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Wood Chemistry
PSE 406
Decay
Introduction to
Deterioration
 Plant matter is
constantly under
attack by fungi,
insects, bacteria,
marine borers and the
weather.
 It is estimated that
roughly 1/10 of the
forest products
generated each year
are destroyed.
PSE 406 Emphasis
 While weathering, insects
and marine borers cause
substantial damage, we are
going to focus on decay
caused by fungi and
bacteria.
 In particular, we are going
to discuss some of the
biology involved but
mostly the chemistry.
Fungi
 The wood deteriorating fungi are organized into
three groups:
» White rot fungi
» Brown rot fungi
» Soft rot fungi
White Rot Fungi
 This group of organisms is known as white rot
because of their ability to degrade lignin.
» The decaying wood looks white.
» Cellulose and hemicelluloses are also degraded.
 Largest number of species belong to
Basidiomycotina.
 White rot fungi typically decay hardwoods
» They will decay softwoods but hardwoods are
their food of choice.
 Simultaneous decay: all the cell components
are degraded simultaneously from lumen
outwards.
 Preferential decay: lignin and hemicelluloses
are removed selectively across the cell wall
leaving cellulose.
White Rot Fungi (2)
hypha
ML?
Cross section of an oak tree with white rot.
The fungus has decayed the sapwood and
dark heartwood turning it white.
Scanning electron micrograph showing the
hypha of a white rot fungus.
A cross section of wood from a white-pocket
area of decayed wood showing delignified
wood cells.
Brown Rot Fungi
 With brown rot fungi, cellulose and
hemicelluloses are degraded with
only limited lignin degradation.
» Decayed wood is brown and crumbly.
 Most species belong to
Basidiomycotina.
 Brown rot fungi typically decay
softwoods.
 Attack starts at the cell lumen and
works outwards.
» Cellulose is rapidly degraded.
Brown Rot Fungi (2)
Brown-rooted wood.
Wood cracks and checks into cubicle pieces.
Scanning electron micrograph of brown-rotted wood.
Only slight pressure causes the wood cell walls to
crumble into minute fragments.
Soft Rot Fungi
 Soft rot occurs in areas where plant
matter is in contact with excessive
amounts of moisture.
 The term soft rots comes from the soft
appearance of the decayed surface.
» When dry the wood surface is cracked.
 Members: Ascomycetes and Fungi
Imperfect.
 Degradation is mainly though cavity
formation in the secondary wall.
 Soft rot fungi attack holocellulose; lignin
protects the plant.
Soft Rot Fungi (2)
Soft rot in wood often appears brown
and can be confused with decay
caused by brown rot fungi.
This micrograph taken of a section from soft-rotted wood
and viewed with a light microscope shows cavities
within the cell walls.
Molds and Blue Stain Fungi
 Wood is often stained by these organisms
with little loss of structural integrity.
» Particularly in softwoods, some strength loss in
hardwoods.
 Molds: Aspergillus, Penicillium etc.
 Blue Stain Fungi: Philaphora, etc.
 These organisms typically attack non lignified
parenchyma cells and pit membranes.
How Does This Happen?
 This is a picture of
highly degraded
wood.
 What you see are the
fungal bodies known
as hyphae. They grow
through the plant
matter like little
worms.
 The organisms arrive
as spores (transported
by a variety of
methods).
Movement of Hyphae
 This SEM picture
shows fungal hyphae
inside hardwood
xylem.
 The hyphae enter the
cells though openings
(pits, etc) or can bore
directly through the
wall (chemically).
How Do Fungi Destroy the
Cell Wall Material?
 This is a very complex
question which is not
well understood.
 The process is
enzymatic. Fungi
possess a wide variety
of cell wall degrading
enzymes:
» Cellulases,
hemicellulases, etc.
How do Enzymes Function?
 Enzymes are very large proteins.
» Enzymes have very specific functions: they cause
chemical reactions to occur in exact fashions.
» A very large number of enzymes have been isolated from
fungi and their functions identified.
 Fungal hyphae release enzymes to degrade cell wall
components: this reaction is extra cellular.
 Enzymes are too large to penetrate into the cell wall
structure and react with cell wall components. How
does degradation occur?
» This is the big unknown question.
Enzyme Function
 There are a large number of fungal enzymes
responsible for the breakdown of each wood
component. Each enzyme plays specific roles:
» Cellobiohydrolase (CBH), acts on the end of the molecule
successively cleaving off the disaccharide cellobiose.
» Endo-beta-1,4-glucanase acts within the chain, breaking it
into smaller units and providing more "ends" for CBH.
» Beta-glucosidase (or cellobiase) which cleaves cellobiose to
two glucose units.
 These enzymes working together produce glucose
which is consumed by the fungi.
What Happens to the Chemicals
Unloved by the Fungi?
 Basically the question is
what happens to all of the
organic material that is not
consumed by the
organisms?
 In this picture, the log is
rotting leaving a pile of
organic material on top of
the soil.
 Does this organic material
simply disappear?
Soil Organics
 The answer to the question on the last slide is
of course not, the organic material doesn’t
disappear it is simple changed into the soil
organics: Fulvic Acids, Humic acids, and
Humins. These materials are classified by
their solubility.
» Fulvic Acids (Acid soluble fraction)
» Humic acids (Alkali soluble fraction/ acid
insoluble)
» Humins (Insoluble organics)
Soil Organics II
 These materials are very important to the soil.
 The amounts of these compounds is very soil
type dependent.
» 60-70% of soils organics are humin, humic acids
and fulvic acids.
» Soil organic matter ranges from 0.5 to 20 % of
the soil material.
Structure of Soil Organics
 These soils organics are large polymers and
thus like lignin structural determination is
somewhat difficult.
» Fulvic acid Mw~2000+, humic acids higher,
humins as high as 300,000?
 These materials are more difficult than lignin
for structural studies because they are
produced from so many different materials
(unlike lignin: 3 possible precursors).
Proposed Humic Acid
Structure
 This is a proposed segment of humic acid by Stevenson*
» Notice the phenolics, the sugars, and the peptides
» It is obvious that this molecule does not arise directly from any
component but is built from pieces of other components.
COOH
HC O
COOH
COOH
HO
OH
OH
N
H
(HC OH) 4
R CH
O
(Sugar)
C O
H
O
O
O
O
N
O
O
NH
R CH
CH 2
NH 2
O
(Peptide)
COOH
COOH
CH 2
O
HO
O
O
O
O
OH
Proposed Fulvic Acid
Structure
 This is a proposed structure for a fulvic acid
fragment by Buffle.
OH
COOH
CH2OH
HOOC
CH3
O
OH
HOOC
COOH
OH
COOH
COOH
O