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(American Journal of Botany. 2002;89:1237-1241.)
© 2002 Botanical Society of America, Inc.

Reproductive Biology

The effects of stigma age on receptivity in Silene alba (Caryophyllaceae)1

Helen J. Young2,5 and Lauren Gravitz4

2Middlebury College, Biology Department, Middlebury, Vermont 05753 USA; 3Barnard College, Biology Department, 3009 Broadway, New York, New York 10027 USA

Received for publication July 31, 2001. Accepted for publication March 1, 2002.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Silene alba, a perennial, dioecious plant, produces flowers that open in the evening and can remain open and receptive to pollination for up to 5 d, though in hot and dry conditions the flowers will wilt during the day only to reopen night after night. In the field, it is visited by two different kinds of pollinators with differential success: moths visit the flowers at night, and their movements result in broad pollen dispersal and large seed production, whereas bees, wasps, and flies visit the flowers in the mornings and have decreased pollination effectiveness. However, this differential success may be due to a decrease in stigmatic receptivity soon after the flowers open. We performed controlled pollinations to determine the effect of stigma age on pollen germination and seed set. We pollinated flowers at 12-h intervals up to 120 h and divided these into two sets: from one set, we removed stigmas 24 h after pollination to examine percentage of pollen germination. The second set of flowers was allowed to produce fruits, and the seeds were counted and weighed. Pollen germination declined significantly with stigma age, but there was no significant effect of stigma age at pollination on the number or mass of resulting seeds. Thus, the decreased pollination success of bees is not due to a decrease in stigmatic receptivity but is most likely a result of pollinator inefficiency.

Key Words: Caryophyllaceae • nocturnal anthesis • Silene alba • Silene latifolia • stigma age • stigma receptivity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Ovules of angiosperms are sessile and require the mobile pollen grains to be transported to the stigma, to germinate on the stigma, to grow down the style, and to fertilize the ovules. Vagaries exist at every stage of this pollination and fertilization process: pollen is groomed from pollinators bodies (Thomson, 1986 ; Harder, 1990 ), pollen passively falls off the anthers during pollinator visits (Harder and Thomson, 1989 ), the stigmatic surface may be clogged with heterospecific pollen, pollen germination may decline with pollen age (Palmer, Travis, and Antonovics, 1989 ; Page and Stucker, 1990 ; Barnes, 1997 ; Proctor, 1998 ) or stigma age (Chang and Struckmeyer, 1976 ; Stosser and Anvari, 1982 ; Morse, 1987 ), and the strength of self-incompatibility may decline with flower age (Stephenson et al., 1992 ; Stephenson, Good, and Vogler, 2000 ). In addition, for plants whose flowers are visited by more than one type of pollinator, the pollinators may vary in the pollen loads they carry, the time they arrive at the flower, and the quantity of pollen they deposit. The ideal scenario involves a pollinator that arrives at a receptive stigma with large quantities of conspecific pollen. Although most pollinators act to increase pollen flow between plants, some are much more efficient than others. Wilson and Thomson (1991) describe two different classes of pollinators in their study of honeybee and bumble bee visitation to Impatiens capensis. "Good" pollinators are efficient both at removing pollen from the anthers of a flower and depositing that pollen on the stigma of another flower. "Bad" pollinators are efficient at removing pollen but less than efficient at depositing it. "Bad" pollinators, when coexisting with "good" pollinators, can act more as antagonists than mutualists to the plant by decreasing the amount of pollen available for the good pollinators to transport (Wilson and Thomson, 1991 ). Because the opportunity for this antagonism exists, it is not uncommon for plants to evolve pollinator-specific traits that maximize visitation by their "good" pollinators.

Silene alba (Caryophyllaceae) is a dioecious, short-lived perennial native to Eurasia. First introduced to North America in the early 19th century, it has since spread throughout the northern United States and Canada (McNeill, 1977 ). The flowers of S. alba are white with a long tubular corolla and produce a strong, sweet scent after dark (H. J. Young, personal observation). Both male and female flowers produce nectar as a reward for their pollinators. The flowers first open in the evenings, but can remain in bloom for up to 7 d if not pollinated (Primack, 1985 ). Once pollinated, however, the flowers begin to wilt within 12 h. It is visited by two different classes of pollinators: nocturnal moths (Shykoff and Bucheli, 1995 ; Young, 2002 ) and diurnal bees, wasps, and flies (Young, 2002 ). Because the flowers open in the evenings, moths are the first to arrive at the flowers, and bees do not arrive until the morning of the following day.

Field studies in Colorado have shown that flowers pollinated by bees had a significantly lower seed set than those pollinated by moths (Young, 2002 ). However, because these pollinators are temporally separated, unless we know how long the stigmas of Silene are receptive, it is difficult to know whether the differential success of bees and moths is a result of pollinator efficiency or due to a rapid decrease in stigmatic receptivity. We suspect at the outset that stigma receptivity will be on the order of several days because female flowers remain open for several days in nature. When pollinators are rare or unpredictable, the costs associated with prolonged flower opening and stigma receptivity may be offset by increased seed production due to increased exposure to pollinators.

While most of the studies that have examined the effects of stigma age on receptivity have found little relation between the two (Chang and Struckmeyer, 1976 ; Kwak and Jennersten, 1986 ; Palmer, Travis, and Antonovics, 1989 ; Gross, 1990 ; Page and Stucker, 1990 ; Shafer, Burson, and Hussey, 1999 ), stigma receptivity (measured by peroxidase activity) did increase with flower age in Collinsia verna (Kalisz et al., 1999 ). In addition, seed set in Asclepias syriaca (Asclepiadaceae) and Mimulus guttatus (Scrophulariaceae) vary significantly with the age of the flower at pollination (Morse, 1987 ; Barnes, 1997 , respectively). Thus, in order to determine whether the decreased seed set from the diurnal pollinators in S. alba was due to decreased pollinator efficiency or decreased stigmatic receptivity, we examined the effects of stigmatic age on receptivity, pollen germination, and seed production.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Silene alba Poiret (= Silene latifolia = white campion) is a dioecious member of the Caryophyllaceae. Female flowers of S. alba usually have a five-lobed stigma (although this trait appears to be plastic and can vary from four to eight), the long styles of which are composed mostly of stigmatic papillae on which pollen can germinate (Lassere, Carroll, and Mulcahy, 1996 ) (Fig. 1). Each female flower is capable of producing more than 500 seeds.



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  		Fig. 1. Drawing of the female flower of Silene alba with the calyx and corolla cut away to show long stigmatic lobes and ovary. Reproduced with permission from Purrington (1993) , Blackwell Publishing, Oxford, UK

 
The plants used in this study were grown from seeds that had been collected in Princeton, New Jersey, USA. Seeds were planted in January 1997 and were divided into different families according to each seed's maternal plant. Seeds were germinated in trays and then transferred to "conetainers" once the cotyledons had fully emerged. The plants were grown under laboratory conditions, with a constant temperature and water supply. They were fertilized two times a week and grown under fluorescent Gro-lux lights. Initially, lights were on for 24 h/d, but as the plants approached flowering they were placed on a cycle of 14 h of light/10 h of dark per day and were no longer fertilized. Flowers were tagged with colored tape on their pedicels immediately after opening so we knew the age of each flower at all times. We used female plants in 12 families.

Gauging stigmatic senescence
Stigmas of flowers of varying ages, from 0 to 5 d old, were examined with a compound microscope at 40x to determine when senescence begins. Senescence was detected by the loss of turgidity of the stigmatic lobes and the papillae on the stigma.

Pollen germination analysis
The flowers used for analyzing pollen germination were pollinated at 12-h intervals up to 120 h old, with the initial pollinations performed soon after the flowers first opened in the evening (N = 3–12 per time interval). They were pollinated using two anthers from a male unrelated to the female plant that were 36 or 48 h old. Preliminary investigations had revealed that pollen younger than 36 h appeared to have a lower germination rate. Two anthers were used for pollinations in this treatment to provide enough pollen to obtain an accurate count of germinated pollen grains but not so much pollen that counting individual grains would be difficult.

The pollinated stigmas were examined to determine the percentage of pollen grains that had germinated. We removed stigmas from the flowers 24 h after pollination and fixed them in 3 : 1 acetic acid : ethanol for 24 h. They were stained with 1% acid fuchsin : 1% fast green (4 : 1) for 24 h and then destained and softened in lactic acid for 12 h (Levin, 1990 ). Stigmas were examined using a light microscope at 100x magnification to count germinated pollen grains and resulting pollen tubes. Using a field of view with more than 150 pollen grains (or more than one field of view for a combined total pollen count of more than 150), we counted the total number of pollen grains and the number of germinated pollen grains. We also counted collapsed pollen grains with no pollen tubes attached and pollen tubes with no pollen grain attached.

Seed set analysis
The protocol was the same as above except that more pollen was applied to the stigmas to ensure full seed set. Pollen from 3–4 anthers of an unrelated male was applied to stigmas until the stigmas were visibly coated with pollen. The first age group of flowers in this treatment was also initially pollinated soon after the flowers first opened and then additional flowers were pollinated at subsequent 12-h intervals (up to 120 h, N = 6–10 per time interval). These flowers were allowed to set fruit, and the resulting seeds were counted and weighed.

Statistical analyses
To measure the effect of stigma age on percentage of pollen germinated, we performed an analysis of variance (ANOVA) using pollen age (36 vs. 48 h) as a fixed block. Percentage of pollen germination was arcsine transformed before analysis. ANOVA was also used to detect female family effects and stigma age effects on seed set and total seed mass. Regressions were performed to determine the relationship between stigma age and pollen germination (arcsine transformed), seed set, and mean seed mass per fruit.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Stigmatic senescence
Stigmas remained turgid and the stigmatic papillae remained firm for the first 4–5 d. After the fifth day, a rapid change occurred in stigma appearance, and the stigma and its papillae appeared to wilt and become visibly flaccid.

Pollen germination analysis
The basic fuchsin/fast green stain revealed two different types of pollen grains. The smallest pollen grains, measuring only 22 µm, have been previously shown to be inviable (Young, 1992 ) and were not included in our pollen counts. The other class of pollen grains were viable and often observed to have pollen tubes emerging from them: they ranged in diameter from 43 to 57 µm. ANOVA revealed no significant variation in percentage of pollen germination among pollen age groups (36 or 48 h old) or stigma age groups (Table 1). However, regression analysis revealed a significant decline in pollen germination with increasing stigma age (Fig. 2; F1,70 = 5.27, P = 0.025, proportion pollen germinating = [–0.0002 x stigma age] + 0.1125; quadratic regression did not result in a better fit). Although pollen germination percentages were low overall, the percentage germinating decreased from about 11% on young stigmas to only 7.5% on 5-d-old stigmas.


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  		Table 1. ANOVA of percentage of pollen germinated of Silene alba in response to pollen age (36 and 48 h) and stigma age. The number of pollen germinating was arcsine transformed before analysis. Type III sums of squares are presented

 


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  		Fig. 2. Effect of stigma age on pollen germination in Silene alba

 
Seed set analysis
The number of seeds per fruit varied greatly, from only 45 seeds to over 600 (N = 88 fruits). There was a significant effect of stigma age and maternal family on seed number (Table 2), but the relationship between stigma age and seed number did not show a linear decline (regression: F1,86 = 0.91, P = 0.34; Fig. 3). There was no significant effect of stigma age on mean seed mass per fruit (Table 2), and the regression between stigma age and mean seed mass per fruit was not significant (F1,86 = 1.81, P = 0.18). The apparent elevated mean seed mass resulting from pollinations of 120-h-old stigmas (Fig. 3) is explained by the low number of seeds within those fruits: each seed was relatively large, but there were few seeds in this treatment. Across all treatments and maternal families, there was a significant negative correlation between seed number per fruit and the mean mass of those seeds (Pearson's correlation, r = –0.46, P < 0.001, N = 88).


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  		Table 2. ANOVA partitioning variation of seed set and seed mass in Silene alba into female plant family and stigma age effects. Type III sums of squares are presented

 


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  		Fig. 3. Effect of stigma age on seed number and mean seed mass per fruit following pollination

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Although pollen germination rate declined with stigma age, the number of seeds produced and their mean mass did not. The observation that stigmas of female flowers remain turgid and their papillae do not begin to senesce until 5 d after opening supports our finding that stigmatic age is uncorrelated with receptivity. Thus, the significantly lower seed set effected by diurnal bees compared to that produced by nocturnal moth visitation in Young's (2002) earlier study may be due to a difference in pollinator efficiency and is not the result of a sudden decrease in stigmatic receptivity.

The question remains why female flowers of S. alba remain receptive for 5 d. Young's study (2002) showing elevated effectiveness of nocturnal pollinators relative to diurnal pollinators was performed by bagging female flowers every day (or night) as long as the flowers remained open. Flowers of each treatment (moth pollinated and bee pollinated) received visitation by insects for 2–4 nights (or days). The per-visit pollen transfer and rate of pollen accumulation on stigmas were not measured. But, because female flowers tend to wilt shortly after pollen deposition, these data suggest that pollen deposition was rare or unpredictable: some flowers were still open after 4 d. Selection may favor prolonged flower longevity and/or stigma receptivity when pollinators are rare or unpredictable. In another moth-pollinated Silene (S. vulgaris), Pettersson (1991) found that although most moths (57%) visiting the flowers did deposit pollen, they delivered fewer pollen grains than were necessary for full seed set.

In the field, the flowers of Silene alba tend to close during the day unless the weather is very cool and humid (Shykoff and Bucheli, 1995 ). This behavior has been attributed to water stress (Primack, 1985 ); however, when they were grown under controlled laboratory conditions with constant temperature and water availability, the flowers appeared to exhibit similar patterns to those observed in natural populations: while they opened fully at night, the following day the blossoms would close slightly. The following night, however, they would open fully once again. After the flowers had been open for more than two nights, however, they did not continue to open and close but remained completely open until they began to senesce. Because this diurnal closing makes the flowers less visible and the stigma less accessible, it is possible that this mechanism has evolved to discourage visitation by diurnal pollinators for several days. If the female has not received pollen on its stigma after several days the flowers remains open all day and night to increase the probability of visitation. Miyake and Yahara (1998 , 1999) and Cruden (1973) have also suggested that the diurnal closure of flowers with nocturnal anthesis protects the pollen from wasteful and inefficient diurnal foraging bees.

Plant–pollinator interactions are complex to study because not all pollinators are equally efficient at effecting pollen transfer. In order to determine the true relationship between Silene alba and its pollinators, much work still needs to be done. A study examining pollen transfer by individual pollinators (as Pettersson did for Silene vulgaris, 1991) would be important in defining pollinator efficiency for each visiting species. An examination of pollen loads and number of visits each flower receives would allow for a more complete understanding of the thoroughness and effectiveness of the pollinating animals. In addition, the order of visitation by different pollinators may be important for both female and male plant reproductive success: if the stigma becomes covered with heterospecific pollen or if the pollen is removed by non-species-specific or wasteful visitors, the presence of these visitors may act to reduce plant fitness (Wilson and Thomson, 1991 ). This effect is compounded when pollen and/or stigmas have limited life spans.


   FOOTNOTES
 
1 The authors thank the National Science Foundation BSR-9106864 and Barnard College for funding and Jackie Donnelly for laboratory assistance. Back

4 Present address: Discover Magazine, 114 5th Avenue, New York, New York 10011 USA Back

5 Author for reprint requests (hjyoung{at}middlebury.edu ) Back


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 ABSTRACT
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 MATERIALS AND METHODS
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 DISCUSSION
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Barnes L. A. 1997 Effects of floral age in Mimulus guttatus (Scrophulariaceae): do things really get better with age?. Senior thesis, Reed College, Biology Department, Portland, Oregon, USA

Chang W. N. B. E. Struckmeyer 1976 Influence of temperature, time of day, and flower age on pollen germination, stigma receptivity, pollen tube growth, and fruit set of Allium cepa L. Journal of the American Society of Horticultural Science 101: 81-83

Cruden R. W. 1973 Reproductive biology of weedy and cultivated Mirabilis (Nyctaginaceae). American Journal of Botany 60: 802-809[CrossRef][ISI]

Gross C. L. 1990 The breeding systems of three co-occurring legumes: Dillwynia hispida, D. uncinata and Pultenaea densifolia (Leguminosae: Papilionoideae). Australian Journal of Botany 38: 207-215[CrossRef]

Harder L. D. 1990 Pollen removal by bumble bees and its implications for pollen dispersal. Ecology 71: 1110-1125[CrossRef][ISI]

Harder L. D. J. D. Thomson 1989 Evolutionary options for maximizing pollen dispersal of animal-pollinated plants. American Naturalist 133: 323-344[CrossRef][ISI]

Kalisz S. D. Vogler B. Fails M. Finer E. Shepard T. Herman R. Gonzales 1999 The mechanism of delayed selfing in Collinsia verna (Scrophulariaceae). American Journal of Botany 86: 1239-1247[Abstract/Free Full Text]

Kwak M. M. O. Jennersten 1986 The significance of pollination time and frequency and of purity of pollen loads for seed set in Rhinanthus angustifolius (Scrophulariaceae) and Viscaria vulgaris (Caryophyllaceae). Oecologia 70: 502-507[CrossRef][ISI]

Lassere T. B. S. B. Carroll D. L. Mulcahy 1996 Effect of pollen competition on offspring quality at varying stages of the life cycle in Silene latifolia Poiret (Caryophyllaceae). Bulletin of the Torrey Botanical Club 123: 175-179[CrossRef][ISI]

Levin D. A. 1990 Sizes of natural microgametophyte populations in pistils of Phlox drummondii. American Journal of Botany 77: 356-363[CrossRef][ISI]

McNeill J. 1977 The biology of Canadian weeds. 25. Silene alba (Miller) E.H.L. Krause. Canadian Journal of Botany 57: 1103-1114

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Page N. J. R. E. Stucker 1990 An evaluation of factors affecting success of controlled crosses of wild rice (Zizania palustris L.) in the greenhouse. Canadian Journal of Plant Science 70: 677-681[ISI]

Palmer M. J. Travis J. Antonovics 1989 Temporal mechanisms influencing gender expression and pollen flow within a self-incompatible perennial, Amianthium muscaetoxicum (Liliaceae). Oecologia 78: 231-236[CrossRef][ISI]

Pettersson M. W. 1991 Pollination by a guild of fluctuating moth populations: option for unspecialization in Silene vulgaris. Journal of Ecology 79: 591-604[CrossRef][ISI]

Primack R. 1985 Longevity of individual flowers. Annual Review of Ecology and Systematics 16: 15-37[CrossRef][ISI]

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Stephenson A. G. J. A. Winsor T. E. Richardson A. Singh T.-H. Kao 1992 Effects of style age on the performance of self and cross pollen in Campanula rapunculoides.. In E. Ottaviano, D. L. Mulcahy, M. Sari Gorla, and G. Bergamini Mulcahy [eds.], Angiosperm pollen and ovules, 117–121. Springer-Verlag, New York, New York, USA

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Young H. J. 2002 Diurnal and nocturnal pollination of Silene alba (Caryophyllaceae). American Journal of Botany 89: 433-440[Abstract/Free Full Text]




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