Nectar biodiversity: a short review

 Nectaries differ in many aspects but a common feature is some kind of advantage for the plant conferred by foraging of consumers which may defend the plant from predators in the case of extrafloral nectaries, or be agents of pollination in the case of floral nectaries. This minireview is concerned mainly with floral nectaries and examines the following characteristics: position in flower; nectary structure; origin of carbohydrates, aminoacids and proteins; manner of exposure of nectar; site of nectar presentation; volume and production of nectar in time; sexual expression of flower and nectary morphology; nectar composition and floral sexual expression; variability of nectar composition; fate of nectar; energy cost of nectar production. The species of certain large families, such as Brassicaceae, Lamiaceae and Asteraceae, resemble each other in nectary organisation; other families, such as Cucurbitaceae and Ranunculaceae, have various types of organisation. A scheme is presented to illustrate factors influencing nectary and nectar biodiversity. Received July 23, 2002; accepted September 18, 2002 Published online: June 2, 2003     

Effect of rootstocks on floral nectar composition in apple cultivars

 Insect attraction of flowers in apple cultivars is an essential question from the viewpoint of fruit yield, since apple flowers are autosterile and entomogamous. Therefore, it is important to study the two most significant factors of primary attraction, pollen and nectar production. The composition of nectar in apple cultivars on different rootstocks was investigated, putting an emphasis on sugar components. It was stated that the composition of nectar and the ratio of mono- and disaccharides was almost identical and constant in trees on rootstock M.4, whereas on other rootstocks the effects of the season were more pronounced. In triploid cultivars on rootstocks M.9 and M.26 nectar can be characterised by the dominance of sucrose. Relationship was found between free fertilisation rate of cultivars and sugar amount of nectar. In triploid cultivars nectar sugar content was usually lower than in diploid ones, and reached the bee visitation threshold only in some years, influenced also by the effect of rootstocks. Received August 27, 2002; accepted November 16, 2002 Published online: June 2, 2003     

PCR-based method for identifying the S-genotypes of Japanese pear cultivars

 Japanese pear (Pyrus pyrifolia Nakai), a member of the Rosaceae, shows gametophytic self-incompatibility that is controlled by the S-locus. The S-genotype of Japanese pear cultivars is an important factor for crossing and breeding. We report a rapid reliable method to identify these S-genotypes. It consists of PCR amplification of the S-RNase gene from genomic DNA and subsequent digestion of the PCR fragments with S-allele-specific restriction endonucleases. Using this method, we determined the unknown S-genotypes of nine Japanese pear cultivars and selected self-compatible varieties from the offspring of the self-compatible cultivar, ‘Osa-Nijisseiki’. Received: 8 June 1998 / Accepted: 19 October 1998     

Amygdalin in almond nectar and pollen – facts and possible roles

 Nectar and pollen within flowers are usually the primary attractants to floral visitors. Chemical analysis of almond nectar and pollen in this study revealed that they contain the secondary compound amygdalin. Floral display often reflects pollinator characters, and almond flowers are accordingly designated as “bee flowers”. A previous study in Israel showed that when almonds bloom early in the season they attract honeybees, but later in the season the bees shift toward other species that start blooming. In this study, we offered honeybees sugar solutions containing various concentrations of amygdalin. These preference experiments revealed that in mid-summer bees were not selective, whereas early in the summer they were more discriminating, and consumed faster the sugar solutions with the lower amygdalin concentrations. Possible roles of amygdalin in almond nectar and pollen are discussed. Received September 10, 2002; accepted January 17, 2003 Published online: June 2, 2003     

Variability of nectar production and composition in Linaria vulgaris (L.) Mill. (Scrophulariaceae)

 We studied nectar characteristics during the long flowering period (late June to end of November) in two populations of Linaria vulgaris (L.) Mill. spontaneously growing in the Botanical Gardens of Siena University (Tuscany, central Italy). The two populations were close to each other but they differed in blooming period. Plants of population 1 sprouted in May and flowered from the end of June to the end of September. Population 2 sprouted at the end of August and flowered from September to the end of November. Differences in nectar production and composition were found between and within populations. Flowers of population 1 produced a very small amount of nectar (not collectable) that remained on the nectary surface. The quantity of nectar increased in late September, when each flower produced 2–3 μl of nectar that flowed into the spur. Total sugar concentration was 175.8 mg/ml in young flowers. Flowers of population 2 produced 5–8 μl of nectar with a total sugar concentration of 200.9 mg/ml in the young stage. In bagged senescent flowers nectar volume decreased in both populations and nectar sugar concentration decreased down to 11.6 mg/ml in population 2 and increased up to 289.6 mg/ml in population 1. For both populations, the decrease in nectar volume in bagged flowers may have been due to water loss by evaporation. In population 2, the decrease in sugar concentration may have been due to nectar reabsorption that was never observed in population 1. Nectar variability is discussed in relation to insect visits and seed set. Received August 14, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Selective inhibition of the growth of incompatible pollen tubes by S-protein in the Japanese pear

The S-allele-associated proteins (S-proteins) in the styles of the Japanese pear (Pyrus serotina Rehd. var. culta Rehd.) were purified by cation exchange chromatography. Their inhibitory action on the growth of incompatible pollen tubes (pollen tubes bearing the same S- allele as in the style from which the S-proteins were prepared) was characterized in vitro. Germination and tube growth of self-pollen (pollen from the same cultivar from which the S-proteins were prepared) decreased dose-dependently when the S-protein was added to the medium. Tube length was reduced to 10% that of compatible pollen tubes (pollen tubes bearing the S-allele different from that in the style from which the S-proteins were prepared) at 1.5 μg μl¹. S-proteins from Shinsui (S ₄ S ₅ ) also inhibited growth of cross-incompatible Kosui (S ₄ S ₅ ) pollen tubes, but not of compatible Chojuro (S ₂ S ₃ ) pollen tubes. After inactivation of RNase of the S- protein, the inhibitory action of the S-protein disappeared. These results indicate that the S-protein acts directly to inhibit growth of incompatible pollen tubes in Japanese pear styles, and that the RNase activity of the protein is essential for the biological function. However, small amounts of proteins that co-migrated with the S-protein may also play some roles in the inhibition. This is the first report on the selective inhibitory action of S-proteins in Rosaceae. Received: 11 April 2000 / Revision accepted: 28 September 2000     

Characteristics of floral nectaries and nectar in two species of Crataegus (Rosaceae)

 The structure of floral nectaries of Crataegus coccinea and C. crus-galli was examined using light and scanning electron microscopy. The radial length of the floral nectary, measured from longitudinal sections of flowers, was 30% larger in C. crus-galli than in C. coccinea. For both Crataegus species the glandular tissue thickness was similar – approx. 400 μm. Also, the number of stomata per mm² of nectary surface in C. crus-galli was much higher (by 43%) than for C. coccinea. Stomata were situated in deep hollows. For both taxa the period of nectar secretion was 4 days. The mean quantity of total sugar in nectar per 10 flowers of C. crus-galli and C. coccinea was 3.87 mg and 0.33 mg, respectively. Received August 28, 2002; accepted December 17, 2002 Published online: June 2, 2003     

A major function of the tobacco floral nectary is defense against microbial attack

 We have characterized the major nectar protein (Nectarin I) from ornamental tobacco as a superoxide dismutase that functions to generate high levels of hydrogen peroxide in nectar. Other nectar functions include an anti-polygalacturonase activity that may be due to a polygalacturonase inhibiting protein (PGIP). We also examined the expression of defense related genes in the nectary gland by two independent methods. We isolated a sample of nectary-expressed cDNAs and found that 21% of these cDNAs were defense related clones. Finally, we examined the expression of a number of specific defense-related genes by hybridization to specific cDNAs. These results demonstrated that a number of specific defense genes were more strongly expressed in the floral nectary than in the foliage. Taken together these results indicate that the floral nectary gland can have specific functions in plant defense. Received August 8, 2002; accepted January 7, 2003 Published online: June 2, 2003     

Introducing plants for bee-keeping at any cost? – Assessment of Phacelia tanacetifolia as nectar source plant under xeric Mediterranean conditions

 We assessed nectar and honey potential of Phacelia tanacetifolia, an American plant, under Mediterranean conditions. Its flowering occurred during the major flowering season unless intensely irrigated, whereas duration and flower life span were shorter compared to continental conditions. Mediterranean climate limitations to nectar secretion were assessed on plants grown under natural conditions (xeric) vs. regular irrigation. May flowering xeric plants produced less nectar per flower than the irrigated ones, but had the same nectar potential per surface area. On the contrary, at the cost of intense irrigation, July flowering plants produced much higher nectar per flower and per surface area compared to xeric ones. In all flowering periods and sections honeybees were the most constant and numerous visitors, visiting the flowers mainly for nectar, whereas solitary bees were scarce. Based on our results, we suggest that although the plant may be a good nectar source for honeybees in some cases, we have serious reservations for a generalised use in the Mediterranean. Received September 3, 2002; accepted November 18, 2002 Published online: June 2, 2003     

Inheritance and linkage of isozyme loci in pear (Pyrus communis L.)

 The polymorphism of 11 enzymes was analysed in 11 progenies from controlled crosses between pear varieties, using acrylamide and starch electrophoresis gels. Twenty-two loci were identified and segregation was scored for 20 of them. Three pairs of duplicated loci forming intergenic hybrid bands were detected, these correspond to equivalent duplicated genes in apple. A total of 49 active alleles and 1 null allele were identified. Joint segregation analysis revealed three linkage groups, which could all be related to existing groups on the apple map. The conservation of isozyme patterns, duplicated genes and linkage groups indicates a high degree of synteny between apple and pear. Received: 8 July 1996 / Accepted: 19 July 1996     

Nectar as food for birds: the physiological consequences of drinking dilute sugar solutions

 Nectarivory has evolved many times in birds: although best known in hummingbirds, sunbirds and honeyeaters, it also occurs on an opportunistic basis in a varied assortment of birds. We present a phylogenetic analysis of the distribution of nectarivory in birds. Specialised avian nectarivores are generally small, with an energetic lifestyle and high metabolic rates. Their high degree of dependence on nectar as a food source has led to convergence in morphological, physiological and behavioural adaptations. We examine the constituents of nectar which are most important to bird consumers, and how the birds deal with them in terms of physiology and behaviour. There are still unanswered questions: for example, the dichotomy between sucrose-rich nectars in hummingbird-pollinated plants and predominantly hexose-rich nectars in sunbird-pollinated plants appears to have little to do with bird physiologies and may rather reflect patterns of nectar secretion. Received November 28, 2002; accepted January 26, 2003 Published online: June 2, 2003     

Nectar resorption in the spur of Platanthera chlorantha Custer (Rchb.) Orchidaceae – structural and microautoradiographic study

 In the flowers of Platanthera chlorantha nectar is secreted and accumulated in a spur. Previous studies of this species revealed that after the period of secretion and cessation, rapid nectar resorption occurs. The aim of this study was the observation of nectar resorption on the structural level using tritium labelled sucrose. For this purpose, during the peak of nectar secretion, 10 μl of nectar accumulated in the spur was replaced with the same volume of labelled sucrose (10 μCi). Small fragments of spurs were sampled between 12–36 h of incubation, at the resorption phase. Afterwards, they were fixed and embedded in epoxy resin. Material for the microautoradiographic study was prepared with the dipping technique. The cells of secretory epidermis and unicellular secretory papillae had dense, strong stained cytoplasm, a large nucleus and small vacuoles. A characteristic feature of these cells was the presence of numerous starchless plastids, mitochondria and ER profiles. Many vesicles occurred in the close vicinity of the cell wall. In the cuticle covering cell walls no pores or cracks were observed. Presence of [³H] sucrose was detected mainly in the walls of the nectary cells, which would indicate an apoplastic route of resorbed nectar. Received August 3, 2002; accepted November 10, 2002 Published online: June 2, 2003     

Oil secretion and the pollination of Colpias mollis (Scrophulariaceae)

 Colpias mollis is a perennial lithophyte that secretes non-volatile oil as a pollinator reward. Its white to yellow flowers bear two narrow pouches lined with glandular oil-secreting trichomes. The pollinating bee, Rediviva albifasciata (Melittidae), collects oil with its forelegs and midlegs by rubbing them against the gland patches within the flower. The presence of specialised hairs on these legs facilitates oil-collection. The strongly congruent geographic distributions of C. mollis and R. albifasciata and their close morphological fit suggest a long and close association between plant and pollinator. Although C. mollis appears dependent on R. albifasciata for pollination, R. albifasciata is not always dependent on C. mollis for oil. It can also obtain and utilise oil from flowers of the annual herb Hemimeris racemosa (Scrophulariaceae). At the main study site, fruit set of C. mollis was high (75.7%; N=202 flowers) in a dry year when H. racemosa was rare and relatively low (11.4%; N=184 flowers) in a wet year when H. racemosa was common. Received July 15, 2001; accepted June 17, 2002 Published online: November 20, 2002 Addresses of the authors: Kim E. Steiner (e-mail: ksteiner@calacademy.org), Compton Herbarium, National Botanical Institute, Kirstenbosch, South Africs; Current address: Department of Botany, Califormia Academy of Sciences, Golden Gate Park, San Francisco, CA 94118, USA. V. B. Whitehead, South African Museum, P.O. Box 61, Cape Town 8000, South Africa; Current address: 87 Bruke Street, Graaff-Reinet 6280, South Africa.     

Chloroplast DNA study in wild populations and some cultivars of Prunus avium L.

The PCR-RFLP technique was used to detect chloroplast DNA diversity in wild populations of Prunus avium from five European deciduous forests and some cultivars. A study of 10.8% of the total chloroplast genome detected eight insertion-deletion (indel) mutations, distributed over 12 haplotypes. Six haplotypes (H1, H2, H3, H4, H5 and H6) were found in wild populations and eight (H2, H6, H7, H8, H9, H10, H11 and H12) in the cultivars. Only two haplotypes (H2 and H6) are shared by the wild populations and the cultivars. The most-abundant and frequent haplotype in wild populations is H2 (frequency=78%). The wider geographical distribution along with the high frequency reflects its ancient origin. Of the five populations, three are polymorphic. Populations GA (Scotland) and KE (Germany) have unique haplotypes. The total cpDNA diversity in wild populations is h_T=0.40, and a major portion of it is within populations (h_S=0.37). The genetic differentiation among populations was low (G_STC=0.08) and no genetic structure among wild populations was observed. A minimum-length spanning tree, demonstrating relationships among the haplotypes in wild populations, indicated two possible chloroplast lineages. The ten identified cultivars were represented by seven haplotypes; this result proposes the possible utilisation of the PCR-RFLP technique for the characterisation of sweet cherry cultivars. The cpDNA diversity in P. avium should be considered carefully for phylogenetic studies involving this species. Received: 10 July 2000 / Accepted: 19 October 2000     

Floral rewards and pollination in Cytiseae (Fabaceae)

 Cytiseae have been reported to be mostly nectar-lacking, yet some taxa secrete nectar from extrastaminal nectaries. We studied the pollination biology of four shrubby species of Cytiseae (Cytisophyllum sessilifolium (L.) Lang, Spartium junceum L., Genista radiata (L.) Scop., Genista cilentina Valsecchi) which differ for ecology, distribution and population size. All species resulted obliged xenogamous, insect visits being necessary for successful pollination. Bumblebees and solitary bees are the principal pollinators, but also many beetles, some hover-flies, and few bugs visit flowers. Pollinator specificity is low, and this may be the reason of the scarce seed set compared to the number of ovules. Pollen is the main reward, but traces of glucose were detected in all species, at the base of vexillum or on the reproductive column. Nectar production is irregular in time, and apparently unpredictable. We suppose that nectar may play a role in attracting pollinators determining their right position for a successful pollination. Received August 12, 2002; accepted November 25, 2002 Published online: June 2, 2003     

Cognition-mediated coevolution – context-dependent evaluations and sensitivity of pollinators to variability in nectar rewards

 In this paper we emphasize the role of pollinator perception and decision-making processes in mediating floral nectar distribution strategies. Since pollinator choice behavior is guided by how the pollinator perceives and evaluates floral rewards, we hypothesize that plants have evolved strategies that maximize their perceived profitability, through pollinator cognition-mediated coevolution. We focus on two classes of cognitive phenomena, context-dependent evaluations and risk-sensitivity. These phenomena are of interest to psychologists and biologists. Our paper is an attempt to show the value of cross-disciplinary exchange of theories and ideas. A review of the ecology literature suggests that pollinators evaluate variability in nectar volume in proportion to the mean, and thus choice behavior is guided by the coefficient of variation (standard deviation/mean) of the distribution. This functional relationship is predicted by Weber's law, which describes a wide range of psychophysical phenomena. Simulations show that this phenomenon also affects how pollinators perceive skewed nectar distributions. Cognition-mediated coevolution theory should be a fruitful approach to understanding the evolution of pollinator-plant interactions. Received November 12, 2002; accepted January 15, 2003 Published online: June 2, 2003     

Molecular investigation of the genetic base of sugarcane cultivars

 Molecular diversity was analysed among 162 clones of sugarcane using DNA restriction fragment length polymorphism (RFLP). One hundred and nine of them were modern cultivars of interspecific origin; most of them were bred in Barbados or in Mauritius. Fifty three were from Saccharum officinarum species, which is the major source of genes in modern cultivars, prevailing over the part of the genome incorporated from the wild species Saccharum spontaneum. Twelve low-copy nuclear DNA probes scattered over the genome were used in combination with one or two restriction enzymes. A total of 399 fragments was identified, 386 of which were polymorphic. Each sugarcane clone displayed a high number of fragments per probe/enzyme combination, illustrating the polyploid constitution of the genome. Among the S. officinarum clones, those from New Guinea had the largest variability and encompassed that present among clones collected from the Indonesian Islands and those known to have been involved in the parentage of modern cultivars. This is in agreement with the hypothesis that New Guinea is the centre of origin of this species. The clones from New Caledonia formed a separate group and could correspond to S. officinarum clones modified through introgression with other members of the ‘Saccharum complex’. Despite the low number of S. officinarum clones used for breeding cultivars, more than 80% of the markers present in the whole S. officinarum sample were also found in modern cultivars due probably to a high heterozygosity related to polyploidy. Among the cultivars, the two main groups, originating from Barbados and Mauritius, were clearly separated. This appeared essentially due to S. spontaneum alleles present in Mauritian cultivars and absent in Barbadan ones, probably in relation to the regular use of early generation interspecific hybrids in the breeding program employed in Mauritius. Received: 9 November 1998 / Accepted: 19 November 1998     

Asymptotic behaviour of a model of hierarchically structured population dynamics

 A hierarchically structured population model with a dependence of the vital rates on a function of the population density (environment) is considered. The existence, uniqueness and the asymptotic behaviour of the solutions is obtained transforming the original non-local PDE of the model into a local one. Under natural conditions, the global asymptotical stability of a nontrivial equilibrium is proved. Finally, if the environment is a function of the biomass distribution, the existence of a positive total biomass equilibrium without a nontrivial population equilibrium is shown. Received 16 February 1996; received in revised form 16 September 1996