ISA - ISA Certified Arborist Tree Biology and Physiology 1 — Questions and Answers
Question 1: In which direction does the phloem primarily transport photosynthates (sugars) within a tree?
- From roots upward to leaves via transpiration pull
- From leaves and other sources to roots and other sinks (Correct answer)
- Exclusively laterally through ray parenchyma cells
- From the pith outward through the bark
Correct answer: From leaves and other sources to roots and other sinks
Phloem transport follows the source-to-sink model: sugars produced by photosynthesis in leaves (sources) are loaded into the phloem and transported to areas of active growth or storage such as roots, developing fruit, and cambium (sinks). This pressure-flow mechanism moves photosynthates bidirectionally along the phloem but always from high-pressure sources to low-pressure sinks.
Question 2: What is the Casparian strip, and what is its primary role in tree root physiology?
- A waxy band in endodermal cells that forces water and ions to pass through cell membranes before entering the vascular cylinder (Correct answer)
- A layer of cork cells that waterproofs the root exterior
- A ring of sclerenchyma fibers that provides structural support to the root
- A region of meristematic cells that produces lateral roots
Correct answer: A waxy band in endodermal cells that forces water and ions to pass through cell membranes before entering the vascular cylinder
The Casparian strip is a band of suberin deposited in the walls of endodermal cells surrounding the root's vascular cylinder. It acts as a selective barrier, preventing apoplastic (cell-wall) flow of water and dissolved ions, forcing them through the symplast (living cell contents). This allows the tree to regulate what enters the xylem and maintain mineral selectivity.
Question 3: What is the term for the model describing how trees limit the spread of internal decay through the formation of four distinct chemical and structural boundaries?
- Wound wood occlusion
- Compartmentalization of Decay in Trees (CODIT) (Correct answer)
- Lignotuber formation
- Tylosis development
Correct answer: Compartmentalization of Decay in Trees (CODIT)
CODIT (Compartmentalization of Decay in Trees), developed by Dr. Alex Shigo, describes four 'walls' trees establish in response to wounding or infection: Wall 1 resists vertical spread (blocking vessels), Wall 2 resists inward spread (latewood rings), Wall 3 resists lateral spread (rays), and Wall 4, the barrier zone formed by the cambium after wounding, is the strongest and most important.
Question 4: Which process causes water droplets to exude from leaf hydathodes at leaf tips and margins, particularly during periods of high soil moisture and low transpiration?
- Dew condensation on the leaf surface
- Guttation driven by root pressure (Correct answer)
- Cavitation within the xylem vessels
- Osmotic leakage from phloem sieve tubes
Correct answer: Guttation driven by root pressure
Guttation occurs when root pressure forces xylem sap up through specialized pores called hydathodes located at leaf tips and margins. It typically happens at night or early morning when stomata are closed and transpiration is minimal but soil moisture is high. Unlike dew, guttation fluid originates inside the plant and may contain dissolved minerals and sugars.
Question 5: What is the primary role of mycorrhizal associations in tree physiology?
- To fix atmospheric nitrogen directly into usable forms for the tree
- To extend the effective absorptive surface area of roots and improve uptake of water and phosphorus in exchange for photosynthates (Correct answer)
- To protect roots from soil-borne pathogens by producing antifungal compounds
- To break down heartwood cellulose, releasing nutrients back into the soil
Correct answer: To extend the effective absorptive surface area of roots and improve uptake of water and phosphorus in exchange for photosynthates
Mycorrhizal fungi colonize tree roots and extend hyphae far into the surrounding soil, dramatically increasing the absorptive surface area available to the tree. In exchange for carbohydrates (photosynthates) from the tree, the fungi improve the tree's uptake of water, phosphorus, and other minerals. Most forest tree species rely on these mutualistic relationships for healthy growth, especially in low-nutrient soils.
Question 6: What initiates the annual formation of a distinct abscission zone in deciduous trees, leading to autumn leaf drop?
- A drop in soil temperature below 10°C triggers ethylene production in roots
- Decreasing photoperiod (shorter days) sensed by phytochrome triggers hormonal changes that form the abscission zone (Correct answer)
- Frost damage to leaf mesophyll cells causes mechanical separation of the leaf
- Depletion of soil nitrogen causes the tree to shed leaves to conserve nutrients
Correct answer: Decreasing photoperiod (shorter days) sensed by phytochrome triggers hormonal changes that form the abscission zone
Deciduous trees detect shortening day length (photoperiod) through the phytochrome pigment system, which triggers a hormonal cascade — primarily a decline in auxin and a rise in ethylene — that promotes the development of a specialized abscission zone at the leaf petiole base. This zone develops weakened cell walls that eventually allow the leaf to detach cleanly, protecting the tree from winter desiccation.
ISA - ISA Certified Arborist Tree Biology and Physiology 1
In which direction does the phloem primarily transport photosynthates (sugars) within a tree?