Define "mass flow" in nutrient uptake?

• A. Movement of nutrients to roots via the transpiration stream with water, especially mobile nutrients
• B. Active transport of nutrients into root cells
• C. Diffusion of nutrients in soil
• D. Absorption of nutrients by leaves

Answer: (A)

Mass flow is the bulk movement of dissolved nutrients toward the roots, carried by the transpiration-driven flow of water. As water is pulled up through the plant, nutrients that are soluble in that water—especially those that move easily in solution like nitrate and potassium—are delivered to the root surface where they can be absorbed. The rate of this delivery depends on how much the plant transpires: more transpiration means more water movement and more nutrients arriving with the flow.

This is different from active transport, which uses energy to move ions into root cells against a gradient, and from diffusion, which relies on concentration differences and occurs mainly over short distances. It also isn’t about absorption by leaves, since nutrient uptake in this context primarily happens at the roots.