Amots Dafni

Michael Hesse Ettore Pacini (eds.)

Pollen and Pollination

Springer-Verlag Wien GmbH

Prof. Dr. Amots Dafni Laboratory of Pollination Ecology, Institute of Evolution, Haifa University, Haifa, Israel

Prof. Dr. Michael Hesse Institut für Botanik und Botanischer Garten, Universität Wien, Wien, Austria

Prof. Dr. Ettore Pacini Dipartimento di Biologia Ambientale,Universita degli Studi di Siena, Siena, Italy

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machines or similar means, and storage in data banks.

© 2000 Springer-Verlag Wien Originally published by Springer-Verlag Wien New York in 2000 Softcover reprint of the hardcover 1 st edition 2000

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Cover illustration: Esterhazya splendida Mikan var. angustifolia (Scrophulariaceae). Foto: Prof. Dr. Stefan Vogel, Serra da Mantiqueira, Brazil

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With 67 partly coloured Figures

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Special Edition of Plant Systematics and Evolution, Vol. 222, No. 1-4,2000

ISBN 978-3-7091-7248-3 ISBN 978-3-7091-6306-1 (eBook) DOI 10.1007/978-3-7091-6306-1

Preface

Pollen studies make important contributionsto our knowledge in many interdisciplinaryarenas. Pollen identification is widely used inreconstruction of, e.g., vegetation, the climateof the past, and plant biodiversity. Studiesconcerning pollen structure, size and form arekey issues in basic sciences, as, e.g., planttaxonomy and evolution, but are also ofimportance in applied fields as, e.g., plantbreeding . In pollination studies pollen isgenerally used specifically to identify foodsources of visitors and to reconstruct theirforaging routes. Fewer have been devoted topollen collection mechani sms and to the struc­ture and content of pollen in relation to itsfunction.

A computerized search of the currentliterature reflects these general trends. A searchdone on December 1st 1999 showed thefollowing: In Agricola the term pollen wasmentioned 13354 times, pollination 7184, thecombination pollen + pollination 36, andpollen + ecology 17. In Biosis the resultswere: 15515, 3985, 57 and 3; and in Bioscience53886, 20187, 502 and 38, respectively.

The present volume , filling a gap , is anattempt to produce a comprehensive interdis­ciplinary text as a bridge between scientistswho study various aspects of pollen biology:physiologists, botanists, ecologists, zoologistsand neurophysiologists. We aim to considerthe functional ecology of pollen in relation toits structure and constituents as a frameworktoward s a better understanding of evolution­ary processes that mold pollination biology.

The 16 papers, all of which were commis­sioned especially for the present volume, couldbe roughly classified, by their interdisciplinary

nature, into three main themes: pollen struc­ture and constituents, pollen evolutionaryecology and the pollen-pollinator interface.Several papers overlap somewhat or areperhaps even somewhat contradictory andreflect the author's own ideas and experience.Some could be understood more deeply byconsulting other closely related articles . Thereader is strongly referred to the respectiveliterature list of each article.

The last steps of anther ripening and pollendevelopment (Pacini) and the mature pollenwall structure (Hesse) are key factors tounderstand pollen dispersal mechanisms inbiotic pollin ation (Stroo) as well as abioticpollination (Ackerman). Pollen size, shape,wall stratification and the micro-topographyof its surface as well as the composition andstructure of the pollenkitt have an importantrole in pollen -connecting mechanisms (Thorp)and also in the dispersal mechanism in whichelectrostatics is an important factor. Pollenelectrostatics (Vaknin et al.) is reviewed fromtheoretical as well as practical aspects inrelat ion to natural and agricultural systems.Special attention is devoted to the importanceof the flower's morphological feature s thatconfer adaptiveness to electrostatic pollination.

Abiotic pollination is discussed by Ack­erman, who surveyed definitions and termi­nology of the phenomenon. Pollen physicalcharacteristics are evaluated in relation to itsfunctional ecology and the evolutionary rela­tionships between plant morphology and fluiddynamics. His findings are closely related tothose on pollen structure (Hesse), electrostatics(Vaknin et al.), and content (Pacini, Nepi andFranchi) as well as to viability and longevity

VI

(Dafni and Firmage) and adaptation to specialpollinators (Stroo).

Pollen constituents are viewed from twocomplementary angles: pollen cytochemistry(Nepi and Franchi) and pollen nutritionalvalue (Roulston and Cane) . Pollen cytochem­istry (Nepi and Franchi) deals with the appli­cation of cytochemical methods to maturepollen with special reference to reserve mate­rial. The chapter on pollen cytochemistryserves as a basic background to understandingpollen physiological behaviour and chances forgermination and fertilization . The chemistry ofthe pollenkitt is a key factor in understandingpollen dispersal (Ackerman; Hesse et al.) aswell as in pollen odour (Dobson and Berg­strom) and colour (Lunau). The inner pollencomposition is related to its viability (Pacini ;Dafni and Firmage) as well as to its value as afood for consumers (Roulston and Cane) .

Pollen viability and longevity is discussedfrom practical, ecological and evolutionaryviewpoints. The article by Dafni and Fir­mage surveys the advantages and disadvan­tages of the existing methods to asses pollenviability and examines the current hypothesesthat link pollen longevity and pollinationbiology .

Pollen may attract flower visitors by visual(Lunau) as well as by odoriferous cues(Dobson and Bergstrom). In general , ourattention is focused on petals (and sepals) asthe main attractants of flowers. Lunau drawsour attention to the importance of visualpollen signals. While considering evolutionary,ecological, sensory-physiological and behavi ­oral aspects of flower-pollinator interaction,the various strategies of angiosperms forattracting pollinators to the site of rewardsare elucidated with special reference to pollen.Floral odours are important cues even as along distance signals that flowers emit. Dobsonand Bergstrom discuss pollen odours in rela­tion to bee foraging behavior, location ofpollen sources, discrimination of pollenamounts and specific recognition of flowerresources. Their work includes a review of thecurrent knowledge as well as new experimental

Preface

evidence and chemical analyses concerningpollen odours.

Cruden deals with the intriguing questionof why there are so many pollen grains? Thisarticle examines the selective forces that affectpollen number.The author analyses the rela­tionships among other floral traits of animal­pollinated plants, including pollen size, stigmaarea and depth, and the pollen-bearing area ofthe pollinator that may affect pollen number,and also provides a model to examine howchange in one trait may elicit change in othertraits.

Pollen as a food for its consumers is awidely neglected topic . The article by Roul­ston and Cane covers the mechanisms ofpollen digestion by animals; the efficiency ofremoval of pollen content and the taxonomicdistribution of pollen content. All these as­pects are discussed in relation to adaptivehypotheses that associate pollen chemistrywith pollinator rewards. Roulston and Canesurveyed the digestion and nutrient content ofpollen, which is an outcome of pollen cyto­chemistry (Nepi and Franchi). They discussmechanisms of pollen digestion and its effi­ciency and the range and taxonomic distribu­tion of pollen nutrients. All these addressadaptive hypotheses concerning pollenchemistry and pollinator reward . Pollen di­gestibility is, to a large extent, an outcome ofthe pollen wall structure (Hesse) and content(Pacini) .

The various mechanisms of pollen collec­tion by bees are surveyed by Thorp. Pollen isan essential food of many pollinators especial­ly bees due to its nutritive contents (Roulstonand Cane ; Pacini) especially bees. Thorp sur­veys the various structures and behavioraladaptations for acquiring and transportingpollen by bees.

Pollen structure and packaging have co­evolved in relation to transfer functionalityand pollinator's specificity. Stroo reviews thepollen morphological evolution in bat- polli­nated flowers. An analysis of pollen size,shape , aperture number and type and orna­mentation type is used to test previous hy-

Preface

potheses concerning the characteristics of bat­pollinated pollen. Orchid pollinaria structure isdiscussed in relation to its functionality (John­son and Edwards) . Aspects as attachment tovectors, mechanisms of pollen deposition,pollen longevity (see also Dafni and Firmage),pollen:ovule ratio (see also Cruden), spatialdispersal patterns and outcrossing in relationto pollen export are also discussed. Thiscontribution is a key to elucidating Roubik'spaper on a new deceptive mechanism inorchids involving Meliponini bees as pollina ­tors . It is assumed that the relationship isbased either on mimicry of rewarding resourc­es or on a bee pheromone.

Various thread-forming structures in pol­len anthers are discussed by Hesse et al. inrelation to their role in pollination ecology.These unique structures may function as pol­len-connecting vectors in forming pollen dis­persal units and /or playing a role in pollenpresentation.

Beetles are important pollen-eaters, Bern­hardt reviews the convergent evolution andadaptive radiation of beetle- pollinated angio­sperms. His survey stresses the diversity ofbeetle-pollinated flowers and their shared andpeculiar adaptations contributing to the syn­drome of cantharophily and its evolution.

VII

We would like to acknowledge here allthose who have made possible the publicationof this volume . We are very grateful to all ourcontributors for their effective co-operation,and to all our reviewers for their carefulrevisions and for their efforts to return quicklytheir comments and suggestions . We wouldlike to thank especially the staff of ourpublisher, Springer-Verlag in Vienna, for thehelp and support during the preparation ofthis special issue of Plant Systematics andEvolution. It is hoped that this interdisc­iplinary volume will promote multifacetedstudies leading to a better understanding ofpollination evolutionary ecology. Floral re­wards , such as pollen and nectar, are only oneaspect of the multidimensional pollinationkaleidoscope. Any attempt to evaluate therole of pollinators attraction and behaviourhas to be considered in relation to thereward's content, composition, and physicaland chemical properties, all this in relation toenvironmental conditions as well as theagent's efficiency as a disperser of a functionalpollen.

A. Dafni, M. Hesse and E. Pacini,Guest Editors

Contents

Hesse, M.: Pollen wall stratification and pollination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Pacini, E.: From anther and pollen ripening to pollen presentation , 19

Nepi , M., Franchi, G. G.: Cytochemistry of mature angiosperm pollen 45

Dobson, H. E. M., Bergstrom, G.: The ecology and evolution of pollen odors . . . . . . . . . . . . . . . . 63

Lunau, K.: The ecology and evolution of visual pollen signals . . . . . . . . . . . . . . . . . . . . . . . . . . .. 89

Dafni , A., Firmage, D.: Pollen viability and longevity : practical , ecologicaland evolutionary implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 113

Vaknin, Y, Gan-Mor, S., Bechar, A., Ronen, B., Eisikowitch , D.: The roleof electrostatic forces in pollination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Cruden, R. W.: Pollen grains : why so many? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Ackerman, J. D.: Abiot ic pollen and pollination: ecological, function al,and evolutionary perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

Roulston , T. H., Cane , J. H.: Pollen nutritional content and digestibility for animals " 187

Thorp, R. W.: The collection of pollen by bees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 211

Stroo, A.: Pollen morphological evolution in bat pollinated plants . . . . . . . . . . . . . . . . . . . . . . . . 225

Johnson, S. D., Edwards, T. J.: The structure and function of orchid pollinaria . . . . . . . . . . . . . . . 243

Roubik, D. W.: Deceptive orchids with Meliponini as pollinators . . . . . . . . . . . . . . . . . . . . . . . . . 271

Hesse, M., Vogel, S., Halbritter, H. : Thread-forming structuresin angiosperm anthers: their diverse role in pollination ecology . . . . . . . . . . . . . . . . . . . . . . . . 281

Bernhardt, P.: Convergent evolution and adaptive radiationof beetle-pollinated angiosperms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321