ISA Tree Biology and Physiology 3 — Questions and Answers
Question 1: Which wood characteristic is associated with the earlywood formed at the beginning of a growing season?
- Large-diameter, thin-walled cells with wide lumens (Correct answer)
- Small-diameter, thick-walled cells with narrow lumens
- Cells with abundant tyloses and resins
- Dense cells with high lignin content
Correct answer: Large-diameter, thin-walled cells with wide lumens
Earlywood (springwood) is characterized by large-diameter, thin-walled tracheids or vessels that efficiently conduct large volumes of water during peak growing season.
Question 2: What is the primary carbon-fixing reaction in C3 photosynthesis used by most temperate trees?
- CO2 is fixed by PEP carboxylase into a 4-carbon compound
- CO2 is fixed by RuBisCO into a 3-carbon compound (Correct answer)
- CO2 is stored in vacuoles at night and released during the day
- CO2 is fixed exclusively in bundle sheath cells
Correct answer: CO2 is fixed by RuBisCO into a 3-carbon compound
In C3 photosynthesis, RuBisCO fixes CO2 into a 3-carbon compound (3-phosphoglycerate) as the first stable product; this pathway is used by most broadleaf and conifer trees.
Question 3: Which layer of the cell wall is shared between adjacent cells and acts as the cementing layer?
- Primary wall
- Secondary wall
- Middle lamella (Correct answer)
- Tertiary wall
Correct answer: Middle lamella
The middle lamella, composed mainly of calcium pectate, is the outermost layer shared between adjacent cells and holds them together.
Question 4: What phenomenon occurs when a tree's transpiration demand exceeds the soil's ability to supply water, leading to wilting?
- Cavitation cascade
- Permanent wilting point (Correct answer)
- Hydraulic failure
- Osmotic adjustment
Correct answer: Permanent wilting point
The permanent wilting point is the soil moisture level at which the tree can no longer extract sufficient water and wilting becomes irreversible without rewatering.
Question 5: In which tissue are sieve tube elements and companion cells found?
- Primary xylem
- Secondary xylem
- Phloem (Correct answer)
- Pericycle
Correct answer: Phloem
Sieve tube elements and companion cells are the defining cell types of phloem, which conducts photosynthates from source tissues to sink tissues.
Question 6: What is 'girdling' and why is it fatal to trees?
- Root compaction that limits water uptake
- Complete removal of bark around the circumference, cutting off phloem transport (Correct answer)
- Excessive pruning that reduces photosynthetic capacity
- Crown lifting that exposes roots to desiccation
Correct answer: Complete removal of bark around the circumference, cutting off phloem transport
Girdling removes a complete ring of bark, severing the phloem and preventing sugar transport from the canopy to roots, which eventually starves and kills the root system.
Question 7: Which of the following describes the function of mycorrhizal fungi on tree roots?
- They parasitize the root cortex, causing necrosis
- They extend effective root surface area and enhance nutrient uptake (Correct answer)
- They fix atmospheric nitrogen directly for the host tree
- They produce growth regulators that replace root tips
Correct answer: They extend effective root surface area and enhance nutrient uptake
Mycorrhizal fungi form mutualistic associations with tree roots, dramatically extending their effective surface area and improving uptake of water, phosphorus, and other nutrients.
ISA Tree Biology and Physiology 3
Which wood characteristic is associated with the earlywood formed at the beginning of a growing season?