5.4 Tree Defense Responses to Biotic Injury: Constitutive, Induced & Structural
Key Takeaways
- The hypersensitive response is localized programmed cell death that walls off a biotrophic pathogen by killing the host cells around the infection site.
- Tyloses are balloon-like outgrowths of parenchyma cells into vessel lumens; they occlude wide earlywood vessels and are the primary vascular defence of ring-porous species such as oak.
- Constitutive defences are pre-formed and always present, whereas induced defences are synthesized after attack and therefore consume stored carbohydrate.
- CODIT describes the compartmentalization of decay by three pre-existing anatomical walls plus Wall 4, the barrier zone laid down by the cambium after wounding.
- A tree in negative carbon balance cannot fund induced defence, which is why site correction is a defence intervention and not merely a vigour measure.
The third task under Biotic Disorders asks the arborist to evaluate tree responses to different types of biotic injury. This is the biology that explains why a treatment works, why a stressed tree cannot be saved by a product, and why some wounds compartmentalize and others do not.
Constitutive versus Induced Defence
| Class | Timing | Metabolic cost | Examples |
|---|---|---|---|
| Constitutive (pre-formed) | Present before attack | Paid during normal growth | Bark and periderm; cuticle and epicuticular wax; leaf hairs (trichomes); lignified cell walls; pre-formed resin ducts and latex canals; heartwood extractives |
| Induced | Synthesized after detection | Paid from stored non-structural carbohydrate at the moment of attack | Hypersensitive response; tyloses and gums; phenolic and terpenoid accumulation; wound periderm and woundwood; barrier zone (CODIT Wall 4); volatile organic compound signalling |
The exam-critical consequence: induced defence is expensive and is funded from stored starch. A tree in chronic negative carbon balance — compacted rooting volume, repeated defoliation, construction root loss — has depleted reserves and mounts a weak or delayed response. This is the mechanistic bridge between site stress and secondary pest colonization, and it is why correcting the rooting environment is a genuine defence intervention.
The Hypersensitive Response
The hypersensitive response (HR) is rapid, localized programmed cell death at and immediately around the infection site. The host recognizes a pathogen effector, generates reactive oxygen species, and deliberately kills its own cells. Because biotrophic pathogens require living host tissue, killing that tissue starves the invader before it can spread.
- Visible expression: discrete, sharply delimited necrotic flecks or spots, often surrounded by a narrow chlorotic halo.
- The paradox to understand: HR necrosis is not the pathogen destroying the plant; it is the plant successfully containing the pathogen. Small, sharply bounded lesions that stop expanding often indicate a resistant interaction.
- HR is generally ineffective against necrotrophic pathogens, which kill host tissue and feed on the remains — for those, killing host cells helps the pathogen.
Associated systemic responses include the accumulation of pathogenesis-related proteins and a broad, longer-lasting elevated defensive state in uninfected tissue.
Vascular Occlusion: Tyloses, Gums and Resins
When a vascular pathogen or a wound opens the xylem, the tree seals the conduit.
- Tyloses are balloon-like outgrowths of adjacent ray or axial parenchyma cells that push through pit membranes into the vessel lumen and physically plug it. Tylose formation is favoured in wide-diameter vessels, which makes it the dominant occlusion mechanism of ring-porous hardwoods such as oak, elm, ash, and black locust.
- Gums and gels (polysaccharide and phenolic deposits) occlude narrower vessels and dominate in diffuse-porous species and in many conifers.
- Resin and pitch flow is the conifer analogue, both constitutive (pre-formed ducts) and induced (traumatic resin ducts formed after attack).
Why this matters clinically. Occlusion is defensive but hydraulically costly: the tree trades water transport for containment. In oak wilt, the vigorous tylose response of a red oak group member occludes so much of its earlywood that the crown wilts and dies within weeks, whereas white oak group members compartmentalize more effectively and often survive. The same mechanism explains why root graft severance is a decisive oak wilt tactic — it removes the transmission pathway that bypasses the tree's own occlusion.
Structural and Chemical Responses
| Response | Mechanism | Field expression |
|---|---|---|
| Increased lignification | Deposition of lignin in cell walls near the injury | Hardened, brittle tissue at the lesion margin |
| Corky layers / wound periderm | New periderm formed beneath the injury | A raised, sealed edge; separation of dead tissue |
| Abscission zones | Programmed shedding of infested leaves or twigs | Premature leaf drop; self-pruning of infested tips |
| Phenolics and tannins | Antimicrobial and antifeedant secondary metabolites | Dark staining in wood adjacent to wounds |
| Terpenoids and essential oils | Toxic and repellent volatiles | Aromatic tissue; conifer oleoresin |
| Woundwood | Cambium-derived tissue closing over a wound margin | The characteristic rolled ridge around a pruning cut |
| Volatile organic compounds (VOCs) | Airborne signalling from damaged tissue | Attracts parasitoids and predators; primes neighbouring tissue |
VOC signalling deserves particular note because it explains a component of biological control: damaged foliage releases herbivore-induced volatiles that recruit parasitoids and predators of the feeding insect. Conserving those natural enemies — the core of the conservation biological control tactic — depends on that signalling pathway remaining intact.
CODIT as the Response to Wounding
Compartmentalization is the structural expression of all of the above. The four walls, in the order of their strength, are:
| Wall | Anatomical basis | Direction resisted | Relative strength |
|---|---|---|---|
| Wall 1 | Plugging of vertical conduits (tyloses, gums) | Vertical spread up and down the stem | Weakest |
| Wall 2 | Latewood cell walls of the annual ring | Inward spread toward the pith | Moderate |
| Wall 3 | Ray parenchyma | Lateral spread around the stem | Strong |
| Wall 4 | Barrier zone — a distinct layer of cells laid down by the cambium after wounding | Outward spread into wood formed after the injury | Strongest |
Wall 4 is the one to understand precisely. Walls 1–3 are pre-existing anatomy repurposed; Wall 4 is new tissue created in response to the wound. It is the strongest barrier and it is why decay from an old wound rarely enters wood formed after that wound. It is also mechanically the weakest plane in the stem, being a discontinuity, which is why a large old wound can be a separation surface even where the tree has compartmentalized successfully.
Compartmentalization is a boundary-setting process, not healing. Trees do not repair damaged wood; they isolate it and grow new tissue around it. This distinction underlies correct pruning practice: a cut just outside the branch collar preserves the branch protection zone — a pre-formed chemical boundary at the branch base — while a flush cut destroys it and opens the stem to a decay column.
Species Differences in Compartmentalization
Species vary widely in compartmentalization ability, and this drives real decisions:
- Strong compartmentalizers — many oaks in the white oak group, hophornbeam, black locust, some maples — tolerate larger wounds and older wounds without extensive decay columns.
- Weak compartmentalizers — birch, willow, poplar, silver maple, horse chestnut — develop extensive decay from comparatively modest wounds.
A pruning specification for a weak compartmentalizer should minimize both the number and diameter of cuts that expose wood beyond the branch protection zone. The current pruning standard's requirement that cuts exposing heartwood be minimized is the standard's expression of exactly this biology.
Tying It Back to the Diagnosis
When a scenario describes a tree failing to respond — a canker expanding rather than being walled off, a wound not forming woundwood, a decay column extending far beyond the wound — the correct reasoning is not "the pathogen is unusually aggressive." It is: the tree lacked the reserves or the cambial activity to build the boundary. Look for the predisposing site factor.
A pathologist reports that a resistant cultivar responds to inoculation with small, sharply delimited necrotic flecks that stop expanding within days, while the susceptible cultivar develops expanding water-soaked lesions. What does the resistant cultivar's response represent?
Why does a member of the red oak group typically wilt and die within weeks of oak wilt infection while a white oak group member often survives the same pathogen?
Which statement about CODIT Wall 4 is correct?
A consulting arborist observes that a mature horse chestnut on a compacted, root-severed site has failed to form woundwood around a three-year-old wound and that decay now extends well beyond the original injury. Which explanation is most defensible?