Wood ChemistryPSE 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.

Cross section of an oak tree with white rot. The fungus has decayed the sapwood and

dark heartwood turning it white.

White Rot Fungi (2)

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.

hypha

ML?

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.

* http://helios.bto.ed.ac.uk/bto/microbes/armill.htm

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.

HOOH

COOH COOH

OH

O

O

N

CHRCOOH

O

O

O

NHCHCH2

R

NH2

OH

N

CH2

O

OC O

(HC OH)4

HC O

O

OH

O

COOH

COOHHOOO

O

H

(Peptide)

(Sugar)

Proposed Fulvic Acid Structure

This is a proposed structure for a fulvic acid fragment by Buffle.

COOH

HOOC

HOOC

OH COOH

OH

CH2OH

CH3

COOHO

COOH

O

OH

Formation of Soil Organics

There is a tremendous amount not known about this process.

This figure (borrowed from a website) shows 4 proposed routes to humic substances.