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Reproductive Biology |
2Departamento de Biología de Organismos y Sistemas (Unidad de Ecología), Universidad de Oviedo, E-33071 Oviedo, Spain; 3Bournemouth University, School for Conservation Sciences, Dorset House, Talbot Campus, Fern Barrow, Poole, BH12 5BB UK
Received for publication December 8, 2000. Accepted for publication March 8, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Araceae • Arum italicum • between-plant distance • floral display • flowering dynamics • flowering synchrony • inflorescence number • pollination context
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Floral biology studies usually test the assumption that floral traits have some adaptive value to pollination and reproductive success (Waser, 1983
). Shape or size of floral structures (Campbell et al., 1991
), nectar rewards (Mitchell, 1993
), and floral display size (combining both size and number of flowers) (Andersson, 1991
) are known to influence rates of pollinator visitation and subsequent pollen export and/or receipt.
The hypothesis that floral traits play a significant role in pollination and reproductive success assumes either implicitly or explicitly that additional confounding factors are not present or can be safely neglected. However, reproductive success of particular individuals in one population can be more related to the context in which those individuals reproduce than to their intrinsic properties. For example, several species require disassortative mating between members of two groups (e.g., pin–thrum, male–female). In such species, relative abundance of plants in every group (Wyatt and Hellwig, 1979
; House, 1992
) or the quality of the neighbors as pollen donors (e.g., their flower number; House, 1993
) can affect reproductive success. In addition, negative effects of small population size and plant isolation on fecundity have been documented for species with diverse breeding systems (Widén and Widén, 1990
; House, 1992
; Kearns, Inouye, and Waser, 1998
). Weather can also be an important determinant of fitness. Low temperature decreased pollinator activity and seed set in two subarctic species (Bergman, Molau, and Holmgrem, 1996
). We will refer to all the abovementioned factors as the flowering context. Despite their disparity they share two characteristics: they potentially influence the pollination and reproductive success and they are not factors on which natural selection can act (as opposed to intrinsic properties such as floral morphology or display).
Within-season timing of events can also influence the fitness of particular individuals in a population. Variation in pollination success can be a result of weak flowering synchrony in hermaphrodites (Augspurger, 1981
; Guitián and Sánchez, 1992
), as well as variation in floral sex ratio in monoecious (Le Corff, Ågren, and Schemske, 1998
) or gynodioecious (Williams, Kuchenreuter, and Drew, 2000
) species. The pollination context, such as pollinator visitation rate (Totland, 1994
) or weather (Totland, 1994
; Bergman, Molau, and Holmgrem, 1996
), may also vary over short time scales. Vagaries of the pollination context and its temporal variation can have strong consequences for reproductive performance of species producing one or few flowers. Long floral longevity (Primack, 1985
) or high flowering synchrony (Rathcke and Lacey, 1985
) of single- or few-flowered species can be considered as adaptations to reduce the effect of the pollination context in reproductive success.
Previous studies have shown that external factors can override the effects of floral traits on fitness (Herrera, 1993
; Mitchell, 1994
; O'Connell and Johnston, 1998
), although pollination context has been an understudied topic. Arum italicum Miller (Araceae) is a good system to study how interactions among floral traits, flowering synchrony, and pollination context affect pollination success. The inflorescence of A. italicum is a spadix bearing separated whorls of female and male flowers in its lower portion. The upper half of the spadix consists of a club-shaped sterile appendix. A large green bract surrounds the spadix. At blooming this bract unfolds, showing the appendix but still forming a chamber around the fertile flowers (see Fig. 1 in Méndez, 1998
). This inflorescence is the pollination unit (Fægri and van der Pijl, 1979
). Most reproductive plants produce a single inflorescence in the axil of the third or fourth leaf (M. Méndez, unpublished data), but up to five inflorescences can be produced, in subsequent leaf axils, by large individuals. Arum italicum is pollinated by deception (Dafni, 1984
) of small nematoceran flies. Anthesis occurs over two consecutive days. The species is temporally dioecious sensu Cruden and Hermann-Parker (1977)
. In the evening of the first day, the inflorescence opens and female flowers are functional (female phase of anthesis). Pollinators are attracted to the foul and urinous odor produced by the appendix. Heat production by thermogenic respiration in the appendix (Meeuse, 1975
) volatilizes the attractive substances. Insects are trapped in the lower chamber. In the afternoon of the following day, heat production has stopped, stigmas wither, anthers on male flowers dehisce (male phase of anthesis), and escaping insects are covered with pollen. For further details on Arum pollination, see Fægri and van der Pijl (1979)
. Pollination by deception in A. italicum implies that selective pressures on inflorescence traits can be strong, because of the need for attracting pollinators in a short temporal period. In addition, the low number of inflorescences produced by reproductive plants and the very short functional period makes successful pollination potentially dependent on a strong flowering synchrony with other conspecifics (Fridlender, 1999
; Ollerton and Díaz, 1999
). On the other hand, functional dioecy in A. italicum makes pollination potentially susceptible to the relative abundance of, and distance between, male and female phase plants. In addition, pollen availability for the blooming plant depends on the number of insects trapped by previous inflorescences, which is a function of their appendix length (Méndez and Obeso, 1992
).
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; Uemura et al., 1993
; Yafuso, 1993
). But, as far as we know, no previous study has related intraspecific differences in heat production to individual differences in pollination or fruit initiation. In addition, we address the relative role of flowering synchrony and pollination context on infructescence and fruit initiation. Specific questions addressed are: (1) are inflorescences attractive structures related to fruit initiation?, (2) do flowering synchrony and pollination context influence fruit initiation?, (3) what is the relative importance of pollination context on fruit initiation as compared with inflorescence traits or flowering synchrony?, (4) does production of several vs. one inflorescence increase successful fruit initiation by buffering against a changing pollination context? |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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500 ramets). There, A. italicum was growing along a roadside jointly with other ruderal forbs and grasses, Rubus sp., and Salix atrocinerea. As a representative of a middle size population, we used an experimental population located in Avilés (43°31' N, 5°54' W). It consisted of 67 plants planted as tubers originally collected in a riparian forest in Arlós (43°29' N, 5°54' W; for further details see Méndez, 1997
). Plants occurred within a rectangular plot, spaced 30 cm apart. The greatest distance between two plants within this plot was 4.5 m. A small group of plants was naturally growing 10 m apart from the plot; the only inflorescence in this group overlapping anthesis with the plants in the experimental population was included in our data set. Aside from that small group, this population was at least 1 km from any suitable habitat for A. italicum populations. The isolation of this experimental population allowed us to accurately measure the donor and competitor number (see below) while keeping density and flowering dynamics similar to the one in natural populations.
Variables measured
Both sites were visited daily during the flowering period, February through June. Daily observations in Ablaneda were made until 9 May; subsequent antheses were scarce (<15 inflorescences) (Fig. 1) and widely spaced in time.
We noted rain in a qualitative way (rainy vs. not rainy) for each sampling day. We also counted the number of inflorescences in female (f) and male (m) phases. From these counts we obtained the daily variation in the number of newly open (female phase) inflorescences and daily variation in the number of "donors" (i.e., m) and putative "competitors" (i.e., f – 1) that each newly open inflorescence faced. In Ablaneda, the large population size made it impractical to examine all the plants. Instead, we monitored all the plants within a 25 x 1.5 m plot (>200 plants, 40–50% of the plants at this site). Arum italicum is fully self-compatible (A. Díaz, personal observation), which makes all pollen donors suitable as mates independent of genetic relatedness to the focal inflorescence.
For each newly open inflorescence we measured the following variables: (a) appendix length, from its tip to the joint with the stipe, to the nearest millimeter; (b) appendix length of its nearest "donor" (an estimate of neighbor quality as pollen donor); (c) distance to nearest "donor," to the nearest centimeter. In Ablaneda, we searched for the nearest "donor" both within and outside the plot monitored. When inflorescences opened on days when no "donors" were in bloom, we noted the time elapsed from the most recent day in which "donors" had bloomed. The data on variables (b) and (c) above for these cases refer to the most recent day in which "donors" were present (usually the previous day; Table 1). Final sample size (inflorescence number) is shown in Table 1 and corresponds to 82 plants in Ablaneda (a subsample of all the flowering plants within the plot monitored) and 43 in Avilés (all the flowering plants).
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After anthesis, we estimated pollination success as infructescence and fruit initiation (sensu Stephenson, 1981
), i.e., whether, and how many, flowers in the inflorescence initiated their fruit development. Thirty days after anthesis, we scored any inflorescence appearing dry and shriveled as unsuccessful and any inflorescence showing fruit development as an infructescence. We scored inflorescence status after 30 d because bagged inflorescences (i.e., unpollinated) dried and abscised by that time in a previous assay; despite being self-compatible, we never observed self-pollination in A. italicum. We excluded inflorescences from analyses if abscission occurred <21 d after anthesis. This early abscission, related to the presence of an unidentified pathogen, made it not possible to discern whether growth of fruits or ovules had occurred. For every infructescence, we analyzed fruits and recorded whether they developed (contained seeds or expanded ovules) or not (dry, brown fruits with undeveloped ovules).
Statistical analyses
We considered two successive components of female fecundity: (1) the probability of infructescence initiation and (2) the percentage of fruit initiation within an infructescence, i.e., number of fruits initiated per flower, expressed as a percentage. First, the relative influence of variables measured on the probability of infructescence initiation was identified by means of logistic regression (Hosmer and Lemeshow, 1989
). We tested the significance of variables using the likelihood-ratio test (Trexler and Travis, 1993
).
Second, we identified variables that influenced the percentage of fruit initiation within an infructescence using multiple linear regression. We treated two discrete variables (number of "donors" and number of "competitors") as continuous for these analyses (Sokal and Rohlf, 1981
), and we modeled "rain" as a dummy variable (Zar, 1999
). No transformation was applied to these or any other variable, because the analysis of residuals did not show significant departures from the assumptions of the regression. No strong colinearity was found between independent variables, according to the variance inflation factor (all values below 10) or condition index (all values below 30) (Philippi, 1993
).
Disagreements exist on the best method (enter, forward, backward, or stepwise) of selecting variables in multiple regression. We ran logistic and linear regressions using both enter and stepwise methods, as performed by the statistical package SPSS-PC 4.0 (Noru
is, 1990
). For logistic regression, we got identical results by using either enter or stepwise methods; for linear regression, the stepwise method gave more conservative (i.e., less significant variables) results than the enter method. In the following, we will give results using the enter method, signaling differences with stepwise method when necessary.
Throughout the text, values are given as means ± 1 SD.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The overall percentage of infructescence initiation was 73.6% (N = 106) in Ablaneda and 65.5% (N = 58) in Avilés. These percentages were not significantly different (G test: G1 = 1.163, P = 0.281). Other summary statistics are presented in Table 1. We found that 11% of inflorescences in Ablaneda (N = 106) and 50% in Avilés (N = 58) opened on days when no "donor" was blooming. These percentages were significantly different (G test: G1 = 29.169, P < 0.0001). From those inflorescences, 67% (N = 11) initiated fruit in Ablaneda and 48% (N = 29) did in Avilés. These percentages did not differ significantly (G test: G1 = 1.174, P = 0.279).
Relative influence of inflorescence traits, flowering synchrony and pollination context on infructescence and fruit initiation
At Ablaneda, the logistic regression analysis showed a positive relationship, as indicated by the sign of the partial correlation coefficient (R), between appendix length and the probability of infructescence initiation (Table 2). No other variables were significantly related to the probability of infructescence initiation in this population. In the multiple regression analysis, percentage of fruits initiated per infructescence was positively related to number of "donors," appendix length, and appendix length of nearest "donor" (Table 3). Distance to nearest "donor" was negatively related to fruit initiation (Table 3). In the more conservative regression analysis using the stepwise method, only number of "donors" and appendix length were significantly related to fruit initiation (Table 3). Sign and magnitude of coefficients of partial correlation for these two variables were similar in both models.
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Consequences for single- vs. multi-inflorescence plants
Percentage of multi-inflorescence plants (range: 2–4 inflorescences) was 63.9% (N = 122) in Ablaneda and 51.1% (N = 43) in Avilés. In multi-inflorescence plants, inflorescences were produced sequentially and their antheses were separated by 33.7 ± 8.3 d (range: 7–56 d, N = 43) in Ablaneda and 31.6 ± 8.3 d (range: 18–52 d, N = 25) in Avilés. Production of several inflorescences by the same plant extended the flowering period both at the plant and the population level (Fig. 1). At the population level, a second peak of flowering was apparent.
At both sites, multi-inflorescence plants overlapped their flowering with significantly more plants than single-inflorescence ones (Table 4A). In addition, probability of infructescence initiation was significantly higher for multi-inflorescence plants in Avilés (Table 4B) and close to significantly higher in Ablaneda (Table 4B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Bown, 1988
; Vogel, 1990
; Young, 1990
; Gottsberger and Silberbauer-Gottsberger, 1991
; Gibernau et al., 1999
). Most previous studies on the importance of such structures for pollinator attraction have been qualitative (e.g., Lack and Díaz, 1991
), but there is some quantitative evidence of the positive influence of size or number of those structures on pollinator attraction (Young, 1990
; Gibernau et al., 1999
). In the present study, appendix length was positively related to both the probability and amount of fruit initiation. In addition, fruit initiation increased with temperature excess, a component of floral display understudied until now in Araceae. A previous paper also showed a positive relationship between appendix length and pollinator visits in this species (Méndez and Obeso, 1992
).
Some evidence exists regarding the influence of pollination context, such as between-plant distance or weather, in pollen transfer or reproductive success in Araceae. Fruit set decreased as distance to male inflorescences increased in Arisaema triphyllum (Rust, 1980
; but see Bierzychudek, 1982
) and Amorphophallus johnsonii (Beath, 1996
). Also, cool weather made midges pollinating Arum maculatum torpid (Kite, 1995
). In A. italicum, fruit initiation decreased with increasing between-plant distance in Ablaneda, although this result was not consistent between regression models. On the other hand, any daily variations in pollinator abundance or activity due to rain seemed to have little effect on infructescence initiation. Temperature could be a more important factor, but it was not measured in this study. Everything else being the same, we expected that infructescence or fruit initiation would increase for inflorescences having "donors" with long appendices, because these should attract more pollinators (Méndez and Obeso, 1992
) and, consequently, be better sources of pollinators for newly opened inflorescences. This was the result for Ablaneda, but the opposite effect occurred in Avilés. In all cases, the effect of appendix length of nearest "donor" was weak and its significance changed among regression models. Further observations should be made in order to test the importance of this aspect of "donor quality" for fruit initiation.
Flowering synchrony affected reproductive success in Arum maculatum (Ollerton and Díaz, 1999
). In the same way, in the present study number of "donors" was positively related to either infructescence or fruit initiation at both sites studied. This was not surprising, because of the high daily variation in number of "donors." However, some ability of buffering against this variability seemed to be present in A. italicum because infructescence initiation also occurred on days in which no "donors" were in bloom. This fact is opposed to findings of Beath (1996)
for Amorphophallus johnsonii. In the latter species, no successful fertilization occurred unless pollen was transferred on the same day of anthesis. Results in A. italicum inflorescences could be due either to carryover of viable pollen on pollinators for 1 or 2 d or to pollen arrival from other populations. Unfortunately, data on pollen longevity in this species are lacking to test this.
The relative effect of inflorescence traits, flowering synchrony, and pollination context was population specific. The present study was correlative; thus, it is difficult to disentangle which population characteristics were responsible for such differences. One tentative explanation is offered here that should be tested by means of future experimental manipulation. In Ablaneda, a large population, probability of overlapping antheses was higher than in Avilés (89 vs. 50%, see RESULTS), and this could mean both a lower uncertainty in the Ablaneda pollination context and a lower influence of number of "donors" on infructescence initiation. This, combined with a lower average appendix length, could give better opportunities for the effect of inflorescence traits to become apparent.
A higher flowering synchrony would apparently be advantageous in A. italicum (Méndez, 1998
). A similar result was obtained by Ollerton and Díaz (1999)
for A. maculatum. However, flowering synchrony was not high in A. italicum (maximum of 11 simultaneous antheses in Ablaneda, a population in which 256 antheses were recorded; see Fig. 1) or other Araceae studied (Dieffenbachia longispatha—Young, 1988, 1990
; Philodendron solimoesense—Gibernau et al., 1999
). At present it is not known which proximate cues or ultimate mechanisms can be responsible for that asynchrony. Benefits of an increased flowering synchrony in A. italicum could be overridden by an increase in competition for pollen, as indicated by the negative effect of number of "competitors" on fruit initiation found in Avilés.
Another way to cope with environmental uncertainty is to produce several inflorescences (Burd, 1995
). But despite its benefits for multi-inflorescence plants, this is a limited possibility, because inflorescence number in A. italicum is size dependent (Méndez and Obeso, 1993
). Production of additional inflorescences could potentially be dependent on the pollination success of the first inflorescence but this possibility could not be explored with the data available. On the other hand, an increased inflorescence longevity could also buffer against a variable pollination context. Although many Araceae inflorescences exhibit a short functional period (Gibernau et al., 1999
; Ollerton and Díaz, 1999
), extended flowering also exists (Wada and Uemura, 2000
). To what extent phylogenetic constraints (Ollerton and Díaz, 1999
) or a high maintenance cost (Primack, 1985
) of an energetically expensive floral display affect inflorescence longevity in Araceae should be addressed by future studies.
Finally, although we have mainly discussed the evidence concerning Araceae, the interaction among floral characters, flowering synchrony, and pollination context can be important for fitness in many other species producing one or a few flowers (Totland, 1994
) or requiring disassortative mating between members of two groups (Wyatt and Hellwig, 1979
; House, 1992
). The pollination context could potentially affect selective pressures on flowering synchrony or floral characters (Schemske and Horvitz, 1989
) differently in populations of varying densities or sex ratios, as well as influence genetic neighborhoods (Young, 1988
). This will be a fruitful topic for future studies.
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FOOTNOTES |
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4 Author for reprint requests, current address: Department of Plant Ecology, University of Uppsala, Villavägen 14, SE-752 36 Uppsala, Sweden (e-mail: marcos.mendez{at}ebc.uu.se
).
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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}G), and level of significance are given