Plant toxicology, fourth edition

Tai Lieu Chat Luong


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Plant
Toxicology
Fourth Edition


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BOOKS IN SOILS, PLANTS, AND THE ENVIRONMENT

Editorial Board
Agricultural Engineering

Robert M. Peart, University of Florida,
Gainesville

Animal Science

Harold Hafs, Rutgers University, New Brunswick,
New Jersey

Crops

Mohammad Pessarakli, University of Arizona, Tucson

Irrigation and Hydrology

Donald R. Nielsen, University of California, Davis

Microbiology

Jan Dirk van Elsas, Research Institute for Plant
Protection, Wageningen, The Netherlands

Plants

L. David Kuykendall, U.S. Department of Agriculture,
Beltsville, Maryland
Kenneth B. Marcum, Texas A&M University,
El Paso, Texas

Soils

Jean-Marc Bollag, Pennsylvania State University,
University Park, Pennsylvania

Tsuyoshi Miyazaki, University of Tokyo, Japan

Soil Biochemistry, Volume 1, edited by A. D. McLaren and G. H. Peterson
Soil Biochemistry, Volume 2, edited by A. D. McLaren and J. Skujins
Soil Biochemistry, Volume 3, edited by E. A. Paul and A. D. McLaren
Soil Biochemistry, Volume 4, edited by E. A. Paul and A. D. McLaren
Soil Biochemistry, Volume 5, edited by E. A. Paul and J. N. Ladd
Soil Biochemistry, Volume 6, edited by Jean-Marc Bollag and G. Stotzky
Soil Biochemistry, Volume 7, edited by G. Stotzky and Jean-Marc Bollag
Soil Biochemistry, Volume 8, edited by Jean-Marc Bollag and G. Stotzky
Soil Biochemistry, Volume 9, edited by G. Stotzky and Jean-Marc Bollag


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Page C

Organic Chemicals in the Soil Environment, Volumes 1 and 2,
edited by C. A. I. Goring and J. W. Hamaker
Humic Substances in the Environment, M. Schnitzer and S. U. Khan
Microbial Life in the Soil: An Introduction, T. Hattori
Principles of Soil Chemistry, Kim H. Tan
Soil Analysis: Instrumental Techniques and Related Procedures,
edited by Keith A. Smith
Soil Reclamation Processes: Microbiological Analyses and Applications,
edited by Robert L. Tate III and Donald A. Klein

Symbiotic Nitrogen Fixation Technology, edited by Gerald H. Elkan
Soil-–Water Interactions: Mechanisms and Applications, Shingo Iwata
and Toshio Tabuchi with Benno P. Warkentin
Soil Analysis: Modern Instrumental Techniques, Second Edition,
edited by Keith A. Smith
Soil Analysis: Physical Methods, edited by Keith A. Smith and Chris E. Mullins
Growth and Mineral Nutrition of Field Crops, N. K. Fageria, V. C. Baligar,
and Charles Allan Jones
Semiarid Lands and Deserts: Soil Resource and Reclamation,
edited by J. Skujins
Plant Roots: The Hidden Half, edited by Yoav Waisel, Amram Eshel,
and Uzi Kafkafi
Plant Biochemical Regulators, edited by Harold W. Gausman
Maximizing Crop Yields, N. K. Fageria
Transgenic Plants: Fundamentals and Applications, edited by Andrew Hiatt
Soil Microbial Ecology: Applications in Agricultural and Environmental
Management, edited by F. Blaine Metting, Jr.
Principles of Soil Chemistry: Second Edition, Kim H. Tan
Water Flow in Soils, edited by Tsuyoshi Miyazaki
Handbook of Plant and Crop Stress, edited by Mohammad Pessarakli
Genetic Improvement of Field Crops, edited by Gustavo A. Slafer
Agricultural Field Experiments: Design and Analysis, Roger G. Petersen
Environmental Soil Science, Kim H. Tan
Mechanisms of Plant Growth and Improved Productivity: Modern Approaches,
edited by Amarjit S. Basra
Selenium in the Environment, edited by W. T. Frankenberger, Jr.
and Sally Benson
Plant–Environment Interactions, edited by Robert E. Wilkinson
Handbook of Plant and Crop Physiology, edited by Mohammad Pessarakli
Handbook of Phytoalexin Metabolism and Action, edited by M. Daniel

and R. P. Purkayastha


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Soil–Water Interactions: Mechanisms and Applications, Second Edition,
Revised and Expanded, Shingo Iwata, Toshio Tabuchi,
and Benno P. Warkentin
Stored-Grain Ecosystems, edited by Digvir S. Jayas, Noel D. G. White,
and William E. Muir
Agrochemicals from Natural Products, edited by C. R. A. Godfrey
Seed Development and Germination, edited by Jaime Kigel and Gad Galili
Nitrogen Fertilization in the Environment, edited by Peter Edward Bacon
Phytohormones in Soils: Microbial Production and Function,
William T. Frankenberger, Jr., and Muhammad Arshad
Handbook of Weed Management Systems, edited by Albert E. Smith
Soil Sampling, Preparation, and Analysis, Kim H. Tan
Soil Erosion, Conservation, and Rehabilitation, edited by Menachem Agassi
Plant Roots: The Hidden Half, Second Edition, Revised and Expanded,
edited by Yoav Waisel, Amram Eshel, and Uzi Kafkafi
Photoassimilate Distribution in Plants and Crops: Source–Sink Relationships,
edited by Eli Zamski and Arthur A. Schaffer
Mass Spectrometry of Soils, edited by Thomas W. Boutton
and Shinichi Yamasaki

Handbook of Photosynthesis, edited by Mohammad Pessarakli
Chemical and Isotopic Groundwater Hydrology: The Applied Approach,
Second Edition, Revised and Expanded, Emanuel Mazor
Fauna in Soil Ecosystems: Recycling Processes, Nutrient Fluxes,
and Agricultural Production, edited by Gero Benckiser
Soil and Plant Analysis in Sustainable Agriculture and Environment,
edited by Teresa Hood and J. Benton Jones, Jr.
Seeds Handbook: Biology, Production, Processing, and Storage: B. B. Desai,
P. M. Kotecha, and D. K. Salunkhe
Modern Soil Microbiology, edited by J. D. van Elsas, J. T. Trevors,
and E. M. H. Wellington
Growth and Mineral Nutrition of Field Crops: Second Edition, N. K. Fageria,
V. C. Baligar, and Charles Allan Jones
Fungal Pathogenesis in Plants and Crops: Molecular Biology and Host Defense
Mechanisms, P. Vidhyasekaran
Plant Pathogen Detection and Disease Diagnosis, P. Narayanasamy
Agricultural Systems Modeling and Simulation, edited by Robert M. Peart
and R. Bruce Curry
Agricultural Biotechnology, edited by Arie Altman
Plant–Microbe Interactions and Biological Control, edited by Greg J. Boland
and L. David Kuykendall
Handbook of Soil Conditioners: Substances That Enhance the Physical
Properties of Soil, edited by Arthur Wallace and Richard E. Terry


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Environmental Chemistry of Selenium, edited by William T. Frankenberger, Jr.,
and Richard A. Engberg
Principles of Soil Chemistry: Third Edition, Revised and Expanded, Kim H. Tan
Sulfur in the Environment, edited by Douglas G. Maynard
Soil–Machine Interactions: A Finite Element Perspective, edited by Jie Shen
and Radhey Lal Kushwaha
Mycotoxins in Agriculture and Food Safety, edited by Kaushal K. Sinha
and Deepak Bhatnagar
Plant Amino Acids: Biochemistry and Biotechnology, edited by Bijay K. Singh
Handbook of Functional Plant Ecology, edited by Francisco I. Pugnaire
and Fernando Valladares
Handbook of Plant and Crop Stress: Second Edition, Revised and Expanded,
edited by Mohammad Pessarakli
Plant Responses to Environmental Stresses: From Phytohormones to Genome
Reorganization, edited by H. R. Lerner
Handbook of Pest Management, edited by John R. Ruberson
Environmental Soil Science: Second Edition, Revised and Expanded,
Kim H. Tan
Microbial Endophytes, edited by Charles W. Bacon and James F. White, Jr.
Plant–Environment Interactions: Second Edition, edited by Robert E. Wilkinson
Microbial Pest Control, Sushil K. Khetan
Soil and Environmental Analysis: Physical Methods, Second Edition, Revised
and Expanded, edited by Keith A. Smith and Chris E. Mullins
The Rhizosphere: Biochemistry and Organic Substances at the Soil–Plant
Interface, Roberto Pinton, Zeno Varanini, and Paolo Nannipieri
Woody Plants and Woody Plant Management: Ecology, Safety,
and Environmental Impact, Rodney W. Bovey

Metals in the Environment, M. N. V. Prasad
Plant Pathogen Detection and Disease Diagnosis: Second Edition, Revised
and Expanded, P. Narayanasamy
Handbook of Plant and Crop Physiology: Second Edition, Revised
and Expanded, edited by Mohammad Pessarakli
Environmental Chemistry of Arsenic, edited by William T. Frankenberger, Jr.
Enzymes in the Environment: Activity, Ecology, and Applications,
edited by Richard G. Burns and Richard P. Dick
Plant Roots: The Hidden Half, Third Edition, Revised and Expanded,
edited by Yoav Waisel, Amram Eshel, and Uzi Kafkafi
Handbook of Plant Growth: pH as the Master Variable,
edited by Zdenko Rengel
Biological Control of Major Crop Plant Diseases,
edited by Samuel S. Gnanamanickam
Pesticides in Agriculture and the Environment, edited by Willis B. Wheeler


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Mathematical Models of Crop Growth and Yield, Allen R. Overman
and Richard Scholtz
Plant Biotechnology and Transgenic Plants,
edited by Kirsi-Marja Oksman Caldentey and Wolfgang Barz
Handbook of Postharvest Technology: Cereals, Fruits, Vegetables, Tea,

and Spices, edited by Amalendu Chakraverty, Arun S. Mujumdar,
G. S. Vijaya Raghavan, and Hosahalli S. Ramaswamy
Handbook of Soil Acidity, edited by Zdenko Rengel

Additional Volumes in Preparation
Humic Matter: Issues and Controversies in Soil and Environmental Science,
Kim H. Tan
Molecular Host Resistance to Pests, S. Sadasivam and B. Thayumanavan


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Page i

Plant
Toxicology
Fourth Edition
Edited by

Bertold
Hock
Professor of Cell Biology
and Dean of the Center of Life and Food Sciences
Technische Universität München
Freising, Germany

Erich
F.
Elstner
Professor and Head of the Institute of Phytopathology
Technische Universität München
Freising, Germany

Marcel Dekker

New York


Although great care has been taken to provide accurate and current information,
neither the author(s) nor the publisher, nor anyone else associated with this
publication, shall be liable for any loss, damage, or liability directly or indirectly
caused or alleged to be caused by this book. The material contained herein is not
intended to provide specific advice or recommendations for any specific situation.
Trademark notice: Product or corporate names may be trademarks or registered
trademarks and are used only for identification and explanation without intent to
infringe.
Library of Congress Cataloging-in-Publication Data
A catalog record for this book is available from the Library of Congress.
ISBN: 0-8247-5323-2
This book is printed on acid-free paper.
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Marcel Dekker, 270 Madison Avenue, New York, NY 10016, U.S.A.
tel: 212-696-9000; fax: 212-685-4540
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Copyright ß 2005 by Marcel Dekker. All Rights Reserved.
Neither this book nor any part may be reproduced or transmitted in any form or by
any means, electronic or mechanical, including photocopying, microfilming, and
recording, or by any information storage and retrieval system, without permission in
writing from the publisher.
Current printing (last digit):
10

9 8 7 6 5

4 3 2 1

PRINTED IN THE UNITED STATES OF AMERICA


Preface

Plant toxicology is dealing with poisons causing harmful effects in plants.
Not only humans and animals, but also plants are affected by a multitude of
toxins. Therefore plant toxicology is concerned with damages, which are
caused by toxic agents, either accidentally or deliberately. Considering the
growing number of environmental compounds interfering with plant
metabolism and development, and keeping in mind the role of plants as
primary producers of food, it is surprising that the term toxicology has been
confined almost exclusively to humans and animals. If recent damages such

as forest diebacks are taken into account, it is clear that plant toxicology
represents an important branch in biological sciences.
Understanding of toxic processes in plants requires a detailed
knowledge of molecular events when toxic compounds as well as elicitors
during host–pathogen interactions bind to their molecular targets. However,
toxicology involves the entire series of phases that are relevant for the toxic
process, i.e. exposure to toxic material, uptake, distribution, metabolism
and finally secretion. These topics are central to this book.
Another focus is the recognition and possible prevention of damage,
caused by environmental pollutants. Quantification of damage is therefore
crucial. But plant toxicology also deals with negative effects, which are
intended. Agriculture and horticulture provide many examples, such as the use
of herbicides. Questions concerning the uptake, metabolism and detoxification
have to be solved before suitable and justifiable applications can be considered.
Although exogenous compounds, which normally do not occur in the
metabolism of plants (xenobiotics) are central to this book, it should be
iii


iv

Preface

noted that also endogenous compounds of the organism can become
harmful when certain thresholds are exceeded. Such effects may result from
over-fertilization and are therefore taken into account. Even physical factors
such as ionizing radiation or detrimental effects of biogenic origin such as
infestation with parasites have to be considered if damage arises similar to
the impact of xenobiotics.
The boundaries of plant toxicology are relatively wide. They are

primarily determined by practical aspects. This book should also help to
classify observed damage and, if possible, identify. The limitation on
eukaryotic plants as potential target groups will meet practical interests.
Methods of plant toxicology originate primarily from chemistry and
biochemistry. Chemical analysis provides mainly the methods, biochemical
techniques contribute to the elucidation of action mechanisms and
metabolism of toxic compounds. Progress in toxicology largely depends
on the development of new methods and techniques.
It is not sufficient to see plants as isolated organisms. The
consideration of the ecological context is an important requisite for the
evaluation and abolishment of toxic influences. Basics of biological
knowledge are essential and provided where needed.
The editors would like to thank Marcel Dekker, Inc., particularly
Theresa Stockton, who edited and guided the book throughout production.
Indeed we wish to thank the entire staff for their understanding, encouragement, and practical help.
Bertold Hock
Erich F. Elstner


Contents

Preface
Contributors

iii
vii

1.

Characteristics of Plant Life: Hazards from Pollutants

Bertold Hock and Nicola M. Wolf

1

2.

Plant Stress: Avoidance, Adaptation, Defense
Harald Schempp, Susanne Hippeli, and Erich F. Elstner

87

3.

Uptake and Transport of Xenobiotics
Markus Riederer

131

4.

Air Pollution: Trace Gases as Inducers of Plant Damage
Harald Schempp, Susanne Hippeli, Erich F. Elstner,
and Christian Langebartels

151

5.

Limitation of Salt Stress to Plant Growth
Yuncai Hu and Urs Schmidhalter

191

6.

Mineral Element Toxicities: Aluminum and Manganese
Walter J. Horst, Angelika Staß, and
Marion M. Fecht-Christoffers

225

v


vi

Contents

7.

Herbicides
Carl Fedtke and Stephen O. Duke

8.

Molecular Basis of Toxic Effects: Inhibition of
Cellular Pathways and Structural Components
K. Kramer and Bertold Hock

9.

10.

11.

12.

Metabolism and Elimination of Toxicants
K. K. Hatzios

247

331

469

Host–Pathogen Relations: Diseases Caused by Viruses,
Subviral Organisms, and Phytoplasmas
Bala´zs Barna and Lo´ra´nt Kira´ly

519

Interactions Between Host Plants and Fungal
and Bacterial Pathogens
Ingrid Heiser, Joărg Durner, and Christian Langebartels

555

Allelopathy
Astrid Lux-Endrich and Bertold Hock

Index

597

621


Contributors

Balazs Barna Plant Protection Institute, Hungarian Academy of Sciences,
Budapest, Hungary
Joărg Durner Institute of Biochemical Plant Pathology, National Research
Center for Environment and Health, Neuherberg, Germany
Stephen O. Duke
U.S.A.
Erich F. Elstner

U.S. Department of Agriculture, University, Mississippi,

Technische Universitaăt Muănchen, Freising, Germany

Marion M. Fecht-Christoffers
Carl Fedtke

Universitaăt Hannover, Hannover, Germany

Koăln, Germany

K. K. Hatzios Virginia Polytechnic Institute and State University,

Blacksburg, Virginia, U.S.A.
Ingrid Heiser Technische Universitaăt Muănchen, Freising, Germany
Susanne Hippeli
Bertold Hock

Technische Universitaăt Muănchen, Freising, Germany

Technische Universitaăt Muănchen, Freising, Germany
vii


viii

Contributors

Walter J. Horst Universitaăt Hannover, Hannover, Germany
Yuncai Hu

Technische Universitaăt Muănchen, Freising, Germany

K. Kramer

Technische Universitaăt Muănchen, Freising, Germany

Lorant Kiraly Plant Protection Institute, Hungarian Academy of Sciences,
Budapest, Hungary
Christian Langebartels Institute of Biochemical Plant Pathology, National
Research Center for Environment and Health, Neuherberg, Germany
Astrid Lux-Endrich

Technische Universitaăt Muănchen, Freising, Germany

Markus Riederer

Universitaăt Wuărzburg, Wuărzburg, Germany

Harald Schempp

Technische Universitaăt Muănchen, Freising, Germany

Urs Schmidhalter
Angelika Staò
Nicola M. Wolf

Technische Universitaăt Muănchen, Freising, Germany

Universitaăt Hannover, Hannover, Germany
Technische Universitaăt Muănchen, Freising, Germany


1
Characteristics of Plant Life:
Hazards from Pollutants
Bertold Hock and Nicola M. Wolf
Technische Universitaăt Muănchen, Freising, Germany

I.

GREEN PLANTS AS PHOTOAUTOTROPHIC ORGANISMS

Living organisms are characterized by their extraordinary complexity. It is
manifested in the higher plant by a hierarchy of structures comprising
organs, tissues, and cells. The following chapters provide an introduction to
plant organization starting with the cell as the smallest elementary unit.
Emphasis is laid upon plant-specific features, which are discussed with
respect to their susceptibility to environmental contaminants.

II.

FUNCTIONAL ORGANIZATION OF THE CELL

A.

Role of Membranes

Plant cells as eukaryotes are characterized by their compartmentation into
membrane-enclosed reaction spaces. This structure separates different
metabolic pathways but at the same time allows a grouping of connected
biochemical functions. This process allows sophisticated regulations.
The borders of compartments are composed of biomembranes (Fig. 1).
These thin and highly flexible structures determine the architecture of
biological systems. Biomembranes are flat, asymmetrical structures, which
are closed and usually topologically equivalent to the surface of a sphere
or a torus. Along with the basic composition of lipids and proteins there
are variety of individual compositions. The ratio of proteins to lipids varies
within a range of 1:4 to 4:1. Both components are held together by
1


2

Hock and Wolf

Figure 1 Biomembrane with integral and peripheral membrane proteins (1). The
protruding oligosaccharide chains belong to glycoproteins and glycolipids.

hydrophobic interactions. The lipids form a bimolecular layer, which serves
as a barrier and prevents the passage of polar molecules. Integral proteins
penetrate completely or at least partly the lipid bilayer. Conspicuous
examples are the chlorophyll a/b-binding protein of chloroplast membranes
and the adenosine triphosphate (ATP) synthase of mitochondrial and
chloroplast membranes. Peripheral proteins bind at the membrane surface
to integral proteins from which they can be detached easily by appropriate
solvents. They include clathrin, spectrin, and ankyrin of the plasma
membrane. Usually membrane proteins have mediating functions: they
serve as receptors, ion channels, ATP-powered pumps, or transporters. In
many cases enzymatic activities are crucial for the function. This explains
why the two sides of the membrane bilayer are usually different.
Transport through a biomembrane uses one of the three following
mechanisms (Fig. 2):
1. Passive diffusion: Only a few substances are able to penetrate
the lipid bilayer. Examples are gases such as oxygen (O2), nitrogen (N2), or
carbon dioxide (CO2) and some small, uncharged molecules such as ethanol,


Figure 2 Scheme of the membrane transport (2). Passive transport follows an electrochemical gradient as free diffusion or
facilitated diffusion and is mediated by transport proteins. In contrast, active transport requires adenosine triphosphate–
(ATP-)powered pumps and moves ions or small molecules uphill against the electrochemical gradient.

Characteristics of Plant Life

3


4

Hock and Wolf

Figure 3 Scheme of transport proteins, which act as uniporters, symporters, or
antiporters (2).

urea, or benzene, which dissolve easily in the lipid bilayer. In principle this
also holds true for water (H2O) although its diffusion may be accelerated
by transport proteins (aquaporins). Passive diffusion of H2O through the
biomembrane is crucial for osmosis. In contrast, lipid bilayers are practically
impermeable to charged molecules.
2. Facilitated diffusion (catalyzed diffusion): Similar to passive
diffusion this mechanism does not require energy and leads only to a
concentration equilibrium. However, transport proteins (channels and
transporters) are required for this type of membrane transport. Three
options are available (Fig. 3): (a) unidirectional transport (uniport), which
moves only one kind of molecule; (b) symport (cotransport), in which two
molecules or ions are transported in the same direction, and one of them,
for instance, Hỵ, follows a concentration gradient; (c) antiport, in which
two molecules or ions move in opposite directions, one of them following a
concentration gradient. Both antiporters and symporters mediate coupled
reactions in which the energetically unfavorable reaction is coupled to an
energetically favorable reaction.
3. Active transport: The process pumps ions or small molecules
through a membrane against a chemical concentration gradient and/or
electric potential. This ATP-powered transport moves ions such as Hỵ, Kỵ,

Ca2ỵ, and Naỵ in one direction. It is mediated by adenosine triphosphatases


Characteristics of Plant Life

5

(ATPases). The required energy is released by the hydrolysis of ATP to
adenosine disphosphate (ADP) and inorganic phosphate (Pi). This active
transport indirectly drives symport and antiport.
The compartmentation of cells creates huge membrane areas, which
offer extended targets to environmental compounds and toxins approaching
and penetrating the cells. Binding of a variety of substances can lead not
only to impairment of individual functions, but also to destruction of
whole compartments. The abolition of compartmentation always leads to
cell death.
Compartmentation entails a separation of protoplasmic and
nonprotoplasmic spaces: the plasmalemma separates the protoplast from
the cell wall at the outside, the tonoplast from the vacuole at the inside
(Fig. 4). The plasmalemma (plasma membrane) allows the movement of
solutes into the protoplast as well as outward. In addition, it is involved in
signal transduction between the outside and inside and mediates hormonal as
well as light effects. This allows the cell to detect changes of its environment
and react accordingly. For this purpose several receptor types as well as
redox chains are available. A prominent component of the plasmalemma is
the Hỵ-ATPase. This transport ATPase plays a crucial role in the uptake of
nutrients and pH regulation. By means of this ATP-driven Hỵ pumping
the membrane becomes energized, and the electrochemical potential for
the import of solutes through ion channels and carriers is maintained by ATP
hydrolysis. This function is taken in animal cells by the sodium pump (Naỵ/

Kỵ-ATPase). Whereas in this case the pump is usually coupled to Kỵ import
and Naỵ export, proton coupling is used by plants. The proton pump
transports a single Hỵ for each hydrolized ATP and creates in this way a
large electrical potential up to
300 mV (inside the membrane negative) as
well as a proton gradient. The values reach a pH of c. 7.1 at the inner side of
the membrane and at the outside values between 4.5 and 5. A comparison of
the different strategies is illustrated by Fig. 5.
Cells with intensive active transport such as root hairs require between
25% and 50% of their total cellular ATP to keep their proton pumps
running. In addition hydrogen adenosine triphosphatases (Hỵ-ATPases)
play an important role in cell elongation (cf. acid growth theory: Chapter 1,
2D structure and function of the cell wall).
The plasma membrane contains a multitude of further membrane
proteins, depending on the specific cell. In addition to Hỵ-ATPases, cation
and anion channels as well as carriers for sucrose, nitrate, and amino and a
multitude of hormone and blue light receptors have been identified. Even
many pollutants act on components of the plasmalemma, especially on HỵATPases. Many xenobiotics also enter the cell, where they are taken up
directly or after conjugation to glutathione into the vacuole. In other cases


6

Hock and Wolf

Figure 4 Structure of a plant cell: (a) Scheme (3); (b) electron micrograph of
a tobacco mesophyll cell. ER, endoplasmic reticulum. (With kind permission of
Prof. Katherine Esau, University of California.)


Characteristics of Plant Life

7

Figure 4 Continued.

Figure 5 Different strategies of plant and animal cells regarding the uptake of
solute (S) and pH regulation. ATP, adenosine triphosphate; ADP, adenosine
diphosphate; Pi, inorganic phosphate. (From Ref. 4.)

cytochrome P450–dependent hydroxylations occur, followed by further
metabolic steps.
According to the principle of hierarchic structuring the protoplast is
divided into several compartments. The membrane-enclosed spaces are
organelles in a narrower sense. There are organelles bounded by two


8

Hock and Wolf

membranes (cell nucleus, mitochondria, and plastids) and organelles
surrounded by a single-membrane envelope (such as endoplasmic reticulum,
dicytosomes, lysosomes, peroxisomes, glyoxysomes, and microbodies).
These organelles are embedded into the cytosol, which also contains
additional particles (organelles in a wider sense such as ribosomes as well as
fibrous elements belonging to the cytoskeleton). Sometimes the traditional
grouping of the protoplast into cell nucleus and cytoplasm is used.
With respect to cell compartmentation the following rules are
important: an envelope composed of two membranes separates plasmatic
from other plasmatic spaces (P spaces). The P spaces are characterized by

their potential to synthesize nucleic acids and/or proteins. The delimitation
of the nucleus, mitochondria, and plastids against the cytosol falls into this
group. A delimitation by a single membrane separates plasmatic from
endoplasmatic spaces (E spaces). The latter spaces are not capable of nucleic
acid or protein synthesis. The cell wall and vacuole are nonplasmatic
spaces, as are the lumen of the endoplasmic reticulum (ER), dicytosomes,
lysosomes, and microbodies. The symbiont theory of organelle evolution
explains these distinctions: mitochondria and plastids are derived from
prokaryotic cells that have colonized as endosymbionts, the progenitors of
modern eukaryotes.
Cell membranes are subjected to remarkable dynamics, to which
membrane flow substantially contributes. Membrane flow is mediated by an
intensive vesicle stream within the cytosol. There are several options,
depending on the respective cell: the pathway from the ER, which is
connected to the nuclear envelope, via the Golgi apparatus to the
plasmalemma (exocytosis) or to the lysosomal compartment. On the other
hand, components of the plasmalemma, e.g., receptors, are recycled by
endocytosis. By means of membrane flow large amounts of newly
synthesized fatty acids and lipids are transported from the ER to the
plasmalemma. Figure 6 gives an overview of the path of vesicle streams.
The plant cell differs from the animal cell in the existence of three
additional compartments, plastids, vacuole, and cell wall, whereas the other
structures are generally very similar. These additional components provide
specific targets for pollutants, but on the other hand remarkable indifference
to certain xenobiotics that have dramatic effects on animals and humans
prevails.
B.

Chloroplasts, the Photosynthetic Organelles

Green plants as photoautotrophic organisms utilize the energy of sunlight
for the synthesis of high-energy compounds. This transformation of energy
takes place in the chloroplasts (Fig. 7a). Here the light energy is used for


Characteristics of Plant Life

9

Figure 6 Membrane flow between different compartments. ER, endoplasmic
reticulum.

Figure 7 Structure of chloroplasts: (a) Electron micrograph of a mesophyll cell
from a corn leaf (5). (b) Arrangement of thylakoid membranes (scheme) (6).


10

Hock and Wolf

Figure 8 Contribution of light and dark reactions of photosynthesis to CO2
assimilation (7). ATP, adenosine triphosphate; NADPH, reduced nicotinamide
adenine dinucleotide.

the generation of the assimilatory power for the dark reactions, a historic
term for the synthesis of carbohydrates from CO2, for which light energy
itself is not required. Actually the light-driven reactions provide specific
regulatory substances, which restrict the operation of the dark reactions to
the light period. Regulation involves the ferredoxin/thioredoxin system,
Fig. 8 shows this connection. A light-driven reaction chain (light reactions I

and II) requiring intact thylakoids (an endomembrane system of chloroplasts; Fig. 7b) generates the assimilatory power provided by reduced
nicotinamide adenine dinucleotide phosphate (NADPH) and ATP.

1.

Reduced Nicotinamide Adenine Dinucleotide Phosphate

The electrons required for the reduction of the coenzyme, are provided by
an electron transport chain. It accepts electrons from a water-splitting
complex (H2O ! ẵ O2 ỵ 2 Hỵ ỵ 2 e
) and moves them with the aid of
light-activated ‘‘pump station,’’ photosystem II (PS II) and I (PS I), to
nicotinamide adenine dinucleotide phosphate (NADP). The photoreceptor
of the two photosystems is the green pigment chlorophyll a, surrounded
by several so-called antenna pigments. A destruction of these pigments,
for instance, during bleaching reactions, interferes with photosynthesis.
The photosynthetic pigments are embedded in a multiprotein complex.
Photosystem II, which catalyzes the oxidation of H2O and the reduction of
the electron acceptor plastoquinone, contains the homologous polypeptides
D1 and D2. Figure 9 shows a model of photosystem II. The D1 polypeptide
has a fast turnover. Under high light intensities, which exceed the adaptive
level, the degradation is faster than the speed of repair, and photoinhibiton