Water movement in plant roots via potential gradients

Steps:

• Recall that water absorption by roots occurs along a water potential (ψ) gradient, from higher ψ in soil to lower ψ in xylem.
• Identify pathways: symplast (cell-to-cell via cytoplasm) and apoplast (cell walls and spaces).
• Evaluate each option against ψ gradient and root anatomy principles.
• Confirm D aligns with water flowing through symplast driven by decreasing ψ.

Why D is correct:

• Water moves passively through the symplast pathway following the water potential gradient, as defined by the equation ψ = ψ_s + ψ_p + ψ_m, where flow occurs from higher to lower ψ.

Why the others are wrong:

• A: Ions enter the symplast before the Casparian strip but must cross it via symplast to enter the stele, not stopping there.
• B: Soil water has less negative ψ (higher potential) than xylem, driving uptake; the reverse is false.
• C: Xylem sap has more negative ψ than soil water due to transpiration pull, so soil ψ is higher, not more negative.

Final answer: D