Water Potential Gradients Drive Transpiration

Steps:

• Transpiration involves water evaporation from leaf mesophyll cells into air spaces, driven by water potential gradients.
• Water moves via symplast (cell-to-cell) or apoplast (cell wall) pathways to reach evaporation sites.
• Evaporation from cell surfaces into intercellular spaces creates a steep potential gradient, pulling more water.
• Humid conditions reduce the gradient between leaf air spaces and external air, slowing transpiration.

Why B is correct:

• Evaporation from wet cell surfaces into air spaces lowers water potential there, increasing the gradient that draws water via symplast, per the water potential equation (Ψ = Ψ_s + Ψ_p + Ψ_m), where evaporation reduces Ψ in air spaces.

Why the others are wrong:

• A: Humid days decrease, not increase, the external air-to-mesophyll gradient, reducing transpiration.
• C: Water doesn't diffuse from saturated air spaces through guard cells; guard cells regulate stomatal opening for gas exchange, not direct water diffusion.
• D: Apoplast pathway stops at mesophyll cell walls; water doesn't "continue" directly to cells before evaporation without crossing via osmosis.

Final answer: B