Transpiration rate increases in dry air, affecting xylem flow

Steps:

• Identify R as upward xylem sap (water from roots) and S as downward phloem sap (sugars from leaves).
• In humid air, low transpiration limits upward water pull via cohesion-tension.
• Switching to dry air raises evaporation from leaves, increasing transpiration.
• This accelerates upward xylem flow (R) but does not affect phloem loading/unloading for downward flow (S).

Why D is correct:

• Dry air boosts transpiration, greatly increasing upward xylem movement per cohesion-tension theory, while phloem flow remains steady as it depends on source-sink gradients.

Why the others are wrong:

• A: Downward phloem movement unchanged by air humidity.
• B: Upward movement increases, not decreases, due to higher transpiration.
• C: Upward movement greatly increases from enhanced evaporation.

Final answer: D