Casparian Strip Bypassed in sgn3 Mutant

Steps:

• Casparian strip blocks apoplast pathway, forcing ammonium ions into symplast via cytoplasm.
• sgn3 mutant allows apoplast entry without crossing casparian strip.
• Bar charts compare NH₄⁺ uptake in non-sgn3 (wild-type) and sgn3 plants at low/high concentrations.
• Similar uptake rates in both mutants indicate casparian strip has minimal impact.

Why D is correct:

• Uptake data shows comparable NH₄⁺ movement into xylem with/without casparian strip, confirming little effect from its loss.

Why the others are wrong:

• A: Data shows similar uptake, not reduced when forced through cytoplasm.
• B: Data shows no increase when bypassing cytoplasm via apoplast.
• C: Similar rates mean casparian strip does not slow movement.

Final answer: D