ISA Tree Biology & Physiology 2 — Questions and Answers

Question 1: What is translocation and how does it differ from transpiration in trees?

Both terms describe water movement from roots to leaves
Translocation is the movement of sugars and nutrients through the phloem; transpiration is the loss of water vapor through stomata that drives xylem water transport (Correct answer)
Translocation occurs only in roots; transpiration only in leaves
They are two names for the same process

Correct answer: Translocation is the movement of sugars and nutrients through the phloem; transpiration is the loss of water vapor through stomata that drives xylem water transport

Translocation moves photosynthetic products (sugars) and mobile nutrients through the phloem from source (leaves) to sink (roots, growing points). Transpiration is the evaporation of water from leaf stomata that creates the tension driving water up through the xylem.

Trees have two separate transport systems that are often confused. Transpiration is the physical process of water evaporating from leaf mesophyll cells through open stomata. This water loss creates negative pressure (tension) in the leaf xylem that pulls water upward from the roots through continuous water columns — the engine of the transpiration stream. Translocation, also called phloem transport, is a biological process driven by the pressure-flow (Munch) mechanism. Sugars produced by photosynthesis in leaves are actively loaded into sieve tube elements of the phloem, creating an osmotic gradient that draws water in and generates positive pressure (turgor). This pressure drives the sugar solution through the phloem to metabolic sinks — growing shoot tips, roots, developing fruits, and storage tissues. Translocation is bidirectional (up or down depending on source and sink locations), while the transpiration stream is unidirectional (upward).

Question 2: What is the role of stomata in tree physiology and what factors control their opening and closing?

Stomata are structural support cells that strengthen leaves against wind
Stomata are pores on leaf surfaces that regulate gas exchange (CO2 in, O2 and water out), controlled primarily by light, CO2 concentration, humidity, and water stress (Correct answer)
Stomata are root structures that absorb water from soil
Stomata only function during winter dormancy

Correct answer: Stomata are pores on leaf surfaces that regulate gas exchange (CO2 in, O2 and water out), controlled primarily by light, CO2 concentration, humidity, and water stress

Stomata are microscopic pores (formed by pairs of guard cells) primarily on leaf undersurfaces. They control the uptake of CO2 for photosynthesis and the loss of water through transpiration, opening and closing in response to environmental signals.

Stomata are the critical interface between a tree and its atmospheric environment. Each stoma consists of two guard cells that change shape to open or close the pore. When open, CO2 diffuses in for photosynthesis, but water vapor escapes (transpiration) — the fundamental dilemma of terrestrial plant life. Guard cells respond to multiple signals: light triggers opening (via blue light receptors and photosynthesis in guard cell chloroplasts); high CO2 inside the leaf promotes closure (sufficient CO2 already available); low humidity and dry wind increase the vapor pressure gradient, accelerating water loss and triggering partial closure; water stress (low leaf water potential) causes ABA hormone production that forces stomatal closure to conserve water, at the cost of reduced photosynthesis. Temperature, air pollutants, and pathogen infection also influence stomatal behavior. Trees regulate thousands to millions of stomata to balance carbon gain against water loss throughout the day.

Question 3: What is the difference between ectomycorrhizae and endomycorrhizae (arbuscular mycorrhizae)?

Ectomycorrhizae are harmful parasites; endomycorrhizae are beneficial symbionts
Ectomycorrhizae form a fungal sheath around roots without penetrating cells; endomycorrhizae penetrate root cell walls to form arbuscules inside cells — both are beneficial (Correct answer)
Ectomycorrhizae only occur on dead trees; endomycorrhizae on living trees
There is no functional difference between the two types

Correct answer: Ectomycorrhizae form a fungal sheath around roots without penetrating cells; endomycorrhizae penetrate root cell walls to form arbuscules inside cells — both are beneficial

Ectomycorrhizae (ECM) form a fungal mantle around root tips and grow between cells (Hartig net) without penetrating cell walls. Endomycorrhizae (AM) penetrate root cell walls to form arbuscules — branched exchange structures inside cells. Both are mutualistic.

These two major mycorrhizal types differ in structure, host range, and ecology. Ectomycorrhizae (ECM) are formed by Basidiomycetes and Ascomycetes (mushroom-forming fungi) primarily with oaks, pines, birches, beeches, and other temperate forest trees. The fungus forms a visible mantle (sheath) around fine root tips and grows between cortical cells (the Hartig net) but does not enter cells. ECM change root tip morphology — they become swollen, shortened, and often branched. Endomycorrhizae, specifically arbuscular mycorrhizae (AM, formerly VAM), are formed by Glomeromycetes with the majority of plant species including many trees (maples, ashes, tulip trees, most tropical trees). AM fungi penetrate root cortical cell walls and form arbuscules — highly branched structures within cells that are the primary sites of nutrient exchange. AM do not change root appearance externally. Both types dramatically enhance phosphorus uptake and provide other benefits including drought resistance and pathogen protection.

Question 4: How does girdling (complete removal of bark around the trunk) kill a tree?

It exposes the heartwood to decay organisms that quickly destroy the trunk
It severs the phloem, preventing sugars produced in leaves from reaching the roots, causing root starvation and eventual death of the entire tree (Correct answer)
It allows too much water to evaporate from the exposed sapwood
It immediately stops water transport from roots to leaves

Correct answer: It severs the phloem, preventing sugars produced in leaves from reaching the roots, causing root starvation and eventual death of the entire tree

Girdling removes the bark and phloem, severing the downward translocation pathway. Roots, deprived of sugars from photosynthesis, eventually starve and die. Without functional roots, the above-ground portions then die. Water transport through the xylem continues temporarily because the xylem (wood) is not removed.

Girdling is lethal because it completely interrupts phloem transport — the conduit for photosynthetic sugars moving from leaves to roots. When the bark (including cambium and phloem) is removed around the full circumference, sugars accumulate above the girdle (causing swelling), but roots receive no new food supply. The xylem (sapwood) is interior to the bark and initially remains intact, so water continues to move upward temporarily. The tree may even produce a normal spring leaf-out using stored energy if girdled in winter. However, as root carbohydrate reserves are depleted (typically over weeks to months), roots begin to die, water uptake fails, and the canopy wilts and dies. This is why animals that strip bark (beavers, porcupines), lawn equipment damage that circles the trunk, and tight guy wires can kill trees — they functionally girdle the phloem. Understanding this explains why protecting the root flare and lower trunk from physical damage is so critical.

Question 5: What is photoinhibition and how does it affect trees adapted to shade?

It is the process by which trees block harmful insects using chemical signals
It is the inhibition or damage to the photosynthetic apparatus caused by excessive light intensity, particularly harmful to shade-adapted species suddenly exposed to full sun (Correct answer)
It is the normal cessation of photosynthesis during winter dormancy
It only occurs during drought stress, not from light alone

Correct answer: It is the inhibition or damage to the photosynthetic apparatus caused by excessive light intensity, particularly harmful to shade-adapted species suddenly exposed to full sun

Photoinhibition occurs when the photosynthetic apparatus receives more light energy than it can process, generating reactive oxygen species that damage chlorophyll and photosystem proteins. Shade-adapted trees are especially vulnerable to sudden exposure to intense light.

Photosynthesis has a maximum rate determined by the capacity of the electron transport chain and Calvin cycle. When light energy absorbed by chlorophyll exceeds what these systems can process, the excess energy produces reactive oxygen species (ROS) — highly destructive molecules that damage chlorophyll, membrane lipids, and photosystem II reaction center proteins (particularly the D1 protein). Sun-adapted leaves have protective mechanisms: carotenoid pigments that dissipate excess energy as heat (xanthophyll cycle), higher concentrations of antioxidant enzymes, and thicker leaves with more chloroplasts. Shade-adapted species have thinner leaves with fewer protective mechanisms, optimized instead for maximum light capture in low-light conditions. When understory trees lose their overhead canopy (due to storm damage, logging, or adjacent construction), the sudden light increase can cause severe photoinhibition — leaf scorching, bleaching, and dieback. Gradual exposure over one or more seasons allows some adaptation.

Question 6: What is the process of abscission and what triggers it in deciduous trees?

Abscission is the growth of new branches in spring, triggered by rising temperatures
Abscission is the controlled shedding of leaves (and sometimes fruits or branches) through formation of a specialized separation layer, triggered by shortening day length and hormonal changes (Correct answer)
Abscission refers to root pruning that occurs naturally during drought
Abscission is the annual shedding of bark in species like sycamore

Correct answer: Abscission is the controlled shedding of leaves (and sometimes fruits or branches) through formation of a specialized separation layer, triggered by shortening day length and hormonal changes

Abscission is the active, genetically controlled process by which trees shed leaves (and other organs). In temperate deciduous trees, it is triggered primarily by shortening day length (photoperiod), which initiates hormonal changes leading to the formation of an abscission zone at the leaf base.

Leaf abscission in deciduous trees is a sophisticated, genetically programmed process. The primary trigger is photoperiod — shortening day length in autumn is detected by phytochrome photoreceptors. This initiates a cascade of hormonal changes: auxin production in the leaf blade decreases, while ethylene production increases. The shift in the auxin-to-ethylene ratio activates genes in the abscission zone — a pre-formed layer of small, thin-walled cells at the base of the petiole. These cells produce enzymes (cellulases and pectinases) that dissolve the middle lamella between cells, weakening the zone. Simultaneously, a protective layer of suberin forms on the stem side to seal the wound after leaf fall. Before abscission, the tree resorbs valuable nutrients (nitrogen, phosphorus, potassium) from the leaves, which is why leaves change color — chlorophyll is broken down, revealing carotenoids (yellows) and anthocyanins (reds) that may protect the leaf's remaining photosynthetic machinery during nutrient recovery.

ISA Tree Biology & Physiology 2

What is translocation and how does it differ from transpiration in trees?