Understanding Cambial Behaviour The key to wood quality · Pinus banksiana, resinosa, and strobus...
Transcript of Understanding Cambial Behaviour The key to wood quality · Pinus banksiana, resinosa, and strobus...
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Understanding Cambial Behaviour
The key to wood quality
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� A brief history� Terminology� Dormancy and reactivation� Growth of derivatives and wall formation� Pitting and plasmodesmata
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� A brief history
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Nehemiah Grew (1641-1712)
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� Grew’s drawing of elm (detail)
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Charles Francois Brisseau-Mirbel
� Proposed that cambium was a tissue rather than a sap (1808)
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Mirbel’s (1827) diagram of elm (from Larson, 1994)
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Cambial cell theories
� Hartig (1853)- Back to back theory
� Phloem initial� Xylem initial
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Cambial cell theories
� Sanio (1863)- Single initial theory
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Cambial cell theories
� Raatz/Mischke (1892) – Multiple initial theory
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� Oblique orientation of plane of division
� Kinoplasmic fibres (microtubules)
� Kinoplasmosomes (phragmoplast
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� From Bailey (1923)
� Anticlinal pseudotransverse division
� Transverse division
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Length of cambium/cambial age (from Bailey 1923)
� A: Conifer or vessel-less dicot. (12 species)
� B: Less specialised dicot. (10 species)
� C: Highly specialised dicot. (10 species)
� D: Dicot. with storeyedcambium (10 species)
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Vacuolation in cambial cells (Bailey 1930)
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Pinus radiataActive cambium
Nomenclature
Cambium?
Cambial Zone?
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Butterfield (1975) IAWA Bulletin 13 – 14
Cambium “a multiseriate zone of periclinallydividing cells lying between the differentiatingsecondary xylem and phloem, with a distinctinitial capable of both periclinal and anticlinaldivisions lying somewhere within each radial fileof cells”
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Cambium according to Butterfield
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Schmid (1976) IAWA Bulletin 51-59
“Cambium” equivalent to the “initiating layer”
“Cambium” applied to the entire differentiating region might lead to the conception that the cambium is a multiseriate layer of initials”
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Cambium according to Schmid
?
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The difficulty of identifying the initial means the terms have been used interchangeably
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Pinus radiataMid-winter
A slowly dividing meristem
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Pinus radiataMid-winter
Cambium
Butterfield
Schmid ?
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Humpty Dumpty
From “Through the Looking Glass –and what Alice found there”
by Lewis Carroll“When I use a word”, Humpty Dumptysaid in rather a scornful tone, “it means just what I choose it to mean –neither more nor less”
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� Can an initial ever be identified with certainty?
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� Aesculus hippocastanum� February
Cells appear similar across thecambium
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Boundary parenchyma phloemside
Boundary parenchyma xylemside
Identifying an initial
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A. hippocastanum TEM
Boundary parenchyma (phloem side)
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A. hippocastanum TEM
Phloem cells in suspended orslow development
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A. hippocastanum TEM
Fusiform Initial
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A. hippocastanum TEM
Boundary parenchymaxylem side
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Sieve element/companion cellpair in a state of arresteddevelopment
Initial
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Boundary parenchyma
Companion cell precursor
Sieve/element precursor
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Phloem boundary cell
Previous season’s phloem:
Sieve tube member
Companion cells
9 February
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� Dormancy and reactivation
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Dormant Cambium
Fragmented vacuome
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Storage materials in dormant fusiform cells
Spherosomes (lipids)
Protein bodies
“Thick” cell walls
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9 February Fusiform initial Ray initial
Starch
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Cambial reactivation in Aesculus
� Activity can be detected in the cytoplasm of cambial zone cells long before any signs of activity are displayed by the tree.
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Active dictyosome in aboundary layer cell ofdormant cambium
23 February
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� Developing and mature coated vesicles
� 8 March
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� Reactivation (16 March)
� Expanding phloem precursors
� Fusiform initial
� Boundary parenchyma
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Dividing phloem mother cell (16 March)
New tangential wall
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� 13 April
� Boundary parenchyma
� Cytoplasm confined to a thin parietal layer
� Boundary parenchyma
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� 23 April
� Developing phloem cells
� Dividing initial� Xylem mother cell
� New xylem elements
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� Typically in the Reading area, Aesculus bud-break occurs in late March, with leaves fully emerged by late April
� Xylem formation appears to begin coincidentally with leaves beginning to export photosynthate
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Reactivation sequence
� Larson (1994):
� Xylem production first – 26 species � Phloem production first – 21 species� Simultaneous production – 10 species
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Observations are inconsistent between authors
� Acer pseudoplatanus, Quercus rubra , Pinus sylvestris and Vitis vinifera appear in the list of xylem reactivators and phloem reactivators
� In the Pinaceae: � Pinus halepensis and rigida are xylem reactivators� Pinus banksiana, resinosa, and strobus (five authors)
are phloem reactivators � Picea excelsa, rubens and rubra are xylem reactivators,� Picea abies is a phloem reactivator � Picea glauca a simultaneous reactivator.
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� Phloem annual growth rings marked by boundary parenchyma
� The number of phloem cells in each file is similar to the number of over-wintering precursors
� All the phloem for the season is produced at the beginning of the season
Phloem production in Aesculus
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� Pinus radiata
� Active cambium producing both xylem and phloem throughout the season
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� Growth of derivatives and wall formation
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Cell enlargement
Quercus robur
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� 20 April
� Developing xylem cells
� Boundary parenchyma
� Previous year’s latewood fibre
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� Cell tip growing between fibres
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Enlarging vessel element
Boundary parenchyma
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� Developing fibres are compressed and files of cells distorted by vessel enlargement
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Perforation plates
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Secondary wall formation
�� This the classic This the classic representation of the representation of the wall of a cell that has wall of a cell that has all possible wall layers:all possible wall layers:
�� the ML+P+S1+S2+S3 the ML+P+S1+S2+S3 +HT+W wall zones+HT+W wall zones
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Cell wall Layers
Helical thickening (tertiary layer)
Middle lamella
Primary wall
S1S2
S3
Cell lumen
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� Earliest stages of cellulose deposition forming the S1 layer
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Microtubules and cellulose orientation
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Tubulin in a fusiform cambial cell of Aesculus (B), and developing fibres (C, D, E)
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Tubulin in developing fibres in Populus
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Tubulin in tension wood fibres of Populus
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The significance of microfibril angle
� The relationship between MFA and axial stiffness according to Cave (1968)
� There is a 5 fold increase in stiffness when MFA shifts from 40 to 10 degrees
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Corewood in a 125-year Old Tree
� Juvenile wood (large microfibril angle)
� Mature wood� (small microfibril angle)
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Corewood in a 25-year Old Tree
� Juvenile wood� Mature wood
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The Problem of High Microfibril Angle
� Corewood is too flexible to be used as high grade timber
� Any improvement that would reduce the amount of low grade timber would result in significant financial gain for the producer and result in more efficient use of forest land
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Consequences for the Tree
� High microfibril angle in corewood makes young trees flexible and able to withstand high winds
� Only small reductions in angle may be feasible without affecting survivability
� These may, however, still give significant increases in the quality of corewood
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� Pitting and plasmodesmata
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Formation of pits
Aesculus hippocastanum vessel wall
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Cambium pit fieldsSorbus aucuparia
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Pit fields in enlarging fibres
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Pinus radiata Cambium pit fields
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Pit fields in enlarging tracheids
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� Interference contract micrograph of a TLS through the radial wall of a tracheid of Pinus radiata
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Pinus radiata
� Pit fields in radial walls of enlarging tracheids
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Developing torus
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Pinus radiata
� Beginning of formation of the torus
and pit border
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Functional and aspirated bordered pits
P. radiata
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Vessel pitting seen from the middle lamella in Aesculus
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Vessel-vessel wall in Aesculus hippocastanum
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Plasmodesmata in radial wall of cambium in Aesculus
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Fibre-fibre pit in Aesculus
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Pits in a tangential wall between ray parenchyma
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Plasmodesmata in pit fields
� Absent: Vessels and tracheids (conifer and angiosperm) to any other cell type
� Present: Fibres to fibres and parenchymaParenchyma to parenchyma and fibres
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Plasmodesmata in developing vessel walls of hybrid aspen
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� Microtubules and pit formation
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Tubulin in a developing Aesculus vessel
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Tubulin in young vessel elements in Aesculus
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Plasmodesmata in pit membranes in some members of the Rosaceae
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Sorbus aucupariaRLS 2µm thick
Plasmodesmata appear in section as black spots
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Pit fields in developing fibres of Sorbus
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Conclusions
� Microscopy reveals that cambial behaviour varies between species
� A knowledge of ultrastructural changes during differentiation of xylem is essential to understanding wood formation processes
� The cytoskeleton and plasmodesmata are important factors in the control of xylem differentiation
� Cambial behaviour ultimately governs wood structure and quality
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The End