

Collins and WeinÐPolygonu

coexistence of these Polygonum species.

shade tolerance. International Journal of Plant Science 159:

ACKNOWLEDGEMENTS

This research was supported by Financial Assistance AwardNumber DE-FC09-96SR18546 between the US Depart-ment of Energy and the University of Georgia's Savannah

River Ecology Laboratory.

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