USABO Plant Physiology and Photosynthesis 2 — Questions and Answers
Question 1: What triggers the conversion of the inactive phytochrome form (Pr) to the active form (Pfr)?
- Far-red light at 730 nm
- Blue light at 450 nm
- Red light at 660 nm (Correct answer)
- UV-A light at 370 nm
Correct answer: Red light at 660 nm
Phytochrome Pr absorbs red light at ~660 nm and undergoes a conformational change to the biologically active Pfr form, promoting germination and photomorphogenic responses.
Question 2: Which plant hormone primarily promotes cell elongation by activating plasma membrane H+-ATPases to loosen cell walls via expansins?
- Cytokinin
- Abscisic acid (ABA)
- Ethylene
- Auxin (IAA) (Correct answer)
Correct answer: Auxin (IAA)
Auxin (IAA) activates H+-ATPases that acidify the apoplast, activating expansins that break hydrogen bonds in the cell wall and allow turgor-driven elongation.
Question 3: What is the primary role of abscisic acid (ABA) in plant responses to water deficit?
- Promoting stomatal opening to maximize CO2 uptake during stress
- Triggering stomatal closure by inducing K+ efflux from guard cells (Correct answer)
- Stimulating aquaporin expression to increase root water uptake
- Increasing mesophyll conductance for CO2 diffusion
Correct answer: Triggering stomatal closure by inducing K+ efflux from guard cells
ABA is synthesized in wilted leaves and signals guard cells to efflux K+ and anions, reducing turgor and causing stomatal closure to limit transpirational water loss.
Question 4: According to the cohesion-tension theory, what property of water is most critical for maintaining continuous water columns in xylem?
- High specific heat capacity
- Polarity enabling mineral solvation
- High cohesion between water molecules via hydrogen bonds (Correct answer)
- Density differences between liquid and vapor phases
Correct answer: High cohesion between water molecules via hydrogen bonds
Water's strong intermolecular hydrogen bonds create cohesion, allowing transpiration at the leaf to pull unbroken water columns up through xylem against gravity.
Question 5: What is the primary function of the Casparian strip in the root endodermis?
- Providing structural support to cortical cells during drought
- Blocking apoplastic water movement and forcing symplastic passage (Correct answer)
- Facilitating apoplastic flow of water into the xylem
- Actively pumping minerals from the cortex toward the epidermis
Correct answer: Blocking apoplastic water movement and forcing symplastic passage
The hydrophobic suberin lamellae of the Casparian strip block the apoplastic pathway, forcing water and ions to cross the endodermal plasma membrane where selective transport can occur.
Question 6: In photoperiodism, which molecular sensor in leaves measures the length of the critical dark period?
- Cryptochrome, which accumulates during long nights
- Stomatal guard cells that track daytime light duration
- Chlorophyll, which degrades during darkness as a timer
- Phytochrome, which slowly converts from Pfr to Pr in darkness (Correct answer)
Correct answer: Phytochrome, which slowly converts from Pfr to Pr in darkness
Phytochrome in leaf tissue slowly reverts from Pfr to Pr during uninterrupted darkness; the length of the dark period (measured by Pfr depletion) determines flowering responses in photoperiodic plants.
Question 7: Gibberellins promote stem elongation primarily by which mechanism?
- Activating DELLA protein repressors to sustain growth inhibition
- Promoting both cell division and cell elongation by targeting DELLA repressors for degradation (Correct answer)
- Inhibiting ABA synthesis to release dormancy constraints
- Stimulating cell elongation exclusively without affecting division
Correct answer: Promoting both cell division and cell elongation by targeting DELLA repressors for degradation
Gibberellins trigger DELLA protein degradation via the GID1-SLY1/GID2 ubiquitin pathway, derepressing growth genes that promote both cell division in intercalary meristems and cell elongation.
USABO Plant Physiology and Photosynthesis 2
What triggers the conversion of the inactive phytochrome form (Pr) to the active form (Pfr)?