In an experiment, a radioactive tracer introduced into the leaves of a well-watered plant was detected moving downward through the stem toward the roots. This movement is best explained by

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mass flow of assimilates driven by a turgor pressure gradient in the phloem. The downward movement of organic assimilates (such as sucrose) from leaves (source) to roots (sink) occurs in the phloem and is explained by the mass flow hypothesis: high turgor pressure at the source end (due to active loading of sugars) drives bulk flow of solution toward the lower-pressure sink

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A) transpiration pull acting on water columns in the xylem

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B) mass flow of assimilates driven by a turgor pressure gradient in the phloem

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C) root pressure forcing solutes upward from the roots to the shoots

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D) diffusion of solutes along a concentration gradient in the xylem vessels

In an experiment, a radioactive tracer introduced into the leaves of a well-watered plant was detected moving downward through the stem toward the roots. This movement is best explained by

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