Composition and Nectary Surface Changes in Eucalyptus

Original article

Influence of floral visitation on nectar-sugar composition and nectary surface changes in Eucalyptus

AR Davis

Department of Biology, 112 Science Place, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2

(Received 23 July 1996; accepted 29 January 1997)

Summary — Floral nectaries and their production of major nectar carbohydrates were studied in three species of Eucalyptus in Australia. In E cosmophylla, E grandis and E pulverulenta, the nectary is located on the inner surface of the hypanthium, below the stamen filaments. Nectary surfaces possessed hundreds of modified stomata that were solitary, distributed uniformly, asynchronous in development, and served as exits for nectar flow. Nectar yields per bagged flower were greatest in E cosmophylla and least in E grandis, correlating with flower size but not nectary stomatal density. The nectar of E pulverulenta was sucrose-rich, but hexose-rich for the others. Few changes in nectar carbohydrate composition were detected between flowers whether protected or continually exposed to visitors (eg, honeybees), and whether young or old, indicating an overall constancy in composition for the long period of nectar availability.

Eucalyptus / honeybee / modified stomata / nectar carbohydrate / nectary surface

INTRODUCTION ical analyses of Australian honeys derived from individual eucalypt species are avail- Floral nectar from Eucalyptus trees provides able (Chandler et al, 1974). However, little Australia’s most important source of honey research has been devoted to Eucalyptus production (Brimblecombe, 1946; Clem- nectar, particularly within Australia (Bond son, 1985), allowing the country’s average and Brown, 1979; Moncur and Boland, honey yields per bee colony to rank among 1989). the highest in the world (Anonymous, 1985, 1987). Today, their potential for honey and Although the overwhelming majority of timber production, as well as their orna- species secrete floral nectar of relatively mental value, has fostered cultivation of constant composition (Percival, 1961), some eucalypts in many other countries (Pellett, changes may occur. The carbohydrate com- 1923; Lovell, 1926; Vansell, 1941; Barbier, position may change with floral age, whether 1951; Lupo and Eisikowitch, 1990). Chem- collected from species having short-lived (≤ 24 h) flowers (Freeman, 1986; Petanidou to investigate further the mechanism of nec- et al, 1996), flowers of intermediate (2-4 tar escape. days) duration (Loper et al, 1976), or long- lived (5-8 days) flowers (Christ and Schnepf, 1988; Kronestedt-Robards et al, MATERIALS AND METHODS 1989). An increased amino acid concentration is common in nectar of aging flowers Plant material (Gottsberger et al, 1990; Petanidou et al, the of flo- 1996). Additionally, composition The trees investigated, one per species, were ral nectar may be altered by animal visits growing as ornamentals at The Australian (Corbet, 1978; Willmer, 1980; Freeman and National University in Canberra. E cosmophylla Wilken, 1987). Flowers of eucalypt species F Muell (native to South Australia; Penfold and between of the are 4-18 Willis, 1961) grew buildings generally long-lived, lasting days of and the Research School 1975; Department Forestry post-anthesis (Ashton, Hodgson, of Biological Sciences. Both E grandis Hill 1976; Núñez, 1977; Griffin and Hand, 1979; (Maiden) (coastline of Queensland and New Moncur and Boland, 1989; Lupo and South Wales; Penfold and Willis, 1961) and E Eisikowitch, 1990; Ellis and Sedgley, 1992). pulverulenta Sims (southeastern NSW; Peters et However, the constancy of Eucalyptus nec- al, 1990) were studied near the western border tar composition either during flower phe- in the Culture Area of the Department of Botany. Of these species, E grandis especially is recog- nology, or following animal visits, has not nized as an important honey plant (Penfold and been examined in Australia. Willis, 1961; Clemson, 1985). Nectar escape in most Myrtacean flowers occurs from the inner surface of the hypanthium, in a region extending from the base of Floral stages the staminophore to the upper surface of the E and ovary (Carr and Carr, 1987, 1990; Beard- cosmophylla E pulverulenta typically possessed three buds umbel 1 and sell et al, 1989; Moncur and Boland, 1989; per (fig 7); E grandis had seven. Five floral stages (I-V; see figs 2, O’Brien et al, Structures referred to 1996). 5 and 8) were designated. Flowers entered stage as stomates stomata (Davis, 1968, 1969), I following dehiscence of the bud cover (oper- (Beardsell et al, 1989), stomatal-like (Mon- culum; fig 2 top left), when the stamens com- cur and Boland, 1989), modified stomata menced unfolding from their natal position occu- (O’Brien et al, 1996) and pore cells (Carr pying the hypanthial region. The protandrous flowers at II had most stamens still with and Carr, 1987, 1990) now have been stage their filaments bent inward; only a minority of detected on the surfaces of floral nectaries innermost stamens still occupied the hypanthium. from over 40 Myrtacean (mostly Eucalyp- Continual unfolding of innermost stamens cou- tus) species. On the nectary surface of pled with outstretching of the outermost stamens E stellulata, Davis ( 1969) reported that these characterized stage III. A strong fragrance was structures remain permanently open, evident by this stage. At stage IV, stamens were whereas Carr and Carr (1987) postulated extended fully to form an obvious ring encir- cling the central style. Stage-V flowers still pos- that are able to open and close, and they sessed all stamens, but in a collapsed ring owing may regulate nectar flow through them. to the shrivelled filaments.

The objectives of this study were to deter- To prevent visits by birds (honeyeaters) and insects to mine, in three Eucalyptus spp in Australia, large (primarily Apis mellifera; fig 1) some flowers, inflorescences tree were various nectar characteristics the many per including when anthesis was imminent, of protected (fig 1) constancy nectar-carbohydrate composi- by enclosure within bags (mesh size approx tion flower as following visits and flowers 2.5 mm; see fig I 1 of Davis, 1992) fitted over age; and to examine the nectary surfaces, wire frames. In E grandis, dehiscence of the operculum was preceded by its change from Contents of sucrose, glucose and fructose per green to yellow (fig 5 top left; Hodgson, 1976); nectar sample were expressed as percentages. predictors of anthesis in the other species were Means were calculated for each floral stage and not obvious. compared by pooled, two-tailed t tests (P = 0.05). Ratios of glucose to fructose (G/F), and sucrose to hexose [S/(G+ F)], were also compared statistically. Nectar collection and analysis

Small numbers of flowers were harvested peri- odically from May-August 1990, and immedi- Examination of nectary surfaces ately carried into the laboratory nearby. In total, five (occasionally four or six) flowers per stage Mature, pre-secretory buds and post-secretory (both exposed and bagged) per tree were taken flowers of each species were harvested and for nectar collection. Nectar was removed using immediately prepared for low-temperature scan- a technique (Davis and Gunning, 1991) com- ning electron microscopy (SEM). Sliced buds bining Drummond Microcaps® (1-10 μL) and and flowers were mounted nectary-side up with then filter-paper wicks (McKenna and Thom- Tissue Tek®/colloidal graphite (1:1) on stubs son, 1988) to soak up residual nectar. Nectar- and plunged into liquid N2 slush (-230 °C). The solute concentrations were assayed immediately flash-frozen specimens were then transferred by expelling nectar from a capillary onto Belling- under vacuum into a Hexland 1000A CryoTrans ham and Stanley refractometers (Tunbridge cold stage attached to a Cambridge Stereoscan Wells, UK), then corrected to 20 °C. Concen- 360 SEM. Nectaries were coated with gold, trations were expressed as g per 100 mL using the viewed directly at 15 kV and -180 °C, and pho- formula of Búrquez and Corbet (1991). Wicks tographed with Ilford SEM film. were allowed to air-dry before storage in clean labeled vials at room temperature. Total nectar volume per flower was calcu- RESULTS lated by summation of filled capillaries, and that of the wicked nectar (< 2 μL) estimated from Nectar volumes, concentrations and sugar concentration data (above) and sugar yields (below). quantities as flowers aged Contents of sucrose, glucose and fructose in Flower duration was shortest in E nectar were assayed enzymatically (Kronestedt- grandis, Robards et al, 1989). All enzymes were of ana- taking 4-6 days from anthesis to stage IV lytical grade (Boehringer-Mannheim; Sigma). and about 6 days more to stage V. In E cos- Briefly, sample wicks were placed in known vol- mophylla, stage III was reached 4-6 days umes (0.5-1.0 mL) of distilled water in dispos- after operculum dehiscence and stage V 7-9 able tubes and on Eppendorf centrifuge agitated later. In III was a Vortex Genie@. After 30 min elution time on days E pulverulenta, stage reached IV and ice, tubes were centrifuged before 5.0-200 μL 6-10-days, stage 12-days, V aliquots were introduced into separate cuvettes stage 17-days post-anthesis, respectively. 100 mM Tris-HCl buffer 4.5, for containing (pH In all species, nectar was absent in flow- sucrose determination; 7.6, for hexoses), pH ers with but was 30 mM ATP, 5 mM NADP and 0.175 U glu- opercula just dehiscing, cose-6-phosphate dehydrogenase (EC 1.1.1.49). collectable by stage I (figs 14-16, upper Depending on the assay, 5-μL aliquots of inver- plots) when flowers of E cosmophylla pos- tase (β-fructosidase, EC 3.2.1.26), hexokinase sessed 20-50 times more than the others. (EC 2.7.1.1) or phosphoglucose isomerase (EC In that species, nectar secretion preceded were added to initiate the reaction. Final 5.3.1.9) anther dehiscence. At stage I, all E cosmo- volumes were 800-900 cuvette. Reduc- μL per flowers secreted nectar, to tion of NADP+ to NADPH was measured phylla compared spec- and E trophotometrically (Eppendorf) at 340 nm using E pulverulenta (70%) grandis (17%). a chart recorder and compared to standards of Thereafter, nectar was available from stages freshly-prepared carbohydrate solutions. II-V in each species.

Patterns of nectar-standing crop were in the others. At these peak levels, flowers similar for all species; flowers exposed con- of E cosmophylla averaged 4.4 and 8.9 times tinually to animal visits contained volumes more nectar than E pulverulenta and that were not significantly different across E grandis, respectively. Bagged flowers of stages II-IV (figs 14-16). By stage V, nec- stage V contained less nectar than stage IV, tar yields declined. but this difference was statistically signifi- In flowers that opened within bags and cant (P < 0.05) only for E grandis. were continually inaccessible to large visi- Nectar-sugar quantities per floral stage tors, copious volumes of nectar accumu- (figs 14-16, bottom plots) corresponded lated (figs 14—16). Nectar yields were great- closely to the patterns for nectar volumes. est at stage III in E pulverulenta, and at IV This relationship occurred because, at each floral stage within a species, nectar-solute 16.0-37.3 g/100 mL (exposed) and concentrations were not significantly dif- 14.1-29.6 g/100 mL (bagged) in E cosmo- ferent between bagged and exposed flow- phylla. In Egrandis, concentrations varied ers. In flowers, of bagged average quantities from 14.8-68.2 g/100 mL (exposed) and nectar sugar were maximal at stage III In E pul- (E pulverulenta) or IV (others), with E cos- 19.1-41.9 g/100 mL (bagged). concentrations from mophylla secreting 1.2 and 5.6 times more verulenta, ranged sugar per flower than E pulverulenta and 17.8-30.6 g/100 mL (exposed) and E grandis, respectively. 23.3-49.6 g/100 mL (bagged). Overall, nec- Average nectar-solute concentrations at tar-solute concentrations were usually low- the five floral stages ranged from est at stage I and highest at stages IV and V. Nectar-sugar composition in exposed and plot). The same held true for exposed flow- protected flowers, as flowers aged ers. In E grandis, the levels of the different sugars remained highly consistent for In E cosmophylla, flowers that were pro- bagged and exposed flowers (fig 15). Sim- tected from visits by netting showed no sta- ilarly, no statistically significant differences tistically-significant changes in quantity of occurred in E pulverulenta, whether bagged each nectar carbohydrate (glucose, fructose, or exposed, as flowers progressed through sucrose) as flowers aged (fig 14, middle stages I-V (fig 16). For both E cosmophylla and E grandis, the sucrose content of nectar the stamens: E cosmophylla (250-450 pm), was usually lower than each hexose (figs 14 E grandis (250 pm), E pulverulenta (fig 9; and 15; table I), whereas nectar sucrose in 200-250 μm). In all species, the modified E pulverulenta (fig 16) averaged 33% over stomata were usually solitary and distributed all flowering stages (table I). When the regularly (figs 6, 9 and 10). Stomatal fre- S/(G+F) ratios were compared between quency remained constant in pre- and post- bagged and exposed flowers for each floral secretory flowers and averaged 348 ± 15 stage in all three species, and then overall (SE, n = 7; E cosmophylla), 299 ± 10 (n = 3; (table I), no statistically significant differ- E grandis) and 163 ± 7 (n = 3; E pulveru- ences were detected. lenta) per mm2 of nectary surface. For all the content species average of glu- In all species, modified stomata of vari- cose in nectar exceeded fructose ous could be detected differences in the developmental stages (figs 14-16). Significant in a In ratio of G/F between and single nectary epidermis. pre-secre- exposed bagged tory buds could be found immature (figs 3 flowers occurred at II in E cosmo- stage left and 10) and maturing (figs 3 right and phylla 14; P < 0.05) and stage III in (fig 11) stomata, and those with open (figs 4, 10 E pulverulenta (fig 16; P < 0.01). When and 12) pores, as well as modified stomata mean data were combined for composition with occluded pores (figs 10 and 13). Sim- all floral stages (table I), nectar of bagged ilarly, this asynchrony in development was flowers of had a E pulverulenta significantly detected in nectaries 6). G/F ratio than that of flow- post-secretory (fig greater exposed The occluding material of light electron den- ers (P < 0.02). sity on the surface of the guard cells of E pulverulenta (fig 13) may correspond to the small red dots apparent with the naked features as flowers Nectary aged eye, especially in fresh flowers of stages IV and V. The nectary surface spanned the inner hypanthium from the style base to near the In E cosmophylla the nectary surface had staminophore and bore modified stomata, a dull sheen and possessed numerous wax except for a zone of variable width below globules (figs 3 and 4) that were absent post- secretion. In E grandis and E pulverulenta but it was not disclosed which floral stages the nectary appeared shiny and, except for were involved nor whether this change was many small flecks of wax in mature buds statistically significant. It would be inter- of the former, lacked wax throughout (figs 6, esting to determine whether pre-nectar of 10-13). Eucalyptus flowers travels two separate routes (which final nectar com- Nectary color changed from mature bud may impact position) through the nectary (Nichol and to stage III to stage V, as follows: E cos- and if so, what of — Hall, 1988), proportion mophylla pale green to bright yellow to the nectar travels each route, yellow-orange; E grandis — bright lemon- particularly of differential rates of secre- yellow to butterscotch (see Clemson, 1985) during periods tion I or V, versus III or IV). That to light butterscotch with mottled black (eg, stage G/F ratios were above (fig 5, bottom right); E pulverulenta—pale unity (1.2-1.4) sug- gests that the secretory process in these euca- green to bright orange to butterscotch with lypts is more complicated than simple inver- red or brown mottling (fig 8, bottom right). sion of sucrose to equal quantities of the hexoses.

DISCUSSION For reasons unknown, G/F ratios occasionally were significantly different, though Floral nectar characteristics not in a consistent pattern. In E cosmophylla (stage II) exposed flowers had a higher G/F ratio than bagged ones, but in E pulveru- Nectar traits of these species differed in sev- lenta (III) the reverse was true. In E cos- eral respects. Both maximal nectar volume mophylla, variability in nectar-sugar com- and sugar production per flower were related position was always greater from exposed to flower size, being highest in E cosmo- flowers than bagged (see SE values in phylla and lowest in E grandis. The ten- table I), but was irregular for the others. dency for nectar-solute concentration to Willmer (1980) found changes in amino- increase during stages I-IV may relate to acid composition of nectar following vis- exposure of standing nectar to the atmo- its; however, amino acids were not detected sphere, during the progressive unfolding of in nectar (Núñez, 1977). the stamens during flower phenology. In eucalypt stage V, further evaporation of water from Microorganisms can occur in nectar and nectar and a diminishment in rate of nectar may affect its composition (Lüttge, 1961; volume secreted, probably contributed to Gilliam et al, 1983), and algae have been the higher concentrations observed. detected on nectaries of eucalypts (Carr and Carr, 1987). Although not sought, uniden- Nectar-sugar composition remained con- tified green-pigmented algae were found stant per species, in contrast to other species on nectaries of two with long-lived flowers (see Introduction). macroscopically bagged flowers of E grandis, nectar collec- Individual levels remained similar, as during sugar tion. the did the ratios G/F and S/(G+F). However, carbohydrate composi- Carbohy- tion of these nectars did drate composition appeared most consistent algal-contaminated not differ from others of III or IV. for nectar of E grandis, which contained the stage Evidently the algal spores entered the nectar lowest sucrose content (14%), and most vari- of these two flowers by passing through the able for E pulverulenta, in which sucrose mesh airborne, or on the bodies of thrips, levels were highest (33%). In California, common inhabitants of all flowers studied. Vansell (1941) reported that for E globu- lus, the ratio between invert and total sugars Overall, the S/(G+F) ratios for nectar of varied over 5 days from 1:1.566 to 1:2.192, E cosmophylla and E grandis averaged between 0.1-0.499 (’hexose-rich’; Baker Nectary surfaces and Baker, 1983), but were in the range 0.5-0.999 (’sucrose-rich’) for E pulveru- The floral nectaries differed in color and lenta. These designations generally agree amounts of surface wax. Changes in color, with observations of floral visitors to these such as the corolla, affect foraging behaviour species. For instance, short-tongued bees (Weiss, 1995). In the absence of prominent like A mellifera are regular visitors of taxa petals, changes in nectary appearance during that produce nectar having a wide range of flower phenology may provide visual for- sucrose/hexose ratios, but particularly those aging cues to eucalypt visitors (O’Brien et which are hexose-dominant or -rich (Baker al, 1996). However, the frequency of mod- and Baker, 1983). Similarly, species polli- ified stomata on the nectary did not change nated principally by honeyeasters (birds) during phenology, nor was it correlated with have hexose-dominant or -rich nectar, only nectar volume or carbohydrate production. and These nectar-car- (Baker Baker, 1983). The terms ’modified stomata’ (Fahn, are similar to bohydrate compositions pre- 1979; Davis and Gunning, 1992) are used vious of other in Cali- analyses eucalypts here to represent the pore structures of Euca- fornia (Vansell, 1941, 1944; Baker and lyptus nectaries. They share specific devel- Baker, 1990), Argentina (Núñez, 1977) and opmental features (eg, breakthrough; Carr Australia (Moncur and Boland, 1989) which and Carr, 1987) and likeness with foliar ranged from hexose-dominant to sucrose- stomata, but evidently are unable to close rich. However, here the G/F ratios exceeded their pores by guard-cell movements (Davis, unity. Non-eucalypt nectars of the Myr- 1969). Instead, modified stomata (even on taceae are hexose-rich or -dominant (Perci- buds) can be found with pores occluded val, 1961; Beardsell et al, 1989; O’Brien et (Davis and Gunning, 1992). Occlusion al, 1996). apparently involves release of hydrophobic material(s) from the guard cells to seal the The phenomenon of net reabsorption of pore. Unknown is whether an occluded pore nectar carbohydrates is well established in completely restricts the passage of exudate various taxa (Pedersen et al, 1958; Shuel, from the gland, or disallows any re-entry 1961; Corbet and Delfosse, 1984; Búrquez during nectar reabsorption. It would prove and Corbet, 1991) and now for Eucalyptus. interesting to determine whether modified Interestingly, no changes in sugar compo- stomata without overlying nectar droplets sition occurred here during the period of (see Beardsell et al, 1989) are possibly nectar reabsorption. Therefore, net recla- immature or occluded. Unlike the stamens mation of uncollected nectar did not occur and style, the nectary does not abscind (Carr selectively, for any of the three carbohy- and Carr, 1990). Hence, it is possible that occlusion serves to deter drates in particular, but instead as a mixture pathogen entry (Shuel, 1961; Freeman, 1986). Flowers of through nectary pores to the developing seeds below. stage V appeared to resorb nectar at a rate inversely proportional to the volumes of Low-temperature SEM minimizes arte- IV E and stage (eg, compare cosmophylla facts (Robards, 1984) and here provided no E grandis, figs 14 and 15) that accumulated evidence that nectar flow in eucalypts is reg- by protection from visitation. Contactable ulated by pore aperture movements (Carr nectary-surface area available for reabsorp- and Carr, 1987). Rarely were modified tion limits the process (Búrquez and Cor- stomata of the nectary observed to have bet, 1991). How pore occlusion influences guard cells almost meeting, or touching, reabsorption remains to be determined. across the pore. Instead, complete pore clo- sure could occur by total occlusion. Inter- nectaries of Vicia, the plasmodesmata estingly, partial stomatal occlusion (as polar detectable in primary pit-fields of guard- flaps and pseudo-outer stomatal ledges) also cell walls of immature modified stomata, has been detected in leaves of certain euca- were no longer complete at stomatal matu- lypts, where it involves deposition of new rity (Davis and Gunning, 1992). Further cuticle and wall (Carr and Carr, 1980). Even evidence against the modified stomata mak- after nectar secretion had ceased, modified ing a direct contribution as part of a sym- stomata with open pores were still found. plastic secretory pathway for nectar in that Furthermore, pre-secretory buds possessed species, is considerable (Davis and Gun- open pores on their nectaries. Therefore, ning, 1993). the modified stomata did not control floral nectar secretion of these species, because commencement of nectar release did not ACKNOWLEDGMENTS coincide with synchronous, initial opening of pores of the modified stomata, nor did It is a pleasure to thank Dr P Macnicol, Phy- secretion cease as a result of stomatal clo- totron, CSIRO Division of Plant Industry, Can- berra, ACT, for his excellent introduction to sure. At any floral modified stomata enzy- stage, matic of BES could be found on the surface in all analysis carbohydrates. Gunning, nectary J Jacobson and RW King kindly labo- of four break- provided ontogenetic stages: immature, ratory space and invertase. The technical exper- ing through, open and occluded. This asyn- tise with low-temperature SEM of R Heady and chronous pattern in development of modi- M Ciszewski, Electron Microscopy Unit, RSBS, fied stomata, itself sides against stomatal also was appreciated. D Dyck assisted greatly regulation of nectar flow. Indeed, like the with preparation of the figures. A research grant (ANU-1H) from the Australian Research nectary (fig 6), the leaf of E grandis shows Honey Council, and a from in between Postgraduate Scholarship asynchrony development nearby NSERC of Canada, provided the necessary funds. stomata (fig 20 of Carr and Carr, 1991). In an evolutionary sense, early land plants bore stomata before leaves, and stomata in the Résumé — Influence de la visite florale different of plant kingdom possess degrees sur la composition glucidique du nectar et These close functionality (Ziegler, 1987). sur les modifications de la surface necta- similarities between leaf and struc- nectary rifère de l’Eucalyptus. Les eucalyptus tures in favor utilization of the Eucalyptus (Eucalyptus spp) constituent la source de ’modified stomata’ existing terminology, nectar floral la plus importante pour la pro- Davis and and (Fahn, 1979; Gunning, 1992) duction de miel en Australie. Leur nectar a cells’, instead of cells’ (Carr ’guard ’pore fait pourtant l’objet de peu de recherches. and Carr, 1987, 1990). Les nectaires floraux et la production des Further studies are required to investi- principaux sucres du nectar ont été étudiés gate the anatomy and ultrastructure of Euca- chez trois espèces d’eucalyptus poussant à lyptus nectaries. A key question (Carr and Canberra, Australie : E cosmophylla, E Carr, 1987) still to be addressed in euca- grandis et E pulverulenta. Chez ces trois lypts is whether the nectary guard cells have espèces, le nectaire est situé à la surface plasmodesmata and are involved in sym- interne de l’hypanthium, sous le stamino- plastic transport of nectar. There is insuffi- phore (fig 9). La microscopie électronique à cient evidence from Chamelaucium unci- balayage à basse température a montré que natum, the only Myrtacean species whose la surface des nectaires possédait des cen- nectary modified stomata have been stud- taines de stomates modifiés et distribués ied by transmission electron microscopy relativement uniformément (figs 6, 9, 10), à (O’Brien et al, 1996). However, in floral raison de 163 (E pulverulenta) à 348 (E cosmophylla) stomates/mm2. Le développe- Au stade V, une réabsorption nette du nec- s’est ment des stomates modifiés est asynchrone tar produite (figs 14—16) et la constance dans la du nectar (figs 6, 10), chaque nectaire examiné au composition glucidique cette n’ait lieu stade présécréteur et postsécréteur possé- implique que réabsorption dant à la fois des stomates modifiés imma- de façon sélective pour aucun des trois mais sous forme d’un tures (figs 3 gauche, 6, 10), en cours de sucres, plutôt mélange Il est nécessaire de maturation (figs 3 droite, 11), avec un pore complexe. poursuivre les recherches étudier la fermeture des ouvert (figs 4, 10, 12) ou fermé (figs 10, pour stomates modifiés et le rôle les 13). On n’a pas trouvé de preuve que le sto- joué par obturés dans l’écoulement et la réab- mate modifié s’ouvre et se ferme pour régu- pores du nectar. ler la sécrétion nectarifère. Il est en revanche sorption évident que la fermeture du pore se fait par occlusion et non par des mouvements de Apis mellifera / Eucalyptus / nectar/ glu- cellules stomatiques. Grâce à une technique cide / nectaire / stomate combinée capillaire-mèche, le nectar a été prélevé à cinq stades floraux (I-V) sur chaque espèce (figs 2, 5, 8) : depuis les Zusammenfassung — Einfluß des Blü- fleurs fraîchement écloses qui venaient de tenbeflugs auf die Zusammensetzung der perdre leur operculum jusqu’aux fleurs de 12 Zucker im Nektar und Änderung der à 17 jours dont les étamines étaient forte- Oberflächenbeschaffenheit der Nekta- ment flétries. À chaque stage, le nectar a été rien bei Eucalyptus ssp (Myrtaceae). prélevé sur des fleurs ensachées (fig 1) Obwohl Eukalyptusbäume in Australien die quand l’operculum était déhiscent (fig 2 en wichtigste Quelle von Blütennektar zur haut à gauche) et sur des fleurs laissées libres Honigproduktion darstellen, ist Eukalyp- d’accès aux visiteurs, principalement tusnektar bislang nur wenig untersucht wor- l’abeille domestique Apis mellifera (figs 1, den. Die floralen Nektarien sowie die Sekre- 7) et le guêpier (oiseaux). Les rendements en tion der wichtigsten Nektarkohlenhydrate nectar et les quantités de sucre par fleur wurde bei drei Eukalyptusarten in Canberra, ensachée avaient un niveau maximum chez Australien untersucht. Bei E cosmophylla, E E cosmophylla (fig 14), intermédiaire chez grandis und E pulverulenta ist das Nekta- E pulverulenta (fig 16) et minimum chez E rium auf der inneren Oberfläche des Blü- grandis (fig 15), conformément à la taille tenbechers unterhalb der Staminophore gele- de la fleur mais pas au nombre de stomates gen (Abb 9). Rasterelektronenaufnahmen des nectaires. L’analyse enzymatique du bei niedrigen Temperaturen zeigten, daß die glucose, du fructose et du saccharose dans Oberflächen der Nektarien hunderte von les échantillons de nectar a montré qu’E modifizierten Stomata besitzen. Diese lie- cosmophylla et E grandis sont riches en gen zumeist einzeln und sind relativ gleich- hexose (0,1 < S/(G + F) < 0,499 ; tableau I), mäßig verteilt (Abb 6, 9, 10). Ihre Dichte tandis que E pulverulenta est riche en sac- beträgt 163 (E pulverulenta) bis 348 (E cos- charose (0,5 < S/(G + F) < 0,999 ; tableau I). mophylla) pro mm2 der Oberfläche. Die Ent- Le rapport glucose/fructose se situait autour wicklung dieser modifizierten Stomata de 1,2-1,4 pour les trois (tableau I). On a erfolgte nicht gleichzeitig (Abb 6, 10). Bei trouvé peu de changements dans la compo- allen untersuchten Nektarien kamen diese sition glucidique du nectar selon que les sowohl vor als auch nach der Sekretion fleurs étaient jeunes ou vieilles, ensachées ou unreif (Abb 3 links, 6, 10) durchbrechend à l’air libre (figs 14-16), ce qui indique une (Abb 3 rechts, 11), mit offenem (Abb 4, 10, constance dans la composition tout au long 12) oder verstopftem Porus vor (Abb 10, du processus de secrétion des stages I-IV. 13). Es gab keine Anzeichen, daß sich die Stomata zur Regulierung des Nektarflusses drei Zucker ist, sondern die gesamte öffnen oder schließen. Stattdessen wurden Mischung betrifft. Zur Klärung der Ver- die Poren durch Verstopfung verschlossen, stopfung der modifizierten Stomata und zu offensichtlich nicht durch Bewegung der der Rolle der verstopften Poren während Begleitzellen. Mit einer Kapillar-Docht- des Nektaraustritts und der Reabsorption Kombinationstechnik wurde Nektar von werden weitere Untersuchungen benötigt. jeder Art aus fünf Blühstadien (I-V) gesam- melt (Abb 2, 5, 8). Diese Stadien umfassten Apis mellifera / Eucalyptus / Nektar / frischgeöffnete Blüten, die gerade ihren Zucker / Stomata / Nektarien Knospendeckel verloren hatten, bis zu meh- rere Tage älteren Blüten mit stark verwelk- ten Staubgefäßen. 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