Fabaceae) in Southeast Queensland, Australia

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Plant Species Biology (2009) 24, 11–19 doi: 10.1111/j.1442-1984.2009.00235.x

Pollination biology of the sclerophyllous shrub Pultenaea villosa Willd. (Fabaceae) in southeast Queensland, Australia

JANE E. OGILVIE,1 JACINTA M. ZALUCKI and SARAH L. BOULTER Centre for Innovative Conservation Strategies, Grifﬁth University, Nathan, Queensland 4111, Australia

Abstract

The pollination biology of the common shrub Pultenaea villosa Willd. was examined in a subtropical dry sclerophyll forest in eastern Australia. We determined floral phenology and morphology, the timing of stigma receptivity and anther dehiscence, nectar availability, the plant breeding system, and flower visitors. The shrub’s flowers are typical zygomorphic pea flowers with hidden floral rewards and reproductive structures. These flowers require special manipulation for insect access. A range of insects visited the flowers, although bees are predicted to be the principle pollinators based on their frequency on the flowers and their exclusive ability to operate the wing and keel petals to access the reproductive structures. Nectar and pollen are offered as rewards and were actively collected by bees. Nectar is offered to visitors in minute amounts at the base of the corolla. In Toohey Forest, P. villosa flowers in spring and is the most abundant floral resource in the understory of the forest at this time. The breeding system experiment revealed that P. villosa requires outcrossing for high levels of seed set and that the overlap of stigma receptivity and pollen dehiscence within the flower suggests the potential for self-incompatibility. Keywords: bees, dry sclerophyll forest, melittophily, Mirbelieae, self-incompatible. Received 6 July 2008; accepted 10 November 2008

Introduction markings. The only exceptions appear to be species in the genera Leptosema and some species in Gastrolobium, where The fabaceous tribe Mirbelieae is a large and diverse flowers are entirely red and elongated (Crisp 1994; Chan- group of approximately 690 species in 25 genera that is dler et al. 2002; Crisp et al. 2005). Members in the Mirbe- endemic to Australia, except for two species that extend to lieae group appear to be largely self-incompatible, relying Papua New Guinea and the Lesser Sunda Islands (Crisp on animal pollinators to transfer pollen to compatible et al. 2005). Species in this tribe are mostly ericoid shrubs stigmas for successful seed set (six out of seven species in that make up a conspicuous component of the understory which the breeding system has been examined; Gross in sclerophyll eucalypt-dominated open forest and wood- 1990, 2001; Young and Brown 1998; but see Rymer et al. land and in heath communities, to which they are largely 2002). In particular, the flowers appear to be mainly ento- confined (Crisp et al. 2005). The floral morphology among mophilous and bees are the most frequent visitors and members of the Mirbelieae is remarkably uniform, con- most likely pollinators (Beardsell et al. 1986; Gross 1992, sisting of mostly zygomorphic pea flowers that are yellow 2001; Young & Brown 1998; Rymer et al. 2002, and see to orange in color, with reddish nectar guides and table 5 in Armstrong 1979). There are exceptions of course, for example, Stonesiella (Pultenaea) selaginoides (Hook. f.) Crisp & P. H. Weston is most likely to be polli- Correspondence: Jane E. Ogilvie nated by nitidulid and clerid beetles (Lynch 1999), and Email: [email protected] 1Present address: Department of Ecology and Evolutionary species in Leptosema and some species in Gastrolobium that Biology, University of Toronto, 25 Harbord St, Toronto, ON M5S have red elongated pea flowers are most likely to be 3G5, Canada. pollinated by birds (Crisp et al. 2005), for example,

Gastrolobium formosum (Kippist ex Lindl.) G. Chandler & ceous species in the genera Eucalyptus, Corymbia and Crisp is pollinated by honeyeaters (Keighery 1984). Angophora (Coutts & Dale 1987). Botanical nomenclature Pultenaea villosa Willd., a member of the Mirbelieae, is a follows that of Henderson (2002). All methods described perennial shrub that is common and widespread in the hereafter were carried out at each of the four sites. understory of dry sclerophyll forests of southeast Queen- sland and eastern New South Wales (Stanley & Ross 1983; Floral phenology and morphology Henderson 2002; Ryan 2003). In these fire-prone dry sclerophyll forests, P. villosa shrubs are killed during fire, but The flowering phenology of the P. villosa populations at require the heat of fire to break through the hard seed coat each site in Toohey Forest was monitored over the entire of soil-stored seeds to trigger seed germination (Coutts 2006 flowering season, from August until it largely ceased 1987; Hill & French 2003). In the absence of fire, the shrub in November. A permanent transect, measuring 100 m in has a lifespan of 6–8 years, at which point populations length and 10 m in width, traversing a representative area begin to decline. As a result of this germination response of the habitat was established at each site (Dafni 1992). The to fire, P. villosa often dominates regularly burnt areas and transects were visited every 2 weeks and the number of can be a good indicator of fire frequency and time since flowering individuals and open flowers per plant was fire at a site (Coutts 1987; Ryan 2003). Despite a good counted. understanding of this aspect of the plant’s life history, The floral development of 102 P. villosa flowers from 20 nothing is known of its pollination biology. shrubs (five per site) was followed from opening to senes- In the present study, we describe the pollination cence. Between three and seven buds from a branch on biology of P. villosa in dry sclerophyll forest. Specifically each shrub were individually labeled with numbered we: (i) examine the floral morphology and determine the merchandise tags. Visitors were excluded from buds floral phenology at both the population and flower levels, through the use of small plastic acetate exclusion cages including the timing of stigma receptivity and anther covered with fine mesh bags (see Boulter et al. 2006 for dehiscence; (ii) assess the availability of the nectar reward the design). The progress of each bud was monitored within flowers; (iii) measure the level of self-incom- daily in the morning and afternoon and was recorded as patibility by experimentally testing the plant breeding ‘closed bud’, ‘splitting bud’, ’open flower’ or ‘senescent system; and (iv) observe flower visitors to evaluate likely flower’. From this data an average measure of the floral pollinators. lifetime, or floral longevity, was made. Accompanying notes were made of the position of the reproductive structures when they were visible, the wilting order of Methods the floral organs, the presence or absence of odor, and Study species any visible changes in the floral organs. At each visit the temperature and relative humidity were measured using Pultenaea villosa (Fabaceae: Faboideae: Mirbelieae) is a a whirling psychrometer (G. H. Zeal, London, England). perennial shrub 1–3 m in height. The shrub has zygomor- Floral longevity in hours was compared among sites phic pea flowers that are axillary and yellow-orange in using a general linear model anova with least squares color, with reddish nectar guides at the base of the upper means to account for unbalanced data; anovas were standard petal. The shrub flowers from autumn until carried out using the statistical program SAS (SAS Insti- spring, with its main peak of flowering in spring (Stanley tute 2003). Type III sums of squares were used in the & Ross 1983). Pultenaea villosa produces turgid pods con- analysis (Sokal & Rohlf 1995). taining one to two seeds (Stanley & Ross 1983). The timing of stigma receptivity and anther dehiscence over the floral lifetime was determined by testing 255 flowers of varying known ages from five shrubs at each of Study site the four sites. To test stigma receptivity the minute style We studied P. villosa during spring flowering and summer was removed from the flowers, placed in a glass capillary fruiting in 2006 in Toohey Forest, Brisbane, southeast tube and a 3% hydrogen peroxide solution was added to Queensland, Australia, a remnant subtropical dry sclero- test for peroxidase activity (Kearns & Inouye 1993). The phyll forest of approximately 640 ha. The area has a sub- stigma was viewed under a field hand-held microscope tropical climate with summer dominant rainfall and an and the presence of bubbling at the style tip was used to average annual rainfall of 1151 mm (Dale & Stock 1987). indicate stigma receptivity (Kearns & Inouye 1993). When Four sites were selected that had substantial numbers of the flowers were collected the number of dehisced anthers P. villosa; Pultenaea Track, Nathan, Toohey Ridge and was recorded to determine the timing of pollen release Mimosa Ridge. All sites were characterized by woodland and any possible overlap with stigma receptivity within or open forest with the tree layer dominated by myrta- the flower.

A further 80 labeled and bagged flowers were collected made and any open flowers were removed. The six treat- and placed in 70% ethanol 24 h after they had opened to ments were as follows: obtain basic morphological measurements. After return- 1 Open pollination: flowering branches were left open ing to the laboratory, the flowers were dissected and mea- to natural pollinators and the flowers were surements were made of corolla length, style and stamen unmanipulated. length, standard petal length and width, wing petal 2 Spontaneous selfing: flowering branches were covered length, and corolla tube depth to obtain mean flower with an exclusion cage and fine-mesh bag to exclude measurements. visitors and the flowers were unmanipulated. 3 Induced selfing: flowering branches were covered with an exclusion cage and fine-mesh bag to exclude Nectar production visitors, and the flowers were emasculated and then hand pollinated with pollen from within the flower. The floral nectar of P. villosa was sampled to assess the 4 Geitonogamy: flowering branches were covered with volume and concentration of nectar available to floral visi- an exclusion cage and fine-mesh bag to exclude visitors. Nectar was sampled from a total of 80 flowers, four tors, and the flowers were emasculated before anthesis flowers from five shrubs per site that were bagged prior to and hand pollinated with pollen from a few flowers on opening to exclude flower visitors. Individual flowers the same plant. were numbered with merchandise tags and monitored for 5 Cross-artificial: flowering branches were covered with timing of flower opening and age. Nectar was allowed to an exclusion cage and fine-mesh bag to exclude visi- accumulate for 24 h (Gross 1992) and at this point flowers tors, and flowers were emasculated before anthesis were removed and immediately sampled. One microliter and hand pollinated with pollen from a few flowers of water was placed at the base of the corolla tube to dilute from different plants. the minute and viscous amount of nectar to enable extrac- 6 Cross-natural: flowering branches were left open to tion (Gross 1992). The solution was gently mixed and then natural pollinators and flowers were emasculated with extracted using a 10 mL microsyringe to calculate the no supplemental hand pollination. volume of nectar. This solution was then placed on the All flowers were visited daily and all open flowers surface of a hand-held 0–32% BRIX refractometer (Atago, received the allocated pollen. This continued until the last Tokyo, Japan) for a reading of sugar concentration. The % flower to open was pollinated twice. The flowers were BRIX measurement (or gram solute per 100 gram solu- visited 2 months later and the presence of a developed tion) was then converted to milligrams of sugar per flower pod was used to indicate successful pollination and the (Bolten et al. 1979; Gross 1992). The nectar volume and seeds within each pod were collected and counted. The sugar concentration per flower were compared among proportion of flowers setting fruit was compared among sites using a one-way anova, after the sugar concentra- the six treatments, four sites and plant identity within tion data were transformed using a log(x + 1) transforma- sites using a general linear model anova with least tion (Sokal & Rohlf 1995). After the initial analysis it was squares means to account for unbalanced data and a obvious that the nectar sugar concentration differed Tukey–Kramer test to pinpoint the differences. The pro- among sites and that this was most likely the result of a portional data were arcsine transformed using a modified single shrub; thus, shrub identity was then included as a version of the Freedman and Tukey arcsine transforma- factor nested within site in a nested anova. tion recommended by Zar (1999). Type III sums of squares were used in the analysis (Sokal & Rohlf 1995). In addi- tion, we calculated the mean seed : ovule ratio for open- Plant breeding system pollinated flowers. To test for the level of self-incompatibility in P. villosa a set of six experimental treatments were applied, modified Flower visitors from the protocol proposed in Dafni (1992). These experimental treatments involved combinations of visitor exclu- We documented visitors to flowers during observational sion, emasculation and hand pollination. Each set of six surveys conducted in September and October 2006 on treatments was randomly split across two shrubs to clear and warm days between 0900 and 1700 hours. Each ensure sufficient flowers, as P. villosa plants have only a flowering shrub was observed for 10 min and every few main branches. Three sets of treatments (six shrubs) visitor was identified as it visited the flowers. A record each at Nathan and Mimosa Ridge and two sets of treat- was made of the number of flowers visited and the time ments (four shrubs) each at Pultenaea Track and Toohey spent at the plant by each visitor. To distinguish pollina- Ridge were carried out. For each set of treatments, six tors from flower visitors, additional notes were made on budding branches were selected, an initial bud count was whether the visitor sipped nectar, collected pollen and

2 1600 Pultenaea Track The yellow-orange corollas (6.32 Ϯ 0.08 mm in length) 1400 Nathan were composed of an upright standard petal Toohey Ridge (4.61 Ϯ 0.07 mm in length and 5.35 Ϯ 0.09 mm in width) 1200 Mimosa Track often with reddish nectar guides at the base, two lateral 1000 wing petals (4.93 Ϯ 0.08 mm in length) that enclosed two 800 fused keel petals, together the wing and keel complex 600 that concealed the gynoecium and androecium. The Ϯ 400 corolla tube measured 2.35 0.04 mm in depth. Within the wing and keel complex, 10 free stamens encircled the 200 style with anthers and stigma held closely together just No. flowering plants per 1000 m 0 below the tip of the petals (Fig. 2). The stamens were of varying lengths around the style (the shortest was 2-Sep 1-Nov 24-Jul Ϯ Ϯ 12-Oct 11-Dec 31-Dec 22-Sep 13-Aug 21-Nov 4.29 0.10 mm and the longest was 6.07 0.50 mm) Ϯ 2006 and the style measured 5.60 0.10 mm in length; that is, the style and the longest stamens were similar in length. Fig. 1 Flowering phenology of Pultenaea villosa populations at Receptivity testing indicated that the stigma was usually each of the four sites in Toohey Forest from August to December receptive at bud splitting (approximately 12–24 h before 2006. flower opening), and remained receptive, in general, until senescence (Fig. 3). Pollen was released from longi- tudinal slits in the anthers and the majority of anthers touched the stigma. Visitors were collected when possible were dehisced at flower opening and were entirely with a hand net and killing jar for later identification. At dehisced within 6 h of opening (Fig. 3). The timing of each observation period the temperature and relative stigma receptivity and anther dehiscence overlapped humidity were measured using a whirling psychrometer within the flower. (G. H. Zeal). Voucher specimens of the bees collected were lodged at the Queensland Museum. In this paper, we only present data on the identity of the visitors. Further analy- Nectar production sis of the density dependence of pollinator visitation will Nectar was produced in minute amounts by P. villosa and be made in a later paper. was located at the bottom of the corolla tube around the base of the androecium and gynoecium. Some flowers Results sampled had no detectable nectar present. After 24 h, the mean nectar volume per flower across the sites was Floral phenology and morphology 0.93 Ϯ 0.03 mL and this amount did not differ among the The P. villosa populations in Toohey Forest commenced four sites (one-way anova, P > 0.05; Fig. 4a). The sugar flowering in the middle of August and continued until concentration of the flowers was also low and varied mid-November in 2006 (Fig. 1). The main peak of flower- greatly, ranging from 0 to 0.16 mg of sugar per flower with ing occurred in early to mid-October and the pattern of a mean of 0.019 Ϯ 0.003 mg or 2.06 Ϯ 0.45% BRIX across flowering was similar among sites (Fig. 1). the sites. The sugar concentration per flower was similar Pultenaea villosa flowers opened in the morning and among the sites (nested anova, P > 0.05; Fig. 4b), but the remained open for approximately 3 days (70 Ϯ 1h sugar concentration differed among shrubs within sites; [mean Ϯ standard error], across sites). Floral longevity these differences arose from a single shrub at Toohey did not differ significantly among the sites (one-way Ridge with a high nectar sugar concentration (nested anova, P > 0.05). On opening, the standard petal would anova, P = 0.0003). pull up from the wing and keel complex and come to rest perpendicular to it. Throughout the lifetime of the flower Plant breeding system the style and stamens would remain enclosed within the keel and wing complex and were only exposed when Approximately 28% of P. villosa flowers were successfully insects manipulated the petals open. The flowers pro- pollinated and produced pods when left open to natural duced no scent detectable by nose. On senescence, the vectors (Fig. 5). The mean seed : ovule ratio for flowers set standard petal was the first to drop, followed by the wing in the open-pollination treatment was 0.84 Ϯ 0.04. The and keel petals, and finally the stamens. The style proportion of fruit set in all within-plant pollination treat- remained and if successfully pollinated and fertilized ments, that is, induced selfing, geitonogamy and sponta- became part of the seed pod. Each flower had two ovules neous selfing, was significantly lower than the remaining with up to two seeds produced per pod. cross-pollination treatments (two-way anova, P < 0.0001

Fig. 2 (a) Position of the stamens and style within the wing and keel complex with the front wing removed and (b) with all petals removed. Scale bar = 2 mm.

Time (h)

Fig. 3 Timing of stigma receptivity and anther dehiscence within the ﬂowers of Pultenaea villosa over the lifetime of the ﬂower. A darkened style or anther indicates that it is receptive or dehiscing pollen, respectively. The number of anthers dehisced at a particular time are means to the nearest whole number, with n = 28, 41, 22, 86, 8, 30, 7, 11, 11 for -12, 0, 6, 24, 30, 48, 54, 72 and > 72 h, respectively. Stigma receptivity is based on the presence or absence of bubbling at the stigma.

and Tukey–Kramer tests, P < 0.05). The pollination Flower visitors success of flowers from the cross-natural treatment did not differ significantly from the open-pollinated flowers, Pultenaea villosa plants were visited by a total of 324 indicating that emasculation had no adverse effect on pol- insects during 30 h of observations. Visitors to P. villosa lination. Flowers cross pollinated artificially had signifi- flowers were from a diverse range of taxonomic groups, cantly lower fruit set than open-pollinated flowers, but a including bees (both social and solitary), wasps, ants, similar fruit set to cross-natural flowers (Fig. 5). beetles, flies and hard bugs (Table 1). Many of these

therefore be potential pollinators. For example, the muscid flies, wasps, ants, chyrsomelid beetle and bug observed crawled over the flowers without contacting the reproductive structures. Buprestid beetles were often seen eating the petals. The bombyliid flies hovered above the flowers, attempted to land and often inserted their proboscis down at the base of the standard petal. In addi- tion, early on in the flowering season, thrips were observed in the keel of the flowers (J. E. Ogilvie, pers. obs., 2006). Eleven bee species were observed on the flowers. Five of these species were common visitors, whereas the remaining visitors were rarely observed (Table 1). The introduced European honeybee Apis (Apis) mellifera Lin- naeus (Apidae) was by far the most frequent visitor, accounting for 54% of bee visits (Fig. 6). The remaining common bee visitors (observed on more than two occa- sions) were native solitary bees from the family Halic- tidae, including Lipotriches (Austronomia) sp. 1, Lipotriches (Austronomia) sp. 2, Lasioglossum (Chilalictus) convexum Smith (Walker 1995), and the native social bee Trigona (Heterotrigona) carbonaria Smith from the family Apidae (Fig. 6). Ϯ Fig. 4 Mean ( standard error) of nectar (a) volume and (b) sugar Bees were the only visitors capable of manipulating the per flower in Pultenaea villosa at the four different sites in Toohey lower petals open. The larger bees would alight on the Forest (n = 20). wing and keel complex and insert their proboscis down below the standard to sip nectar. In this action the keel would move downward and the reproductive column would stay fixed so that the anthers and stigma would become exposed. Using force and with the action of their legs the bees could collect pollen. After suitable rewards were collected the bees would pull away from the flower and the keel would retract back to its original position. Smaller bees, more specifically T. carbonaria, foraged at the tip of the flowers by prying the petals open to collect pollen and contact the stigma.

Discussion Pultenaea villosa presented a typical set of floral traits asso- ciated with bee pollination or ‘melittophily’ (Fægri & van der Pijl 1979), which are traits also typical of the Mirbe- Fig. 5 Mean (Ϯstandard error) proportion of flowers producing pods per branch from the six different pollination treatments lieae tribe (Crisp et al. 2005). The flowers have striking used to test the breeding system of Pultenaea villosa. Different yellow-orange petals, diurnal anthesis, nectar guides, letters indicate significant differences using general linear offer pollen and nectar rewards, and produce nectar at the models with least squares means and the Tukey–Kramer test base of the corolla (Proctor & Yeo 1972; Fægri & van der (P < 0.05). Pijl 1979). Bees were certainly common flower visitors, but for flower visitors to be considered likely pollinators they must demonstrate the ability to both actively collect insects were infrequent visitors. All bees (superfamily pollen and contact the stigma when visiting the flowers. Apoidea) combined were by far the most frequent taxo- In the case of P. villosa, the morphology of the flowers nomic group of visitors. Only a select few of the flower meant that not all observed visitors could achieve this. visitors were able to gain access to the hidden rewards Pollinators need to be capable of operating the wing and and reproductive structures of the P. villosa flowers and keel complex to access the concealed reproductive struc-

Table 1 Flower visitors by taxonomic group to Pultenaea villosa during 10-min observations of shrubs throughout the day

Mean no. individuals Total no. individuals Collect Sip Contact Taxonomic group per 10 min during 30 h of observation pollen nectar stigma

Apis mellifera (Apidae) 0.817 147 ++ + Trigona carbonaria (Apidae) 0.089 16 ++ Lasioglossum (Chilalictus) convexum (Halictidae) 0.139 25 ++ + Lipotriches (Austronomia) sp. 1 (Halictidae) 0.372 67 ++ + Lipotriches (Austronomia) sp. 2 (Halictidae) 0.067 12 ++ + Unknown solitary bee sp. 1 0.005 1 ++ + Unknown solitary bee sp. 2 0.005 1 ++ + Unknown solitary bee sp. 3 0.005 1 ++ + Unknown solitary bee sp. 4 0.005 1 ++ + Unknown solitary bee sp. 5 0.011 2 ++ + Hyleoides sp. (Colletidae) 0.005 1 ++ + Family Vespidae 0.033 6 Family Formicidae 0.033 6 Family Buprestidae 0.078 14 Family Chrysomelidae 0.005 1 Family Bombyliidae 0.111 20 - Family Muscidae 0.005 1 Family Syrphidae 0.005 1 Suborder Heteroptera 0.005 1

+, indicates that the specified action was consistently performed by that flower visitor; -, indicates that the action was performed occasionally, but not at all visits to flowers by that flower visitor.

Fig. 6 Pollinators at ﬂowers of Pultenaea villosa. (a) Apis mellifera (Apidae), (b) Trigona carbonaria (Apidae) and (c) Lipotriches (Austronomia) sp. 2 (Halictidae). Scale bar = 5 mm.

tures, which requires a great amount of force contact the ventral side of the bee. After the bee departed, (Westerkamp 1997). In general, bees appear to be the only the wing and keel complex would spring back into its visitors capable of applying the force needed to access original position. This pollination mechanism, which we flowers of this form (Fægri & van der Pijl 1979; observed in P. villosa, is known as ‘tripping’ and is the Westerkamp 1997), and indeed bees were the only visitor most basic and widespread pollination mechanism in pea capable of operating the flowers of P. villosa. This suggests flowers (Fægri & van der Pijl 1979; Arroyo 1981; that there is a morphological ‘match’ between bees and Westerkamp 1997). these particular pea flowers (Waser 1983). Once it landed On the part of the flower, concealing the reproductive on a flower, a bee would force itself into the flower to structures maximizes the conservation of pollen and forage and in doing so it would depress the wing and keel nectar. However, the trade-off is that it requires special complex to expose the stamens and style, which would manipulation on the part of the flower visitor to gain

Plant Species Biology 24, 11–19 © 2009 The Authors Journal compilation © 2009 The Society for the Study of Species Biology 18 J. E. OGILVIE ET AL. access to the floral rewards and reproductive structures source of floral resources for anthophilous insects, in par- (Arroyo 1981; Westerkamp 1997). Although this might not ticular bees, at this time of year. Although pollen was not always be the case. In the legume Anthyllis vulneraria quantified in the present study, it is likely to be an impor- subsp. vulgaris, Bombus spp. are nectar robbers and access tant reward collected by foraging bees, as female solitary nectar illegitimately by avoiding ‘proper’ manipulation of bees and worker social bees forage for both nectar and the flower, yet they still promote legitimate pollination pollen to provision their nests. (Navarro 2000). Despite such an exception, it is expected Floral morphological differences in plants are often that by imposing a mechanically constraining morphol- found in different populations and in different habitats ogy, the flower will restrict access to particular, although for the same species (e.g. Ehlers et al. 2002). In the present supposedly more effective, flower visitors (Heinrich 1975; study, no such differences were found across populations. Tucker 2003). Providing that visitors are flower constant The floral morphology of P. villosa was consistent across (Chittka et al. 1999), this pollination mechanism is all sites. The only difference detected was the concentra- believed to promote outcrossing (Fægri & van der Pijl tion of sugar at one site. This difference, however, can be 1979; Arroyo 1981; Westerkamp 1997). It would be an directly attributed to one individual shrub, which had interesting exercise to test the hypothesis that the wing nectar of a high sugar concentration. The reasons for this and keel complex of the pea flower restricts access to more individual’s high nectar sugar concentration are not effective pollinators by removing the keel of the flowers known, but might relate to localized effects. To determine and comparing flower visitation, visitor composition and if the floral characteristics of P. villosa are consistent the subsequent seed set of these flowers to intact flowers. throughout the distribution of this species, further com- Pultenaea villosa is a xenogamous plant, that is, it parative studies are needed, but we believe that the data requires the services of pollinators to transfer and deposit presented here is representative of this species. pollen among plants for high levels of pollination and The present study has demonstrated that P. villosa is seed set. Although stigma receptivity and anther dehis- dependent on pollinators for seed set and that bees are cence overlapped within the hermaphroditic flowers, likely to be the only visitors capable of effecting pollina- making selfing a physical possibility, the breeding system tion. The P. villosa populations studied in Toohey Forest experiment revealed that very few seeds are set from self- exist in an already fragmented system and the introduced pollen. This suggests the presence of a biochemical self- honeybee (Apis mellifera) is a common flower visitor. Frag- incompatibility mechanism (Dafni 1992). It is possible, mentation and introduced pollinators are known threats however, that maximal pollen germinability could have to the pollination success of native plants (Goulson 2003; occurred after anther dehiscence and hence after stigma Tscharntke & Brandl 2004); however, it is difficult to receptivity within a flower (Navarro 1997). However, we speculate on the possible impact of either of these pro- regard this dichogamy unlikely because very little seed cesses on the P. villosa populations examined without was set from flowers hand pollinated with self-pollen. knowledge of the pollination condition prior to their Self-incompatibility in P. villosa is supported by the preva- establishment. lence of self-incompatible species in the Mirbelieae, with most published accounts of plant breeding systems finding a reliance on pollen vectors to effect seed set Acknowledgments (Gross 1990, 2001; Young & Brown 1998; but see Rymer The authors thank the Centre for Innovative Conservation et al. 2002). Strategies and the Griffith School of Environment, Griffith Both pollen and nectar were offered by P. villosa as University, for financial support. This study was com- floral rewards to flower visitors. Nectar was produced in pleted as part of J. E. Ogilvie’s honors program. The very small amounts and at low concentrations by the authors thank Yvonne Buckley and Janet Chaseling for flowers so that the amount of sugar available for foraging advice on statistical analysis and Roger Kitching for his visitors was minute. The production of nectar by flowers advice and comments on earlier drafts of this manuscript. from the Mirbelieae taxa Dillwynia spp. was also small and We are also grateful to Chris Burwell at the Queensland in Pultenaea densifolia it was negligible in sclerophyll Museum and Ken Walker at Museum Victoria for identi- mallee communities in South Australia (Gross 1992). fying the bees and Brett Harris for his help preparing the These flowers were visited by solitary bees (Family Col- figures. letidae) that foraged for both nectar and pollen. As P. villosa plants produced many flowers at the patch level and were the most common flowering plants in the References understory layer in the forest at this time, the sheer mass Armstrong J. A. (1979) Biotic pollination mechanisms in the Aus- of flowers present suggests that nectar was still a valuable tralian flora—a review. New Zealand Journal of Botany 17:467– reward and that the plant is likely to be an important 508.

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