Final answer:
The fastest transport through the xylem in a tree is achieved by negative pressure potential in the leaf mesophyll, which pulls water up as part of the cohesion-tension theory.
Step-by-step explanation:
The condition resulting in the fastest transport through the xylem in a tree, assuming an adequate water supply in the roots, is C. negative pressure potential in the leaf mesophyll.
Water transport through the xylem relies on negative pressure potential generated by transpiration in the mesophyll cells of the leaves. This process is a part of the cohesion-tension theory of sap ascent, where water evaporates from these cells, creating a tension that essentially 'pulls' water upwards from the roots.
The negative pressure at the air-water interface in the leaf mesophyll creates the greatest tension to drive water through the xylem, facilitating efficient transport to areas where it is needed for photosynthesis and other processes. In contrast, positive pressure potential often results from osmotic flow into the xylem from the phloem, which is less effective for moving water up the tall structures of trees.