Lecture 5 - Translocation in Phloem - Botany PDF Download

 Page 1


Translocation in the phloem 
 
1 
Institute of Life Long Learning, University of Delhi 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
Discipline: Botany 
Paper: Plant Physiology 
National Coordinator: Prof. S.C. Bhatla 
Lesson: Translocation In The Phloem 
Lesson Developer: Dr. Rashmi Shakya 
Department/College: Department of Botany, Miranda 
House 
Lesson Reviewer: Prof. S.C. Bhatla 
Department/College: Department of Botany, University of 
Delhi 
Language Editor: Madhurima Kahali 
Department/College: University of Delhi, South Campus 
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL 
 
 
Page 2


Translocation in the phloem 
 
1 
Institute of Life Long Learning, University of Delhi 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
Discipline: Botany 
Paper: Plant Physiology 
National Coordinator: Prof. S.C. Bhatla 
Lesson: Translocation In The Phloem 
Lesson Developer: Dr. Rashmi Shakya 
Department/College: Department of Botany, Miranda 
House 
Lesson Reviewer: Prof. S.C. Bhatla 
Department/College: Department of Botany, University of 
Delhi 
Language Editor: Madhurima Kahali 
Department/College: University of Delhi, South Campus 
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL 
 
 
Translocation in the phloem 
 
2 
Institute of Life Long Learning, University of Delhi 
 
 
 
Table of Contents 
Chapter: Translocation in the Phloem 
? Introduction 
? Translocation Pathways 
? Translocation of Sugar Takes Place in Sieve Elements 
? Mature Sieve Elements Are Specialized Structures 
? Companion cells Support Functioning of Sieve Elements 
? Ordinary Companion Cells 
? Transfer Cells 
? Intermediary Cells 
? Path of Translocation: Source to Sink 
? Specific translocation Pathways are Often Complex 
? Materials Translocated in the Phloem  
? Analysis of Phloem Sap Revealed Translocation of Sucrose, Amino 
Acids, Hormones, and Some Inorganic Ions 
? Rates of Movement of Materials in the Phloem 
? Pressure-Flow Model: The Mechanism Explaining Phloem Translocation  
? Confirmation of Predictions made by Pressure-Flow Model 
? Prediction 1: The sieve plate pores must be unobstructed 
? Prediction 2: Simultaneous transport of photosynthate in two 
directions i.e. bidirectional transport is not possible in a single 
sieve element 
? Prediction 3: Rate of translocation in sieve elements is 
insensitive to the energy supply to the path tissues 
? Prediction 4: Positive pressure gradient is necessary for bulk 
flow of solution 
? Phloem Loading 
? Transport of photosynthate from mesophyll cells to the sieve 
elements can take place through Apoplastic or Symplastic Pathway 
Page 3


Translocation in the phloem 
 
1 
Institute of Life Long Learning, University of Delhi 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
Discipline: Botany 
Paper: Plant Physiology 
National Coordinator: Prof. S.C. Bhatla 
Lesson: Translocation In The Phloem 
Lesson Developer: Dr. Rashmi Shakya 
Department/College: Department of Botany, Miranda 
House 
Lesson Reviewer: Prof. S.C. Bhatla 
Department/College: Department of Botany, University of 
Delhi 
Language Editor: Madhurima Kahali 
Department/College: University of Delhi, South Campus 
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL 
 
 
Translocation in the phloem 
 
2 
Institute of Life Long Learning, University of Delhi 
 
 
 
Table of Contents 
Chapter: Translocation in the Phloem 
? Introduction 
? Translocation Pathways 
? Translocation of Sugar Takes Place in Sieve Elements 
? Mature Sieve Elements Are Specialized Structures 
? Companion cells Support Functioning of Sieve Elements 
? Ordinary Companion Cells 
? Transfer Cells 
? Intermediary Cells 
? Path of Translocation: Source to Sink 
? Specific translocation Pathways are Often Complex 
? Materials Translocated in the Phloem  
? Analysis of Phloem Sap Revealed Translocation of Sucrose, Amino 
Acids, Hormones, and Some Inorganic Ions 
? Rates of Movement of Materials in the Phloem 
? Pressure-Flow Model: The Mechanism Explaining Phloem Translocation  
? Confirmation of Predictions made by Pressure-Flow Model 
? Prediction 1: The sieve plate pores must be unobstructed 
? Prediction 2: Simultaneous transport of photosynthate in two 
directions i.e. bidirectional transport is not possible in a single 
sieve element 
? Prediction 3: Rate of translocation in sieve elements is 
insensitive to the energy supply to the path tissues 
? Prediction 4: Positive pressure gradient is necessary for bulk 
flow of solution 
? Phloem Loading 
? Transport of photosynthate from mesophyll cells to the sieve 
elements can take place through Apoplastic or Symplastic Pathway 
Translocation in the phloem 
 
3 
Institute of Life Long Learning, University of Delhi 
? Plants with intermediary cells show Symplastic pathway for phloem 
loading 
? Translocation Pathway is correlated with Plant Type and Climate 
? Phloem Unloading 
? Unloading of transport Sugars can take place via Symplastic or 
Apoplastic Pathways 
? Allocation and Partitioning of Photosynthates 
? Control of Allocation in Source Leaves 
? Competition takes place among various sink tissues for available 
translocated photosynthates 
• Summary  
• Exercise/ Practice 
• Glossary 
• References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
Page 4


Translocation in the phloem 
 
1 
Institute of Life Long Learning, University of Delhi 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
Discipline: Botany 
Paper: Plant Physiology 
National Coordinator: Prof. S.C. Bhatla 
Lesson: Translocation In The Phloem 
Lesson Developer: Dr. Rashmi Shakya 
Department/College: Department of Botany, Miranda 
House 
Lesson Reviewer: Prof. S.C. Bhatla 
Department/College: Department of Botany, University of 
Delhi 
Language Editor: Madhurima Kahali 
Department/College: University of Delhi, South Campus 
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL 
 
 
Translocation in the phloem 
 
2 
Institute of Life Long Learning, University of Delhi 
 
 
 
Table of Contents 
Chapter: Translocation in the Phloem 
? Introduction 
? Translocation Pathways 
? Translocation of Sugar Takes Place in Sieve Elements 
? Mature Sieve Elements Are Specialized Structures 
? Companion cells Support Functioning of Sieve Elements 
? Ordinary Companion Cells 
? Transfer Cells 
? Intermediary Cells 
? Path of Translocation: Source to Sink 
? Specific translocation Pathways are Often Complex 
? Materials Translocated in the Phloem  
? Analysis of Phloem Sap Revealed Translocation of Sucrose, Amino 
Acids, Hormones, and Some Inorganic Ions 
? Rates of Movement of Materials in the Phloem 
? Pressure-Flow Model: The Mechanism Explaining Phloem Translocation  
? Confirmation of Predictions made by Pressure-Flow Model 
? Prediction 1: The sieve plate pores must be unobstructed 
? Prediction 2: Simultaneous transport of photosynthate in two 
directions i.e. bidirectional transport is not possible in a single 
sieve element 
? Prediction 3: Rate of translocation in sieve elements is 
insensitive to the energy supply to the path tissues 
? Prediction 4: Positive pressure gradient is necessary for bulk 
flow of solution 
? Phloem Loading 
? Transport of photosynthate from mesophyll cells to the sieve 
elements can take place through Apoplastic or Symplastic Pathway 
Translocation in the phloem 
 
3 
Institute of Life Long Learning, University of Delhi 
? Plants with intermediary cells show Symplastic pathway for phloem 
loading 
? Translocation Pathway is correlated with Plant Type and Climate 
? Phloem Unloading 
? Unloading of transport Sugars can take place via Symplastic or 
Apoplastic Pathways 
? Allocation and Partitioning of Photosynthates 
? Control of Allocation in Source Leaves 
? Competition takes place among various sink tissues for available 
translocated photosynthates 
• Summary  
• Exercise/ Practice 
• Glossary 
• References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
Translocation in the phloem 
 
4 
Institute of Life Long Learning, University of Delhi 
 
 
Learning outcomes: 
The student will be able to: 
? Understand the importance of translocation pathways and learn about the 
specialized structures involved in it 
? Understand the path of translocation 
? Study about the materials that are translocated through the phloem 
? Understand the rate of movement of materials 
? Learn about the Pressure-Flow model 
? Understand phloem loading and unloading 
? Learn about allocation and partitioning of photosynthates in phloem 
 
 
 
 
 
 
 
 
 
 
Page 5


Translocation in the phloem 
 
1 
Institute of Life Long Learning, University of Delhi 
 
                                                                                              
 
Lesson Prepared Under MHRD project “National Mission on 
Education Through ICT” 
Discipline: Botany 
Paper: Plant Physiology 
National Coordinator: Prof. S.C. Bhatla 
Lesson: Translocation In The Phloem 
Lesson Developer: Dr. Rashmi Shakya 
Department/College: Department of Botany, Miranda 
House 
Lesson Reviewer: Prof. S.C. Bhatla 
Department/College: Department of Botany, University of 
Delhi 
Language Editor: Madhurima Kahali 
Department/College: University of Delhi, South Campus 
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL 
 
 
Translocation in the phloem 
 
2 
Institute of Life Long Learning, University of Delhi 
 
 
 
Table of Contents 
Chapter: Translocation in the Phloem 
? Introduction 
? Translocation Pathways 
? Translocation of Sugar Takes Place in Sieve Elements 
? Mature Sieve Elements Are Specialized Structures 
? Companion cells Support Functioning of Sieve Elements 
? Ordinary Companion Cells 
? Transfer Cells 
? Intermediary Cells 
? Path of Translocation: Source to Sink 
? Specific translocation Pathways are Often Complex 
? Materials Translocated in the Phloem  
? Analysis of Phloem Sap Revealed Translocation of Sucrose, Amino 
Acids, Hormones, and Some Inorganic Ions 
? Rates of Movement of Materials in the Phloem 
? Pressure-Flow Model: The Mechanism Explaining Phloem Translocation  
? Confirmation of Predictions made by Pressure-Flow Model 
? Prediction 1: The sieve plate pores must be unobstructed 
? Prediction 2: Simultaneous transport of photosynthate in two 
directions i.e. bidirectional transport is not possible in a single 
sieve element 
? Prediction 3: Rate of translocation in sieve elements is 
insensitive to the energy supply to the path tissues 
? Prediction 4: Positive pressure gradient is necessary for bulk 
flow of solution 
? Phloem Loading 
? Transport of photosynthate from mesophyll cells to the sieve 
elements can take place through Apoplastic or Symplastic Pathway 
Translocation in the phloem 
 
3 
Institute of Life Long Learning, University of Delhi 
? Plants with intermediary cells show Symplastic pathway for phloem 
loading 
? Translocation Pathway is correlated with Plant Type and Climate 
? Phloem Unloading 
? Unloading of transport Sugars can take place via Symplastic or 
Apoplastic Pathways 
? Allocation and Partitioning of Photosynthates 
? Control of Allocation in Source Leaves 
? Competition takes place among various sink tissues for available 
translocated photosynthates 
• Summary  
• Exercise/ Practice 
• Glossary 
• References/ Bibliography/ Further Reading 
 
 
 
 
 
 
 
 
 
 
Translocation in the phloem 
 
4 
Institute of Life Long Learning, University of Delhi 
 
 
Learning outcomes: 
The student will be able to: 
? Understand the importance of translocation pathways and learn about the 
specialized structures involved in it 
? Understand the path of translocation 
? Study about the materials that are translocated through the phloem 
? Understand the rate of movement of materials 
? Learn about the Pressure-Flow model 
? Understand phloem loading and unloading 
? Learn about allocation and partitioning of photosynthates in phloem 
 
 
 
 
 
 
 
 
 
 
Translocation in the phloem 
 
5 
Institute of Life Long Learning, University of Delhi 
 
 
Introduction 
During the evolution of early land plants, their upright growth led to the differentiation of 
the tissue responsible for gathering water and inorganic solvents, from those responsible for 
photosynthesis. This led to the differentiation of plant body into two distinct portions: aerial 
portion, which was photosynthetic and the lower non-photosynthetic portion, which was 
dependent on photosynthetic part for its survival. Phloem evolved as a means to transport 
photosynthates (sugars, sucrose in particular and other metabolic products) downwards into 
the lower parts of the plants. This process of transport is known as translocation. The word 
phloem has been derived from the Greek word phlois which means “bark”.  
Contrary to functional xylem, phloem is a living vascular tissue. Sugars and other nutrients 
are loaded into it by molecular pumps in the plasma membrane; these pumps actively 
transport certain molecules into the phloem in some areas (referred to as source) and take 
them out in other areas (referred to as sink). Thus, transport in phloem is very dynamic.  
Translocation Pathways 
Both xylem and phloem extend throughout the plant body for the transport of water and 
photosynthates, respectively. In vascular bundles, the phloem is generally located on the 
outer side and it constitutes the inner bark in plants in which secondary growth takes place. 
Phloem is a complex tissue consisting of conducting cells and its associated cells, and 
supportive cells. The conducting elements of the phloem are sieve elements which are of 
two types: sieve cells, found in gymnosperms (e.g. Conifers) and seedless vascular plants 
(e.g. ferns), and sieve tube members present in angiosperms (flowering plants). All sieve 
elements have groups of pores which are either uniformly distributed on the surface of cell 
or are slightly aggregated at the ends. In angiosperms, companion cells are closely 
associated with sieve elements whereas in non-angiosperms, albuminous cells are found to 
be closely associated. In some plants the phloem tissue includes certain supportive cells 
which are generally of two types- sclereids and fibres for protection and strengthening of 
tissue, and laticifers which are latex containing cells. However, only the direct involvement