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Reproductive Biology |
2USDA Forest Service, Pacific Northwest Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331 USA; 3Department of Biology, Saint Louis University, St. Louis, Missouri 63103 USA; 4Department of Educational Studies, Saint Louis University, St. Louis, Missouri 63103 USA
Received for publication January 8, 2004. Accepted for publication August 26, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: beargrass • Melanthiaceae • pollen • pollination • seed production • self-incompatibility • Xerophyllum tenax
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Although forest management practices as well as extensive commercial harvest may be affecting the species' reproduction, growth, and survival (Dimock, 1981
; Mosley, 2000
), little knowledge exists of its reproductive ecology (Bradley, 1984
). While the floral phenology and flowering pattern of its inflorescence have been described (Long, 1981
; Utech, 1978
; Maule, 1959
) its pollination ecology, self-isolation mechanisms (sensu Bernhardt and Thien, 1987
), and rates of fruit and seed set remain largely anecdotal.
Xerophyllum tenax (beargrass or Indian basket grass) is of cultural and economic importance in northwestern North America (Vance et al., 2001
). To Native American basket weavers of California and the Pacific Northwest, X. tenax historically was and continues to be a valued plant (Turner, 1998
; Rentz, 2003
). Leaves are carefully selected, gathered and dyed for use in basketry (Lobb, 1990
; Moerman, 1998
). Burning off areas where beargrass grows continues to be a traditional practice used to produce the pliable and less pigmented leaves preferred for basketry (Rentz, 2003
). The management and protection of these resources are supported by the USDA Forest Service on lands under their jurisdiction (Vance et al., 2001
). In recent years beargrass leaves have grown in commercial value and are harvested for the floral industry. These leaves are sold to processors for export as raw material to Asian and European markets for dried floral crafts and decorations. The market potential has been estimated for thousands of tons of leaves at over $US 1 million (Blatner and Schlosser, 1998
). Illegal harvest has been extensive and damaging as flowering shoots are often destroyed. In one year over 100 tons of illegally harvested leaves were confiscated from the Willamette National Forest (Mosley, 2000
). Interpreting the floral biology of X. tenax is necessary to understand its conservation and to develop appropriate management of this species.
Furthermore molecular analyses have changed the morphologically based interpretation of the phylogeny of the monocotyledons in general and Xerophyllum and its allied genera in particular. Rudall et al. (2000)
maintain Xerophyllum within the Liliales but their strict consensus tree now places this genus within the family Melanthiaceae. Genera placed within the Melanthiaceae s.s. are synapomorphic for dorsal composite vascular bundles in their flowers, a pollen grain with an operculate sulcus and the presence of Veratrum-type alkaloids. However, while Xerophyllum spp. bear many small, whitish flowers on a much elongated, racemose-paniculate inflorescence, this genus is not a sister of Veratrum and Zigadenus with similar modes of floral presentation. Instead, Xerophyllum is a sister genus of Paris and Trillium. In these two genera the short flowering shoot usually terminates in a large but solitary and often sessile flower. Recent work by Sage et al. (2000)
has also found an early acting, stigmatic self-incompatibility in Trillium spp. This self-incompatibility mechanism is rare in the monocotyledons in general and is currently restricted to lineages in only four other families. Consequently additional work on the floral biology of Xerophyllum is also required to compare phenotypic characters between this taxon and its sister genera to determine the divergence of pollination mechanisms and breeding systems within this lineage.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Xerophyllum tenax consists of a short rhizome and tightly connected shoots. Each vegetative shoot of X. tenax arises from a basal meristem, producing grass-like, keeled, rigid leaves with a life span of several years (Bradley, 1984
). In openings the long-lived plants may become large with multiple shoots of which one or two may produce an inflorescence. The onset or length of flowering at different locations varies with differences in seasonal temperatures, aspect, and elevation (Long, 1981
). In the Oregon Cascades flowering may be initiated as early as April and continue into August. Flowering is often most prevalent in plants growing in forest openings created by disturbance such as wildfire; however, flowering becomes less frequent and may disappear altogether as the forest canopy recloses (Maule, 1959
; Simpson, 1990
).
Floral morphology
The inflorescense is a mass-flowering terminal raceme of small, white to cream-colored flowers on pedicels 2–5 cm long maturing successively from the bottom to the top (Maule, 1959
). The flowering phase is initiated when an inflorescence elongates on an axis that may extend as high as 15 dm. The lower flowers have distinctive bracts that become reduced in length and adnate to the pedicels of the upper flowers. The flower pedicels become elongate and erect at maturity, so that midway through the flowering season, the raceme appears as a conical or cylindrical cluster of opened flowers topped by unopened floral buds. Each flower consists of six oblong to ovate tepals without basal glands. We detected a distinct odor (see Floral attractants and rewards under RESULTS) when the majority of the flowers in the raceme were open. The gynoecium is tricarpellate with three free, recurved styles. The carpels are unilocular, each carpel having a maximum of four ovules (Utech, 1978
). The capsule is ovoid, acute, and about 5–7 mm long and it undergoes loculicidal dehiscence when seeds mature in late summer. After the capsule dehisces and seed has been released, the shoot senesces (Hitchcock and Cronquist, 1978
; Utech, 1978
).
Study sites and design
Two similar but geographically separated sites were selected for the experimental pollination study series and for the observation/collection of insect pollinators. A third site not used for experimentation was used only to collect pollinating insects. These three sites are in the central Cascade Range of Oregon on the USDA Forest Service, Willamette National Forest. The plant association at these sites is Abies amabilis/Rhododendron macrocarpum/Xerophyllum tenax. At all sites flowering occurs usually from late May through early July. The first experimental study site, at about 1020 m elevation, is on a shallow slope of the Hackleman Creek drainage within 100 m of the road. The coniferous stand had been logged on part of the site in the early 1990s and trees were scattered in clumps on grassy openings. The second experimental site is situated on a shallow slope of Camp Creek drainage at about 1120 m elevation and also logged in the early 1990s. Removal of a part of the overstory at both sites created sufficiently large openings that favored X. tenax flowering. These two sites were about 13 km apart in order to ensure that separate populations were being sampled. The third site, used exclusively for insect collection, was Browder Ridge at about 1350 m elevation where the coniferous overstory was also removed in the early 1990s.
The controlled pollination study was carried out on the two widely separated sites noted above. Four treatments were applied to 24 independent plants selected at each site and randomly assigned. Treatment descriptions follow. (1) Self: stigmas of tagged flowers on re-bagged inflorescences received no additional pollen. (2) Open: stigmas of tagged flowers that opened that morning on unbagged inflorescences were exposed to insect pollinators for 24 h. (3) Short-distance hand cross-pollination (SDP): stigmas of bagged inflorescences received pollen from a solitary inflorescence of a neighboring plant within the same population. (4) Long-distance hand cross-pollination (LDP): stigmas of bagged inflorescences received pollen from a solitary inflorescence of a plant from a disjunctive population at the other site (13 km).
Insect identification and observations of floral development and scent occurred during two flowering seasons from 2001 through 2002. The hand cross- and self-pollination study occurred during flowering season of 2002. Micrographs were taken from collections made in 2001 and 2002.
Flower and floral forager observations
Individual flowers on inflorescences were labeled with jeweler's tags and observed while wearing optical glass magnifiers (Opti Visor). Insect foraging behavior was recorded for 2 d in early July 2000, and 3 d in late June 2001 and 2002, representing approximately 22 h of observation. Optical glass magnifiers made it possible for the observer to note whether the forager contacted the stigmas and the anthers while foraging.
To determine whether visitors carried significant quantities of pollen of X. tenax (>25 grains/insect) insects observed foraging on flowers and/or manipulating floral organs with their legs or mouth parts were collected in butterfly nets and killed in jars poisoned with fumes of ethyl acetate on each observation day. Removal, staining, mounting, identification, counting and recording of individual pollen grains carried on insect bodies followed Bernhardt and Weston (1996)
. We used Calberla's fluid (Ogden et al., 1974
) to stain the exine of the pollen walls with basic fuchsin. Insect body length was recorded by measuring the body from its labrum to the apex of the abdomen prior to pinning. Pinned specimens were sent for identification and vouchers were deposited in respective collections: Coleoptera (J. Chemsak, Division of Insect Biology, University of California, Berkeley, California, USA), Diptera (F. C. Thompson, Entomology Division, Smithsonian, Washington, D.C., USA) and Hymenoptera (C. D. Michener; Snow Entomological Museum, Kansas State University, Lawrence, Kansas, USA).
Pollination experiments
Twenty-four flowering plants were randomly assigned to the Open, Self, short-distance cross-pollination (SDP), and long-distance hand cross-pollination (LDP) treatments at each site in late May– early June 2002. For all treatments except the Open treatment, an inflorescence on each plant was isolated in a nylon mesh bag before the flower buds opened.
For determining natural rates of successful pollination Open-treated (unbagged) flowers (each sampled within 24 h of anthesis) were collected from inflorescences at both experimental sites every second day over a 10-d period during mid-June (at peak flowering). For the Self, SDP, and LDP treatments, bags were removed every day and 3–6 flowers were sampled from each inflorescence on their first day of anthesis, hand-pollinated and then labeled with jeweler's tags. Hand-pollinations were made at this time with the aid of an optical glass binocular magnifier, but pollen was applied to the stigmatic surfaces until it was visible to the naked eye. These treatments required re-bagging the inflorescence for an additional 24 h prior to harvesting, fixing, and preserving whole flowers. Pollination success was determined by the numbers of pollen tubes in pistils that penetrated the micropyles of ovules. All flowers were too small to emasculate prior to hand-pollination so the SDP and LDP treatments represent "cross-pollen enhancements" as in Lipow et al. (2002)
.
Collected flowers were fixed in a 3 : 1 glacial acetic acid : 95% ethanol solution for 2 h before decanting the fixative and storing flowers in 70% ethanol. Individual flowers were softened in a 5% aqueous solution of sodium sulphite at room temperature for 24 h. Each pistil was then excised and taken through three consecutive baths of distilled water. Each washed pistil was placed on its own glass slide, the styles were teased apart with forceps or dissecting probes, and then the entire organ was spread and squashed in 2–3 drops of decolorized aniline blue, labeled, and refrigerated a minimum of 7 d. To view pollen tubes in the pistil under epifluorescence we used a Carl Zeiss Incident Fluorescence Microscope with a violet exciter filter as in Bernhardt et al. (1980)
. Viewing techniques and micrography followed Goldblatt and Bernhardt (1990)
.
To determine the number of germinated pollen grains that had grown pollen tubes two pollen tube counts were taken for each pistil. Because the ovary of each pistil bears three separate styles, the number of pollen tubes observed to penetrate the transmission tissue in each style was counted. The total counts from each style were pooled and pollen tubes per style calculated. The number of pollen tubes penetrating ovary tissue was similarly determined and recorded separately.
Open-pollination vs. self-pollination effects on seed set
Bagged stalks bearing infructescences that received no cross-pollination and stalks with open-pollinated flowers were collected in August when capsules were fully developed but had not yet dehisced. At this stage the capsules were sufficiently mature to dissect each ovary to see if they contained seeds. In the laboratory, each infructescence of 11 flowering stalks from the Camp Creek site (the covering of one infructescence was too damaged to use in analysis) and 12 from the Hackleman Creek site were measured to the nearest millimeter and divided into three equal sections. Sampled capsules were dissected, and ovules were observed under a dissecting microscope. The basal, medial, and axial portions were analyzed separately to determine the phenology of flowering in which the flowers basally located on the inflorescence reached anthesis earlier than the axial flowers. The number of flowers was inferred based on counting each pedicel and bract. The capsules were also counted so that ratio of capsules to flowers could be determined. In each (basal, medial, and axial) section of the infructescence, 20 capsules were randomly selected and the length of each measured to the nearest millimeter; seeds were extracted, counted, and the length determined to the nearest millimeter. Total number of seeds and average number of seeds per capsule were determined for the capsules collected from each infructescence of the bagged and unbagged flowering stalks.
Statistical analysis
Tests were conducted using Statgraphics Plus statistical software version 5.1. (Manugistics, 2000
). The pollen tube data were tested for equal variances and distribution departures from normality (Kolmogorov-Smirnov [KS] test for goodness of fit). The Student's t test and KS-test D statistic were used on the pollen tube data to determine significant differences among treatments without assumptions of equal variances. For the seed set analysis, potential site differences were assessed in a two-way analysis of variance (ANOVA); we found no significant differences in the variables tested. Because there were no interactions, each site was treated separately. Significant differences in means were assessed by Tukey's Honestly Significant Difference (HSD). Levene's test of variance was used. Where parameters were not met for the test of variance, the nonparametric Kruskal Wallis test was used. Regression analysis was used to relate capsule length to number of filled seed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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As the perianth expanded in an opening bud, 1–2 of the six anthers were dehiscent, while the three, often appressed, styles curve upwards in suberect positions (Fig. 1A). Within the next two-four hours all remaining anthers dehisced and the style arms began to curve downwards (Fig. 1B). The receptive stigmatic surface on each of the three styles ran from the blunted tip of the style down to its base where the style "arm" connected to the top of the ovary forming a narrow ridge. As the pistil aged each style arm coiled, beginning at the tip, exposing a now upwardly curved segment of the stigmatic surface while concealing more terminal portions of the stigmatic surface within the coil (Fig. 1C). While an individual flower's receptivity is measured in days, each inflorescence may bear from 150 to 400 flowers. Anthesis occurs successively along the peduncle, beginning with flowers in the basal portion of the raceme and progressing acropetally so that the same inflorescence may have receptive flowers for up to 2 wk.
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Floral attractants and rewards
Perianth segments and pistils were white to the human eye, while dehiscent anthers released yellowish, cream-colored pollen. Ovaries commonly turned pink-burgundy purple after the perianth and androecium withered or when the outer whorls were dried by excessive heat.
Scent was variable. In three inflorescences sampled in the field, the floral odor was sweet and agreeable; to the second author it was reminiscent of cultivated lilacs (Syringa). In the remaining 12 inflorescences the odor was musty-acrid. Flowers were removed from eight inflorescences at random and placed in a clean, capped glass vial. The lid was removed and the accumulated odor was described at 5-, 10-, and 30-min intervals. Musty-acrid notes dominated with undertones of sweet notes. The odor of these bottled flowers most resembled cultivated privet (Ligustrum). Floral nectar was not detected at any stage in the floral life-span.
Foraging insects and their pollen loads
Prospective pollinators represented three insect orders and varied considerably in size, yet almost all foragers examined carried the pollen of X. tenax (Table 1). Taxa within the order Diptera (true flies) provided the most numerous and diverse group of foragers. Flies represented six families, but the majority of identified taxa (0.91) belonged to the family of hover flies (Syrphidae). Hover flies were observed probing the stamens and style arms with their probosces while they foraged. These insects remained on an inflorescence for only a few seconds before flying to a second inflorescence. A total of 17 fly specimens were measured with lengths varying from 8 mm (e.g., Parasyrphus relictus) to 17 mm (Laphria sp.). The male-to-female sex ratio of collected hover flies visiting X. tenax was low. Of 69 hover flies evaluated, 0.277 were males. Only one specimen of Cheilosia hoodiana (Syrphidae), the most commonly collected fly, was a male.
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Bees (Hymenoptera) on X. tenax represented four families but were the least frequently observed of all visitors. A total of 14 bee specimens were measured ranging in size from 11 mm (e.g., Coelioxys sp.) to 18 mm (Bombus fernaldi). A solitary specimen of Megachile vidua was the only male captured. All females, excluding Bombus fernaldi, were observed clutching and scraping anthers for pollen. The ventral portions of the bees' bodies bounced against the style arms while they scraped or vibrated anthers. The solitary specimen of Bombus fernaldi was observed to extend its tongue into the inflorescence as if it were attempting to take nectar from the pedicels, but this is a brood-parasitic species (syn. Psithyrus fernaldi) that never collects pollen for its offspring. The collected specimen represented the only observation of this species on X. tenax for the duration of the study.
Of the 138 insect foragers collected on flowers of X. tenax 110 specimens (0.797) carried pollen of X. tenax exclusively (Table 1). Insects bearing mixed loads carried a maximum of two extra pollen types mixed with the pollen of X. tenax. The most common pollen grains, in order of frequencies, were identified as Ceanothus velutinus, general rosiid-type (Amelanchier alnifolia, Fragaria spp., Rubus spp.—all flowering at the three sites), and Rhododendron macrophylum. These species are common co-flowering associates of X. tenax in the Western Cascades (Ross and Chambers, 1988
).
Pollen-pistil interactions in unbagged controls
The stigmatic surfaces of all styles (N = 243) of the unbagged, naturally pollinated pistils (Open) bore hydrated pollen grains, but the amount of pollen per stigmatic ridge varied from 14 to 790 grains. Pollen grains were deposited from the blunted, circular, stigma tip down to the base of the stigmatic ridge above the ovary (Fig. 2A). However, the vast majority of these grains either failed to germinate and were covered in callose crusts, or produced short, irregular tubes, lacking callose plugs that failed to penetrate the stigma surface.
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There was a greater number of penetrating pollen tubes in the stigmas and ovules of pistils that received SDP and LDP (Fig. 2C, D) than those counted in Open or Self treatments (Fig. 2A, B). In styles and ovaries of pistils that received the SDP treatment, the mean number of penetrating pollen tubes were approximately 24 and 20 tubes, respectively. In styles and ovaries of pistils that received the LDP treatment the mean numbers of penetrating pollen tubes were approximately 17 and 15 tubes, respectively. No significant difference was detected in mean number of pollen tubes counted in styles or ovaries between the SDP- and the LDP-treated pistils (Table 2).
Fruit and seed set
Observations of the infructescences and fruits at both sites showed distinct differences between those that developed from flowers that were naturally pollinated (Open) and those bagged (Self). Although there was no significant difference in the number of flowers per inflorescence, we detected a significant difference in mean length between bagged and unbagged infructescences at both sites (Table 3). The size of infructescence may be a function of capsule development as the open pollinated flowers on the (Open) treated inflorescences produced larger, but not significantly more capsules (Table 3). We observed that development was arrested in capsules sampled from bagged inflorescences. Those found to be 
1 mm in length were not included in any capsule analysis as they were obviously undeveloped capsules. Even with this exclusion of undeveloped capsules, the analysis indicated that there was no significant difference detected in the mean capsule-to-flower ratio among the bagged and unbagged flowers (Table 3).
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50% or more of the total. Either total lack of any pollen reaching the anthers or autogamy was possible for flowers that had been bagged. It is likely no seed (or capsule) development occurred if there was no pollination and aborted development occurred with autogamy.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), but direct observation, insect collections, and pollen load analyses indicate that flies, beetles and bees do not appear to contribute equally to the cross-pollination of these flowers. Although female bees carried the greatest numbers of pollen grains on their bodies and in their pollen baskets (P. Bernhardt, unpublished data), they were the least frequent foragers over two seasons. We observed medium-large beetles on inflorescences every day we were in the field and noted they moved sluggishly, at best, among neighboring inflorescences of X. tenax. Flies probably effected the majority of cross-pollinations. Among these three sites, species within the family Syrphidae were the most frequent and "faithful" pollen vectors. Note that 0.88 of the hover flies collected on X. tenax carried only the pollen of X.tenax.
The diversity of flies on the flowers of montane plants in North America is usually explained on the basis of elevation and prevailing climate. Bees are less common and become less active under cooler, wetter regimes. Consequently plants distributed at higher elevations show shifts towards fly-pollination. Our own unpublished observations, though, suggest that a wide variety of solitary bees and eusocial Bombus spp. foraged daily on C. velutinus, Fragaria spp., R. macrophyllum, and Pedicularis racemosa. Despite the mass-flowering mode of presentation, X. tenax is probably unattractive to the majority of common, montane bees as the light colors, disagreeable odors and nectarless condition of its flowers fails to reflect the suite of characters most often associated with bee-pollination. Floral presentation in X. tenax is simply much closer to the classic descriptions of fly-pollinated syndromes (Faegri and van der Pijl, 1979
) or the mass-flowering, "brush mode" of beetle-pollination (see review in Bernhardt, 2000
).
There is precedence for the skewed sex ratios of some insect foragers. The greater proportion of foraging female bees is predictable. Male bees do not, as a rule, collect pollen for the offspring they sire and are probably not attracted to the nectarless flowers of X. tenax. Flowers that have abundant pollen may be more attractive to some female hover flies as a source of lipids and amino acids to invest in eggs; to male flies not having that use for pollen, nectarless flowers would be less attractive. In contrast the higher ratio of males of the beetle C. chrysocoma on inflorescences followed a pattern noted by Dafni et al. (1990)
and Goldblatt et al. (1998)
for hairy, flower-visiting scarabs. Male flower beetles often assemble and wait for the arrival of unfertilized female beetles on preferred flowers (Bernhardt, 2000
), which suggests that an attribute not related directly to floral food may influence the sex ratio of some beetles found on the flowers.
Therefore, floral presentation has diverged significantly within the lineage that includes Paris, Trillium, and Xerophyllum. The first two genera produce much larger flowers and, in most cases, a single flower terminates each peduncle. Little is known of pollination systems in Paris but pollination in Trillium appears to alternate between species pollinated by flies vs. those pollinated by bees (including queens of Bombus spp.) and several Trillium spp. are known to secrete nectar (see review by Irwin, 2000
). The trend within this lineage (sensu Rudall et al., 2000
) suggests a bifurcation in floral presentation. The mass-flowering of many small, nectarless flowers of X. tenax attracts a broader range of potential pollen vectors than the much larger (often nectariferous) flowers of Trillium spp. that may attract a less diverse, but more specialized, spectrum of pollinators belonging to a single order.
More significantly, several species within this lineage retain the same, early-acting self-incompatibility response. The stigmatic ridge of X. tenax recognizes and rejects pollen produced by the same plant as do the stigmas of some Trillium spp. (Sage et al., 2000
). While this self-incompatibility response is atypical for monocotyledons in general, it may be more conservative within these allied genera than their more variable modes of insect-pollination as Trillium populations show either early-acting SI or are self-compatible (Irwin, 2000
; Sage et al., 2000
). As a few bagged flowers of X. tenax set seeds in the absence of cross-pollen, early-acting SI may be "leaky," a feature common to other, unrelated taxa with early-acting systems (Richards, 1997
). A less likely possibility is that these two populations of X. tenax show a small but persistent rate of agamospermy.
This means that while short-tongue flies and beetles are indicative of a "mess and soil" mode of pollination as described by Faegri and van der Pijl (1979)
, our pollen tube analyses suggest at least that some members of these orders are fairly efficient agents of cross-pollination. Most vectors must transfer a minimum of 1–8 viable grains of pollen from one plant to a minimum of one flower on a second genet within 24 h after the flower buds first open as almost 95% of all insect-pollinated pistils contained at least eight normal, penetrating tubes. While less than two tubes penetrated ovules within 24 h, we must remind ourselves that the tubes in the styles showed normal development and probably represented more than one insect visit several hours apart. These tubes would have probably reached the ovary had each one been allowed to grow the full 24 h as in the case of both sets of single-deposition, manually manipulated cross-pollinations. As all flowers were harvested 24 h after the perianth expanded our results reflect insect visitations limited to the first 25–33% of the actual floral life-span.
Technically, X. tenax cannot be labeled as a "pollen-limited" species either, because even the stigmatic ridges of bagged flowers remain liberally coated with grains due to mechanical or wind-driven self-pollination. However, due to the early-acting self-incompatibility system relatively few adhering grains produce tubes that penetrate pistil tissue. Although insects transport pollen between compatible genets, pollen tube and fruit and seed set analyses show clearly that they don't deposit enough compatible grains to fertilize every ovule in the same ovary. This may also be because some insect pollinators are likely to transfer additional incompatible grains as they visit more than one flower in succession on the same inflorescence. Therefore this geitonogamous transfer of grains may decrease the effectiveness of an insect's cross-pollination service. Cross-pollinations made by hand from single sires selected from presumably different genets result in far more pollen tubes penetrating ovules than the vast majority of cross-pollinations perpetuated by insects. Consequently this species may be described as "compatible-pollen limited" for, although this plant is a copious pollen producer and attracts many vectors that cross-pollinate, in 95% of all flowers seed set remains relatively low.
The species is further limited by a fairly substantial light requirement to produce the large racemes. It is adapted to the variation and unpredictability of climate in late May and early June in its montane habitat by producing an inflorescence with many flowers that open sequentially over the span of several weeks. Despite that flowering trait, we observed at one site almost all flowers damaged by a late frost. In addition, flowers are browsed presumably by large ungulates (Young et al., 1939
; Simpson, 1990
). As long as a population remains intact, plants within a population maintain their genetic structure through rhizomatous regeneration and growth. However, if a population is reduced in number by severe fire (Bradley, 1984
) or other disturbances that would displace whole plants, this study suggests that demographic recovery through outcrossing may be slow. Seeds do not germinate readily in cultivation and have long stratification requirements (Smart and Minore, 1977
), so it is unlikely that hand-planting seedlings currently represents a feasible technology. The rhizomatous habit may maintain individual genets particularly when environmental conditions do not favor flowering. However, after natural disturbance such as fire, X. tenax with an incompatibility system and copious pollen production as an attractant can ensure sufficient outcrossing to effect gene flow and remixing of alleles. In addition, by providing food that attracts numerous and diverse pollinators, X. tenax is assuming an important functional role in a montane pollinator system.
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FOOTNOTES |
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5 Author for correspondence (Tel: 541 750-7302; Fax: 541 750-7329; nvance{at}fs.fed.us
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Bernhardt P. R. B. Knox D. M. Calder 1980 Floral biology and self-incompatibility in some Australian mistletoes of the genus Amyema (Loranthaceae). Australian Journal of Botany 28: 437-51[CrossRef]
Bernhardt P. L. B. Thien 1987 Self-isolation and insect pollination in the primitive angiosperms: new evaluations of older hypotheses. Plant Systematics and Evolution 156: 159-176[CrossRef][ISI]
Bernhardt P. P. Weston 1996 The pollination ecology of Persoonia (Proteaceae) in eastern Australia. Telopea 6: 775-804
Blatner K. A. W. E. Schlosser 1998 The floral and Christmas greens industry of the Pacific Northwest. Project Report. USDA Forest Service, Pacific Northwest Research Station, Portland, Oregon, USA
Bradley A. F. 1984 Rhizome morphology, soil distribution, and the potential fire survival of eight woody understory species in western Montana. Master's thesis, Department of Biology, University of Montana, Missoula, Montana, USA
Dafni A. P. Bernhardt A. Shmida Y. Ivri S. Greenbaum C. O'Toole L. Losito 1990 Red bowl-shaped flowers; convergence for beetle pollination in the Mediterranean region. Israel Journal of Botany 39: 81-92[ISI]
Dimock II E. J. 1981 Herbicide and conifer options for reforesting upper slopes in the Cascade Range. PNW-Research Paper-292, USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, Oregon, USA
Faegri K. L. van der Pijl 1979 The principles of pollination ecology, 3rd ed. Pergamon, New York, New York, USA
Franklin J. F. C. T. Dyrness 1973 Natural vegetation of Oregon and Washington. PNW- General Technical Report-8, USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, Oregon, USA
Goldblatt P. P. Bernhardt 1990 Pollination of Nivenia (Iridaceae) and the presence of heterostylous self-compatibility. Israel Journal of Botany 39: 81-92
Goldblatt P. P. Bernhardt J. C. Manning 1998 Pollination of petaloid geophytes by monkey beetles (Scarabaeidae; rutellinae: Hoplini) in southern Africa. Annals of the Missouri Botanical Garden 85: 214-230
Hitchcock C. L. A. Cronquist 1978 Flora of the Pacific Northwest: an illustrated manual. University of Washington Press, Seattle, Washington, USA
Irwin R. E. 2000 Morphological variation and female reproductive success in two sympatric Trillium species: evidence for phenotypic selection in Trilllium erectum and T. grandiflorum (Liliaceae). American Journal of Botany 87: 205-214
Lipow S. R. P. Bernhardt N. C. Vance 2002 Comparative rates of pollination and fruit set in widely separated populations of a rare orchid (Cypripedium fasciculatum). International Journal of Plant Sciences 163: 775-782[CrossRef]
Lobb A. 1990 Indian baskets of the Pacific Northwest and Alaska. Graphic Arts Center, Portland, Oregon, USA
Long R. 1981 Some Liliaceae of British Columbia. Davidsonia 12: 85-88
Manugistics. 2000 STATGRAPHICS 5 Plus. Manugistics, Inc., Rockville, Maryland, USA
Maule S. M. 1959 Xerophyllum tenax, squawgrass, its geographic distribution and its behavior on Mount Rainier, Washington. Madroño 15: 39-48
Moerman D. E. 1998 Native American ethnobotany. Timber Press, Portland, Oregon, USA
Mosley J. 2000 Man pleads guilty in beargrass thefts. June 15, 2000, The Register-Guard, Eugene, Oregon, USA
Ogden E. C. G. S. Raynor J. V. Hayers D. M. Lewis 1974 Manual of sampling airborne pollen. Hafner Press, London, UK
Rentz E. D. 2003 Effects of fire on plant anatomical structure in native Californian basketry materials. Master's thesis, Department of Biology, San Francisco State University, San Francisco, California, USA
Richards A. J. 1997 Plant breeding systems, 2nd ed. Chapman and Hall, London, UK
Ross R. A. H. L. Chambers 1988 Wildflowers of the Western Cascades. Timber Press, Portland, Oregon, USA
Rudall P. J. K. L. Stobart W. P. Hong J. G. Conran C. A. Furness C. G. Kite M. W. Chase 2000 Consider the lilies: systematics of liliales. In K. L. Wilson and D. A. Morrison [eds.], Monocots: systematics and evolution, 347–357. C.S.I.R.O. Collingwood, Victoria, Australia
Sage T. L. V. Pontieri R. Christopher 2000 Incompatibility and mate recognition in monocotyledons. In K. L. Wilson and D. A. Morrison [eds.], Monocots: systematics and evolution, 270–276. C.S.I.R.O. Publishing, Collingwood, Victoria, Australia
Simpson M. L. 1990 The subalpine fir/beargrass habitat type: succession and management. Master's thesis, College of Graduate Studies, University of Idaho, Moscow, Idaho, USA
Smart A. W. D. Minore 1977 Germination of beargrass (Xerophyllum tenax [Pursh] Nutt). The Plant Propagator 23: 13-15
Turner N. J. 1998 Plant technology of first peoples in British Columbia. University of British Columbia Press, University of British Columbia, Vancouver, British Columbia, Canada
Utech F. H. 1978 Comparison of the vascular floral anatomy of Xerophyllum asphodeloides (L.) Nutt. and X. tenax (Pursh) Nutt. (Liliaceae-Melanthioideae). Annals of the Carnegie Museum of Natural History 47: 147-167
Vance N. C. M. Borsting D. Pilz J. Freed 2001 Special forest products: species information guide for the Pacific Northwest. PNW-GTR-513, USDA Forest Service, Pacific Northwest Research Station, Portland, Oregon, USA
Waser N. M. L. Chittka M. V. Price N. M. Williams J. Ollerton 1996 Generalization in pollination systems and why it matters. Ecology 77: 1043-1060[CrossRef]
Young V. A. W. L. Robinette 1939 A study of the range habits of elk on the Selway Game Preserve. Bulletin Number 9. School of Forestry, University of Idaho, Moscow, Idaho, USA
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