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

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

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.

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.

RESULTS

DISCUSSION

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.