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(American Journal of Botany. 2002;89:1599-1603.)
© 2002 Botanical Society of America, Inc.

Reproductive Biology

Why does the flower stalk of Pulsatilla cernua (Ranunculaceae) bend during anthesis?1

Shuang-Quan Huang2,3,5, Yoshitaka Takahashi3 and Amots Dafni4

2College of Life Sciences, Wuhan University, Wuhan 430072, China; 3Department of Livestock and Grassland Science, National Agricultural Research Center for Western Region, Oda, Shimane 694-0013 Japan; and 4The Institute of Evolution, University of Haifa, Haifa 31905, Israel

Received for publication November 30, 2001. Accepted for publication May 7, 2002.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Flower stalks of Pulsatilla cernua, an early spring herb in north temperate Asia, changed position from erect to pendulous and back to erect during 6–10 d anthesis. We tested three possible explanations for this movement. Our results showed that (1) this movement is unlikely to be a mechanism to attract pollinators or enhance pollen output, because no pollinator preference was observed between erect and pendulous flowers and we found no buzz-pollination in this species; (2) hand self-pollination yielded higher seed set than open pollination in the field, but spontaneous selfing rarely occurred. Among open-pollinated flowers, seed set was depressed by emasculation, indicating that in the presence of insects, self-pollen provided reproductive assurance in this protogynous and self-compatible species. However, the change in flower orientation cannot be explained as reproductive assurance in that even self-pollination largely depended on pollinator visits rather than gravity. (3) A pollen germination experiment indicated that pollen damage by water is serious in this species. We deduced that the bending of the flower stalk during anthesis was to avoid rain damage to pollen grains in this species. During the 3–6 d period of pollen presentation, the petals elongated and were covered with unwettable hairs. Together with flower stalk movement, this was enough to protect the organs inside the flower from rain. This movement of the flower stalk seems to be important to maintain pollen viability in a rainy habitat with a scarcity of pollinators.

Key Words: adaptation • flower orientation • flower stalk • pollen viability • pollination • Pulsatilla cernua • rain • Ranunculaceae


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Floral traits such as color, shape, size, and symmetry have been interpreted as adaptations to pollinators (e.g., Waser, 1983 ; Stanton, Snow, and Handel, 1986 ; Nilsson, 1988 ; Dafni and Kevan, 1997 ; Neal, Dafni, and Giurfa, 1998 ). However, the roles of extrafloral structures in pollination have largely been ignored. For example, in some species, movement of the floral stalk during anthesis may influence pollination and reproductive success (Eisikowitch and Rotem, 1987 ).

One aspect of flower orientation, heliotropism, has been investigated (e.g., Kevan, 1975 ). Sun tracking increases intrafloral temperature and pollinator activity at low ambient temperatures. This cannot explain the movement of the flower stalk in Pulsatilla cernua (Ranunculaceae), an early spring plant, in which the flower stalks bend distinctly during anthesis but which occasionally brings the flowers upright to track solar radiation. One-fifth of the flowers of P. alpina remained in the upright position, and it was reported to be heliotropic during periods of direct solar radiation (Luzar and Gottsberger, 2001 ).

In this study, we explore why the flower stalk of Pulsatilla cernua bends during anthesis. We have tested several adaptive possibilities as follows. (1) Is the movement attractive to pollinators? What floral orientation do pollinators prefer to visit? If buzz-pollination exists, a pendulous orientation might increase pollen output (Corbet, Chapman, and Saville, 1988 ). (2) If the flowers are self-compatible and pollen limited due to a scarcity of pollinators, a pendulous position might bring reproductive assurance by enhancing self-pollination. (3) Does the flower stalk bend to avoid rain? Rain can damage pollen viability and constrain pollination success (Corbet, 1990 ; Dafni, 1996 ; Jacquemart, 1996 ; Bynum and Smith, 2001 ). Pulsatilla cernua's flowering period, from April to May, is also the rainy season.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Study species
Pulsatilla cernua (Thunb.) Spreng. (Ranunculaceae) is a perennial herb distributed in Northeastern China, Korea, and Japan. It is a common early spring flower in China but is an endangered plant in Japan because of a rapid decline of populations in the last two decades (Naito and Nakagoshi, 1994 ; Environment Agency of Japan, 2000 ). Individual plants may produce several flowers each year. The six petals are dark purplish red and are covered with white silky-villose hairs. The androecium, with numerous stamens, is bright yellow at the beginning of anthesis and becomes paler with age. The outer short club-shaped stamens are always sterile (staminodes) and secrete nectar, as in the other Pulsatilla species (Jonsson, Rosquist, and Widen, 1991 ). The pistils are numerous and borne on a conical receptacle. Each pistil has one ovule with a long purple style covered with hairs. After pollination, the style will continue to grow, reaching 4–5 cm when the achenes mature.

Flower morphology during anthesis
Thirty flowers of cultivated plants, derived from seeds collected from the field, were observed daily in pots outdoors in April 2000. Six floral parameters of each flower were recorded to describe flower morphological changes during anthesis as follows (Fig. 1): length of stem (S1), length of flower stalk (S2), length of the longest petal (PL), corolla width (CW), height of the longest pistil above the androecium (SH), and the angle from erect to the direction of the pistils ({alpha}). The status of the stamens and floral nectar was examined when we observed these flowers.



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  		Fig. 1. Parameters measured for Pulsatilla cernua during anthesis. S1 = length of stem; S2 = length of flower stalk; PL = length of the longest petal; CW = corolla width; SH = height of the longest pistil above androecium; and {alpha} = the angle from erect to the direction of pistils

 
Pollinator behavior
Observations of pollinator behavior were carried out from April to May in five populations, two in northeastern China in 2000 and three in Japan in 2001. We observed each population on at least 5 d in the field. Various insects were observed visiting the flowers of Pulsatilla cernua in these five populations (S.-Q. Huang et al., unpublished manuscript). Visitors carrying pollen of P. cernua observed moving between the flowers are considered as pollinators in this study. To assess whether visitors prefer pendulous to upright flowers, we recorded the number of visits to flowers at different stages of anthesis and the visitors' behavior within flowers.

Pollination treatments
To examine incompatibility and the possibility of self-pollination in this species, three pollination treatments were carried out at Sugadaira, Nagano Prefecture, Japan (36°31' N, 138°25' E) at an elevation of 1400 m above sea level in 2000. To investigate spontaneous self-pollination, eight flowers were covered by nylon mesh bags to exclude pollinators before the flowers opened. To investigate hand self-pollination, ten bagged flowers were artificially pollinated by self-pollen after anther dehiscence. All bags for these two treatments were removed after about 10 d, just after anthesis ended. To investigate emasculation, the anthers were removed from ten flowers after flowers had opened before anther dehiscence. About 20 d after these treatments, tagged fruits were collected individually and seed number was determined. Twenty naturally pollinated flowers were used as a control for open pollination. At Sugadaira, P. cernua was the earliest flowering species observed in this semi-natural grassland from late April to mid-May.

Pollen germination
To test the behavior of pollen grains under rainy conditions, Dafni's (1992) method of pollen germination was followed. Pollen grains of dehisced anthers were placed on concave glass slides in sucrose solutions with concentrations of 0, 5, 10, 15, and 20% by mass. Pollen grains that had germinated or burst were counted under a light microscope after 5–8 h and again the next day when germination had ceased. In each germination sample we counted at least 2000 pollen grains. This germination experiment was repeated ten times.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Flower morphology during anthesis
Anthesis of a single flower lasted from 6 to 10 d. During anthesis all flower stalks of the cultivated plants bent markedly, though a few flowers in the field did not bend the flower stalk. Accompanying the movement of the flower stalk, flower orientation changed from 0° to 180° and back to 0° with age. According to the angle of flower from vertical ({alpha}), seven flowering stages were identified (Fig. 2, Table 1). Most flowers were not vertically oriented, and only 16% (N = 150) of the flowers were erect when they began to open. When the upper stamens of the androecium dehisced, the flower angle was usually ≥90°. When pendulous flowers began to become upright in the later stages of anthesis, all anthers had opened, the filaments had begun to wilt, and the flowers no longer contained nectar (Table 1). At the same time, the petals stopped growing and the corolla width decreased rapidly, but the flower stalks and the pistils grew longer quickly (Fig. 3).



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  		Fig. 2. Floral orientation during anthesis. Seven stages were identified from A to G according to the degree of flower stalk bending. See Table 1 for floral traits at different stages

 

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  		Table 1. General floral development and the percentage of a total of 494 recorded visits provided by various insects at different stages of anthesis

 


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  		Fig. 3. Changes of five parameters (means ± SD) relative to flower development with the movement of flower stalks

 
Pollinator behavior
Bumble bees (Bombus spp.) and solitary bees were the main pollinators. Some syrphids also visited the flowers but they seldom carried pollen grains. Bumble bees and solitary bees were observed foraging for nectar and carrying pollen grains. The bumble bees pushed their bodies into the flower between petals and the androecium and seemed to forage mainly for nectar. When they crept more or less inside the flower, pollen grains became attached to their legs and abdomen. These pollen grains could be transferred to the stigmas during a continuous visit to the flower of P. cernua. Bees were observed moving around the androecium to collect pollen grains as well as foraging for nectar. Buzz-pollination by bumble bees or solitary bees was not observed in this species.

Pollinators preferred to visit flowers at early stages with nectar (Table 1), but did not prefer downward to upward flowers. Visit frequencies (mean ± 1 SE) to upward and downward flowers were 0.295 ± 0.038 and 0.216 ± 0.031 visits/h (F1,350 = 2.6, P = 0.11) by bumble bees; and 0.551 ± 0.034 and 0.526 ± 0.035 visits/h (F1,646 = 0.26, P = 0.61) by solitary bees. During 5 d of observation from 7 to 12 May at the Sugadaria population, we observed only 22 visits to 20 flowers in a 2 x 2 m quadrat.

Pollen grain behavior in water
On average, 68.6% of pollen grains germinated in the 10% sucrose solution, which seemed to be the optimal solution for germination (Fig. 4). Pollen grains burst in distilled water as well as in low sucrose concentrations. The rates of both germination and bursting in ten germination experiments were significantly different between 0 and 10% sucrose solutions (F1,18 = 322, P < 0.0001; F1,18 = 150, P < 0.0001, respectively).



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  		Fig. 4. Percentage (means + SD) of pollen germination and bursting of Pulsatilla cernua in sucrose solution with various concentrations

 
Pollination treatments
The pollination experiments showed that P. cernua was a self-compatible species. Hand self-pollinated flowers yielded higher seed set than those that were open pollinated (F1,28 = 5.99, P = 0.021), which indicated that pollen limitation existed in this species. However, seed set was very low (1.5%) in the bagged flowers through spontaneous self-pollination (Fig. 5). In open pollinated flowers, emasculation decreased seed set significantly (F1,28 = 11.16, P = 0.0024).



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  		Fig. 5. Seed sets (means + SD) of three pollination treatments and open pollination in a field population of Pulsatilla cernua

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Pollination mechanism of Pulsatilla cernua
Protogyny is a common character in Pulsatilla (Kratochwil, 1988a ; Jonsson, Rosquist, and Widen, 1991 ). Anthesis of P. vulgaris (Kratochwil, 1988b ) and the five species of Pulsatilla studied by Jonsson, Rosquist, and Widen (1991) lasted longer than 10 d and the period of pollen presentation varied from 1.5 to 10 d. The duration of anthesis may be affected by weather and pollination in other species of Pulsatilla (Kratochwil, 1988a ; Jonsson, Rosquist, and Widen, 1991 ) as well as in the P. cernua that we observed. Pollen presentation in P. cernua lasted 3–6 d (Table 1). Numerous stamens (100–300 per flower; S.-Q. Huang et al., unpublished data) dehisced gradually from top to bottom of the androecium. Dichogamy is incomplete in that there is an overlap between the female and male phases in this genus (Kratochwil, 1988a ; Jonsson, Rosquist, and Widen, 1991 ). However, insect pollen vectors were necessary even for self-pollination in P. vulgaris (Kratochwil, 1988a ) and P. pratensis (Torvik, Borgen, and Berg, 1998 ). In this genus, plants lack specialized pollinators. A total of 37 Hymenoptera species was recorded visiting a population of P. vulgaris (Kratochwil, 1988a ). Most visitors to P. pratensis were Bombus species in two populations in Norway; they foraged for nectar and some behaved as nectar thieves (Torvik, Borgen, and Berg, 1998 ). No buzz-pollination by bumble bees has been reported in this genus. The benefit from buzz-pollination is unlikely to provide an explanation for the downward orientation of flowers.

In spring early flowering species are commonly influenced by unpredictable weather and infrequent pollinator visits (Schemske et al., 1978 ). Prolonging of pollen presentation and extending pollen longevity should enhance paternal reproductive success. The very low seed set of enclosed flowers indicated that seed production largely depended on insect visits in P. cernua. Seed set by open pollination at Sugadaira was 49.3%. Because hand pollination of flowers increased seed set, seed production of P. cernua is pollen limited. Limited seed set due to the scarcity of pollinators has been reported in several species of Pulsatilla (Jonsson, Rosquist, and Widen, 1991 ; Torvik, Borgen, and Berg, 1998 ). For reproductive success, it seems important for this early spring species to maintain pollen viability for a long time to wait for visits by vectors (Dafni and Firmage, 2000 ). In cases where pollinators are rare, a "sit-and-wait" strategy of increased floral longevity may be the only means for the flower to fulfill its reproductive role (Ashman and Schoen, 1994 ).

Why do the flower stalks bend?
Flower stalk movement of P. cernua changed flower orientation during anthesis. Such movement is widespread in this temperate genus flowering in spring (Kratochwil, 1988a ; Jonsson, Rosquist, and Widen, 1991 ; Torvik, Borgen, and Berg, 1998 ), but its adaptive functions have not been studied. This movement could not be constrained when we tried to prevent the flowers from bending. In the later stages, the pendulous flowers became upright. This might be a postpollination adaptive strategy to improve seed dispersal (Verbeek and Boasson, 1995 ). Pollen transfer from the anthers to stigmas depended almost entirely on insects, but downward-facing flowers were not more attractive to insects than erect flowers. The bending of the flower stalk seems unlikely to increase insect visits and pollen output in P. cernua. It cannot be explained as a mechanism of reproductive assurance by self-pollination, allowing self-pollen to fall on the stigmas, because pollen deposition to self-stigmas also largely depended on pollinator visits as mentioned above.

Pulsatilla cernua pollen was seriously damaged in water, with a high proportion of bursting (Fig. 4). This was also observed in P. vulgaris (Kratochwil, 1988a ). The movement of the flower stalk changed flower orientation when pollen was released and was accompanied by the growth of petals that protected reproductive organs from rain. The petal, covered with dense unwettable hairs, may also be an auxiliary device (like an umbrella) to reduce rain damage.

Angiosperm flowers have mechanisms to attract pollinators and to achieve successful reproduction in unfavorable abiotic environments. It is generally considered that rainy weather harms fertility in entomophilous flowers during anthesis (Corbet, 1990 ; Dafni, 1996 ), but empirical tests are few. Rain may cause irreversible damage to pollen grains (Corbet and Plumridge, 1985 ; Jacquemart, 1996 ), prevent pollen germination on the stigma, reduce pollen availability for collecting (Bynum and Smith, 2001 ), dilute flower nectar and decrease pollinator visits (Corbet, 1990 ; Dafni, 1996 ), and depress several aspects of maternal fitness (Bynum and Smith, 2001 ). Changing flower orientation will protect the inner organs from direct exposure to the rain (Corbet, 1990 ), as will cleistogamy, resistance of pollen grains to humidity (Eisikowitch and Woodell, 1975 ), and closure of the flower during rain (Corbet, 1990 ; Dafni, 1996 ). In a recent study, Bynum and Smith (2001) observed that floral movements in response to thunderstorms in Gentiana algida improved reproductive success by reducing pollen loss by rainwash. The flowers closed within minutes of an approaching thunderstorm and reopened after direct sunlight returned. There would also be a risk of losing pollen by rainwash in Pulsatilla because pollen grains were easily dislodged by water.

Avoiding rain damage should be necessary for plants flowering in areas or seasons of high precipitation. In contrast, rain may help pollination in some taxa (Runions and Owens, 1996 ). We have concentrated on the movement of the flower stalk, an extrafloral organ, which protects pollen grains from rain. Further experimental study is needed to understand the adaptive functions of extrafloral organs that contribute to pollination.


   FOOTNOTES
 
1 The authors thank Yan-Wen Zhang, Chen-Chuan Liu, Bo Zhang, Tao Zhang, Norio Otaki, Yuko Kurihara, Ichiroku Hayashi, and Yasuyuki Ide for their help in the field and laboratory; Sarah A. Corbet for correcting the English and reviewing the earlier draft; and Dwight Kincaid and an anonymous reviewer for their helpful comments on the manuscript. This study was partly supported by a grant from Ministry of Agriculture, Forestry and Fisheries, Japan and by JSPS Research Fellowships for S.-Q. H. Back

5 Author for reprint requests (hsq1971{at}affrc.go.jp , or sqhuang2001{at}hotmail.com ) Back


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TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
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Bynum M. R. W. K. Smith 2001 Floral movements in response to thunderstorms improve reproductive effort in the alpine species Gentiana algida (Gentianaceae). American Journal of Botany 88: 1088-1095[Abstract/Free Full Text]

Corbet S. A. 1990 Pollination and the weather. Israel Journal of Botany 39: 13-30

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Dafni A. P. G. Kevan 1997 Flower size and shape: implication in pollination. Israel Journal of Plant Science 45: 201-212

Eisikowitch D. R. Rotem 1987 Flower orientation and color change in Quisqualis indica and their possible role in pollinator partitioning. Botanical Gazette 148: 175-179

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Waser N. M. 1983 The adaptive nature of floral traits: ideas and evidence. In L. Real [ed.], Pollination biology, 242–286. Academic Press, New York, New York, USA





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