Massimo Nepi

Nectaries and nectar.

Contributing Authors. Preface. 1. Introduction E. Pacini, S.W. Nicolson. 1.1 Evolutionary origins. 1.2 Secretions analogous to nectar. 1.3 Floral and extrafloral nectarines. 1.4 Nectar components. 1.5 Organization of this volume. 2. A Systematic Survey of Floral Nectaries G. Bernardello. 2.1 Introduction. 2.2 Nectaries in gymnosperms. 2.3 Nectaries in angiosperms. 2.3.1 Diversity. 2.3.1.1 Nectar presentation. 2.3.1.2 Structure. 2.3.1.3 Fate. 2.3.1.4 Symmetry. 2.3.1.5 Number. 2.3.1.6 Colour. 2.3.2 Factors influencing nectary diversity. 2.3.3 Basic types of floral nectarines. 2.3.4 Nectariferous spurs. 2.3.5 Patterns of variability in nectarines. 2.3.5.1 Asteraceae. 2.3.5.2 Brassicaceae. 2.3.5.3 Cucurbitaceae. 2.3.5.4 Euphorbiaceae. 2.3.5.5 Ranunculaceae. 2.3.5.6 Solanaceae. 2.3.6 Nectaries and deceit pollination. 2.3.6.1 Apocynaceae. 2.3.6.2 Bignoniaceae. 2.3.6.3 Orchidaceae. 2.3.7 Relictual nectarines in anemophilous species. 2.3.8 Distribution of nectary types. 2.3.8.1 Early-branching lineages. 2.3.8.2 Magnoliids. 2.3.8.3 Early-branching monocots. 2.3.8.4 Monocots. 2.3.8.5 Commelinids. 2.3.8.6 Ceratophyllales. 2.3.8.7 Eudicots. 2.3.8.8 Core Eudicots. 2.3.8.9 Rosids. 2.3.8.10 Eurosids I. 2.3.8.11 Eurosids II. 2.3.8.12 Asterids. 2.3.8.13 Euasterids I. 2.3.8.14 Euasterids II. 2.3.9 Evolutionary trends. 3. Nectary Structure and Ultrastructure M. Nepi. 3.1 Introduction. 3.2 Nectary structure and ultrastructure. 3.2.1 Epidermis. 3.2.1.1 Secretory trichomes. 3.2.1.2 Nectary-modified stomata. 3.2.2 Nectary parenchyma. 3.2.2.1 Patterns of plastid development in nectary parenchyma cells. 3.2.3 Subnectary parenchyma. 3.2.4 Nectary vasculature. 3.3 Gynopleural (septal) nectarines. 3.4 Extrafloral nectarines. 3.5 Nectary histochemistry. 4. Nectar Production and Presentation E. Pacini, M. Nepi. 4.1 Introduction. 4.2 Nectar secretion mechanism and models of nectary function. 4.3 Dynamics of nectar components. 4.3.1 Nectar reabsortion: resource recovery and homeostasis. 4.3.2 Nectar standing crop. 4.4 The source of nectar components. 4.5 Ecophysiological significance of parenchyma plastids. 4.6 Nectar presentation. 4.6.1 Floral nectarines. 4.6.2 Extrafloral nectarines. 4.7 Fate of nectar and nectarines. 4.8 Variability of nectar characteristics. 4.8.1 Environmental variables. 4.8.2 Intraspecies variability. 4.8.3 Interpopulation differences. 4.8.4 Variability and experimental design. 5. Nectar Chemistry S.W. Nicolson, R.W. Thornburg. 5.1 Introduction. 5.2 Water. 5.2.1 Nectar concentration. 5.2.2 Chemical and microclimatic influences on nectar concentration. 5.2.3 Viscosity and feeding rates. 5.3 Sugars. 5.3.1 Constancy of sugar composition within species. 5.3.2 The use of sugar ratios can be misleading. 5.3.3 Is sugar composition determined by floral visitors or common ancestry? 5.4 Inorganic ions. 5.5 Amino acids. 5.5.1 Non-protein amino acids. 5.5.2 Nectar amino acids are under the control of environmental factors. 5.5.3 Contribution of amino acids to the taste of nectar. 5.6 Proteins. 5.6.1 Proteins in leek nectar. 5.6.2 Nectar redox cycle. 5.7 Other nectar constituents. 5.7.1 Lipids. 5.7.2 Organic acids. 5.7.3 Phenolics. 5.7.4 Alkaloids. 5.7.5 Terpenoids. 5.8 Conclusion. 6. Molecular Biology of the Nicotiana Floral Nectary R.W. Thornburg. 6.1 Introduction. 6.2 The ornamental tobacco nectary. 6.3 Developmental processes. 6.3.1 Origin of the floral nectary. 6.3.2 Conversion of chloroplasts into chromoplasts. 6.3.3 Filling of the nectary. 6.4 Protection of the gynoecium. 6.5 Gene expression. 6.5.1 Macroarray analysis indentifies defence genes. 6.5.1.1 Role of hydrogen peroxide in plant stress and defence. 6.5.1.2 Role of ascorbate in plant stress and defence. 6.5.2 EST analysis. 6.5.3 Nectary-specific gene expression. 6.6 Nectary molecular biology in other species. 6.6.1 Other n

Nectar biodiversity: a short review

Abstract. 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.

Nectary structure and ultrastructure

It is easy to define nectaries from a functional point of view: they are plantsecreting structures that produce nectar, but it is difficult to provide a general definition. From the anatomical point of view nectaries vary widely in ontogeny, morphology, and structure (Fahn, 1979a, 1988; Durkee, 1983; Smets et al., 2000), both between species and within species, depending on flower sexual expression or flower morph in heterostylous and heteroantheric species (Nepi at al., 1996; Küchmeister et al., 1997; Fahn & Shimony, 2001; Pacini et al., 2003). Intraspecific morphological differences exist between flowers of the same plant and between plants of the same species with different ploidy (Davis et al., 1996), and morphological characters may be

Pollen carbohydrates and water content during development, presentation, and dispersal: a short review.

Summary.Pollen accumulates starch reserves during development and the final stage of ripening. Before the anther opens, starch is totally or partially converted to pectins, glucose, fructose, sucrose, and to some unknown polysaccharides. Pollen is exposed to dispersing agents in an arrested developmental state which differs according to pollen water content. Pollen is classified as partially dehydrated or partially hydrated. The final water content may be reached before or after anther opening. Especially during exposure and dispersal, partially dehydrated pollen may interconvert soluble and insoluble reserves, modifying internal turgor pressure and hindering water loss or gain. Partially hydrated pollen is commonly devoid of mechanisms to conserve viability in time but has the advantage of quickly emitting pollen tubes on reaching the stigma.

Types of carbohydrate reserves in pollen: localization, systematic distribution and ecophysiological significance

Summary Dehiscing pollen grains of 901 species belonging to 104 dicot families and 15 monocot families were scored for starch reserves. Starch grains showed different physico-chemical properties i.e. colour after iodine - potassium iodide staining and birefringence or otherwise under polarized light. Further tests performed in a limited number of species revealed other kinds of carbohydrate reserves in the cytoplasm but outside plastids. From these observations, it results that carbohydrate reserves may be stored in plastids only (amyloplasts), in the cytoplasm but not in plastids, or in both. These kinds of pollen reserves are only partly in line with systematics, as only certain families consistently show the same type of reserve. These and previous findings suggest that the presence of polysaccharides in the cytoplasm prevents rapid decrease in viability due to desiccation. In this sense, our findings are in line with ecophysiological adaptations such as the respective pollination syndrome.

NECTAR SECRETION, REABSORPTION, AND SUGAR COMPOSITION IN MALE AND FEMALE FLOWERS OF CUCURBITA PEPO

Nectar volume and sugar composition of male and female flowers of Cucurbita pepo L. (squash), a vine native to tropical Mexico, were studied in an Italian botanical garden. Flowers opened at dawn and closed at noon. Both sexes were extremely rewarding compared with most bee‐pollinated flowers, producing 22–40 mg sugar/flower in 6 h. Female flowers produced significantly more nectar sugar than did males, mainly because of a higher concentration of sugars in nectar (440 vs. 325 mg/mL). The temporal pattern of secretion was similar in the two sexes, and both nectars were sucrose rich. Sugar composition did not vary during anthesis. By 6 h after flower closure, nectar volume and sugar concentration had decreased drastically, especially in female flowers. Cucurbita pepo has the ability to reabsorb most or all unconsumed nectar.

Water status and associated processes mark critical stages in pollen development and functioning

BACKGROUND The male gametophyte developmental programme can be divided into five phases which differ in relation to the environment and pollen hydration state: (1) pollen develops inside the anther immersed in locular fluid, which conveys substances from the mother plant--the microsporogenesis phase; (2) locular fluid disappears by reabsorption and/or evaporation before the anther opens and the maturing pollen grains undergo dehydration--the dehydration phase; (3) the anther opens and pollen may be dispersed immediately, or be held by, for example, pollenkitt (as occurs in almost all entomophilous species) for later dispersion--the presentation phase; (4) pollen is dispersed by different agents, remaining exposed to the environment for different periods--the dispersal phase; and (5) pollen lands on a stigma and, in the case of a compatible stigma and suitable conditions, undergoes rehydration and starts germination--the pollen-stigma interaction phase. SCOPE This review highlights the issue of pollen water status and indicates the various mechanisms used by pollen grains during their five developmental phases to adjust to changes in water content and maintain internal stability. CONCLUSIONS Pollen water status is co-ordinated through structural, physiological and molecular mechanisms. The structural components participating in regulation of the pollen water level, during both dehydration and rehydration, include the exine (the outer wall of the pollen grain) and the vacuole. Recent data suggest the involvement of water channels in pollen water transport and the existence of several molecular mechanisms for pollen osmoregulation and to protect cellular components (proteins and membranes) under water stress. It is suggested that pollen grains will use these mechanisms, which have a developmental role, to cope with environmental stress conditions.

Partially hydrated pollen: taxonomic distribution, ecological and evolutionary significance

Abstract. The problem of the water content of pollen is reconsidered, especially the distinction between “partially hydrated pollen” (PH pollen), pollen with a water content greater than 30%, and “partially dehydrated pollen” (PD pollen), which has a water content of less than 30%. Both types have been found even in systematically contiguous groups or the same genus. Partially hydrated pollen, encountered in at least 40 families of angiosperms, has the advantage of germinating quickly, normally in a few minutes to less than an hour. Dispersal of highly hydrated pollen also occurs in orchids but for a different reason, i.e. to enable packaging of massulae. The disadvantage of pollen dispersed with a high water content is that water is readily lost and the pollen may desiccate and die unless it has biochemical or anatomical devices to retain water or phenological strategies, such as flowering when temperatures are not too high and when relative humidity is high. Most pollen of Gymnosperms and Angiosperms studied has, however, been found partially dehydrated.

The complexity of nectar: secretion and resorption dynamically regulate nectar features

In this paper, we review the phenomenon of nectar resorption, focusing on its physiological and ecological meaning. Nectar resorption is a phenomenon that has long been known but was rarely reported until the1990s. It has more recently been demonstrated in several species by various direct and indirect methodologies. It has generally been demonstrated in senescent flowers as a phenomenon separate in time from, and independent of, nectar secretion. The significance of this type of resorption is generally recognized as a resource-recovery strategy, recycling at least some materials invested in nectar production. Nevertheless, nectar resorption can occur concomitantly with nectar secretion. Nectar production is therefore best considered as a unified process comprising nectar secretion and resorption. The modulation of these two opposite phases allows nectar concentration to be maintained in a range suitable for pollinators (nectar homeostasis). The mechanism of nectar resorption at the cell level has received little attention, and its molecular basis can only be hypothesized on the basis of recent studies concerning sugar sensing.

Dilute Nectar in Dry Atmospheres: Nectar Secretion Patterns in Aloe castanea (Asphodelaceae)

Aloe species commonly flower during the winter dry season in southern Africa and produce abundant dilute nectar. We investigated variability in nectar production and availability in Aloe castanea because evaporation is more likely from its open flowers than from the tubular flowers of most other Aloe species. The greatest variability in nectar production was associated with flower age, and weather conditions and individual plants also had significant effects. However, when unscreened flowers of similar age were sampled throughout a clear day, nectar volumes and concentrations were remarkably constant, and concentrations did not exceed 10% w/w. Variability in concentration could be reduced by reabsorption of sugars, but there was no evidence of reabsorption after addition of relatively concentrated nectar (26.6%) to flowers. It appears that rapid secretion throughout the day accounts for the constant low concentration.