USABO Plant Physiology and Photosynthesis 2

Question 1

What triggers the conversion of the inactive phytochrome form (Pr) to the active form (Pfr)?

Accepted Answer

Red light at 660 nm

Answer

Far-red light at 730 nm

Answer

Blue light at 450 nm

Answer

UV-A light at 370 nm

Question 2

Which plant hormone primarily promotes cell elongation by activating plasma membrane H+-ATPases to loosen cell walls via expansins?

Accepted Answer

Auxin (IAA)

Answer

Cytokinin

Answer

Abscisic acid (ABA)

Answer

Ethylene

Question 3

What is the primary role of abscisic acid (ABA) in plant responses to water deficit?

Accepted Answer

Triggering stomatal closure by inducing K+ efflux from guard cells

Answer

Promoting stomatal opening to maximize CO2 uptake during stress

Answer

Stimulating aquaporin expression to increase root water uptake

Answer

Increasing mesophyll conductance for CO2 diffusion

Question 4

According to the cohesion-tension theory, what property of water is most critical for maintaining continuous water columns in xylem?

Accepted Answer

High cohesion between water molecules via hydrogen bonds

Answer

High specific heat capacity

Answer

Polarity enabling mineral solvation

Answer

Density differences between liquid and vapor phases

Question 5

What is the primary function of the Casparian strip in the root endodermis?

Accepted Answer

Blocking apoplastic water movement and forcing symplastic passage

Answer

Providing structural support to cortical cells during drought

Answer

Facilitating apoplastic flow of water into the xylem

Answer

Actively pumping minerals from the cortex toward the epidermis

Question 6

In photoperiodism, which molecular sensor in leaves measures the length of the critical dark period?

Accepted Answer

Phytochrome, which slowly converts from Pfr to Pr in darkness

Answer

Cryptochrome, which accumulates during long nights

Answer

Stomatal guard cells that track daytime light duration

Answer

Chlorophyll, which degrades during darkness as a timer

Question 7

Gibberellins promote stem elongation primarily by which mechanism?

Accepted Answer

Promoting both cell division and cell elongation by targeting DELLA repressors for degradation

Answer

Activating DELLA protein repressors to sustain growth inhibition

Answer

Inhibiting ABA synthesis to release dormancy constraints

Answer

Stimulating cell elongation exclusively without affecting division