Factors constraining fruit set in Mandevilla pentlandiana (Apocynaceae

Botanical Journal of the Linnean Society (1999), 129: 187–205. With 6 figures

Article ID: bojl.1998.0216, available online at http://www.idealibrary.com on

Factors constraining fruit set in Mandevillapentlandiana (Apocynaceae)

CAROLINA TORRES AND LEONARDO GALETTO∗

Instituto Multidisciplinario de Biologıa Vegetal (UNC-CONICET), Casilla de Correo 495,5000 Cordoba, Argentina

Received January 1998; accepted for publication September 1998

The reproductive success of Mandevilla pentlandiana was studied to disclose its reproductivestrategy, and to determine the links between nectar production, breeding system, fruit setand inflorescence size. The plant produces many inflorescences with a large number offlowers but initiates few fruits (9%). This vine is self-compatible but not autogamous. Giventhat no significant differences could be detected considering many traits (ripe and abortivefruit sets, fruit quality, and seedling survival) between the pollination treatments (self-, cross-and natural-), the low natural fruit set was not related to pollen limitation. Fruits were notdistributed at random within inflorescences (earlier fruits had the highest probability ofmaturation) but there were no significant differences in fruit quality according to differentfruit positions. Conversely, the time of fruit initiation influenced most of the fruit-traits.Many developing fruits were aborted (20%). An increase in the probability of abortion wasdetected when the whole inflorescence was hand pollinated. In addition, a positive correlationwas detected between the abortions and the number of ripe fruits which developed beforethem. Looking at our data from an evolutionary perspective, we argue that a theoreticalinflorescence size, corresponding to the intersection point between the mean values of fruitnumber and fruit set per inflorescence, can be assumed to indicate the optimum inflorescencesize that maximizes equally both female and male functions. Comparison between thetheoretical and the observed mean inflorescence size suggests, that for M. pentlandiana, pollendonation may be the primary evolutionary factor behind excess flowers.

1999 The Linnean Society of London

ADDITIONAL KEY WORDS:—abortion – breeding system – fruit quality – inflorescencesize – seed set – self-compatibility – surplus flowers.

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . 188Mandevilla pentlandiana . . . . . . . . . . . . . . . . . . 189

Methods . . . . . . . . . . . . . . . . . . . . . . . . 189Phenology . . . . . . . . . . . . . . . . . . . . . . 189Breeding system . . . . . . . . . . . . . . . . . . . . 189Fruit and seed production . . . . . . . . . . . . . . . . . 192Statistical analysis . . . . . . . . . . . . . . . . . . . 192

∗Corresponding author. Email: [email protected]

1870024–4074/99/030187+19 $30.00/0 1999 The Linnean Society of London

C. TORRES AND L. GALETTO188

Results . . . . . . . . . . . . . . . . . . . . . . . . 192Phenology . . . . . . . . . . . . . . . . . . . . . . 192Breeding system . . . . . . . . . . . . . . . . . . . . 193Fruit and seed production . . . . . . . . . . . . . . . . . 194

Discussion . . . . . . . . . . . . . . . . . . . . . . . 196Acknowledgements . . . . . . . . . . . . . . . . . . . . 202References . . . . . . . . . . . . . . . . . . . . . . . 203

INTRODUCTION

In plants, the number and quality of offspring may be limited by extrinsic factors,such as pollination and flower and seed predation, or intrinsic factors, such asgenotype and stored resource content (Stephenson, 1981, 1992; Wiens et al., 1987;Lee, 1988). The nature of the relationships between the amount and kind of pollenreceived by the flowers and the subsequent fruit and seed production has importantconsequences in terms of evolutionary theory. Flower and inflorescence traits suchas nectar production and standing crop, nectar sugar content, and inflorescencesize, can influence reproductive success by affecting pollinator behaviour (e.g.Wyatt, 1982; Cruden, Hermann & Peterson, 1983; Zimmerman, 1988; Pleasants &Zimmerman, 1990; Mitchell & Waser, 1992). Consequently, the ways in whichselection affects these and other pollination-related plant traits also depend on therelationships between pollen movement and plant fitness (e.g. Waser, 1983; Mitchell,1994; Galen, 1996).

The study of causes and consequences of mortality from fertilization to seed andfruit maturation is also important in understanding the reproductive strategies ofplants. In response to varying environmental conditions, plants may adjust theresources committed to seed production by altering, among other traits, the numberof flowers that differentiate and the number of fruits or seeds that abort (e.g. Lloyd,1980; Stephenson, 1981; Wiens, 1984; Stocklin, 1997). The question arises as towhich developing fruit mature and which abort. An underlying premise is that theusual conflicting demands of pollination and fruit set have led to an evolutionarycompromise that maximizes an individual’s contribution to the next generation(Wyatt, 1980). Depending upon the species, pollinated flowers and immature fruitsmay selectively ripen on the basis of the order of pollination, the number of developingseeds, pollen source, plant architecture (Diggle, 1997), or some combination ofthese (Stephenson, 1981).

The present study is the second part of a project planned to evaluate the natureof the relationships between plant nectar traits (Torres & Galetto, 1998), and todetermine the links between nectar production, breeding system, fruit set, andinflorescence size in Mandevilla pentlandiana (A.DC.) Woodson. We address thefollowing questions: (1) Is natural seed production of M. pentlandiana pollen limited?(2) What are the effects of the pollen source on fruit production, fruit traits, andseedling survival? (3) Are fruit set and fruit abortion related to inflorescence size?(4) How does flower position within an inflorescence influence fruit production andfruit abortion? (5) How does fruit position within an inflorescence and seasonal timeof fruit ripening affect fruit quality? (6) Which is the most successful inflorescencesize within the population?

REPRODUCTIVE BIOLOGY OF MANDEVILLA PENTLANDIANA 189

Mandevilla pentlandiana

Apocynaceae is a group of about 2100 species of lianas or, less frequently, trees,shrubs or herbs. They are mostly tropical and are frequent sources of drugs andpoisons (Mabberley, 1987). Mandevilla Lindl. comprises 110 species that grow inwarm regions of Central and South America (Woodson, 1933). Mandevilla pentlandianais one of eight species that live in Argentina. It is a vine that grows specially inmountain habitats of central and northern Argentina, Bolivia, and south of Brazil(Ezcurra, 1981).

This species has tubular, perfect, white-greenish flowers (Fig. 1A) which last 3–4days, and contain a relatively concentrated, sucrose-rich nectar. The complex flowerstructure allows an extremely precise pollination mechanism. The anthers are fusedto the style head and dehisce introrsely by longitudinal slits before anthesis. Thestructure of the style head is thickened and specialized for pollination and thereceptive stigmatic zone is located at its base. The conspicuous nectary is locatedaround the bicarpellary ovary. This floral morphology precludes within-flower self-pollination (Galetto, 1997). Flowers are visited by bumblebees, honeybees andhummingbirds during daylight hours; no visitors were seen during the evening(Torres & Galetto, 1998).

Around 60 plants of native populations from Cordoba Province, Argentina (Dept.Colon: Cabana, and Dept. Calamuchita: Los Aromos) were used to obtain bothexperimental and field data. Plants were followed over a two year period (September1994–September 1996).

METHODS

Phenology

Additional natural populations from Cordoba Province were considered to com-pare phenological data (Dept Calamuchita: La Bolsa, Dique Los Molinos, Embalsede Rio Tercero; Dept Punilla: Cuesta Blanca; Dept Ischilin: Cerro Colorado; DeptColon: Villa Warcalde, Mendiolaza, Dique La Quebrada). Leafing, flowering, andfruiting periods were recorded for two growing seasons (1994–5 and 1995–6). Wenoted the start and end dates of each event as well as the period of seed dispersal.

Breeding system

Pollen ovule (P/O) ratio. The total number of pollen grains was counted from onemature undehisced anther squashed in aniline blue in lactophenol. This numberwas multiplied by 5 to obtain the total number of grains per flower. A total of 12anthers belonging to different flowers and plants was examined. Total number ofovules (from the same flowers used for pollen counts) was determined with the aidof a magnifying glass. Aniline blue in lactophenol (1%wt/v) was used to estimatepollen stainability and viability (Kearns & Inouye, 1993). Darkly stained grains wereconsidered viable. We scored 100 grains per sample.

Effects of the pollen source. The effect on fruit and seed production of self versus crosspollen was studied using three bagged inflorescences of 20 plants. At the moment


Figure 1. Mandevilla pentlandiana. A, inflorescence; B, inflorescence with two ripe and two abortive fruits;C, detail of B showing the abortive fruits; D, detail of a dehisced fruit showing the comose seeds. Scalebars=1 cm for A, B, and 0.5 cm for C, D. The arrows indicate the abortive fruits.


when treatments were applied, we used open flowers (3–5) in each of the 60 racemesassigned to the following manipulative protocol (20 for each treatment): (i) to testfor autogamy and autonomous self-pollination, the flowers were not manipulateduntil the end of the flowering season; (ii) to test for self-compatibility, we coated thestigmatic surface with pollen from the same flower; pollination bags were replacedafter treatment; (iii) to test for cross-compatibility, we used the previous method butwith pollen from different plants >100 m apart. We randomly tagged flowers (15–27per inflorescence) in the same and additional plants to act as open pollinationcontrols (n=45 plants).

To hand-pollinate a flower, a fine bristle was inserted into the corolla tube of arecently open flower, passing between two adjacent anthers. After moving it up anddown once or twice, the bristle picked up fresh pollen that could be used to pollinatethe same or a different flower. Self-pollination was easily achieved artificially byinserting a clean, thin bristle into the corolla tube and forcing it to pass betweentwo anthers. First, as the bristle is moved downwards, a mixture of mucilage andpollen sticks to it. Second, as the bristle is moved upwards, it rubs the edge of thestyle-rim. A portion of the mixture is scraped from the bristle and thus depositedon the receptive area (i.e. the lower side of the style-rim; Galetto, 1997; Torres &Galetto, 1998). Cross-pollination was done by pollinating a flower with a bristlepreviously inserted into a flower that belonged to a different plant. Nevertheless,and as a consequence of both the flower structure and the pollination system (Torres& Galetto, 1998), cross-pollination may result in both self- and cross-pollen beingtransferred to the stigma. Flowers cannot be emasculated without damaging thestigma, because anthers are firmly attached to the style.

We collected fruits of treated and control flowers at maturity just before seeddispersal. Fruit set (no. of ripe fruits/no. of pollinated flowers) and preemergentreproductive success (PERS=(fruits flowers−1) (seeds ovules−1); Wiens et al., 1987)were calculated for the three pollination treatments. Since the seed–ovule ratio tendsto overestimate PERS (because it considers that all seeds are viable), when calculatingit we included germination data for seeds. The index to measure self-incompatibilitywas calculated as follows: ISI=fruit set from selfing/fruit set from crossing (Dafni,1992).

The relative fruit quality was investigated by comparing: fruit length; fruit, pericarpand total seed mass; number of seeds per capsule, and seed germination betweenpollination treatments (self, cross and natural). Mass was registered using an analyticalbalance. Seed number and seed mass were recorded separately for each of the twofollicles of the fruits. Seeds for each fruit were placed into Petri dishes, lined withfilter paper, and watered regularly at room temperature for 1 week. After this periodand to obtain the seed viability percentage of each fruit, the germinated seeds werecounted and divided by the number of seeds used.

In order to evaluate other potential effects on fruit quality, additional data wererecorded when harvesting each control fruit: (a) inflorescence fruit position (i.e. fruitsthat are ripened at the bottom, middle or top of the inflorescences), and (b) seasonaltime of fruit ripening (i.e. fruits that reached their maturity during May, June orJuly).

Seedling survival. The emerged seedlings from both types of mating (i.e. self and cross)were placed in pots filled with local soil at the beginning of the second growingseason (September 1995) and maintained in natural field conditions at Los Aromos.


The following spring (September 1996), the plants were finally transplanted. Per-centage of seedling survival per type of mating was recorded 8, 16, and 24 monthslater (1995–7). In addition, the inflorescence position of the fruit from which seedlingsoriginated was considered when analysing data.

Fruit and seed production

The number of fruits (ripe and aborted) and floral scars were counted in 192unmanipulated racemes (3–5 per plant). The inflorescence fruit (ripe and aborted)position was recorded by numbering all the floral scars from the bottom to the topof the inflorescence. Ripe fruit set (no. of matured fruits/no. of inflorescence flowerscars) and the abortive fruit set (no. of aborted fruits/no. of inflorescence flowerscars) were calculated for each of the 192 inflorescences. Samples included racemesfrom (a) all positions on c. 45 plants, (b) two reproductive seasons: 1994–5 and1995–6, and (c) five different populations.

In seven individuals, three randomly chosen inflorescences per plant were usedto evaluate further the effects of both pollination intensity and pollen source on fruitset. The inflorescences remained bagged during the whole flowering period. Allflowers of each inflorescence were hand-pollinated as they opened throughout theflowering season. In all plants the pollination treatments were applied selecting oneraceme for each one: (i) 100% of the raceme flowers were pollinated with pollenfrom plants placed >500 m apart (cross-pollination); (ii) 100% were pollinated withtheir own pollen (self-pollination); (iii) 50% were cross-pollinated and the remaining50% self-pollinated (mixed pollination; two of these inflorescences were lost due toinsect damage). For the latter treatment, the self- and cross-pollinations were madesimultaneously by dividing vertically the open flowers of the inflorescence (i.e. rightand left side), in order to avoid nourishment fruit effects. The flowers were taggedfor identification after each pollination. Bags were replaced after each hand-pollination interval (i.e. 2–3 days).

Statistical analysis

One- and two-way analyses of variance (ANOVA) were combined with Tukey–Kramer test for multiple comparisons at P<0.05. Analogous non-parametric methods(Kruskal–Wallis and Wilcoxon ranks tests) were used when the assumptions requiredfor parametric methods were not met (Sokal & Rohlf, 1995). Correlation analyseswere done using Pearson’s and Spearman’s (non-parametric) coefficients. Means oforiginal data plus or minus one standard deviation are given in the text and tables.The statistical program package SPSS (1992) was used for these analyses.

RESULTS

Phenology

The inflorescences are simple and lateral, constituting dense racemes 6–25 cmlong (Fig. 1A). Flowering begins in early December and ends in mid-March. The


T 1. Breeding system, fruit set, fruit quality, and reproductive success in Mandevilla pentlandiana.ns=not significant. PERS=pre-emergent reproductive success

Variables FlowersStatistical

Natural-pollinated Self-pollinated Cross-pollinated results

No. of flowers 989 98 80No. of fruits 81 5 7Fruit set 0.082 0.051 0.087 v2=1.05 nsFruit length (cm) 13.78±2.58 14.50±1.58 13.07±2.23 F[2,91]=0.48 nsFruit mass (g) 1.43±0.45 1.35±0.40 1.54±0.53 F[2,92]=0.31 nsPericarp mass (g) 0.77±0.21 0.72±0.38 0.89±0.30 F[2,92]=1.06 nsSeed mass per fruit (g) 0.65±0.27 0.62±0.22 0.65±0.25 F[2,92]=0.03 nsNo of seeds per fruit 99.02±23.10 84.60±20.17 82.86±18.52 F[2,92]=2.43 nsSeed viability (%) 76.08±25.39 79.99±6.88 87.44±11.38 F[2,64]=0.74 nsPERS 0.0597 0.0334 0.0613 v2=0.95 ns

earliest developing fruits are found in late December and the abortive and ripefruits during the middle of February and April, respectively (Fig. 1B, C). By the endof July all capsules have undergone dehiscence. Each follicle has longitudinaldehiscence along the ventral suture, releasing dry, apical comose, wind-dispersedseeds (Fig. 1D).

Plants lose their leaves during the fruiting period, and are subjected to frost duringwinter, with new shoots produced at the beginning of the following spring. Thephenology data were similar in all the populations analysed which are from thenorth-west, south-west and centre of Cordoba Province.

Breeding system

The flowers produced an average of 103.9±10.5 ovules equally distributedbetween the two carpels, and 4769±1226 pollen grains per flower. The resultingP/O ratio was low (45.24±11.58). The percentage of pollen stainability was high(97.28±3.04%).

Mandevilla pentlandiana is self-compatible (ISI=0.58), but non-autogamous. Ifunvisited, M. pentlandiana flowers do not set fruit, as indicated by the lack of fruitingin inflorescences bagged to exclude pollinators (n=600 flowers, no fruits). Thereare no significant differences between fruit set of hand-pollinated flowers (self- andcross-pollinated) with that of natural-pollinated flowers (Table 1). Likewise, thereare no significant differences in the PERS among the pollination treatments (Table1). No significant differences could be detected when evaluating several fruitcharacteristics among the pollination treatments (Table 1).

The seeds quickly germinated 2–3 days after sowing; this was independent oftheir origin (self versus cross). Selfed and crossed seedlings showed similar survival8, 16 and 24 months after germination (Table 2). Although statistical analysis didnot show significant differences, selfed plants showed higher survival percentagesthan crossed ones (Table 2). In addition, no significant correlation was detectedbetween plant survival and the position of the fruits within the inflorescence (r=−0.36, P<0.28; r=−0.31, P<0.37; r=−0.06, P<0.86; for 8, 16 and 24 monthsrespectively, n=11).


T 2. Seedling survival and pollen source in Mandevilla pentlandiana. The results indicate seedlingsurvival percentages; ns=not significant. The F value for the interaction between the factors in the

two-way ANOVA is F[2,27]=0.097, P<0.908

Seedling survival Treatments(number of months afterseed germination) Self-pollination Cross-pollination

8 16.93±8.29 10.43±7.97 F[2,27]=1.09 ns16 15.61±8.19 10.43±7.9724 10.94±7.00 7.36±7.03

F[1,27]=3.53 ns

Fruit and seed production

The inflorescences produced 33.6±16.9 (n=192) functional flowers, yet mostracemes matured only 3.1±1.9 fruits (n=192). In other words, only 9% of theflowers set ripe fruits. In spite of this low fruit set, the absolute seed output per plantwas usually high. Because of large flower production of plants and seed sets perfruit, medium-sized individuals often yield thousands of seeds.

There was large variation in inflorescence size, although middle-sized in-florescences were the most common (range 20–40 flowers; Fig. 2 A). Most of theinflorescences developed 1–4 ripe fruits (77.6%; n=192) while the remainder (22.4%)had 5–10 fruits per inflorescence (Fig. 2 B).

Total seed number per fruit was positively correlated with both fruit length andfruit mass. In addition, fruit length was positively correlated with fruit-, pericarp-,and total seed-mass (Fig. 3). In contrast, seed viability did not show any significantcorrelation when tested against the other variables measured.

Inflorescence size and production of ripe and abortive fruits. The larger the inflorescence sizethe greater was the number of both ripe and abortive fruits per inflorescence (Fig.4). Nevertheless, when fruit set is considered at different inflorescence size intervals,distinct patterns become evident for ripe and abortive fruit sets. A decline in therelative ripe fruit set is observed as the inflorescence displays a greater number offlowers (Fig. 4). On the other hand, the abortive fruit set remains constant independentof inflorescence size (Fig. 4). Ripe fruit set is not correlated with abortive fruit set(r=−0.11 P<0.13, n=182).

Flower position and fruit set. Ripe and aborted fruits were not distributed at random withinthe inflorescences. The closer the position to the bottom of the inflorescence of apollinated flower the greater the chance of a fruit maturing and also aborting (Fig.5). Furthermore, when fruit set is considered at three different positions within theinflorescence (i.e. inflorescence flower positions: bottom, middle, and top; Fig. 6), it isnoted that ripe and abortive fruit sets of bottom flowers are low (0.108±0.049 and0.024±0.013, respectively), but significantly higher when compared to the middleflowers (0.020±0.014 and 0.005±0.005, respectively) or to the top ones that almostnever set a fruit (0.002±0.003 and 0.001±0.002, respectively, Fig. 6). When con-sidering total fruit production, it was evident that bottom flowers have the highestprobability of initiating both ripe and abortive fruits (83 and 79.6%, respectively),compared with middle (15.5 and 16.1%, respectively) and top flowers (1.5 and 4.4%,respectively; Fig. 6).


60

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70–8030–40 60–70

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A

Figure 2. Flower and fruit production per inflorescence in Mandevilla pentlandiana. Frequency of thenumber of (A) flowers per inflorescence, (B) fruits per inflorescence.

Abortive fruits and fruit abortion rate. Abortive fruits were usually 0.5–2 cm long and didnot show signs of damage (Fig. 1C). They simply turned from green to black andstopped growing without abscising. Partially aborted fruits (only one of the twofollicles) were observed occasionally. An average of 0.79±1.62 abortive fruits (n=182) per inflorescence was recorded. In other words, 20% of the total initiated fruitswas aborted. The abortive fruit number per inflorescence is not correlated with thenumber of mature fruits (rs=0.08 P<0.36; n=144). Nevertheless, a positive significantcorrelation is evident (rs=0.16 P<0.04; n=144) between the number of abortionsand the number of mature fruits developed before them.

Pollination intensity and effects of the pollen source. Inflorescences in which all flowers werehand-pollinated set fewer fruits than those in which only 3–4 flowers were randomlyhand-pollinated or those that were naturally pollinated (cf. Tables 1 and 3). Self-versus outcross pollination treatments showed identical fruit set values (Table 3).

Fruit position within an inflorescence and fruit quality. When fruit quality is considered atthree different positions within the inflorescence (i.e. bottom, middle, and top), nosignificant differences are seen for the considered variables (Table 4).


Figure 3. Correlations between fruit and seed traits of Mandevilla pentlandiana.

Seasonal time of fruit ripening and fruit quality. When ripe fruits are divided into setsaccording to their period of dehiscence (May, June and July), significant differencesare found for these variables with the exception of seed viability (Table 5). Earlierfruits (ripened during May) produced the greatest number of comparatively lightseeds. Late fruits ( July) were the heaviest and produced a relatively low number.

DISCUSSION

What determines the limits on the floral display in M. pentlandiana?Pollinators preferentially visit large inflorescences because they provide a larger

signal to attract them and/or offer greater pollen and nectar rewards (e.g. Willson& Rathcke, 1974; Willson & Price, 1977; Stephenson, 1979; Augspurger, 1980;Torres & Galetto, 1998). In addition, even if pollen receipt does not limit seedproduction as in M. pentlandiana, increased pollinator attraction can benefit malefunction if floral mechanisms restrict pollen removal by individual pollinators. Thisis because, with many pollinators following dissimilar foraging paths, an individual


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Figure 4. Fruit production, fruit set, and inflorescence size in Mandevilla pentlandiana. The left axis showsthe mean fruit number (ripe and abortive) per inflorescence (r=0.40, P<0.0001; n=192 and r=0.28,P<0.0001; n=182, respectively) and the right axis shows mean fruit set (ripe and abortive) perinflorescence according to the inflorescence size (r=−0.38, P<0.0001; n=192 and r=0.11, P<0.126;n=182, respectively). The intersection between the figures corresponding to the number of ripe fruitsand the ripe fruit set indicates the optimum theoretical inflorescence size (TIS).

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7131 51

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2111 41 61

Figure 5. Physical location of the fruits within the inflorescence and fruit production in Mandevillapentlandiana. The inflorescence flower scars were numbered consecutively from bottom to top. Thereare significant negative correlations between the position of a pollinated flower and its chances to bothmature (Φ) and abort (Ε) a fruit (r=−0.85, P<0.001 and r=−0.78, P<0.001, respectively; n=73).


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Fru

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%) 8

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Figure 6. Fruit set correlated with the lower physical location in 192 natural-pollinated M. pentlandianainflorescences. The racemes were divided into three subunits: bottom (flower scars numbered 1–24),middle (25–48), and top (49–73). Kruskal–Wallis showed that there are significant differences: H=59.06, P<0.0001 (for ripe fruits; Φ) and H=45.06, P<0.0001 (for abortive fruits; Ε).

T 3. Fruit set in Mandevilla pentlandiana considering both pollination intensity and pollen source.All flowers of the inflorescences were hand-pollinated throughout the flowering season. In the mixedtreatment (50% self and 50% cross) hand-pollinations were performed alternately throughout theflowering season. All experimental inflorescences have approximately the mean number of flowers

(range 25–45 flowers)

Inflorescence traits Treatments

100% self-pollination 100% cross-pollination mixed-pollination

No. of flowers treated 213 215 137(No. of inflorescences used) (7) (7) (5)No. of fruits obtained 3 3 2Fruit set 0.0141 0.0140 0.0146

T 4. Inflorescence fruit position and fruit quality in Mandevilla pentlandiana. Factor levels—countingflowers scars from the inflorescence base to the apex—(fruit position; no. of encountered and measuredfruits): bottom (1–24; n=45), central=(25–48; n=10), and apical=(49–73; n=5). ns=not significant

Fruit quality (variables) Inflorescence fruit position Statistical results

Bottom Central Apical

Fruit length (cm) 14.01±2.14 12.60±1.43 12.50±1.06 F[2,60]=2.99 nsFruit mass (g) 1.45±0.43 1.40±0.40 1.61±0.31 F[2,60]=0.43 nsPericarp mass (g) 0.78±0.23 0.73±0.15 0.91±0.24 F[2,60]=1.16 nsTotal seed mass/fruit (g) 0.66±0.24 0.67±0.28 0.70±0.14 F[2,60]=0.04 nsNo. of seeds/fruit 93.80±24.33 105.80±11.51 95.20±8.20 F[2,60]=1.13 nsSeed viability (%) 81.06±21.77 64.70±23.93 81.75±10.73 F[2,60]=2.43 ns


T 5. Seasonal time of fruit ripening and fruit quality in Mandevilla pentlandiana. Symbols: significantat ∗P<0.05, ∗∗P<0.01, ∗∗∗P<0.001 (A and B indicate the post hoc test results; ns=not significant). n=

no. of fruits measured

Fruit quality (variables) Seasonal time of fruit ripening Statistical results

May (n=17) June (n=12) July (n=35)

Fruit length (cm) 14.14±2.74 12.35±2.28 14.17±2.24 F[2,92]=4.40∗∗ (ABA)Fruit mass (g) 1.32±0.34 1.12±0.41 1.65±0.43 F[2,92]=13.84∗∗∗(AAB)Pericarp mass (g) 0.74±0.18 0.68±0.22 0.85±0.24 F[2,92]=4.86∗∗ (AAB)Total seed mass/ fruit (g) 0.58±0.21 0.44±0.23 0.80±0.22 F[2,92]=20.17∗∗∗ (AAB)No. of seeds/fruit 108.27±20.26 92.25±23.46 91.42±22.32 F[2,92]=5.82∗∗ (ABB)Seed viability (%) 86.88±22.86 79.02±22.47 72.35±23.22 F[2,64]=2.40 ns

plant exports pollen to a larger sample of the population (Harder & Thomson, 1989;Harder & Wilson, 1994; Harder & Barrett, 1996). This species shows a complexflower structure associated with a specialized pollination mechanism that restrictspollen removal: a pollinator arrives at the flower, introduces its proboscis throughone of the five entries to reach the nectar, transfers pollen when removing theproboscis, and, finally, touches a sticky drop with pollen that belongs to only twoof the ten techae (Galetto, 1997; Torres & Galetto, 1998).

On the other hand, when a plant displays numerous flowers simultaneously,pollinators can transfer pollen among flowers on the same plant. Such geitonogamybears two potential mating costs (de Jong, Waser & Klinkhamer, 1993; Snow, Spira& Simpson, 1996): inbreeding depression (in self-compatible species) and pollendiscounting (i.e. pollen used in self-pollination that is not available for outcrossing,potentially reducing the plant’s success as a pollen parent). Thus, self-pollinationoccurring between flowers on the same plant can set an upper limit to theevolutionarily stable size of inflorescences (Schoen & Dubuc, 1990). Nevertheless,our data indicate that M. pentlandiana, a self-compatible species that exposes eachday a small number of the total flowers displayed by an inflorescence throughoutthe flowering period, seems to be capable of surpassing these mating costs. First, itdoes not show inbreeding depression, at least at the levels of fruit- and seed-set,seed germinability, and seedling survival (24 months after hand-pollination, butbefore the reproduction of F1 plants). Finally, and following the line of reasoningstated above, the mechanisms that restrict pollen removal can facilitate low levelsof pollen discounting.

How does physical location of a flower within an inflorescence affect its chance of producing afruit?

Within M. pentlandiana inflorescences, early-formed fruits have the highest prob-ability of maturation. In the absence of pollen limitation, there are two possibilitiesto explain flower-position effects on flower’s probability to set a ripe fruit: (1) thedistance to the parent plant’s pool of resources (Berry & Calvo, 1991), and (2) timeof fruit initiation with respect to other developing fruits competing for a share oflimited resources (e.g. Stephenson, 1981; Wyatt, 1982; Bertin, 1985; Lee, 1988).Although resources in M. pentlandiana might limit fruit set, when a developing fruitexceeds the size for abortion it will ripen showing invariably the same fruit-traits(‘quality’), independent of its position within the inflorescence. Conversely, the time


of fruit initiation influences most of the fruit-traits considered (late fruits present alower number of heavier seeds than earlier ones). We propose two, not mutuallyexclusive, interpretations to explain these results. (1) When fruiting starts the planthas not yet completed both flowering and vegetative growth periods; therefore, theripening of early-formed fruits benefits from the amount of available resources. (2)Pollen loads are larger at the beginning of the flowering season due to (a) greaterpollinator availability, (b) better pollinator efficiency in pollen transfer, and/or (c) ahigher nectar availability (Torres & Galetto, 1998).

Given the significance of seed traits to plant fitness, it is important to understandthe factors that are responsible for the substantial variation that is commonlyobserved. Although variation in seed size can occur among populations and withinindividual plants (Stocklin & Favre, 1994; Vaughton & Ramsey, 1997), a seasonalpattern is evident. Seasonal declines in seed size are often attributed to decreasingavailability of resources over the season (Vaughton & Ramsey, 1997; and referencestherein). In M. pentlandiana the plants appear to distribute their resources (includingsome nectar traits, Torres & Galetto, 1998) over the reproductive season to achievethe highest possible maternal investment (i.e. through a higher seed set early in theseason, and through increasing seed mass later).

What factors limit the production of fruits by M. pentlandiana?Two general and mutually non-exclusive hypotheses (Campbell & Halama, 1993)

can be considered to explain the relationships between flower, fruit, and seedproduction. The ‘pollinator limitation’ hypothesis invokes inadequate pollination asthe major factor underlying low fruit-flower ratios (e.g. Bierzychudeck, 1981;Zimmerman & Pyke, 1988; Burd, 1994). The ‘resource limitation’ hypothesis traceslimited fruit production to a shortage of resources (e.g. Lloyd, 1980; Stephenson,1981, 1992; Lee, 1988; Vaughton, 1993). Finally and at the evolutionary level, afurther ‘sexual selection’ hypothesis attributes low fruit set to conflicts inherent inthe optimization of male (pollen) and female (fruits and seeds) reproductive success(e.g. Bateman, 1948; Stephenson & Bertin, 1983; Lyons et al., 1989; Snow & Lewis,1993).

Pollen limitation. One important question in plant reproductive ecology is the extentto which female reproductive success is limited by inadequate pollen deposition(Carlsson-Graner et al., 1998). The absence of pollen limitation has been confirmedby artificial pollination in M. pentlandiana. For several species, a reduction in thelevel of competition among developing fruits could be experimentally confirmed bypollinating a fraction of an inflorescence’s flowers which might result in an increasein the proportion of the flowers that produce mature fruits (e.g. Lloyd, 1980,Vaughton, 1993; Vaughton & Cartew, 1993). Our results indicate that when thewhole inflorescence was pollinated, there was a higher level of fruit competition forthe available resources. This can be inferred because compared with both partially-pollinated, and naturally-pollinated inflorescences, thoroughly-pollinated onesshowed a lower fruit set. These results thus argue against the possibility of theadjustment to unusually good pollinator resource (Willson & Rathcke, 1974;Stephenson, 1979; Sutherland & Delph, 1984; Sutherland, 1987).

Resource limitation. Abortion may derive from competition among fruits for nutrientssupplied by the parent plant and/or from developmental anomalies induced by


genetic or phenotypic physiological deficiencies (e.g. Bookman, 1984; Stephenson& Windsor, 1986; Sutherland, 1987; Wiens et al., 1987; Lee, 1988; Stocklin, 1997).Both cross- and self-pollinated flowers in M. pentlandiana had similar fruit abortionpercentages. This suggests that abortion as a result of genetic incompatibility mayhave been less important than other mortality factors, such as resource limitation(Lloyd, 1980; Stephenson, 1981, 1992; Lee, 1988; Stocklin & Favre, 1994). Thisidea is reinforced because in M. pentlandiana an increase in abortion probability isfound when the whole inflorescence is hand pollinated, and because there is apositive correlation between the abortions and the number of ripe fruits developedbefore them.

Based on evolutionary considerations, initiation of ‘excess’ fruits and seeds maybe favoured by several environmental factors, including unpredictability of thephysical environment and of pollination quantity and quality, and fruit and seedpredation (Lloyd, 1980). In addition, there is the possibility that selective abortionof fruits and ovules can improve offspring quality (e.g. Stephenson, 1981; Bookman,1984; Stephenson & Windsor, 1986). For these reasons, fruit and seed overinitiationmay represent an adaptive trait. Nevertheless, it is also possible that it is notadaptive and plants may overinitiate fruits simply because selection has favouredthe overinitiation of flowers. In this case the fact of a greater number of pollinatedflowers than mature fruits may have no adaptive significance (Lee, 1988).

Sexual selection. Recent work has suggested that selection for increased pollen dispersalin the face of competition for mates appears to be one major factor in the productionof surplus flowers (Sutherland & Delph, 1984; but see also Campbell, 1989; Wilsonet al., 1994). If the increase in male fitness gained through pollen donation by these‘excess’ flowers is greater than the loss in female fitness caused by not allocatingthose resources to additional fruit maturation, then the production of ‘excess’ flowerswill be favoured (Willson & Rathcke, 1974; Willson & Price, 1977; Sutherland &Delph, 1984; Sutherland, 1987). Thus, this hypothesis attributes large inflorescencesize to selection through male function (Stephenson & Bertin, 1983; Burd, 1998).

Opportunities for male-male competition depend fundamentally on the distributionof pollen grains among flowers. That distribution, in turn, is dependent on thebehaviour of pollinators. Hermaphroditic plants may be adapted to increase malefitness through pollen donation in several ways. One of these is nectar production,which provides plants with a nonstructural mechanism for dispensing pollen (Harder& Thomson, 1989). Nectar is the flower reward of M. pentlandiana and some of thesources of its variability might be mainly linked with female function (e.g. nectarresorption) or with male function (large nectar availability when the flower opens,variation in nectar production as the flower ages, resumption of secretion afternectar removal; Torres & Galetto, 1998). Plants can indirectly influence pollenremoval by controlling nectar production because visit duration varies directly withavailable nectar volume (e.g. Thomson, 1986; Martınez del Rıo & Eguiarte, 1987),and pollen removal varies directly with visit duration (Harder, 1990; Young &Stanton, 1990). In addition, a variable but renewed reward would determineadditional visits to the flower maximizing the amount of pollen removed by way ofmultiple insect probes (Harder & Thomson, 1989). By contrast, nectar ‘saving’would allow the plant to reallocate resources for female function (Pyke, 1991). InM. pentlandiana, resource recovery through nectar resorption can be reallocated tothe developing fruits, especially by the flowers located in the upper 2/3 of the


inflorescence that show almost no fruits (i.e. flowers that mostly contribute to themale function).

There are numerous plant species that regularly produce inflorescences consistingof many flowers but initiate only a few fruits, leaving an apparent floral excess thatmakes no contribution to seed set (Stephenson, 1981; Sutherland & Delph, 1984).Floral overproduction may increase both male and female functions. Surplus flowerscan elevate male fitness by dispersing additional pollen and siring more seeds onother plants (Andersson, 1994). On the other hand, female fitness may be increasedthrough the pollinator attraction, bet hedging, reproductive assurance, and selectiveabortion mechanisms (Burd, 1998).

Since M. pentlandiana is not pollen limited and there is not selective abortion offruits, surplus flowers would elevate female fitness only when there is resourceabundance in exceptionally favourable years and/or making use of the excess flowersas an ovary reserve. Considering that resource costs of floral production andmaintenance can affect a plant’s fruit- and seed-production (Pyke, 1991; Emms,1996), investment costs should eventually nullify the small fitness benefits throughfemale function as the floral surplus increases. Thus, pollen donation may be theprimary evolutionary factor behind excess flowers, even if excess flowers make somecontribution to female fitness as well (Burd, 1998). In this context, we argue that atheoretical inflorescence size, corresponding to the intersection between the figuresof the mean fruit number and the mean fruit set per inflorescence (cf. Fig. 4), canbe assumed to indicate the inflorescence size that maximizes combined female andmale functions. Comparison between the values of this theoretical inflorescence size(TIS) and the mean inflorescence size (MIS) observed, suggests that male, female,or both functions has/have been favoured by selection. In other words, if the TISis close to MIS, female and male functions have been equally favoured. But, if theTIS shows a higher value than the MIS, then female function is favoured (i.e. plantsdisplay small inflorescences with a high fruit set). Conversely, if the TIS is lowerthan the MIS (as in M. pentlandiana population where the values found are 21 (TIS)and 34 (MIS)), large inflorescences with numerous flowers that do not set fruitswould be favoured. According to the arguments stated above, this excess flowerproduction would be attributed to selection acting through male function. Finally,we think that future investigations on the fitness consequences of such nectar traitslinked to female and male functions may contribute to a deeper understanding ofplant-pollinator interactions as well as to provide a more comprehensive picture ofthe evolution of plant reproductive strategies.

ACKNOWLEDGEMENTS

We thank Marcelo Aizen, Sandra Dıaz, Gabriel Bernardello, and Marcelo Cabidofor useful comments and suggestions on early drafts of this manuscript, an anonymousreferee who substantially improved the submitted version, Arnaldo Mangeaud forhelpful collaboration with statistical analyses, ‘Consejo de Investigaciones Cientıficasy Tecnicas de la Provincia de Cordoba’ (CONICOR), ‘Consejo Nacional deInvestigaciones Cientıficas y Tecnicas’, Agencia Nacional de Promocion Cientıficay Tecnologica’, and ‘Secretarıa de Ciencia y Tecnologıa de la Universidad Nacionalde Cordoba’ for financial support, and CONICOR for the fellowship to CT.


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Factors constraining fruit set in Mandevilla pentlandiana (Apocynaceae

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