matrix was made using mean values of the floral attributes for each taxon, and ...... gynoecium) emerged abruptly from t
Plant Systematics and Evolution ? Springer-Verlas 1999
Pl. Syst. Evol. 216: 23–47 (1999)
Printed in Austria
Pollination mechanisms and pollen-ovule ratios in some Genisteae (Fabaceae) from Southwestern Europe JOSEFA LÓPEZ, TOMÁS RODRÍGUEZ -RIAÑO,
ANA
ORTEGA-OLIVENCIA,
JUAN ANTONIO
DEVESA, and TRINIDAD RUIZ Received May 8, 1997; in revised version April 9, 1998
Key words: Leguminosae, Genisteae. – Spain, nectar, pollen, pollination mechanisms, pollen/ovule ratios, anther biomass. Abstract: We studied the biology and floral rewards of 34 taxa of Genisteae from the SW of Europe. Most of the floral attributes show a statistically significant direct relationship. Anther biomass of the lower whorl (1W) is significantly lower than that of the upper whorl (uW), and the ratio of the two (Rv) differs among the taxa. All taxa are polliniferous, and Retama sphaerocarpa also secretes nectar. They can be considered xenogamous or facultative xenogams on the basis of their high pollen/ovule (P/O) ratios. Three principal mechanisms of pollen releasing are identified in this tribe: valvular, pump and explosive; the latter comprises specialized and nonspecialized subtypes. Pollination is sternotribic except in the specialized explosive mechanism, in which it is noto-sternotribic. Thus some pollen serves as food (pollen from the uW, which adheres to the ventral surface of the insect) and part of the pollen fulfils a polliniferous function (pollen from the 1W, which adheres to the dorsal surface). Species that use a pump mechanism have very low Rv values (Rv=0.08–0.26); species with valvular or nonspecialized explosive mechanisms have Rv values between 0.24 and 0.58; those with a specialized mechanism of pollen presentation have high Rv values (0.6–0.76). In contrast to expectations, the highest P/O ratios appear in the specialized explosive system, which allows a single visit from the insect. The tribe Genisteae (ADANS.) BENTH. comprises 14 genera according to POLHILL (1976) and up to 20 according to BISBY (1981). Representatives are present in the Mediterranean region, and throughout Europe and the Canary Islands, and several taxa are located in South Africa, Madagascar and India. Of these genera Lupinus L. is the only one that is widely distributed in the New World. This is the most diverse genus of the group, with nearly 200 species, whereas the rest of the tribe comprises about 273 species (BtsBY 1981). Most members of the tribe are shrubby or subshrubby, and many species of Lupinus are herbaceous. Lupinus is considered a natural group, distinguishable from the rest of the tribe by its morphology, pollination mechanism, root nodules, biogeography, and (to a more limited extent) chemical composition. These features
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led BISBY (1981) to recognize two subtribes within the tribe Genisteae: Lupininae and Genistinae. The Genisteae have typical butterfly-like zygomorphic flowers that are mostly adapted to bee pollination (MEEUSE 1961, FAEGRI & VAN dER PIJL 1979). The different parts of the corolla play an important role with a view to the flower functions. The standard helps to attract the pollinator, the keel protects the staminal column and together with the wings provides a platform for the insects to land on. The wings also operate as levers that raise or lower the carina, and give rise to an attractive structure (ARROYO 1981, STIRTON 1981). The petals are commonly yellow, and less frequently whitish, bluish or purple (Pout-nu 1976). Some petals reflect UV radiation either because of pigment variation or the presence of reflecting hairs (BisBY 1981). Therefore the appearance of the flowers to insects is variable, the greatest differences between genera and species depending on the wing characteristics (KAY 1987). In Lupinus, variegated corollas are frequent, and the contrast may be even more evident after pollination (e.g. L. micranthus Guss.; L. angustifolius L., BISBY 1981; and L. argenteus PURSH, GoRI 1989), a feature that has been noted previously for the genus Chamaecytisus LINK. The most significant functional and taxonomic characteristic of Genisteae is the presence of a typical monadelphous androecium (POLHILL 1976). Unless extrastaminal nectaries exist or autogamy or deception is present, the main attraction of Genisteae flowers to insects is their copious pollen production (Poulin, 1976). In Fabaceae four principal mechanisms of pollen presentation have been recognized: pump, valvular, brush and explosive. Whereas in the first three the flowers can receive several visits, in the last a single visit is effective (DELPINO 1868, LEPPIK 1966). According to LEPPIK (1966) the papilionaceous flower has evolved to greater pollen economy, reaching its biggest success with tripping mechanisms. According to ARROYO (1981), knowledge of the pollen-ovule ratio (P/O), little studied in this family, is useful for detecting possible evolutionary trends in pollen economy. If explosive pollen delivery is the most specialized mechanism (ARROYO 1981), those species with this mechanism would be expected to have low P/O ratios. In the genus Medicago L., whose flowers use an explosive mechanism, P/O ratios are lower than in related genera of the tribe Trifolieae BENTHAM (SMALL 1988). Very little is known of nectar and pollen production in this group (but see HERRERA 1985, 1987; WEBB & SHAND 1985) or of P/O ratios of species of this family. Typifying and quantifying floral rewards in each taxon is of great importance from an applied (e.g. apicultural) point of view, and would shed light on pollinator affinities, whereas knowledge of the P/O ratios provides indirect information about the breeding system. Except for the genus Lupinus (HoRovITz & HARDING 1972, SCHAAL & LEVERICI 1980, SCHAAL 1989) and Chamaecytisus (WEBB & SHAND 1985), no published information exists for the Genistinae. In view of the foregoing, the present study was designed with the following objectives: (1) to elucidate the relationships among the different floral attributes of Genisteae; (2) to typify and quantify the floral rewards (pollen and nectar) offered
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to pollinators, and (3) to determine the P/O ratios of the species, and whether these ratios differ in relation to pollen presentation mechanisms. The work focuses on the Genisteae in the flora of Extremadura (SW Spain), where 36 taxa have been cited (DEVESA 1995). Many of the taxa are important components of Mediterranean forests and scrubs, and are also of interest for apicultural or farming purposes. Better knowledge of their floral and reproductive biology will aid in the management and conservation of these natural ecosystems. Materials and methods Morphology and floral biometry. We studied a total of 88 wild populations from the SW of Spain, belonging to 34 taxa (see Appendix 1). An average of three populations for each taxon and ten flowers per population were studied. Vouchers are conserved in the University of Extremadura Herbarium (UNEX). In each population the colour of the flowers was determined by visual observation in fresh material. Floral size was measured as the maximum length of the flower on its vexilar side, from the base of the calyx to the upper part of the standard in dried material. In dissected flowers we measured ovary and style length and determined the morphology of styles and stigmas. Ovule production per flower was calculated from floral buds preserved in 70% ethanol. Other floral buds were used to study biometric and morphological characteristics of the androecium. Anther size was obtained from two anthers per verticil (B2, D6 from the upper and C4, E8 from the lower whorl; see Fig. 1), and from the F10 or Al anthers when they differed greatly in size from the rest. These anthers were moistened, mounted on a slide and examined with a light microscope. For each anther maximum length (L) and width (W) were obtained, and these values were used to calculate volume. Because the anthers of both whorls were dimorphic, two geometric figures were considered for each flower: (a) a cylinder for the upper whorl (uW), with the formula V= RA, W 2 )/2]/4 (thickness nearly 1/2 the width) and (b) either a cylinder
Fig. 1. Floral diagram of a Genisteae. In the monadelphous androecium, the large ovals represent upper whorl anthers, and the small ones lower whorl anthers
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LÓPEZ
& al.:
or a cylinder and a cone for the lower whorl (1W), with the formula V total = Vcylinder + Vcone = [(piL W2)/2]/4 + [(pi L W 2 )/2]/12. For each taxon the following attributes were also calculated: (1) biomass of the anthers of each whorl (as the average size of the anthers multiplied by the number of anthers); (2) total volume of the whole anthers of the flower (total biomass), and (3) Rv, obtained by dividing 1W anther biomass by uW anther biomass. Pollen size was also studied in acetolysed grains from 89 populations (average of three populations per taxon) with the method of ERDTMAN (1960) modified by HIDEUx (1972). From each slide 15 pollen grains were studied and their volumes were calculated based on the formula for a revolution ellipsoid, 4/3 ra2b, where a is one half of the equatorial axis and b one half of the polar axis. Floral rewards. Nectar. An average of three populations per taxon and 15 flowers per population were studied. Flowers were obtained from flowering branches previously bagged in the field, and were analyzed after 24 h. None of the species produced nectar except Retama sphaerocarpa, although it was reabsorbed or evaporated. Nectar production was estimated in this species in the field in flowering branches from different individuals that had been bagged in nylon nets for 24 h. Production was quantified at 08.00 h (solar time) by measuring the volume with 0.5 µl micropipets, and concentration was measured with two pocket refractometers adapted to small volumes (Bellingham and Stanley Ltd., 0–50% and 40-85%). P o 11 e n . The total production of pollen grains per flower was calculated for the same floral buds as were used to study the ovules. Normally a total of five floral buds per population were studied, or 15 when only one population was available. Direct counting was used when the approximate number of pollen grains per flower was less than 2000, and the dilution method (CRUDEN 1977) when more than 2000 grains per flower were present. For direct counting anthers Al, B2, C4, D6, E8 and F10 (Fig. 1) were each placed in a drop of lactophenol blue on a slide, and squashed gently to spread the pollen. Grains were counted with the help of an eyepiece grid and a manual counter. Production of the uW was calculated with the formula uWP=2(B2+D6)+F10, and that of the 1W with the formula 1WP=A1+2(C4+E8). Total pollen production per flower was considered the sum of the values from the two whorls. With the dilution method the anthers of the two verticils were considered separately in most cases (e.g. Cytisus L. and Adenocarpus DC.), but in some cases (e.g. Lupinus) the 1W anthers were quantified by direct counting while the uW anthers were studied with the dilution method. In all counts those pollen grains that were completely stained with lactophenol blue (fertile) were distinguished from collapsed or weakly stained grains (sterile). Pollen biomass per flower was calculated by multiplying the average number of pollen grains per flower by the average size of the grains. Pollen biomass per verticil was also calculated. Tripping mechanisms and pollen-ovule ratios. Because previous studies deal with only a few species (MULLER 1883, PROCTOR & al. 1996, among others) direct observations were made both in the field and on dried material to determine the mechanism of pollen releasing. Pollen (P) and ovule (0) production per flower were used to calculate the P/O ratio, and these ratios were used to classify the taxa according to the categories proposed by CRUDEN (1977). Statistical analysis. The statistical packages Systat for Windows (Vers. 5; Systat 1992) and Statgraphics for Windows (Vers. 1.0; Statgraphics 1994) were used. A correlation matrix was made using mean values of the floral attributes for each taxon, and these values were subjected to Spearman's rank correlation test. Significance of the values was
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determined with the sequential Bonferroni test at a
