Understanding Cambial Behaviour The key to wood quality.
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Transcript of Understanding Cambial Behaviour The key to wood quality.
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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 storeyed cambium (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 periclinally
dividing cells lying between the differentiating
secondary xylem and phloem, with a distinct
initial capable of both periclinal and anticlinal
divisions lying somewhere within each radial file
of 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 Dumpty said 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 or
slow 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 cell
pair in a state of arrested
development
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 a
boundary layer cell of
dormant 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 parenchyma
Parenchyma 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 aucuparia RLS 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