Advances in agronomy volume 91

Agronomy

D VA N C E S I N

VOLUME 91


Advisory Board
John S. Boyer
University of Delaware

Paul M. Bertsch
University of Georgia

Ronald L. Phillips
University of Minnesota

Kate M. Scow
University of California, Davis

Larry P. Wilding
Texas A&M University

Emeritus Advisory Board Members
Kenneth J. Frey
Iowa State University

Eugene J. Kamprath
North Carolina State University

Martin Alexander
Cornell University

Prepared in cooperation with the
American Society of Agronomy, Crop Science Society of America, and
Soil Science Society of America Book and Multimedia
Publishing Committee
David D. Baltensperger, Chair
Lisa K. Al-Amoodi
Kenneth A. Barbarick

Hari B. Krishnan
Sally D. Logsdon
Michel D. Ransom

Craig A. Roberts
April L. Ulery


Agronomy
D VA N C E S I N

VOLUME 91
Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware
Newark, Delaware

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Contents
CONTRIBUTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi
xiii

GEOCHEMICAL ASPECTS OF PHYTOSIDEROPHOREPROMOTED IRON ACQUISITION BY PLANTS
S. M. Kraemer, D. E. Crowley and R. Kretzschmar
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Strategies of Plant Iron Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Strategy I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Strategy II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. How Much Is Enough? Plant Iron Requirements . . . . . . . . . . . . . . .
IV. Iron-Bearing Minerals and Soluble Iron Species
in the Rhizosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Iron-Bearing Minerals, the Penultimate Iron Source . . . . . . . . . .
B. Iron Complexation by Organic Ligands . . . . . . . . . . . . . . . . . . . .
C. Iron Complexes with Low-Molecular Weight Organic Acids . . . .
D. Microbial Siderophore Complexes . . . . . . . . . . . . . . . . . . . . . . . .

E. Natural Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. The Chemistry of Phytosiderophores
in the Rhizosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Structure of Phytosiderophores. . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Concentrations of Phytosiderophores in the Rhizosphere . . . . . . .
C. Speciation of Phytosiderophores and Iron in Solution . . . . . . . . .
D. EVect of Phytosiderophores on Iron Solubility . . . . . . . . . . . . . . .
E. Adsorption of Phytosiderophores on Iron Oxides . . . . . . . . . . . .
VI. Geochemical Aspects of Iron Shuttling . . . . . . . . . . . . . . . . . . . . . . .
A. EVect of Organic Ligands on Iron Oxide Dissolution Rates. . . . .
B. Thermodynamics and Kinetics of Ligand-Exchange Reactions
with Phytosiderophores as Receiving Ligands . . . . . . . . . . . . . . .
VII. Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v

3
4
4
5
7
9
10
12
14
14
17
18
18

19
20
22
23
25
25
29
37
38


vi

CONTENTS

TAKING STOCK OF THE BRAZILIAN ‘‘ZERO-TILL
REVOLUTION’’: A REVIEW OF LANDMARK RESEARCH
AND FARMERS’ PRACTICE
Adrian Bolliger, Jakob Magid, Telmo Jorge Carneiro Amado,
Francisco Sko´ra Neto, Maria de Fatima dos Santos Ribeiro,
Ademir Calegari, Ricardo Ralisch and
Andreas de Neergaard
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Zero-Till Development in Subtropical Southern Brazil . . . . . . . . .
B. Zero-Till Development in Tropical Brazil . . . . . . . . . . . . . . . . . . .
C. Development of Smallholder Zero-Till Systems . . . . . . . . . . . . . .
III. Individual Issues, Innovations, and Challenges . . . . . . . . . . . . . . . . .
A. Permanent Soil Surface Cover . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Cover Crops, and Crop Rotations and Associations . . . . . . . . . .

C. Soil Organic Matter Build-Up . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Nutrient Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Soil Compaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Soil Acidity and Aluminum Toxicity . . . . . . . . . . . . . . . . . . . . . .
G. Weed Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Pests and Disease Management . . . . . . . . . . . . . . . . . . . . . . . . . .
I. Integrating Livestock and Crops . . . . . . . . . . . . . . . . . . . . . . . . .
J. Suitable Equipment for Resource-Poor Farmers . . . . . . . . . . . . .
IV. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49
51
51
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55
56
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86
88
90
93

93

MECHANISMS AND PATHWAYS OF TRACE ELEMENT
MOBILITY IN SOILS
R. Carrillo-Gonza´lez, Jirka Sˇimu˚nek,
Se´bastien Sauve´ and Domy Adriano
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Mechanisms of Trace Element Mobility . . . . . . . . . . . . . . . . . . . . . .
A. Physicochemical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Biological Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. In Situ Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

112
113
113
121
122


CONTENTS
III. Trace Element Transport Pathways . . . . . . . . . . . . . . . . . . . . . . . . . .
A. DiVusion and Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Preferential Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Colloidal Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Soluble Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Leaching and RunoV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Volatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Factors AVecting Trace Element Mobility and Transport . . . . . . . . .
A. Soil pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Chemical Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Soil Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Fertilizers and Soil Amendments . . . . . . . . . . . . . . . . . . . . . . . . .
E. Redox Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Clay Content and Soil Structure . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Transport Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Variably Saturated Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Solute Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Colloid Transport and Colloid-Facilitated Solute Transport . . . .
VI. Model Applications and Case Studies . . . . . . . . . . . . . . . . . . . . . . . .
A. Single-Component Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Multicomponent Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii
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126
126
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129
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136
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139
140
142

143
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153
155
155
157
161
163
163

THE AGRONOMY AND ECONOMY OF CARDAMOM
(ELETTARIA CARDAMOMUM M.): THE ‘‘QUEEN OF SPICES’’
K. P. Prabhakaran Nair
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Historical Background of Cardamom . . . . . . . . . . . . . . . . . . . . . .
B. Cardamom Production, Productivity: A World View . . . . . . . . . .
II. Cardamom Botany . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Crop Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. Cardamom Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Biosynthesis of Flavor Compounds . . . . . . . . . . . . . . . . . . . . . . .
B. Industrial Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

182
182
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188
189
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215
218
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viii

CONTENTS

IV. The Agronomy of Cardamom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Management Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Establishing a Cardamom Plantation . . . . . . . . . . . . . . . . . . . . . .
E. Shade Management in Cardamom . . . . . . . . . . . . . . . . . . . . . . . .
F. Cardamom-Based Cropping Systems . . . . . . . . . . . . . . . . . . . . . .
G. Cardamom Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Fertilizer Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. The Role of ‘‘The Nutrient BuVer Power Concept’’
in Cardamom Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. The ‘‘BuVer Power’’ and EVect on Nutrient Availability . . . . . . .
B. Measuring the Nutrient BuVer Power and Its Importance
in AVecting Nutrient Concentrations on Root Surfaces . . . . . . . .
C. Quantifying the BuVer Power of Soils and Testing
Its EVect on Potassium Availability . . . . . . . . . . . . . . . . . . . . . . .
D. The Importance of K BuVer Power Determination
in Predicting K Availability to Perennial Crops . . . . . . . . . . . . . .
E. The Commercial Significance of K BuVer Power Determination
in K Fertilizer Management for Perennial Crops . . . . . . . . . . . . .
F. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI. Cardamom Pathology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Major Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Minor Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Integrated Management of Viral Diseases in Cardamom . . . . . . .
VII. Cardamom Entomology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Major Pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Minor Pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Storage Pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VIII. Harvesting and Processing of Cardamom . . . . . . . . . . . . . . . . . . . . .
A. Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Grading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Bleached and Half Bleached Cardamom . . . . . . . . . . . . . . . . . . .
F. Commercial Cardamom Grades in Sri Lanka . . . . . . . . . . . . . . .
G. Grading and Packing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IX. Industrial Processing of Cardamom and Cardamom Products . . . . .
A. Cardamom Seeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Packaging and Storage of Cardamom Seeds. . . . . . . . . . . . . . . . .
C. Cardamom Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

228
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276
276
278
281
281
288
290
290
291
298
317
323
323
329
332
332
333
333
336
340
341
341
347
347
348
348
349

350
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CONTENTS
Storage Powder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cardamom Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Industrial Production of Cardamom Oil . . . . . . . . . . . . . . . . . . . .
Improvement in Flavor Quality of Cardamom Oil . . . . . . . . . . . .
Storage of Cardamom Oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cardamom Oleoresin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Solvent Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Large Cardamom (Nepal Cardamom) . . . . . . . . . . . . . . . . . . . . .
Other Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Economy of Cardamom Production . . . . . . . . . . . . . . . . . . . . . .
A. The Emerging Trends in Cardamom Production . . . . . . . . . . . . .
B. Export Performance of Cardamom. . . . . . . . . . . . . . . . . . . . . . . .
C. Direction of Indian Export Trade. . . . . . . . . . . . . . . . . . . . . . . . .
D. India’s Competitive Position in the International
Cardamom Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Demand and Supply Pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Model Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G. The Forecast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I. Projections of Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pharmacological Properties of Cardamom . . . . . . . . . . . . . . . . . . . . .
A. Pharmacological Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Carminative Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Antimicrobial Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Anticarcinogenetic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Anti-Inflammatory Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Other Pharmacological Studies . . . . . . . . . . . . . . . . . . . . . . . . . . .
G. Toxicity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Other Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I. Cardamom as a Spice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Peep into the Future of Cardamom . . . . . . . . . . . . . . . . . . . . . . . .
A. Potential Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Future Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Large Cardamom (Amomum subulatum Roxb.) . . . . . . . . . . . . . . . . .
A. Habit and Habitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Cultivars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Plant Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Plant Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Crop Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Insect Pest Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G. Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E.
F.
G.
H.
I.
J.
K.
L.
M.

N.

X.

XI.

XII.

XIII.

ix
352
352
354
354
356
356
357
361
362
363
363
366
371
372
373
374
374
375
376

377
379
380
380
381
381
381
382
383
384
385
388
394
396
399
399
403
403
404
407
412
413
414
420


x

CONTENTS

Management of the Chirke and Foorkey Diseases . . . . . . . . . . . .
Harvesting and Postharvest Technology . . . . . . . . . . . . . . . . . . . .
Natural Convection Dryer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Properties and Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XIV. False Cardamom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Elettaria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XV. Specification for Cardamom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

421
422
424
425
426
427
427
430
437
437
441
442

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

473

H.
I.
J.
K.
L.
M.


Contributors
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

Domy Adriano (111), Savannah River Ecology Laboratory, University of
Georgia, Drawer E, Aiken, South Carolina 29802
Telmo Jorge Carneiro Amado (47), Department of Soil Science, Federal
University of Santa Maria, RS 97119-900, Brazil
Adrian Bolliger (47), Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural
University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen,
Denmark
Ademir Calegari (47), IAPAR, Rod. Celso Garcia Cid, Londrina, PR
86001-970, Brazil
R. Carrillo-Gonza´lez (111), Colegio de Postgraduados, Carr. Me´xicoTexcoco km 36.5, 56230 Texcoco, Me´xico
D. E. Crowley (1), Department of Environmental Sciences, University of
California, Riverside, California 92521
S. M. Kraemer (1), Department of Environmental Sciences, ETH Zu¨rich,
CH 8092 Zu¨rich, Switzerland
R. Kretzschmar (1), Department of Environmental Sciences, ETH Zu¨rich,
CH 8092 Zu¨rich, Switzerland
Jakob Magid (47), Plant Nutrition and Soil Fertility Laboratory, Department
of Agricultural Sciences, The Royal Veterinary and Agricultural University,
Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark

K. P. Prabhakaran Nair (179), Indian Council of Agricultural Research,
New Delhi, India
Andreas de Neergaard (47), Plant Nutrition and Soil Fertility Laboratory,
Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark
Francisco Sko´ra Neto (47), IAPAR, Av. Pres. Kennedy, s/n, Ponta Grossa,
PR 84001-970, Brazil
Ricardo Ralisch (47), Department of Agronomy, Campus Universita´rio,
Universidade Estadual de Londrina, Londrina, PR 86051-970, Brazil
Maria de Fatima dos Santos Ribeiro (47), IAPAR, Av. Pres. Kennedy, s/n,
Ponta Grossa, PR 84001-970, Brazil
Se´bastien Sauve´ (111), Environmental Analytical Chemistry Laboratory,
Department of Chemistry, Universite´ de Montre´al, Montreal, QC H1Y
3M4, Canada
Jirka Sˇimu˚nek (111), Department of Environmental Sciences, University of
California, Riverside, California 92521
xi


This page intentionally left blank


Preface
Volume 91 contains four comprehensive reviews on agronomic topics. Chapter 1
is a timely overview on the phytosiderophase promoted iron acquisition by
plants. Chapter 2 is an interesting review on the growing importance of zero
tillage in Brazil. Chapter 3 deals with a worldwide environmental issue, trace
element mobility in soils. Chapter 4 is a comprehensive review on cardamom,
including production and management and medicinal and food uses.
I appreciate the excellent contributions of the authors.
DONALD L. SPARKS

University of Delaware
Newark, Delaware

xiii


GEOCHEMICAL ASPECTS OF
PHYTOSIDEROPHORE‐PROMOTED IRON
ACQUISITION BY PLANTS
S. M. Kraemer,1 D. E. Crowley 2 and R. Kretzschmar1
1

Department of Environmental Sciences, ETH Zu¨rich, CH 8092 Zu¨rich, Switzerland
2
Department of Environmental Sciences, University of California, Riverside,
California 92521

I. Introduction
II. Strategies of Plant Iron Acquisition
A. Strategy I
B. Strategy II
III. How Much Is Enough? Plant Iron Requirements
IV. Iron‐Bearing Minerals and Soluble Iron Species in the Rhizosphere
A. Iron‐Bearing Minerals, the Penultimate Iron Source
B. Iron Complexation by Organic Ligands
C. Iron Complexes with Low‐Molecular Weight Organic Acids
D. Microbial Siderophore Complexes
E. Natural Organic Matter
V. The Chemistry of Phytosiderophores in the Rhizosphere
A. Structure of Phytosiderophores

B. Concentrations of Phytosiderophores in the Rhizosphere
C. Speciation of Phytosiderophores and Iron in Solution
D. EVect of Phytosiderophores on Iron Solubility
E. Adsorption of Phytosiderophores on Iron Oxides
VI. Geochemical Aspects of Iron Shuttling
A. EVect of Organic Ligands on Iron Oxide Dissolution Rates
B. Thermodynamics and Kinetics of Ligand‐Exchange Reactions
with Phytosiderophores as Receiving Ligands
VII. Conclusions and Outlook
References

Iron is an essential trace nutrient for all plants. The acquisition of iron
is limited by low solubilities and slow dissolution rates of iron‐bearing minerals
in many soils. Therefore, iron limitation can be an important nutritional
disorder in crop plants, leading to decreased yields or significant costs for
iron fertilization. However, some species among the group of graminaceous
plants (including wheat and barley) exhibit a rather low susceptibility to iron
deficiency. These species respond to iron‐limiting conditions by the exudation
1
Advances in Agronomy, Volume 91
Copyright 2006, Elsevier Inc. All rights reserved.
0065-2113/06 $35.00
DOI: 10.1016/S0065-2113(06)91001-3


S. M. KRAEMER ET AL.

2

of ligands with a high aYnity and specificity for iron complexation, the so‐

called phytosiderophores. Soluble iron–phytosiderophore complexes are
recognized and transported across the root plasma membrane by specific
transport proteins. This chapter focuses on geochemical aspects of this so‐
called ‘‘strategy II’’ iron acquisition mechanism. The coordination chemistry
of phytosiderophores and their iron complexes in the soil solution are discussed and compared to other organic ligands including low‐molecular
weight organic acids and microbial siderophores. The properties of iron
complexes and iron‐bearing minerals in the rhizosphere are discussed and
compared with regard to their potential as sources of plant available iron. An
important focus of this chapter is the elucidation of the thermodynamics,
mechanisms, and rates of iron acquisition from these sources by phytosiderophores. Thus, we hope to contribute to the understanding of iron acquisition
by strategy II plants in particular and of iron cycling in the rhizosphere
# 2006, Elsevier Inc.
in general.

ABBREVIATIONS
DA‐A
DDA‐A
DFO‐B
DFO‐D1
DMA
DOM
DTPA
EDTA
3‐epiHMA
EXAFS
HMA
HBED
HEDTA
HS
MA

Mi
NA
NHE
NOM
PS

distichonic acid A
deoxydistichonic acid
desferrioxamine B
desferrioxamine D1
deoxymugienic acid
dissolved organic matter
diethylenetriaminepentaacetic acid
ethylenediaminetetraacetic acid
3‐epihydroxymugineic acid
X‐ray adsorption fine structure spectroscopy
hydroxymugienic acid
N,N‐di(2‐hydroxybenzoyl)‐ethylenediamine‐
N,N‐diacetic acid
N‐(2‐hydroxyethyl)ethylenediamine‐N,N0,N0 ‐triacetic
acid
humic substance
mugienic acid
uptake‐rate controlling metal species
nicotianamine
norman hydrogen electrode
natural organic matter
phytosiderophore

PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION

3

I. INTRODUCTION
Iron is a trace nutrient that serves many functions in plant biochemistry
due to its central role as electron donor or acceptor in enzymes. The
acquisition of iron is therefore a matter of survival for plants as well as for
most other known eukaryotic and prokaryotic organisms. Plants are usually
well adapted to the specific nutrient availabilities in their natural habitats.
However, crop plants are often grown in soils where their ability to acquire
iron is insuYcient to overcome the low iron availability that is characteristic
of the substrate. The costs associated with controlling iron deficiency
in crops under these conditions are significant. For example, it has been
estimated that the annual costs for iron deficiency control are as high as
€80–100 million in the Mediterranean area (Abadı´a et al., 2004) and $120
million in the soybean‐growing areas of the North Central United States
(Hansen et al., 2004). However, not all crops suVer the same problem. For
example, it was found that some crop‐relevant species among the group of
graminaceous plants (e.g., barley and wheat) were less susceptible to iron
deficiency than other nongraminaceous species (Ro¨mheld and Marschner,
1986). This chapter adresses geochemical processes that are involved in
eYcient iron acquisition by such plants. The properties of soils that promote
iron deficiency are introduced, and the biogeochemical interactions of plant
roots with the soil that alleviate iron deficiency are discussed. A focus
of this chapter is iron acquisition processes involving the exudation of
phytosiderophores by plant roots.
Plant roots dramatically modify the chemical environment of the rhizosphere and thereby influence processes such as mineral weathering and the
mobility of nutrients and pollutants. The prediction of the impact of complex and dynamic root physiological processes on soil chemistry is thus a
major challenge for biogeochemical research. Ideally, the rewards of this

research include a better understanding of plant nutrient acquisition, pollutant transport, and mineral‐weathering processes. An understanding of these
processes must first consider the chemical substances that are released into
the rhizosphere, the modification of these substances by microorganisms,
and the interactions of organic acids and other ligands in the dissolution of
various minerals. Among the diVerent elements that are required by plants,
the release of organic acids and chelating substances from the plant root is of
particular importance for solubilization of iron and zinc, which are the most
commonly limiting trace elements in alkaline soils that comprise about one‐
third of the world’s land surface area. To overcome iron limitation, graminaceous plants release substances called phytosiderophores that are soluble
organic ligands with a high aYnity for iron.


4

S. M. KRAEMER ET AL.

The geochemistry of iron in the rhizosphere is a ‘‘ground‐truth’’ that any
iron acquisition strategy has to address. Some of the key problems that need
to be overcome to supply iron to a root are related to the following:
The low solubility of iron‐bearing mineral phases.
Their slow dissolution kinetics.
The transport of soluble iron species to the root.
The low bioavailability of certain soluble iron complexes.
Slow iron release from these complexes in ligand‐ or metal‐exchange reactions.
Some of these problems are interrelated, and root responses to iron deficiency may act on several of the factors listed earlier. Also, it is important to
explore synergisms and antagonisms between various known root activities.
Ultimately, iron acquisition or any biogeochemical interaction is a complex
optimization problem that may have more than one solution leading to the
desired result: survival!

II. STRATEGIES OF PLANT IRON ACQUISITION
A. STRATEGY I
Plant roots respond to iron deficiency by various morphological and physiological changes. A range of plants improve iron acquisition by enhancement of
Hþ eZux via a proton ATPase and the exudation of reductants and ligands, as
well as enzymatic iron reduction by plasma membrane‐bound reductases. The
uptake of Fe(II) is facilitated by an inducible Fe(II) transporter (Grotz and
Guerinot, 2003). This iron acquisition strategy is referred to as ‘‘strategy I’’
(Marschner et al., 1986b).
Iron mobilization by proton eZux into the rhizosphere takes advantage
of the eVect of soil solution pH on iron solubility. However, in calcareous
soils the pH is buVered by heterogeneous reactions with the CO2 in the gas
phase and calcite. The atmospheric CO2 partial pressure is about 10À3.5 atm.
Much higher CO2 partial pressures (10À1.2 to 10À0.9 atm) have been observed
in the rhizosphere due to high respiration rates by plant roots and microorganisms (Gollany et al., 1993). This influences the pH and the solubility of
iron and calcium as illustrated in Fig. 1.
The pH of a soil solution in equilibrium with calcite decreases from 8.3 at
atmospheric PCO2 to pH 6.8 at a rhizospheric PCO2 of 10À1.2 atm while the
soluble calcium concentration increases from 0.6 to 3.4 mM. The total
concentration of soluble inorganic iron species at this rhizospheric PCO2 in
equilibrium with ‘‘soil iron oxide’’ (Lindsay and Schwab, 1982) is only 2 nM.


PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION

5

Figure 1 Calculated pH and total concentrations of dissolved inorganic Ca and Fe species as a
function of the CO2 partial pressure in equilibrium with calcite and ‘‘soil iron oxide’’ (Lindsay and
Schwab, 1982). All solubility coeYcients and equilibrium constants listed in Tables IV and V.

Various strategy‐I plant species of the Proteaceae, Casuarinaceae, Mimosadeae, Fabaceae (e.g., Lupinus albus), Myricaceae, and Moraceae families
respond to nutrient limitations by forming cluster (i.e., proteoid) roots that
are particularly eYcient in modifying rhizosphere pH and organic acid concentrations (Dinkelaker et al., 1995; Neumann and Martinoia, 2002). Cluster
root formation has been observed as a response to phosphorus (Dinkelaker
et al., 1995) and iron (Arahou and Diem, 1996; Gardner et al., 1982; Waters
and Blevins, 2000; White and Robson, 1989) limitations.
Strategy I plants promote the release of iron from organic complexes by
enzymatic reduction via membrane‐bound chelate reductases of rhizodermal
cells (Bienfait, 1985; Chaney et al., 1972; Robinson et al., 1999; Waters et al.,
2002). The reduction of iron in soluble complexes leads to the kinetic and
thermodynamic labilization of iron and facilitates its uptake (Marschner,
1995). For in‐depth discussions of the role of membrane‐bound reductases in
root iron uptake, the reader is referred to a number of reviews and textbooks
(Berczi and Moller, 2000; Curie and Briat, 2003; Hell and Stephan, 2003;
Marschner, 1995; Schmidt, 1999, 2003).

B.

STRATEGY II

Graminaceous plant species, including agriculturally important crops,
such as barley, wheat, and corn, respond to iron deficiency by exudation of
iron‐specific organic ligands, the so‐called phytosiderophores (Takagi, 1976;


6

S. M. KRAEMER ET AL.

Figure 2 Schematic representation of important processes in strategy II iron acquisition (not

to scale). PS, phytosiderophores; OrgAc, low‐molecular weight organic acids; MicrSid, bacterial
and fungal siderophores; HS, soluble particulate, or sorbed humic and fulvic substance.

Takagi et al., 1984). The resistance of graminaceous species to iron deficiency is
correlated to their phytosiderophore release rates (Ro¨mheld and Marschner,
1990). In the apoplastic space and the rhizosphere, phytosiderophores can
scavenge iron from a range of iron‐bearing compounds including iron oxides
(Fig. 2).
The iron deficiency induced synthesis and exudation of phytosiderophores,
and the subsequent uptake of iron–siderophore complexes has been described
as the ‘‘strategy II’’ iron acquisition mechanism (Marschner et al., 1986b). This
strategy resembles bacterial and fungal iron acquisition systems involving
microbial siderophores. A large body of work has been devoted to the regulation and molecular level understanding of the plant physiological responses to
iron deficiency. We refer the reader to a number of excellent review and textbooks for detailed information on these subjects (Curie and Briat, 2003; Grotz
and Guerinot, 2003; Reid and Hayes, 2003; Schmidt, 2003). This chapter
focuses on the geochemical aspects of strategy II iron acquisition.


PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION

7

III. HOW MUCH IS ENOUGH? PLANT
IRON REQUIREMENTS
Iron is a constituent of a number of plant enzymes such as heme proteins
(including cytochromes, catalases, and peroxidases), iron–sulfur proteins (including ferredoxin, superoxide dismutase, and aconitase), lipoxygenase, and
so on (Marschner, 1995). Considering the array of functions of iron‐bearing
enzymes, it is not surprising that iron limitation has a range of consequences
including the impairment of various metabolic and biosynthetic functions and
of photosynthesis (Abadia, 1992; Marschner, 1995). The resulting deficiency

syndrome is iron‐deficiency chlorosis. The minimum total iron content of
iron‐suYcient plant leaves is in the range of 50–150 mg kgÀ1 dry weight
(Marschner, 1995).
The speciation of iron in the rhizosphere has an important eVect on iron‐
uptake rates. Hydroponic culture experiments have been very important to
investigate this eVect and to establish free ion activity models (FIAM) in
which uptake rates (V ) can be related to the activity of a rate‐controlling
metal species {Mi} (Hudson, 1998):
V ¼ f ðorganism; chemical environment; physical environment; fMi gÞ
The rate‐controlling iron species is understood as the species, that is
directly taken up by the plant root with the highest uptake rate compared
to other species in the following reaction:
Mi þ X $ MX

ð1Þ

MX ! Mcellular

ð2Þ

where X is the receptor for Mi of the uptake system at the cell surface.
Generally, it is assumed that Mi is the metal aquo complex (the ‘‘free ion’’)
(Chaney et al., 1992; Morel and Hering, 1993; Parker and Norvell, 1999).
The activity of Mi is usually calculated by equilibrium models rather than
measured, due to the inherent diYculty to measure individual species. The
FIAM is based on the assumption that an equilibrium exists between Mi, all
other species in solution, and the binding sites of the transporter at the cell
surface (Hudson and Morel, 1990). The application of the FIAM to total plant
uptake requires that there is no indiscriminate uptake of complexes via breaks
of the Casparian strip etc. However, evidence of such direct uptake pathways

of natural and synthetic ligand complexes exists (Bell et al., 2003, 2005a;
Wang et al., 1993). Further complications arise if chelators of the metal ion
are toxic to plants (Rengel, 1999, 2002). Some excellent reviews and textbooks


S. M. KRAEMER ET AL.

8

Table I
Iron Requirements of Various Plant Species Grown in Hydroponic Culture
Plant species

Ligand

[Fe]tota [M] Assumed Mib {Mi}c [M]

HEDTAd 6 Â 10À6
EDTAe
9 Â 10À6
f
Barley
HBED
10 Â 10À6
Tomato, soybean DTPAg
?
Soybean
–
0.1 Â 10À6
Barley

HEDTA 7.5 Â 10À6
Barley

Fe3þ
Fe3þ
Fe3þ
Fe2þ
Fe3þ

10À18
10À19
10À14.1
10À28
10À7
10À17.5

References
Bell et al., 1991
Bell et al., 2005b
Chaney et al., 1988
Lindsay and Schwab, 1982
Gries et al., 1995

a

[Fe]tot: Total soluble iron concentration in the nutrient solution.
Mi: The rate controlling metal species, here free aquocomplexes of Fe(III) or Fe(II).
c
{Mi}: Activity of a rate‐controlling metal species Mi.
d

HEDTA: N‐(2‐Hydroxyethyl)ethylenediamine‐N,N0 ,N0 ‐triacetic acid.
e
EDTA: Ethylenediaminetetraacetic acid.
f
HBED: N,N‐di(2‐hydroxybenzoyl)‐ethylenediamine‐N,N‐diacetic acid.
g
DTPA: Diethylenetriaminepentaacetic acid.
b

discussing the FIAM are available (Campbell, 1995; Campbell et al., 2002;
Hudson, 1998; Morel and Hering, 1993; Parker and Norvell, 1999; Parker and
Pedler, 1997).
The uptake of iron in strategy II plants proceeds via a high‐aYnity and a
low‐aYnity uptake system (von Wire´n et al., 1995). The high‐aYnity system
involves the enzymatic transport of intact Fe–phytosiderophore complexes
through the plasma membrane by a transporter (Ro¨mheld and Marschner,
1986). This uptake system controls the rate of iron uptake under iron‐
limiting conditions. Therefore, it seems appropriate to assume that in this
case {Mi} is the activity of the Fe–phytosiderophore complex. Unless phytosiderophore concentrations are measured or added to the nutrient solution
to a known level, their concentration is not known and {Mi} cannot be
calculated in equilibrium models. Their concentration in hydroponic culture
experiments will be a function of exudation rates, degradation rates, and the
volume of nutrient solution relative to the root biomass among other factors. In the rhizosphere, siderophore concentrations are influenced by diVusional transport away from the root and advective transport to the root.
During maximum exudation periods, local siderophore concentrations can
reach very high levels as discussed later (Ro¨mheld, 1991). Almost certainly,
local rhizosphere concentrations will be very diVerent from siderophore
concentrations in well‐mixed hydroponic culture experiments. Nevertheless,
iron‐limiting conditions are usually defined as the maximum activity or
concentration of the iron hexaquo complex (i.e., ‘‘Fe3þ’’) at which iron
deficiency chlorosis occurs (see Table I).

PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION

9

The rate of enzymatic high‐aYnity uptake of iron–siderophore complexes
can be described by a Michaelis–Menten type rate law:
r ¼ ½Mi Šrmax =KM þ ½Mi Š

ð3Þ

where r is the uptake rate and rmax is the maximum uptake rate, [Mi] is a
Fe–siderophore complex, and KM is the half saturation constant. The uptake
rate is linearly related to [Mi] at low concentrations of Mi ([Mi] < KM).
At high concentrations ([Mi] < KM), the uptake rate saturates, that is, it
becomes independent of [Mi] and r ¼ rmax (Morel and Hering, 1993). Von
Wire´n et al. (1995) measured a KM of 10 mM Fe–DMA and rmax ¼ 5.5 mmole
Fe per g (dry weight) per 30 min for the iron eYcient Z. mays L. cv. Alice.
Observations of uptake kinetics of Fe–DMA with isolated root plasma
membrane vesicles of the same cultivar were fitted to a Michaelis–Menten
model with KM of 0.6 Fe–DMA and rmax ¼ 5.5 mM Fe minÀ1 mgÀ1 protein.
These observations give an upper limit for Fe–phytosiderophore complex
concentrations that will increase uptake rates via a fast uptake system in the
range of 10À6 to 10À5 M.

IV. IRON‐BEARING MINERALS AND SOLUBLE IRON
SPECIES IN THE RHIZOSPHERE
Phytosiderophores can scavenge iron from a range of iron‐bearing minerals
and soluble iron species. The ultimate sources of iron in soils are primary iron‐

bearing minerals. Most iron‐bearing primary minerals are not stable over
pedological timescales. Iron liberated by weathering of primary minerals in
well‐aerated soils forms secondary iron oxide minerals and is bound to natural
organic matter (NOM). Iron oxides control the activity of the iron–hexaquo
complex by solubility equilibria and the activity of iron hydrolysis species by
coupled equilibria. The low solubility of iron oxides in the neutral pH range is
responsible for low dissolved iron concentrations that induce iron‐deficiency
symptoms in many plant species. However, the solubility of iron in soil solution
is not only determined by hydrolysis species. Soluble organic ligands, including
organic acids, polyphenols, fulvic and humic acids, and bacterial or fungal
siderophores, can significantly increase iron solubility. Organic iron complexation does not increase the concentration of Fe3þ and hydrolysis species if the
system is at equilibrium. However, some of the important biogeochemical
reactions involved in iron acquisition are slow (e.g., the dissolution reaction)
and it is unlikely that soluble iron concentrations in the rhizosphere are controlled by equilibria. In this case, soluble complexes may be important as iron
sources and iron shuttles for strategy I and strategy II iron acquisition. Generally, it is useful to consider most of the solid or soluble iron species that will be


S. M. KRAEMER ET AL.

10

discussed here as intermediates of the complex chemical transformations that
take place before iron is taken up by plant roots.

A. IRON‐BEARING MINERALS, THE PENULTIMATE IRON SOURCE
1.

Mineral Structures and Solubilities

The weathering of iron‐bearing primary minerals leads to the formation

of secondary iron oxides. In most soils, iron oxides are the largest pool of
iron. The iron content of aerated soils is in the range of 0.2–5% (Blume et al.,
2002), which is in the same range as the average crustal abundance of iron
(3.5%) (Taylor, 1964). Important iron oxide minerals in soils are ferrihydrite,
goethite (a‐FeOOH), and hematite (a‐Fe2O3). Ferrihydrite is a poorly ordered phase with variable composition (Cornell and Schwertmann, 2003).
We report dissolution reactions and corresponding solubility constants
based on the simplifying assumption of an Fe(OH)3 Á nH2O stoichiometry.
Based on observations of iron solubility in soils, Lindsay (1979) defined a
hypothetical ‘‘soil iron oxide’’ that (Cornell and Schwertmann, 2003) interpreted as an aged ferrihydrite. The solubility of these minerals is expressed in
terms of a solubility product and coupled hydrolysis reactions in solution.
The dissolution reactions for these iron oxides are as follows:
Ferrihydrite or ‘‘soil iron oxide’’:
FeðOHÞ3 Ã nH2 O þ 3Hþ $ Fe3þ þ ðn þ 3ÞH2 O
þ

3þ

þ

3þ

Goethite: FeOOH þ 3H $ Fe

Hematite: 1=2 Fe2 O3 þ 3H $ Fe

ð4Þ

þ 2H2 O

ð5Þ

þ 1:5H2 O

ð6Þ

The solubility product for these reactions is:
Ks ¼

fFe3þ g
fHþ g

3

implying that the activity of the Fe3þ species in equilibrium with any of
these iron oxides increases by a factor of 1000 for each unit decrease in pH.
The solubility products depend on bulk lattice energies and particle sizes.
Generally, ferrihydrite has a higher solubility product than the more crystalline goethite and hematite. However, the solubility products of goethite and
hematite increase with decreasing particle size and approach the solubility
product of ferrihydrite in the nanometer range (Langmuir, 1969; Trolard
and Tardy, 1987). Goethite and hematite in the nanometer range are
typically found in soils (Cornell and Schwertmann, 2003).


PHYTOSIDEROPHORE‐PROMOTED PLANT IRON ACQUISITION

11

Figure 3 Calculated solubility of iron in equilibrium with ‘‘soil iron oxide’’ (Lindsay, 1979)
and iron speciation as a function of pH. For solubility product constants and equilibrium
constants for hydrolysis species, see Tables IV and V.

The solubility of iron, defined as the total dissolved iron concentration in
equilibrium with a solid phase (IUPAC, 1997), is also determined by coupled
hydrolysis equilibria as shown in Fig. 3.
The dominant hydrolysis species in the pH range between 6.5 and 7.5
is FeðOHÞÀ
2 . The formation of this species is given by Eq. (7) as follows:
Fe3þ þ 2OHÀ $ FeðOHÞþ
2

ð7Þ

Combining Eqs. (7) and (4), we can derive an expression for ferrihydrite
dissolution in the neutral pH range:
FeðOHÞ3 Ã nH2 O þ Hþ $ FeðOHÞþ
2 þ ðn þ 1ÞH2 O

ð8Þ

with a modified solubility product:
Ks0 ¼

fFeðOHÞþ
2g
þ
fH g

ð9Þ

implying that the solubility of the iron oxides in this pH range increases by a

factor of 10 for each decrease of pH by one unit (see also Fig. 1).

2.

Surface Chemistry of Iron Oxides

Iron ions at iron oxide surfaces are coordinated by structural oxo‐ and
hydroxo groups and by adsorbed water or hydroxyl groups. Net positive or
negative surface charge arises primarily from protonation or deprotonation